Another KCS47 Restore...No High Voltage/B+ Down 25V
 

Starting fresh with an unrestored chassis. I figured I'd start in the power section like everyone else does but you need to make some rigging to be able to test and adjust it.

I took a tip from Kevin and built this rig for the 5" electrostatic test CRT.

https://imagizer.imageshack.com/v2/1...923/4JsIp3.jpg

I do not have the speaker out of the cabinet and therefore, I needed to make a jumper. The 375V rail goes into the sound section and anywhere else the 375V needs to go. The jumper is on the speaker so without it, I jumped it on the speaker connector.

https://imagizer.imageshack.com/v2/1...922/Mkysht.jpg


Where the DMM is connected reads 350V and not the 375V in the schematic. The B+ on pin 8 of the 5U4 reads around 25V lower than the approximately 387V in the service data. I did some recapping in the power section and all three electrolytic caps have been replaced with adapt-a-cap substitutes. I covered them with some cardboard tubes I had laying around but they are grounded to the chassis.

I rigged a HV cable from the metal cone energizing socket on the HV cage. I replaced the 4 film caps in this section. The flyback is a Merit direct replacement for the original one that had an open section.

https://imagizer.imageshack.com/v2/1...924/kcmuAh.jpg


I've switched out the HV rectifier, horizontal output and damper tubes numerous times with known good tubes. I get about 3KV on the anode output when I test it at the socket.

My guess is the low HV is a function of the low B+ but I'm not sure of that. I've tried several 5U4s. If this is a problem with a resistor in the voltage divider, why would the voltage be low on pin 8 of the 5U4?

Anyway, below is the section giving me difficulty. I've made no other changes in the chassis other than the capacitor replacements I listed above.

https://imagizer.imageshack.com/v2/1...924/OA4e3z.jpg

I would not consider your B+ low. Figure those stated voltages are +/- 20% or more. Plus you haven't recapped the whole set yet. Some circuits could be drawing more power than typical.
Measure the voltages on the 6BG6 and check the grid drive waveform. Likely your hor. freq. is off.
I wouldn't expect you to get full HV until the whole hor. oscillator and output circuit is gone over.

Okay thank you I’ll do that and report back

The horizontal frequency after a slight adjustment looks right but the waveform is noisy.

https://imagizer.imageshack.com/v2/1...922/fOL24M.jpg

Pin 8 is 293V DC Should be +329V DC
Pin 5 is -4.8V DC Should be -33V DC
Pin 3 is +11V DC Should be =7.2V DC

HV is around 3.8KV. The waveform grid voltage is way off

If you haven't replaced the leaky paper caps in the horizontal circuit, you need to. Same with the vertical. 3.8 kv means it's basically trying to work. But if there's leakage it's messing up the bias on the 6BG6. With so little grid bias the 6BG6 may be pulling a hefty amount of plate current. 110ma according to your voltage reading on pin 3.

Ok I’ll finish replacing the caps and check the resistors in both deflection circuits completely and see what that does.

I think what I’ll do Kevin is post a list of the caps I’ve replaced and a bigger section of the schematic. I’d like to know for education purposes what’s causing this specifically and how I can diagnose issues like this in the future instead of doing a bulk recap in the power and deflection sections. I’d like to be able to say oh that’s what caused it and not well I changed everything and that took care of it.

I get what you are trying to do, but it's been my experience it can get frustrating.

All of the paper caps are leaky.
Many of the electrolytics will be excessively leaky or open.
Many of the resistors will be high.

It's not like troubleshooting decades ago where most of the parts are fine.

Point taken. Thanks!

You can do that but you'll be spending a lot more time getting things corrected. Right now you know the grid of the horizontal output tube is under biased, and it's pulling more plate current than intended. Besides paper caps it's possible the 560pf mica grid coupling cap is leaking positive voltage from the horz oscillator onto the horz output grid. You could try pulling the 6BG6 and see there's any residual voltage remaining on pin 5 of it's socket. Speculating we can assume the higher than normal plate current the 6BG6 is drawing could be pulling a B+ down few volts. That happens because these old sets don't have much if any voltage regulation. And the horz out is probably the biggest consumption of power.

Thanks as always Kevin. Do you mind expanding on this a bit for me? I thought bias voltage just turned a tube on and off. There's a ramped relationship between bias on the grid and the plate output? Is a biasing voltage always a negative voltage? Since -4.8V is a higher voltage than -33 volts DC does that account for the higher current on the plate? Confused as usual by the details. Can you explain to me how a 6BG6 works other than the filament voltage?

You've pretty much got it. The negative bias on the grid(relative to cathode)sets the idle current, or operational point. So in some cases it can be turning on and off, like a switch, and other times it's partially on in neutral, so to speak. As far as I know normal receiving tubes use negative bias on the grid. If that's not clear I'll attempt to elaborate.

