KB PVP20

it can take 24 hours to reform some caps .iwould do one section at a time and monitor the voltage across the cap being reformed.
rob t

Hello
Well I decided to take the plunge and give it a try with some AC connected to it.
The main electrolytic was still hovering around 4ma on one section and 2ma on the other. I thought it would probably work.
I snipped out the mains filter cap and changed my lamp limiter from 40 to 100 watt.
I set the variac to about 150 v and turned on the switch.
Nothing, not a glow on the lamp, no bangs or fizzes – checked the on off contacts on the volume, checked the fuse – that’s OK - and the lamp then started to have a very faint glow. I cleaned the fuse contacts and put it back in and then connected my volt meter to the smoothing cap and waited to see when/if the DC HT voltage appeared.
3 minutes went by and I noticed about 5 volts and slowly rising, it went up to about 60v so I left it for a while to soak at this voltage. I then increased the variac to about 180v . HT started to increase again and rose to about 90v. The lamp was glowing much brighter now. I took the variac up to 240v and the lamp had a good glow and the voltage rose to about 120v..
I decided as the lamp had done its job and shown no major issues to switch it out, so I reduced the variac to 150v and switched out the lamp. I went through the same process of increasing the variac in stages when the HT got to about 140v I could hear a gentle hum and a background hiss, the sound was working.
I let it soak for a few minutes and carried on turning up the variac, eventually I saw a horizontal line across the screen. I went and got the signal generator that Horris had lent me and searched high and low for a 75ohm coax lead to connect it. Found one in the end on the TV downstairs.
Anyway here is the result.

I have adjusted the height to full and the vertical linearity and this is the best I can get
I tried the vertical hold and it seemed to do nothing, the horizontal hold did what I remember they used to do, picture broke up into zig zag lines..
It looks like it is almost there with the major components functioning.
I did not want to leave it like that too long as the bright bottom of the image looked like it could burn the screen.
I played around with the pattern generator and changed the frequency, I could not get the vertical bars back. I turned the generator off and then on again at about 70MHz and switched the channel switch until I found the signal, so it seems to be working on band 1 and 3 OK.

Now I am a bit lost what to do next. Any suggestions on how to get the screen looking right would be welcome.
I have to admit I was sweating profusely by the time I turned it all off, I think it must have been the stress and excitement of the situation. (and my pain killers probably don’t help)

I think I will go and read up on Chris's getting started advice.

Thanks
Mike

Great start, a partial frame collapse I would look at the caps around the frame TMB.

The first "First light" sweat! I remember it well

Edit:
I've uploaded the Trader data to the library in case you did not have it or if others wish to follow.

Hi Chris
I have the Trader sheet 1411 but thanks anyway.
I am afraid my knowledge of TV time base circuits is pretty well zero. Am I correct in assuming I should be looking around the area circled in red.
I would be grateful if you could give me an idea of what order would you use for checking the capacitors and other components.
By the way the HT is 196v at C92

Thanks
Mike

Mike, check the PCL82 - swap it with the sound output - and the value of R106 - if it shows any signs of distress, replace it!

Check C87 first, then the components in the linearity circuit: R97, 101 & 102; C84, 85

Posted by: @anonymous

tried the vertical hold and it seemed to do nothing,

If this is a simple generator, it may have an option for either vertical or horizontal bars, with one of the bars being used in lieu of a sync pulse.

If this is the case, you will need to switch to horizontal bars to see the effect of the vertical hold best - although I would have expected to see some change ...

(Apologies to Horris if his generator produces a full sync train - it was just a passing thought ...)

Had some progress, I now have a full screen of bars.
I checked resistors R97 this was just about right. R016 was reading low at 308 ohms (should be 380 ohms) I left this as it was, is this OK?
A bit of copper track came away when I was trying to get C83 out, but I have left longer leads on and connected further up the track.
The HT has come up slightly to about 200v and the vertical hold is having a little more effect although it is difficult to adjust with horizontal or vertical bars.

I think I might need a test card to go any further.

Thanks for all your help Terry and Chris.
Mike

Hi
Is it better to go up or down to a standard value in this instance.

Apologies about my post above, that should have been R106 not R016

edit
I dont know how much more stress the connections to this board are going to take, I have already had one break as a result of all the pulling away from the chassis to get at the copper side of the board and then refixing back to test. I dont think you can test with the board out as there are connections to the chassis through the screws.

I choose the nearest equiv that sits closest to the old value ( and within tolerance of course) be that up or down. Becomes a bit more critical if you're dealing with 5% tolerances but not in this or most cases.

I dont know how much more stress the connections to this board are going to take, I have already had one break as a result of all the pulling away from the chassis to get at the copper side of the board and then refixing back to test. I dont think you can test with the board out as there are connections to the chassis through the screws.

