RCA 6T75 Restoration (16GP4)

[dobulee]

Lot of questions in there! Let me try to answer:

Replacing caps piece meal was my first thought, but I got some advice in the forum to go ahead and recap the whole thing. Its neither here nor there -- The set showed no signs of life before recapping, even though the electrolytics weren't totally shot.

1) No Reverse Polarity electrolytics
2)Yes, it seems unlikely there is a short on B+ with 475V. In fact w/ 355V center tap on the 5u4, the peak open circuit voltage is only 500ish. So there's very little drop going on. Im not sure what went in the 5u4, i'll check tonight.

3)The CRT base connector on this set has 2 pins which do nothing but connect the filter choke to the rectifier when the connector is inserted. Ie. if you do nothing, the power supply is completely disconnected downstream of the rectifier.

4)Tube TV -- I'm a little confused -- the 5V winding for the 5u4 filament IS the b+ winding. Maybe I mis-interpreted what you're asking. In any case, with everything else disconnected, all the transformer windigns show isolation.

5)I'll try to bring the set up w/ a pair of diodes and a variac. However, since the diodes will drop much less voltage than the 5u4, i'll have to keep the line voltage low. This will also lower the 6.3V filament taps. Need I be concerned?

Solid state switch mode power supplies are my main game, so I'm still getting used to the tube side of things...
I'll poke around some this weekend and see what I can see.

Alex

[Bill Cahill]

Diodes are a no go. The hv will not work below 100-110v ac input.
The picture tube socket isn't the connector to B+. It's two pins on the yoke socket.
This set should be operating at 387v dc on rectifier under full load at a line voltage of 117 volts ac.
Your present dc is way high... Do you have the yoke plugged in?
Also, disconnecting the speaker plug saves the audio out from dammage because it removes the voltage from both the plate, and, screen. However, the more circuits not functionning, the higher your power supply voltage will be.
Also, that single secion boost filter is important. If open, or, shorted, that kills the vertical, and, damper circuits.
Just completely re-cap it.
And, oh, yes. Those 1kv capacitors on audio out., vertical, yoke, horiz. out., and, damper circuits are very important.
Black beuty caps are no good, either.
Get rid of your line caps, and, resistor if you want real relative safety.
The RF it isolated set from is rarely produced anymore.
Cars now run on alternators. Not generators.
And, who uses a diathermy machine anymore?
No real noticable change in rf in signal without the line caps.
I have alot of experience on these sets, so, if you need my help, please feel free to pm, or, e mail me.
Bill Cahill

[dobulee]

Yes, I know the b+ is on the yoke, and yes it was connected. I meant CRT connector as a generic term.

If the power line choke were open, I'd get no voltage on B+ at the CRT.

The entire set is recapped.

What makes you say diodes are a no go? The HV shouldn't care what the line voltage is, as long as the DC voltages come up correct.

If you're making the argument that the powerline is cleaner now than in 1950, I'll disagree -- ask the guy who works switch mode supplies! Just try running a flourescent light on the same outlet...

[dobulee]

I didn't mean that to be Smart Alec-y Not sure if it came off that way!!

[Bill Cahill]

Well, you did. That's exactly why I rarely come here.
I offer help, and, I'm treated as if I know nothing.
I wanted to help you. That's all.
Good luck with the set.
B C

[dobulee]

I have much to learn from everybody's experience with this set, and I welcome and enjoy reading everybody's comments. Thanks for following as I muddle through this.

The engineer in me systematically responds to questions and suggestions. No cold shoulders intended.

I am geniuinely curiuos to know if there is a reason I shouldn't attempt to diode across the rectifier tube -- However, the suggestion to recap specific sections confuses me, as my last post explains that it has already been done. Paper caps, and black beauties included.

[Tube TV]

Sorry , my error on the 5V winding .....

