Managed to get this set in the silent auction for $20 :)


The bad news: 19AP4 is toast

The good news: Cabinet has had a decent amateur refinish job and the knobs were sitting inside a coffee cup tucked between the picture tube and chassis.....but that's not all!

After giving the chassis a once over, I'm convinced this was a working set that had a picture tube failure. The recap job here is artistry. Whoever did this job used some funky 2 x 50 uF 500V UK manufactured caps and mounted them neatly in the chassis. Not really any info out there on them, they say FAC/RD37DUK 95-42.

They make everything look so neat, in the midst of what would be long terminal strips of caps mounted on the chassis if I were the one doing the job. Even the disc caps are all new.

Also, there's a funky relay underneath, with one wire labeled COIL. A quick glance shows that the wires go between the power transformer and the twin 5U4s. Could be some type of surge surpression that someone built in, like a soft start circuit?

Anyway, I'd love to shake the hand of the man who did this work.....

:thmbsp: I'm just about 99% positive that you will find that relay is a "field repair" replacement of a Dumont factory supplied item .

I'm less sure as to it's exact functioning , but my guess is that it was to protect the CRT from being phosphor burned in the event of a sweep failure ?

I don't have a schematic to be able to say. I just can't recall ever seeing anything like this in any set I've ever had my hands in.....

The relay was standard fare for Dumont/Crosley sets. 5000 ohm coil.
Usually bad and often just bypassed. 6AL5 activated. I recently replaced
two.

Managed to get this set in the silent auction for $20 :)


The bad news: 19AP4 is toast


.....


WIll it not wake up at all? On say 8-9 volts--will is give decent emission--or is it just "dead"?

The RA-110 manual is on our site.

Those are guitar amplifier filter caps... manufacture stopped around 2001 or so, but they were common in Marshall and other british made tube amps.

So, what is that tube mounted underneath the chassis?

WIll it not wake up at all? On say 8-9 volts--will is give decent emission--or is it just "dead"?

That's the odd thing. Powered it up on my tester, 6.3V, and let it warm up for a minute. Walked away for a sec, heard a quiet SNAP, and the filament went dead :(

Never had that happen before.

The RA-110 manual is on our site.

Thanks Steve, I was actually reviewing the DuMont service notes and find it odd that they claim that the RA-110 and RA-103 chassis are basically the same, especially since the RA-110 has a separate HV chassis. But since I've never had my hands into an RA-103 before, I can't really say.

So, what is that tube mounted underneath the chassis?

It's a 6AL5....

That's the odd thing. Powered it up on my tester, 6.3V, and let it warm up for a minute. Walked away for a sec, heard a quiet SNAP, and the filament went dead :(

Never had that happen before.

Sorry to hear, that is typical heater burn-out for a very gassy tube... likely the 19AP4 was pretty much "up to air" when you got it. How do the getters look?

jr

I have the same set in blonde. I picked mine up on Ebay for $10 and like yours had a dud crt. I picked up a tested like new crt for $75. The set was recapped in the recent past so I tried it and it works well except that it needs a new focus control. The winding of the control is burnt in the "sweet" spot so I cant get the raster as sharp as it could be.

I have a crt if you decide to keep it.

Oh, I'm keeping it, believe me :D

Might have one lined up, but if not I'll definitely let you know :)

Sorry to hear, that is typical heater burn-out for a very gassy tube... likely the 19AP4 was pretty much "up to air" when you got it. How do the getters look?

jr

I didn't notice anything out of the ordinary or remarkable....they weren't white. But then again when I heard the SNAP and the filament went dead....well.....that was about that.