Thanks
Mike

Make up some fly leads with croc clips, these are essential for this scenario it allows you to have sub assemblies removed but clipped in circuit for testing. Either make your own or you can easily buy them on ebay. This is mentioned in section 7 of the guide

Here is an extreme case of clip in used on a set I worked on a while back. A non PCB set but the principle is sound for extending PCB board out from the set.

Chris

Is the function of R106 to set the cathode volts above earth in order to help the bias of V13B
Is this right;
A higher value for R106 would set the bias more negative and ease the strain on V13B a little???

I tried swapping V12 and V13 as Terry suggested and I did not notice any difference, however V12 looks like it has had a harder life than V13 if the amount of blackening on the glass is anything to go by.

Posted by: @anonymous

R016 was reading low at 308 ohms (should be 380 ohms) I left this as it was, is this OK?

No, it most definitely isn't!

As I said in an earlier post: "... check ... the value of R106 - if it shows any signs of distress, replace it!"

You will be over-running the output valve and the problems will just get worse. The resistor will suffer further distress from the extra current and reduce in value even more and so on a so on ...

As for a value of 380Ω, I think that must be a typo as it is only about 2% off the preferred value of 390Ω - what is it marked (or is it so badly stressed that the rings now read black - black - black ...?)

This was always a common problem. The manufacturer would fit the lowest rated resistor possible which would change value (low values invariably go low) and the valve would suffer and eventually cause enough of a problem to initiate a service call. If it was answered by a bottle basher who didn't check the resistor and change it, the new valve would fail much earlier than it should and so on.

We always used to fit 5W wirewounds here - that way we knew the problem could never recur - but you might want to use something closer to the original. In which case I'd fit a 390Ω 2W carbon resistor.

In view of the overrunning the low value resistor has already caused it would still be a useful sanity check to swap the PCL82s again after you've changed the resistor.

Checking the value of cathode bias resistors in high (ish) power circuits should receive as much attention as checking the grid coupling capacitor, in my humble opinion, but nobody ever refers to it ...

Hi Terry
It looks to me like orange, grey, brown. 380 ohms

I will hunt around for a 390 ohm.

No fear Trevor, having met and chatted I would never ignor any advice from Terry on electronics. His advise is always welcome.

Damn, it looks like Maplin have discontinued their 2 watt range.

thanks
Mike

Posted by: @anonymous

It looks to me like orange, grey, brown. 380 ohms

When I said 'distressed', that wasn't what I had in mind! I don't think I've ever seen a mottled one before! The far end looks a bit blackened, though.

It is a standard Egen resistor and looks as if it might have have a silver tolerance band, like its half watt brethren to the left. 380Ω isn't a standard value and I can't see any logic in making a 10% tolerance special so close to the standard value but I agree that it does look like a grey, rather than discoloured white, band so the jury is still out on that one ...

Egen's ceramic tube construction did mean that their resistors could take a lot more punishment before the paint began to 'cook' - on other types where the base body colour was just paint directly over the carbon element, the body and value bands went into boil and bubble mode much sooner! Looks like Maplin have discontinued their 2 watt range.

Posted by: @anonymous

Damn, it looks like Maplin have discontinued their 2 watt range.

???

 

Posted by: @terrykc

When I said 'distressed', that wasn't what I had in mind! I don't think I've ever seen a mottled one before! The far end looks a bit blackened, though.

Ehh, did you mean sad and under the weather, I assumed you meant burnt, cracked, or knackered. 🙄 
I think the paint flaked off when I wiped it with a bit of WD40, I had to stop wiping or it would all have gone.

Re Maplins, Thanks, 👍 
I hate websites where you have to work hard to find what you want, and you have to guess what the programmer used for the search criteria. I searched for "2 watt resistor" zero results.

Now reserved and will pop out to pick them up in a mo.

Mike ,If you want to bring the PVP20 down to me I can can show real pictures on it via an Aurora.I have also got plenty of the resistors that you need.

Dave

I've often seen white go grey. 390 will be right. It would have to be 1% or better for a 380 to make sense. Not plausible.

Mostly I'm working on battery valve radios, which don't normally* have a "cathode" resistor. But Mains output stages I always check the cold resistance AND the live cathode / Grid / 0V voltages.

(* just replaced a 75 Ohm risen to 80 Ohm on "cathode", actually DL84 filament f-, on Lady Anne. But it wasn't the fault!)

New 390 ohm resistor fitted, iIam just sprucing up the cabinet and then off to see Dave and his Aurora when it is all back together.
Cheers everybody.
Mike

Don't forget the camera!  😉