[Bill Cahill]

I was merely warning about voltages of certain caps. I believe is doing the entire thing first.
The rectifier isn't your problem. The set obviously isn't drawing any current.
I'd start looking for open, or, way off value resistors in B+ Supply, and, to each section of the set. This is a major weak point on these sets.
This set should operate at 387 volts at 117 volts ac.
You said, I think, you were getting 488 volts?
That's clearly NOT a rectifier problem.
TV isn't getting out of surge mode.
Check B+ voltages on all major circuits, such as vert. oscilatro, vert. out. caution, don't try to measure plate of output tube, damper, or, plates of horiz. output, hv rect. tubes.
There is a dangerously high ac component there.
The boost filter negative leade connects to cathode of vertical output tube.
Check the .25A fuse in hv cage to make sure it isn't blown.
The correct value of that fuse is .25A 250volt slo blo.

Yoke failures were common then. If horiz. winding shorts enough your hv will fail.
Did you replace all the paper caps in hv cage, also?
There are 4. Two on yoke, and, two on damper.


I've had alot of resistor failures in these sets. They either open, or, go 100 percent, or, more, off value.
Anything above 20 percent should be replaced.
Bill C

[jeyurkon]

Originally Posted by dobulee

Yes, I know the b+ is on the yoke, and yes it was connected. I meant CRT connector as a generic term.

If the power line choke were open, I'd get no voltage on B+ at the CRT.

The entire set is recapped.

What makes you say diodes are a no go? The HV shouldn't care what the line voltage is, as long as the DC voltages come up correct.

If you're making the argument that the powerline is cleaner now than in 1950, I'll disagree -- ask the guy who works switch mode supplies! Just try running a flourescent light on the same outlet...

I'm still confused about the jumper then. If you had the yoke connected why did you need a jumper?

John

[Bill Cahill]

The jumper is original to the set. It's a disconnect in case yoke is accidentally un-plugged.
That cuts all b+ to set.
iF HIS AC line was open, nothing would work. His tubes are lighting, and, he has B+. The problem is more like the set, or, at least part of it, isn't drawing any current.
I still say resistors are likely at fault.
Bill C

[dobulee]

Hi John -- I was asking about using a jumper when checking things on the bench without the yoke connected. You're right, w/ the yoke connected there is no need.

Bill was right on the money with this one.

The 4k Resistor between 375 and 220V is bad - it reads okay at room temp, but reads close to open if you measure soon after power down. How's that for a positive temperature coeffecient. Fun.

The filament went on the original 5U4 -- but I've double checked isolation between the filament windings and all the other taps -- no problems. The open circuit AC voltage across the 5U4 filament is 5.1. Not sure why things went wrong.

Incidentally, I decided to go for the diode route instead of another 5u4 for now. Since the 5u4 drops somewhere around 40Vpeak at full load, I ball-parked some series resistance (220 Ohm 5 W) for each diode in the full wave rectifier. This turned out to be pretty dang close.

With a bunch of parallel 50k 1W resistors bodged in for now where the 4k Power resistor should be, I finally pulled the variac out of storage, and brought the set up slowly. The 370V BUS leveled out at 381, the 200V BUS at 195, and the 125V Bus at 81V. Can't win 'em all. One of the series resistors is high -- I'll get to it later.

At this point, I could hear the vertical output transformer rev-up!

I put the chassis back into the cabinet, and hooked up an aux audio input, -- sound! Not bad for how far off the biasing is due to the 125V BUS. The 6k6 audio output tube has an interesting blue glow....
At this point, there was still no sign of any HV -- I suspect the horizonal output tube, who's getter flash now looks suspiciously oxidized. I'm not having the greatest of luck with ebay'ed tubes!

Back to the bench, and an oscilloscope.

1st capture is 370V Bus Ripple voltage AC coupled at 10V/division. Ripple is about 4Vpp -- I'll take that!

2nd capture is the vertical drive at the grid of vertical sweep output tube. Looks reasonable, stable, and linear!

3rd capture is Tap C of the horizontal oscillator transformer. Gotta admit, i'm not exactly sure what waveshape I should capture here, but the general frequency is about right - about the 15kHz ballpark, although it's not particularly stable.

That's it for now -- all of the about captures are free running oscillators with no video or RF applied.

Is there anything specific I should check out before some new 6BG6 horizonal output tubes arrive?

Any tips, thoughts or comments appreciated

Alex

[jeyurkon]

Congratulations on the progress! Yeah, Bill has the experience to know and his suggestions are good.