If you look in a technical text they use the term Quiescent Current. Different classes of amplifiers idle at different currents. Class A for example is turned on fairly hard at idle, vs the horizontal output is biased closer to cutoff, and acts more so like a switch. :boring::D

Tubes don't just amplify AC and aren't just on and off. The plate current, with no signal, in anything with a control grid (other than a thyratron) is proportional to the DC grid bias. The exact bias voltage to achieve these following these 3 states varies with the tube type number and wear, but here goes. A tube with strong negative grid voltage (for instance -50V) is in cutoff and no plate current flows, a tube with almost no negative (or even positive grid voltage is in Saturation (maximum plate current it can physically pass... sometimes in excess of ratings is flowing.. often hundreds of mA), and if a tube is biased somewhere between the two it's probably in the linear region where a fraction of a volt grid change results in a mA change in plate current.... often the linear region is somewhat non-linear as it approaches the other regions.
In a single ended class A audio amp DC bias is critical to getting maximum volume...You want bias in the exact center of the linear region or the grid swing will clip either the tops or bottoms off the waveform before maximum achievable amplitude is reached.... Also audio output transformer DC saturation at high current can force reduction in plate current bias because the transformer hits saturation before the tube. In a push pull audio amp you want to avoid class A operation as you waste power and don't get maximum amplitude. In push pull the DC transformer saturation from each tube cancels the other and each tube can handle half the signal, but can be in cutoff for the other half without issue... However a little bit of zero signal plate current is necessary to avoid clipping from a messy handoff between tubes at the 0 crossing of the signal....If that distortion doesn't matter Class B can be used.

Horizontal sweep systems use class C where the load has resonance in the desired frequency range (like a bell) and the tube only conducts for a small portion of the AC signal sufficient to ring bell, keep it ringing and at the right ring strength.... Horizontal output systems Instead of using straight forward DC bias circuits that resistively divide the DC supply get their DC bias power from the signal. They use the grid leak detector principle where grid-cathode rectification of the AC signal creates a DC bias that regulates tube current.

Triodes, terrodes and penthouse act the same as signal amplifiers with regard to DC and AC control grid behavior in single input amplifying or switching applications...The only thing the extra grids do is change the slope of the linear region relationship between grid voltage and plate current...They increase gain which is useful enough to merit the additional cost and complexity. With extra grids you can achieve 2 input amplification or switching which can be seen in color TV color demodulation and some gated sync separator/AGC circuits respectively... Superheterodyne converter and or mixer stages also use this... You shouldn't see this in a horizontal output circuit unless it's the infamous single tube horizontal Muntz circuit in that one model they made...

Pulled the 6BG6 and there's no voltage on the grid (5) and the plate (8) is 410V DC

Yep, tube needs to be in the socket to develop that grid leakage bias voltage Tom mentioned.

Of course. I suggested he do that to verify if any B+ was leaking onto the grid from the horz osc section. Apparently not.

At least now you know where some of your B+ is being lost to. I think once you've replaced the paper caps in the horizontal section your sawtooth will increase in amplitude and the 6BG6 will get it's proper grid leak bias.

Ok gonna finish the horizontal and vertical recap tomorrow and I’ll see what happens.

How many of the paper dielectric caps are still in the chassis? If you haven't replaced them, the B supply will be pulled down by the leaky caps upsetting the bias voltages.

Before powering and chassis of this vintage, you must replace the caps first.

Chris,

I have an idea that might be less painful than the way you're trying to go about this. Identify the circuit location on each of the old caps as you remove them. After you have the TV working with new caps, you can experiment with putting the old ones back, one at a time. Then you can evaluate how they influence the operation. It will be easier to do once you have a working base line. You could even keep a spare chassis on the bench just for experimentation. The more you measure voltages and look at waveforms on your scope the easier it will become to wrap you head around how the circuits interact with leaky or out of tolerance components.

Before the first power up I did a dim bulb series to make it safe. When I got no high voltage I replaced the 3 electrolytic cans with 4 section adapt-a-caps and a few but not all the caps in the power, horizontal and vertical sections. It’s interesting there are so many opinions on this process. Bob has commented here replacing all the caps is the prudent thing to do but I’ve watched restoration videos of his where he baselines an untouched chassis from this era using a gradual power up with his Sencore isolation transformer Variac. Shango usually does the same although his videos are more resurrections than restorations and he has no fear of the magic smoke. I don’t do a power slam at 120V without replacing the cans first because I have seen videos where a short can take out a transformer so I don’t want to risk that. Are you saying I should replace everything or just the cans and all waxies in the 3 main power sections I've been working on? Is leaving the RF and IF caps alone until I get HV and a full deflection raster an ok thing to do or am I still asking for trouble with a shorted cap in the video and sound signal sections? To be honest, I usually have done complete recaps prior to first power up but from what I’ve watched that’s not SOP. Gradually replace the paper and foil caps and check function often before proceeding is the smarter more conservative approach. That’s what I surmised anyway.