Set's still in the garage at the moment. I hafta take 3 doors and one long stairwell railing to get it in the house :)

i have one , started to recap the chassis and got side tracked on other sets , i hope to finish it soon and see what the end result is
the crt was very weak , i got one from another vk member and put it in , the original one i was going to put on the side in the hopes to rejuvenate or rebuild if there is ever a rebuild place again one day anyway the socket came loose and off and as i was carrying it it slipped , my thumb hit the glass nipple , a nice air in rush sound occurred and that was the end of that

The disc caps were originally paper capacitors. May not run as well. They are in critical circuits, and, may cause problems.
The solenoid powers on B+ when set is nearly warmed up so it won't surge as badly, which, surges can damage parts, and, Dumont knew that. By the way, Most early Crosleys were made by Dumont, or, RCA.
Bill Cahill

So I'm in the final stages of restoration now!

Okay, to be fair, it was mostly restored already. Anywho.....

I have one filter cap to do. It's a double 70 uF @ 175V. Weird thing is that there are no polarities marked on the cap, 4 wires, two of which go to ground.

I would normally assume the negative leads are chassis ground, and the positives go to wherever they go, because it's not labeled as a bias cap in the Sams. But, on the chassis ground side of the caps, it almost looks like a "+" scrawled into the solder. I've never seen a filter cap with NO polarity markings on it whatsoever, and 4 wires coming out of the cap, two on each end.

That said...what way does the replacement go in? :dunno:

If I am looking at the correct schematic (Sams 93-4, covers RA-103D/RA-104A/RA-110A), it shows that the positive leads of both 70-mfd caps (C9A and C9B) go to ground. The negative leads go on each side of a choke-resistor combination (L2 and R135).

Phil Nelson

Thanks for double checking me. It confused me because I was under the assumption that only bias caps have a positive ground, and these weren't labeled as bias caps. Right about here is where the gaps in my knowledge become chasms....

That's a weird looking electrolytic. I don't recall seeing one with a pair of leads at both ends like that.

Phil Nelson

Sure it's not on the yoke circuit? Or, just a weird bias?
The can likely isn't part of the electrical circuitry.
Trace your wires.
Bill Cahill

The can likely isn't part of the electrical circuitry

Splain.

Simple.. Electircally, the two capacitors are not connected to the can.
What colors are the wires? Where do they connect?
Bill Cahill

Connected to what can? The two capacitors ARE that yellow cap, one on each end believe it or not. Picture sawing it in half and each half being a stand alone filter cap. Weird as it is, that's what it seems to be.

The thing it's encased in is a U-channel that sticks up on the chassis and holds two 6X4 tubes horizontally. Super weird design for sure.

...I've never seen a filter cap with NO polarity markings on it whatsoever, and 4 wires coming out of the cap, two on each end.

I've seen double ended, cardboard-sleeved filter caps before. 'Cept the leads coming out each end were red & black, clearly indicating polarity.
They were used in Sears 17" portables, in the doubler circuit with seleniums.
The first time I ever replaced one, I (mistakenly) thought the red lead coming out one end went with the black lead coming out the other end:eek:, and proceeded to wire it accordingly. The dang set came on and worked, but with humungous hum bars and hourglassing.
I quickly corrected the goof.

Here's a pic of the one end of the cap. I shouldn't say that it's *not* marked for polarity, because it clearly *is*, but it's not so clear. I had to take this picture 5 times and then enhance it to try to get it so that you could see. There's just a "+" carved in the solder, and it's so light that it's hard to tell it's even there. It's not even really visible on the other end of the cap. Other than the values stamped into the side of the cap, it's otherwise unmarked.

Second pic is the new caps installed where the old one was.

Chassis is in the cabinet and I'm just a 5V4 away from first power up. Ordered one up on ebay :)

Swapped the 19AP4 out of the Emerson I just got. I got a tip to lay the cabinet down on its face to install the tube, and that is dead on. Any other way risks breaking the picture tube.

