Capehart Troubles...

[jpdylon]

My first powerup after the recap revealed a hum in the sound, and no raster. HO tube was dead, so I got another.

Second powerup reveals hum in sound, no raster, but I can hear singing, so at least the oscillator is working....


and then...

it all stops working, leaving nothing but the tubes lit. I power down, check the underside for failed components, overheated parts, etc. All fuses intact. LV rectifier checks good on my tester.

Power back up, and the 5u4 lights up in a shower of sparks for a second. Pulled rectifier and replaced it. Powered up again. Same dead set symptom with tubes lit, but the rectifier is making this buzzing rattle, like internal elements are loose or something. Again, no fuses are blown, transformer is cool, and no excessive current draw from the variac.

WTF happened? I was getting so close to first light...now nothing. I hope the transformer didn't crap out

Right now its down in the mid 30s and my workshop has little to no heat. When I do get a chance to get back out there to tinker, where should I start looking to resolve this problem?

[N9ZQA]

Jordan -

Heater winding for the 5U4 shorted to transformer case or other windings?

I would remove the rectifier from the socket and check the voltage(s) from socket pins 4 and 6 to the center tap of the transformer. Don't know what they should be on that particular set (maybe in the neighborhood of 400-450 VAC) but they should be close to each other.

Another thing you could try - check with an ohmmeter between B+ and center tap to see if anything is pulling the B+ down.

Hope this gives you somewhere to start - I know how much fun it is working on sets in unheated spaces...

Jim

[jpdylon]

THanks for the advice Jim. I'll have to pull the scat and check all the transformer windings and voltages.

[jpdylon]

The problem was a cruddy rectifier socket. Some deoxit and pipe cleaners solved that problem. Now I've got good B+, sound, and vertical sweep, but still no raster.

Damper HO tube and HV rectifier are all new. HO cathode fuse is not blown. I tried adjusting the horizontal drive slightly, but it has no effect. Don't want to adjust it too far, as I believe it controls bias for the HO?

any thoughts?

[N9ZQA]

Well, I'm no expert at these things, seeing as how a defective damper tube led me on a wild goose chase through the innards of the Admiral in a recent thread, but here goes:

1. Do you have drive at the grid of the horizontal output tube? You can check with a meter or a scope; you should see maybe -10 to -20 volts at the grid, or a somewhat sawtooth/square wave combination on the scope.

2. Check the plate cap of the HO tube by holding a neon test light close to the insulated plate lead. If it lights, AC is being produced at the flyback primary.

3. Do you have any cathode current on the HO tube?

Hope this helps; maybe someone else has some ideas.

Jim

[Chad Hauris]

Also, if the neon test lamp does not indicate HV when held near the plate lead, check whether you have B+ through the flyback: Remove the plate cap from the horizontal output tube and connect a voltmeter to the disconnected plate lead. If there is B+ the damper and flyback primary winding is working.

[jpdylon]

damper circuits are working, and the fly is conducting. I get 265vdc at pin 5 of the 6w4 and 249 at pin 3 --just about spot on with the test points in the sams. I also get 265vdc at the disconnected HO plate connector.

Next time I get a break I'll look at the HO circuits.

[kbmuri]

Jordan -

It looks like excellent work so far!

I like your idea of mounting a terminal strip inside the chassis, then using it as a common for all your under-chassis caps. A neat arrangement and excellent physical mounting stability. If you recall, I chose exactly the same spot for my electrolytics, but mounted them tranversely in gobs of electrical tape, and tied all the negs together and soldered the common bunch of leads to the voltage-divider-resistor lug, using the lug as support. It's solid enough, but not great. Your solution is superior.

As far as the horizontal-drive control setting, yeah, cranking it all the way up might be a little hard on the 6BG6, but probably not catastrophically so. Turn it all the way down and you should have nothing at the plate of V20. As you turn the control up, there'll be a go/nogo point at which V20 starts conducting. Just leave it there, that should be enough to give you a bright screen. The only penalty I'm aware of for having it too high is a series of one or more "drive lines" will appear vertically on the left half of the picture, which is annoying but not dangerous, and at this point since you still have no picture, unnoticeable as well. Wouldn't hurt to turn it all the way up, just once, to see if there's any effect on the CRT screen.

It would appear from your photos that you do have a different tuner than the rest of the CX-33's on AK. At least the metal cover hints in that direction. Compare it to retrohacker's photo here:
http://audiokarma.org/forums/attachm...8&d=1146310262

and mine:
http://audiokarma.org/forums/attachm...6&d=1147575217

and notice the big brown bakelite drum turret with contact points distributed about the curcumference. I've never seen this drum turret covered by a metal plate before, either in actual TV pics or in service documents. I think if you pull the metal cover on yours, you'll see a bunch of parallel wafers instead of the brown drum. Page 7 of your Sams 112 vs. Page 10 of your Sams 160, to be more precise.