Due to the ambiguities the English language sometimes presents us with I wasn't sure that you weren't testing with the yoke disconnected. That seemed like it would have been a bad thing to me and could have caused the high readings.

Resistors that change like that can be a real pain.

John

[Bill Cahill]

First, without the yoke, you can't get hv.
Secondly, the correct horiz. freq. on these sets is 15,750.
Yours sounds very low.
The 5U4 is quite capable of getting up the proper voltage as long as the tube is good. And, a tube tester claiming it's good means very little.
If the getter on horiz. out. has changed from clear, it's likely gassy. Not a good thing.
Blue glow on aud. out. is gas. Also not good.
The micas in these sets tend to get leaky, and, will cause nasty things in picture.
In the horiz. the oscilator will not run on correct frequency, and, could have a hunting problem.
In sync circuit, it will make both the vertical, and, horiz. un stable.

Which chassis is this?
Does it have the 6J5 vertical oscilator, or, the 6SN7 vertical oscilator-sync seperator?
If it is the later more deluxe set, there are other things you need to know that sams doesn't include.
The tube chart in cabinet will help there.
Also, if there is a 3 lug terminal strip screwed to the side of chassis near vert. oscilator, it's the more deluxe chassis. They primarily use it with a capacitor going to it.
It's a coupling cap not used on cheaper chassis.

There is a 100 ohm two watt resistor on horiz. out. Cathode that will likely need replacement, as well. They take alot of current, and, tend to cook, causing them to go 50-100 percent high.
That will also affect your hv.
Bill C.

[dobulee]

Hey Bill,

A lot of these questions were answered a few posts ago, but here goes:

When I made the comment about no HV, the chassis was installed in the cabinet, and the yoke attached. No issue there.

I know that horizonal line freq is 15,750, unfortunately I have no really accurate way of measuring. All I have is an out of calibration 20Mhz Analog scope. So saying '15kHz ballpark' is about as accurate as I can get without the timebase being re-calibrated.

Thanks for the tip about the mica's, they're on my 'watch list' when I get there.

A few posts ago, I noted this was the 6SN7 vertical oscillator - sync separator. John was kind enough to give me the proper schematic, as my Sams shows the 6J5.

This set has a 2 lug terminal strip in the location you indicate -- I think it shows in 'recapped' pictures.

The 100 Ohm in the cathode section WAS bad. It is now a 2W Metal Oxide resistor. Its the blue one up there in that tightly crammed corner of the chassis.


For what its worth, I'm trying to hunt down the sagging 120V BUS problem -- my original comment of a resistor being high was based on an incorrect glance at the schematic.
My set does not have the multiple parallel resistors between the busses in the power supply, but instead has single wirewound power resistors in each of these locations. Not sure if this is original, or a replacement. I went through the academic exercise of calculating the 'expected' equivalent resistances from point to point in the power supply based on the parallel resistances in that section, as well as the 'tie' between 120 and 370 that feeds the RF Amplifier. Below is what I got...nothing seems off by more than ~ 20%:

370 to 250: Calculate 3.65k, measure 3.77k
370 to 120: Calculate 11k, measure 10.6k
250 to 120: Calculate 10.9k, measure 10.2k
120 to Com: Calculate 4.28k, measure 3.8k -- this is off about 12% and could pull both 120 and 250 low, but doesn't account for the full difference.

Can anybody confirm these measurements on another chassis?
Alex

[Bill Cahill]

I don't believe you looked in all the right places.
Manny circuits, and, resistors can cause troubles.
I F b+ resistors, All audio resistors, tuner b+ resistors,
B+ resistors in video amp, Sync, vert. oscilator, even to a degree, the agc circuit.
I didn't seee all the posts, so, sorry, I wasn't aware of the earlier information.
Try removing he hv doorknob cap, and, trying set without it. Any higher hv?
If so, the doorknob is bad.
They simply snap in, and, out of clamps.
Be sure of direction it comes out. Even mica seems to develop a memory.
The hv doorknob is mica.
They have been known to fail.
It's 500pf 20kv.
If you need to, you could live without it.
Just not as good hv filtering. But, they are obtainable.
Bill Cahill