The anode is securely on, but on the other side is this ground strap. Where on the chassis should this be making contact?

on my 109A set and another its position is down and the lower bend part or "J" should touch the chassis when you slide the chassis forward, it does not screw to the chassis but just touches it to ground, it looks like it is on backwards so you will need to flip it over and down

First power up. Not perfect, but it focuses in and has great sound. Inadequate width, not very bright, and the picture's not lined up, but it's something :)

Video: https://www.youtube.com/watch?v=Hk2SEgKyDu8&feature=youtu.be

Weird how it's just rolled up on the left...

https://www.youtube.com/watch?v=OELi3jK3RyE&feature=youtu.be

did you replace all of the paper caps yet?
there are a few in the Horiz Sweep area that when bad/leaking will cause this, and also a weak damper tube 5v4 would cause this, if the Horiz drive is turned up too high will cause this also
I would start first making sure all the caps in the Horiz osc/output/flyback are new, there is a resistor r122 and r120 that will change value, check those
But the Capacitors c101,102,103,106,105,107 is very important

All of the paper caps have been done. This set was totally restored when I got it, except for the one filter cap mentioned above. Even the disc caps are new...picture tube was just shot.

I agree, all signs kind of point to the damper circuit especially with the dim raster, but the 5V4 is new, just got it in from ebay (it was the only tube I didn't have). Tested strong before putting it in the set.

I've obviously got multiple issues here....it's got extreme foldover in the horizontal right in the middle of the screen and lacks width. Width control does nothing to improve it, so I'm inclined to think that I've got a bad horizontal output transformer. Gonna have to pull the chassis and ohm that out.

The horizontal drive was maxed out, but even bringing it all the way back to halfway caused no change, and with width also causing no change, that also points me to the horiz output transformer. Sound right?

I have a known good Flyback and Yoke from the 12Qp4 version of that set, but the 19" version has a beefier Flyback part numbers are different.
one of those chassis I had one of those Riveted Wirewound Resistors was bad, but I can't remember which exact one
of Course you know to be careful when measuring Pulsed Voltages around the Sweep area so you don't Damage your Meter

Well, one of those is for the focus coil, and the set does focus, so I can assume that's good. Other is a bleed resistor. Probably will end up having to check them anyway.

Unfortunately the only voltage test adapter I have is for a 7 pin miniature, so that's going to limit me in testing chassis voltages, and I'm sure as hell not doing it from underneath ;)

Maybe it's time to finally buy an octal one. Never really needed one up till now.

Having never seen horizontal roll up before, I went to the shelf and grabbed my trusty GE Tele-Clues binder. Well son of a gun.

Tossed the 5V4 on the tester, and it was perfect, so I got to thinking of what ELSE might cause the damper tube to be out of circuit. Peered underneath, and why HELLO Mr. Capacitor!!! Apparently something came loose from the previous tech that did the work! Yay! :D

Problem solved, right? Nope.

Clip leaded the cap across what I figured was a very obvious connection as seen in the pic. Powered it up and heard a SNAP. Shut it down. Grr.

The one end of the cap is definitely at pin 3 of the octal socket that goes to the HV chassis. It's a .1 uF. Have to do some tracing to figure out what the devil is going on.....

Definitely have the right attachment point according to the Sams. But, found a half dozen OTHER loose solder joints, some barely touching. I swear I gave the whole chassis the once over too, before installing it.

Welp, I have something to do tonight :)

I really like seeing yet another DuMont coming back to life. I love the way they were built.

C105 and 106 have a 1000 volt rating in the Damper circuit, the one is your pic looks way underrated, the snap you heard could have been one of those

That's actually C103. But as far as the SNAP goes, my God....

I stopped counting at 12 badly soldered connections. And I KNOW I checked them before powering up, but maybe not well enough. This wasn't my recap job, so I'm also in the habit of giving each cap a good tug at each end before moving on to the next.

The cap connected to the focus pot was TOTALLY disconnected. What a mess, everything was falling apart. I'm just scratching my head as to HOW. I don't end up buying a recapped set very often, so I guess as long as it's fixed, that's what matters.

Well, we'll give her another go and see what happens....