If you can hear the 15.75 Khz "singing" of the horizontal oscillator, one would think you'd have raster. At this point I might urge you to consider the 16RP4 on eBay (http://cgi.ebay.com/ws/eBayISAPI.dll...=190073211353). They don't show up often, new. Sylvania 85's are great tubes, and given it's in the original carton, it's packed in such a way that it won't bounce around. If you insist on having it double-boxed (which he ought to do anyway), I would expect it to arrive safely. I've gotten a few that way, shipped cross-country, in good shape. Given that you've already measured yours as very weak anyway, you'll want a fresh one at some point. Might as well be now, and you can rule out (probably) the pic tube you have.

But first, do some voltage checks of pin 2 of the CRT while adjusting the brightness control. It ought to go both positive and negative, with respect to ground, depending on where the pot is set -- around -80V at minimum, down to zero and then a little positive, maybe as high as +30V. Depending on how well toleranced R43 and R2 are. If not, your brightness pot or R43 could be bad, leaving you with a black screen, despite having plenty of HV.

Since you've already had one problem with dirty tube sockets, it wouldn't hurt to rock each of the tubes in the video strip around, just in case there's a similar problem there. If you're comfortable with doing this while power is on, you ought to see some flashes or sparkles on screen when you rock the tubes, assuming the CRT isn't shot, but be very careful reaching in there to rock the tubes. Wear gloves and all that.

You're definitely on track, and you should be enjoying your vintage Capehart any time now. As long as you're having fun, go for it!

- Kirk

[kbmuri]

Make sure you've got 400V at pin 10 of the CRT as well. It ought to vary a little with the width control too, 400 to 440 according to Sams. -k

[jpdylon]

Thanks for the advice Kirk. I'll post my findings when i can get to the capehart again. get to help a friend with a porthole tomorrow

[jpdylon]

here's what i have run into

pin 8
sams says 170vdc --I have 190v

Pin 5
sams says -70vdc --i have fluctuating voltage between -1 and -5

Pin 4
sams says -70vdc --i have 0v

pin 3
sams says -65vdc --i have 0v

Drive voltage at pot is 273v


R96 has jumped from 47 to 106 ohms --that's probably one fault. Pin 5 side is -2 volts, other side of R96 is -91 volts

Still can't figure out why I'm getting 0v at pin 3.

[wa2ise]

Originally Posted by jpdylon

here's what i have run into

pin 8
sams says 170vdc --I have 190v

Pin 5
sams says -70vdc --i have fluctuating voltage between -1 and -5

Pin 4
sams says -70vdc --i have 0v

That looks suspicous, maybe a heater-cathode short either inside the tube, or in the tube socket? With the power off, take an ohmmeter measurement between the cathode pin and the chassis ground. Also check resistors R98 and R99, the parallel pair should measure 110 ohms. A short on pin 3 could overheat them and blow them up.

[kbmuri]

Do you have -90VDC (against chassis ground) at the far terminal of R109 (where you soldered the black wire for your terminal strip for the electrolytics)? All of the common negatives of your electrolytics should be there too, -90VDC. This is the -90VDC that feeds pin 3 of your HO tube (dropped to -65VDC thru the R98/R99 pair). If you can see -90VDC at the R109 tap-off, and at the terminals of your strip, then you ought to see it at one side of the R98/R99 pair. The other side of the R98/R99 pair connects to pin 3, where you see 0V. If you have 0V at pin 3 due to a short to ground, then (as WA2ISE says) you'd expect the resistor pair to smoke. If you have 0V there because you've got no -90VDC coming in, then it should be easy to track down from what I just said.

You should verify -90VDC at the R109 lug, as stated above, and also verify -43VDC at the center lug, and make sure the left lug is grounded. Verify the +3.8VDC, +115VDC, +205VDC, and +220VDC points as well, as shown in the sams schematic at R105, 106, 107, 108, and 109.

[Chad Hauris]

That's interesting that there is a -90 volt fixed bias at the grid of the horiz output tube...usually the set relies on grid leak bias only for the tube.

I would remove the tube from the socket and check the grid voltage, this will tell if there is a tube related short. 106 ohms instead of 47 is not good...I would replace the resistor feeding the grid.
Also would be good to take a scope and check for an AC signal at the grid.

If there really is only -2 volts at the 6BG6 grid this is hard on the tube, and I would check it after making repairs to the resistor circuit. I have observed lack of bias voltage destroy horizontal output tubes.

[jpdylon]

I have 136k ohms between cathode (pin 3) and ground. Should be sufficient enough right? This was with the HO in or out of the socket.

I left the Ho tube out and took readings from the PS at the resistors Kirk mentioned. The voltages were higher with the tube out, I didn't take them with the tube in because I noticed that even when i left the set on for 30 seconds to take voltage measurements with the Ho tube in it got hot quickly

with the HO tube out i had -95vdc on pin5 and -99vdc on pin 3. OK, maybe the tube is bad? so i put in another 6BG6 and fire it up. Now I've got -84 on pin5 and -71 on pin3 and i can hear the oscillator singing again --but still no raster.

pin 10 of the CRT reads 270vdc. R103 is reading 300k in circuit, could be the reason for lack of G2 voltage. Good side of R103 is 420vdc, other side (pin 10 on CRT) is only reading 270VDC.

pin 2 of the CRT varies between -95vdc and 40vdc through the range of the brightness control.