Thank you for that suggestion! I actually finished the recap in the vertical and horizontal sections tonight but it was late, I got tired and I didn’t want to risk powering the tv up in that state. I did test some of the caps as I replaced them and one in particular had a very high ESR…somewhere around 75 ohms. In bed and I don’t have the schematic in front of me but it was a 102 paper cap rated at 1000V…C155 🤔 maybe. The rest of the wax caps tested fine accurate with
low ESR. The bumblebee caps were a different story and were all bad. Anyway we’ll see tomorrow if it works, is the same or smokes!

Actually it was C156

No you don't want to replace ceramic caps in the RF and IF unless you later identify a known issue with one. Keep in mind some of the individuals you watch videos of have a vast experience with how these circuits work and lots of doing under their belt. Shango in particular does things different just to go against the grain. Don't me wrong, he's a brilliant technician, but he's also intentionally adding drama for the sake of viewer engagement. I get his humor, so I'm not offended by any of it.

Ok well, I’ve completed the work in the troublesome sections. Hopefully it will be closer to what it’s supposed to be when I power it up in the AM. The jammed up orientation of components under the HV cage is a real pain in these early RCA TVs. Not service friendly for sure.

ESR isn't very useful when it comes to testing paper or film caps. That's primarily useful for finding high resistance or open small electrolytics while they are still in circuit. Completely different than leakage. Although if C156 had high ESR it's probably bad just the same.

I tried it today with every cap replaced in the sections we’ve been talking about and I got a raster on the 5” CRT. That’s the first time that’s happened. HV unfortunately hasn’t changed much, still hovering around 4K. I didn’t get a chance to do any voltage checks or look at any traces. I think there may be an issue with the yoke and how I have it mounted because it’s out 90 degrees. The horizontal changes with the vertical and vice versa. Doh!

Interesting. Is it just rotated 90 degrees? The horizontal and vertical are still acting on the correct axis of the picture, as in you didn't change the wiring on the yoke plug?

I didn’t but I completely disassembled the yoke and the mounting when I fixed the stripped nut so maybe something got messed up. The back cardboard cover was ripped and destroyed so I 3-D printed another. I’ll have to look more closely at it. My recollection is the screw hole on the yoke for the rotation adjustment wasn’t removable but maybe it was and I remounted it 90 degrees out. I definitely did not change any wires on the yoke plug

If you look at the first page of the thread where I have the picture of my version of your CRT test jig maybe you can see if there’s an issue

I can't see the yoke in your picture. Looking at a similar RCA yoke I have here I don't see any way to rotate it unless you loosened the steel clamp around the yoke windings and forcefully rotated it. But why? :scratch2:

I got it. There’s a band that holds the nut for the adjustment screw and there’s a screw that can be loosened separating the band so the yoke can be rotated. I adjusted it and now the picture is oriented correctly.

Can’t tell you why I did that other than when I serviced it I took it completely apart.

F#*K!!!! I turned it on and tried to decrease the width using the pot on the back of the chassis and when I turned it I heard a big arcing pop. Now there's less than an amp draw and no picture. Damn it.

Pot could have been dirty/intermittent or previously damaged. I think it would be a good idea to keep your scope on the grid of the 6BG6 when powering this up so you have some indication of what you're feeding the horizontal output with. You might want to ohm out your flyback windings to see if it went open, again. Hopefully not. Before this happened did you ever get back to looking at the waveform and bias on the 6BG6 grid?

The width control is a little unconventional on this KCS47. It varies B+ going to the plate of the first triode section of the horizontal oscillator. Check that the 1/4 amp fuse feeding the horizontal circuitry is still good.

I remember when I resurrected a KCS-47 (I wanted it gone and wanted to get some YouTube content out of it first) that the width pot would spark when blown on...I think they're prone to tin whiskers. You may want to confirm that it's not being shorted (terminal to terminal or terminal to case) by whiskers...And also that no fuses went when it sparked.

Keep in mind I'm not at home so I can't do any troubleshooting, but the fuse occurred to me as well...but not for the same reason. First, I assume if the 375V circuit is blown, I would still get something close to the 0.75 amp draw the TV had after the pop correct? Second, the yoke is OK...I ohmed it out and it seems fine. I think I know how this happened. The jumper I put on the speaker plug to connect the 375V to the rest of the set was an alligator clip wire. Every time I worked on this, I had the speaker cable and jumper dangling off the workbench so it wouldn't contact anything accidently. Last night it was on my disorganized and tool filled bench and when I turned the chassis to the side to get to the rear controls, the jumper was close to the metal base of a gooseneck desk lamp I was using as a work light. I think it may have contacted the metal base and shorted the 375V line, taking out the fuse. The usual stupid, stupid, stupid on my part. We'll see. I'll check it tonight.

Equal chance that happened Tom. Never dealt with tin whiskers before but I certainly know of them and what they can do. Would a big shot of contact cleaner (Deoxit Fader) into the width pot possibly clear them out?