Powered up. Lost audio, raster, and the tuning eye. Fuses are both good, all tube filaments are lighting, including the picture tube. Dammit.

I'll admit that I'm at a loss as to where to even start from here.

1) Get a bright light and look for a solder bridge or burned through insulation during the resoldering.

2) Measure B+.

3) If no luck, download the Sams on ETF and check socket resistance measurements.

Jas.

Kind of what I had in mind. I was hoping to find a common component for an easy fix, but I should know by now that never happens :)

My assumption that the snap I heard was coming from the loose end of the cap that should have been going to the focus pot with voltage finding its way to chassis ground, though it wasn't grounded out.

Time to start from scratch.

OK
Calm down
Drink a beer or 5
Have Volt/Ohmmeter probe in hand
check all Wire wounds, check Voltages on power supply Caps for a Start
Have another Beer
Visually Check the Caps in the Damper and audio output to make sure they are correct Values and VOLTAGE Ratings
Also Remember that soft Start Circuit with Relay does it operate? I just sent someone here a good one

I was thinking about Candohm resistors. Does the 110 use them? They were all bad in my RA-113.

Well, when I say "start from scratch", I don't say that in a panicked way, just basically means that I pull the chassis and start testing some components. First on my list are the candohms. I replaced them all in my RA-113 as well. Even if they're not bad initially, I've had them fail or drift badly after time.

Still want to shake this guy's hand?:D

Sure, with a charged capacitor tucked neatly between my fingers :D

I know what happened here. DuMont caps are the gooeyest things in the world, and my bet is that he didn't realize that the goo on the wires isn't exactly something that promotes solder adhesion. The filter cap connections are fine, along with anything that had a wire connection. When reconnecting everything, I just sanded the old chassis wires with some folded 400 grit paper pulling it forward over the wire, and used some flux. All of the connections are nice and solid now.

With the BANG I heard, I would have guessed that the result would be obvious, but it's not.....at least not to the extent that I've examined it. The chassis is on the bench now, but attending a wedding and getting the old van ready to pass state inspection took precedence this weekend :)

Well, made *some* progress......

It missed my attention the first time around, but C3A was somehow disconnected. The previous tech used these funky bits of waferboard with copper coating on either side to turn the old positive terminals on caps into mounting points. Not exactly what I would do, but it is what it is.

I saw this pair of filters sitting near the terminal, and they didn't LOOK as if they were disconnected, but apparently they were. They're each 1000 uF 50V. What made me scratch my head is the values on them. I traced out the entire circuit and decided that it must be the replacements for the values that are stated on the Sams, which I can't make heads or tails of. See for yourself.

Anyway, it's connected now and no change in the set. Onward to the candohms.

You can plug those numbers from Sams into this reactance calculator.

http://www.sengpielaudio.com/calculator-XLC.htm

1.5 ohms @ 120 Hz is about 884 uF which you could round up to a standard value of 1,000 uF. They are used as filter caps on the horizontal and vertical centering controls.

Checked all the candohms and larger resistors (wirewounds). All in spec.

Brought me to thinking about the power relay under the chassis.

When I was cleaning up the mess of connections I discovered, there were 2 terminals on the relay that were bridged with a couple pieces of wire from the multistrand wire going to the relay. Thinking that it was just a sloppy job, I separated the wires, but now I find myself wondering if that was what caused the set to actually work in the first place.

One of the wires that was connected goes to pin 8 of the 5U4s, the others I can't see where they go, but I assume that those two complete that particular circuit. From what I can find, the relay should show 5000 ohms across the coil, I have 3800.

If this relay were dead, and with those strands of wire connected, could this have caused the set to work before, and not now?

Those caps still look under rated to me. Anything less than 1KV is trouble waiting to happen. One may have blown.
Further, I doubt very much it's the flyback.
Is there a high voltage paper cap across the caps of hv rectifiers? If so, replace. I believe they are rated at 10KV.
Bill Cahill

One of the wires that was connected goes to pin 8 of the 5U4s, the others I can't see where they go, but I assume that those two complete that particular circuit. From what I can find, the relay should show 5000 ohms across the coil, I have 3800.

If this relay were dead, and with those strands of wire connected, could this have caused the set to work before, and not now?

Makes sense, if the relay or driver circuit were bad, the connection might have been bridged on purpose to provide B+ to the set.

jr

Yep, that was it :)

Had the chassis on the bench, so on my lunch break I jumpered the connection, and used the variac to power it up.

It's sure an improvement from where I was when I had a picture last, but it's still not right. Contrast control does nothing, and neither does width. I can get it wider, but then I have to max out the horizontal drive and the whole picture darkens up in the center. Focuses fine. Not very bright, but I did notice that the picture is getting better just with tweaking and use, as everything is beginning to "wake up".

Getting there, getting there :) . Think it's time to re-test tubes before I go further. One candohm was working but was out of spec, so I'll place that on my next Mouser order.

. . . . width. I can get it wider, but then I have to max out the horizontal drive and the whole picture darkens up in the center. . . . .

After tubes, the first thing I'd check would be the cathode resister on the horizontal output tube.

If your other checks do not find the problem. . . .

I notice that the Sam's that you are using lumps the 12 inch RA-103 with the 19 inch RA-110 chassis. I see that Rider keeps them separate.

I have no idea what Du Mont changed (other than the yoke) for the bigger CRT, but there is a slight chance that the person who recapped your set used the more readily available Sam's instead of either Du Mont or Rider diagrams and accidentally created an image size problem.

Jas.

This is the relay that was retrofitted under the chassis. Look like it should work?

http://www.digikey.com/product-detail/en/K10P-11A15-120/PB321-ND/254538?WT.srch=1&WT.medium=cpc&WT.mc_id=IQ66882655-VQ2-g-VQ6-45013740195-VQ15-1t1-VQ16-c

This is the relay that was retrofitted under the chassis. Look like it should work?

The P&B relay is within specs on amperage, under specs on voltage, but probably is OK. (Switching 440 + DC with a 120V RMS AC rating.)

Apparently Du Mont did not publish the pull-in current requirement :no: and that makes it impossible to answer without a test.

What should be a safe way to check the relay is to remove both 5U4's but leave the two 6X4 bias rectifiers in. Unsolder both switched leads. * (The lead going to the 5U4 will have 440 AC on it, so cap it.)

Put a battery and same voltage test lamp in a loop circuit with the switched relay contacts. Power the set up, still with 5U4's out, and remove and replace the 6AL5 power delay tube. The relay should click with each operation, possibly with a slight delay. Test lamp should be on with tube in, and out with tube out.

If relay clicks, but does not light the lamp, the relay is bad. If relay does not click, the trouble probably is in the switching circuit, but there is also a slight chance it could be a bad relay, or the relay coil pull-in rating is incorrect.

With that said, I've never done a relay test on a Du Mont, but have used similar methods on other industrial equipment when I did not have component information or a spare relay. If anyone sees anything I just wrote potentially harming the set, please holler now!

Jas.

* Oops! Clearly an oversight above, hope there are no more.
Before someone points it out, with the 5U4's removed, the only voltage on the socket filament pins is 5 V AC since the 440 AC is on the plates. I would still cap the hot wire before running the test.

Was suggested by a member in PM a while ago (honestly don't recall who) that suggested that I look at the yoke for the problem. Bought a new yoke for the set, and it's sat since about 2 weeks after my last post on this.

Now that I've got a yoke tester and have learned how to use it (very easy BTW), I tested both the new yoke and the old. Looks like that's (at least some of) the issue.

Here's where I'm at. Aftermarket yokes always make me scratch my head, but with the supplied diagrams usually I can figure it out. This one's confusing me a bit.

Below are the pics of both the new and the old yokes. The old yoke has colors red, red/black, green, and tan/black. The new yoke's wiring is green, green/black, tan, and tan/black.

As well the new yoke has caps in it while the old doesn't. Resistors are the same.

Appreciate a little advice :)

Looks like the green are your vertical and the red (faded to tan*) are your horizontal.
I'd try it with the caps first and snip them if they are causing any issues. On yoke swaps I usually do the first power up with clip leads so I can quick turn it off and swap them if I got a winding connected backwards.

*I recently repaired a 50's amp for a guy and one of the tan wires had a stripe of black letters running along it that consisted of the word RED repeated over and over...I had my self a good laugh when I noticed it.

Probably best to follow the wires to the actual coils to identify H vs V. The H coils consist of two closed loops, one above and one below the neck. The V coils are on the sides. If the picture ends up upside down or left-right mirrored, all you have to do is swap the leads of the particular deflection coil that is producing the problem.

(Edit: I didn't see Tom C's post before I replied - there is some duplication in our responses.)

The good news: Looks like I got it right!

The bad news: HV faded out. Stops at the top of the 1B3. Next is a doorknob cap and a resistor, so one of those is shot.

Not giving up yet :)

Is the 1B3 filament glowing?

I'm pretty certain it was, actually.

i would try the doorknob or doorknobs if it has more then one , that is if your not too buried in the snow, and you do now have a fly tester , simple to use , remove these tubes , the horz out , the hv rect , the damper , one wire goes to the plate cap wire of the horz out and the other to the plate cap wire of the hv rect , before connecting the wires you turn it on , set it to shorts , adjust the needle to the iron core line on the meter , hook wires up to the plate cap wires without touching and you will get your results , the fly will probably sing too.

Flyback was buzzing ever so slightly :)

Passed the continuity test easy, but the shorts test is not looking so good :sigh:

And I did disconnect half of the width coil as per the tester instructions.

Kinda crispy looking too isn't it?

Yeah, kinda :(

Why is it that sets I fix for other people are easy jobs, and MINE are SUCH a pain in the ass? :tongue:

At least you don't gotta tell folks that X$ of work and caps into it you found some blown part that could be nearly impossible to find...I dread that possibility; which is why I rarely work on other peoples stuff.

And along those lines, neither Stancor nor Thordarson list a flyback for this set.

Welp, looks like I'm gonna pull the picture tube and put it back in the Emerson from which I stole it, and restore that one. I'll stick this one aside until a parts set happens to become available.

Merit lists HVO-90 as the replacement. Possibly one will turn up.

I haven't seen anyone ask this. I generally test the transformer in service by removing the 1b3 cap and see if I can draw an arc. I would take a pointed screwdriver and connect one end of a clip lead to the screwdriver and the other end to ground. If I can pull a 1/2" to 1" arc, I am satisfied the flyback transformer is okay.

I saved myself looking for a transformer on my old RCA 8TC271. The doorknob capacitor had shorted. I did the same later on my Zenith color TV to find the arc was healthy and the rectifier tube was bad. This was only after I had moved and acquired the spare transformer!

I haven't seen anyone ask this. I generally test the transformer in service by removing the 1b3 cap and see if I can draw an arc. I would take a pointed screwdriver and connect one end of a clip lead to the screwdriver and the other end to ground. If I can pull a 1/2" to 1" arc, I am satisfied the flyback transformer is okay.

Wow. Are you serious about connecting it to ground?
The only way i've ever done it or seen it done is to draw an "air arc" with an ungrounded but well insulated screwdriver.
The air arc is just about as long anyway since it's RF. And it doesn't risk overloading the xfmr as a straight-to-ground current path might.

Tim
that reading isn't the worst , if you lay the fly tester on its back does it show a different reading ? if the meter went all the way down it would be bad , i've seen good flys read what you are getting , i thought i had a bad fly once because of a reading like that and went through the headache of replacing the fly and it had the same reading , the problem was the doorknob
mike

Wow. Are you serious about connecting it to ground?
The only way i've ever done it or seen it done is to draw an "air arc" with an ungrounded but well insulated screwdriver.
The air arc is just about as long anyway since it's RF. And it doesn't risk overloading the xfmr as a straight-to-ground current path might.

Yes, I'll ground the screwdriver. I just make sure I do not ground the HV Plate lead.

I just want to ensure the current goes to ground and not through me! One should be able to draw an arc when the tip is about 1/2" away. This is a quick test and should harm the circuit if a tube is used

Evidently it doesn't hurt anything. Guess it's just the thought of arcing it straight to ground gives me the willyjeebers.
:yikes:

I test HV with a neon bulb. I was getting HV (although not as much as I'd expect) all the way to the cap of the 1B3, then nothing after. The resistor tests good. Maybe I'll replace the doorknob before I pronounce the flyback dead. Where do I source those, anyway? Or what style do I use? Never replaced a doorknob before.

Without a HV meter, the neon bulb is the best way to check HV, They glow with anything
above 50V, and if you do some experimenting with the bulb and a working set, then you
have an idea how close you have to be to a HV source for it to glow.... A working HV
source will set it glowing X inches from the 2nd anode wire, x inches from the transformer,
x inches from the Horiz plate cap.... It was the first device we in tv repair school made,
and learned to use to measure HV at several spots in a tv.

Electricity will only travel from a positive - negative potential to the oppisite potential..
Same for an arc. If you don't ground the screw driver, then the next lowest potential
is through all high resistance paths to ground.... Like you and the handle.....

Like lightning, very high potential will overcome the high resistance of distance and
the poor conducting properties of air...

The neon tester we made in schooll was to test electrical outlets, It was made out
of a piece of house wire, a resistor, and the neon bulb, which was tucked into a piece
of the wire's insulation... Pretty neat little thing. So we held it by the wire to test for
HV, kinda held it backwards... But you have to be aware that the magnetic field, or
electrostatic field will be making the light glow, and inducing a voltage into the wire
as well.... So if you are not careful you will get a taste of it.... Or get too close and
get a spark to jump and get a bigger shock....

.

Grounding the screwdriver when testing for HV at a rectifier tube plate is definitely a BAD idea.

In most sets, the voltage at the plate of the HV rectifier has a DC component of a few hundred volts, in addition to the high voltage AC pulses. Providing a DC path to ground will take the B+ (from the HOT tube plate), and short it to ground through the HV winding on the flyback (200-300 ohms?) and the (low) resistance of the arc itself. This can easily burn the flyback winding open, as it is not designed to have any appreciable DC current running through it, and it is wound with very fine wire.

A grounded screwdriver should only be used to (briefly) check for DC high voltage at the CRT anode lead, and then only if you don't have access to a real HV meter or probe.

Grounding the screwdriver when testing for HV at a rectifier tube plate is definitely a BAD idea.

In most sets, the voltage at the plate of the HV rectifier has a DC component of a few hundred volts, in addition to the high voltage AC pulses. Providing a DC path to ground will take the B+ (from the HOT tube plate), and short it to ground through the HV winding on the flyback (200-300 ohms?) and the (low) resistance of the arc itself. This can easily burn the flyback winding open, as it is not designed to have any appreciable DC current running through it, and it is wound with very fine wire.

A grounded screwdriver should only be used to (briefly) check for DC high voltage at the CRT anode lead, and then only if you don't have access to a real HV meter or probe.

The original statement was letting the hv arc to a screwdriver to ground....
No direct contact made.... Just a quick arc, no 1/2 hour welding job, popping
pop corn, roasting a turkey stuff like that..... One quick Zap! Yup there is HV, move on....

.

The original statement was letting the hv arc to a screwdriver to ground....
No direct contact made.... Just a quick arc, no 1/2 hour welding job, popping
pop corn, roasting a turkey stuff like that..... One quick Zap! Yup there is HV, move on....

.

Understood.

But why take the risk, even briefly, when there is no need for the screwdriver to be grounded to make that test?

High voltage RF will arc to an ungrounded metal object just fine, without the danger of the DC following the arc path to ground and overloading the transformer winding.

I've never had the guts to play the arc-cap-to-screwdriver game. Neon bulbs are cheap.

Yep, I quit playing the "arc-cap-to-screwdriver game" 35 years ago, when I got zapped good fashion, in a Customer's home, from a 23" Philco tube color set, in a metal cabinet! I'll never forget lessons learned from that one:)!

Understood.

But why take the risk, even briefly, when there is no need for the screwdriver to be grounded to make that test?

High voltage RF will arc to an ungrounded metal object just fine, without the danger of the DC following the arc path to ground and overloading the transformer winding.
This. Thank you Sir. :rockon:

Never had a problem with RF. The anode lead, on the other hand will usually whack you pretty solid. For that, I use a high voltage probe.

Tim
not sure where to get a doorknob , i did buy some 15kv 470pf caps from mouser that will sub , you could always pull one from another set that you know is good just to test , or if you disconnect the one in the dumont and bring the set up on the variac and try to feed the set with 100 to 105 volts ac from the variacit should be safe and you may get an answer if the doorknob is at fault , had one take out hv in a rca , a dumont that uses three doorknobs had one that was bad and it caused all kinds of issues with not enough hv and a dim image............ i just noticed you still have the horz output tube in the socket when testing the fly , your supposed to remove it and the hv retifier and the damper , but the damper probably isn't in that hv box , i have the same set and have yet to restore it
mike

Maybe I'll replace the doorknob before I pronounce the flyback dead. Where do I source those, anyway? Or what style do I use? Never replaced a doorknob before.I can only recall replacing one doorknob. Anyhow, Surplus Sales of Nebraska has some. You might need to adapt the mounting a little -- it has been a while.

http://www.surplussales.com/Capacitors/Trans_Coup_Caps/cap_trans.html

(Hey, I didn't say they were cheap :) )

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

There's always a bunch on ebay.

http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR12.TRC2.A0.H0.Xdoork nob+capacitor&_nkw=doorknob+capacitor&_sacat=0

Replaced the doorknob, same result. Since my flyback tester told me that the flyback was shorted, and since everything else has been replaced, I'm going to stick with that diagnosis.

And now, the search begins.....

Replaced the doorknob, same result. Since my flyback tester told me that the flyback was shorted, and since everything else has been replaced, I'm going to stick with that diagnosis.

And now, the search begins.....

Was your flyback tester a ringer that can detect shorted windings more accurately than a DC resistance reading on an ohmmeter would?

Yup, it's an Eico 944 flyback/yoke tester.....

Just for a bit of easy help, Ill take a resistance reading on the DuMont Savoy chassis I have. And post it here as an edit.
I have a Heathkit Flyback-yoke-HV probe meter that is similar in that it has 6 selectable ringwaves to test with.

Tim:

I have an RA-110 that is not yet restored and I pulled the HV cage and measured the fly. Mine looks toasty like yours and measures nearly the same as yours on the EICO 944 shorts test. I guess either they are both bad or both good. Resistance measurements for the primary winding are: 1 <84 ohms> 2 <286 ohms> 3. Secondary is 7.3 ohms. If the schematic is correct for the 110, then the primary on mine definitely has shorted turns from 1 to 2. It could also be that the resistance measurements are only for the 103 fly. When Dave posts his readings we will have more data.

I also pulled Rider 3 which has one page on the 110 and have attached a photo, including the note regarding the RA-110 flyback difference from the RA-103. The 103 uses the same fly as used in the early 630 type sets and clones. Thordarson FLY-1, Merit HVO-3, Stancor 8127, Triad D-11, etc. As a test you could probably sub one of those in and see what it does.