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Zelandeth 9th May 2007 11:07 pm

ITT VC51/1 Gremlin Hunting - Stage 1.
1 Attachment(s)
I warn you right now...there's a lot of numbers in this post!

Okay folks, finally delving properly into the guts of my KB with a view to getting it working well, rather than just showing signs of life.

I've listed below the voltage measurements I took yesterday, what they're meant to be according to the service data - and what I think is the cause for any anomalies - would be most grateful. Leaky capacitors are quite easy to spot like this when you've got a circuit diagram and a voltmeter handy...well...sometimes!

Visible symptoms are a picture which with the brightness control turned down is okay - but advancing it causes the picture to wash out, I doubt it's the tube itself, as each line making the picture up is still clearly visible - there's no loss of focus - it definitely looks like a video problem. I've attached an (old) screenshot.

V1 and V2 are in the VHF tuner, and there's neither any voltages quoted on the service data, or any easy way to measure we'll skip those two.

It's just occured to me that V3 and V4 appear to have gone AWOL entirely as far as this set is concerned...I know there are a couple of semiconductor diodes on this wonder if they were used in a previous version.

Numbers in brackets are what they're meant to be - then what I measured.

V5 - EF183, Common IF amp.
Pin 7: (180V) 198V.
Pin 8: (65V) 44.4V. Diagnosis - C36 leaky.

V6 - EF184, Vision IF amp.
Pin 7: (165V) 130.2V.
Pin 8: (165V) 130.2V. Diagnosis - C141 somewhat leaky (D'OH! It's ones in the electrolytic can).

V7 - PCL84, Video amp & Vertical Synch amp.
Pin 2: (17.5V) 10.4V. Diagnosis - possibly also C141 being leaky.
Pin 3: (15V) 4.27V. Not quite so obvious this one...

V8 - PCF80, Sync Separator & Vertical Oscillator.
Pin 1: (70V) 81.4V.
Pin 6: (40V) 8.3V. C141 yet again methinks...

V9 - PCL85, Vertical Oscillator & Vertical Output Amplifier.
Pin 1: (60V) 79.3V.
Pin 2: (-18V) Correct.
Pin 6: (220V) 223V. Think I can call that one correct.

V10 - 6BW7, Sound IF Amplifier.
Pin 7: (180V) 202V.
Pin 8: (220V) 204V.

V11 - PCL86, Audio Amplifer.
Pin 3: (230V) 236V.
Pin 5: (215V) -31mV. ??? The audio output tranny's primary ain't O/ the sound works perfectly. Reckon this may be my meter having trouble with frequencies. Measurement was taken at least three times to rule out me being an idiot.
Pin 7: (5.3V) 5.7V.
Pin 9 (105V) 110.1V.

V12 - PCF802, Horizontal Oscillator.

Pin 1: (210V) 216V.
Pin 2: (-20V) -18.8V.
Pin 3: (130V) 149.2V
Pin 6: (163V) 147.2V
Pin 9: (3V) 1.1V - C110 leaky?

V13 - PY801, Reclaim (Efficiency?) Diode.
Pin 9 - 230V exactly as it should be.

V14 - PL36, Horizontal Output Amplifier.
Pin 4: (200V) 62.9V. C127 Leaky. Tend to think that could cause some odd symptoms in itself.
Pin 5: (-60V) -43.3V

And that's it!

I know there are a couple of dodgy resistors about (I've replaced one, and another is an unravelling but still intact wirewound which will be replaced once I find one for it)

Think next step is to replace all the caps to the chassis and see what happens.

If anyone can point me in a direction for some of the dodgy voltages above I'd be most grateful. I'm learning this tech from the ground up really, so second opinions are very much valued. I know a lot of you guys are very skilled at this fault finding business!


Steve_P 10th May 2007 12:51 am

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
Well... From V8 upwards will not cause this sort of problem.

Have you tried changing D6. Also is the 230v line OK. If yes, then check those resistors. Also C59.

Mind you, changing all the caps is not a bad idea. Leave the canned ones alone for now though!


Steve P

Mike Phelan 10th May 2007 8:06 am

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
I think that for now, one fault at a time so you can ignore everything except from the vision detector to the CRT.
You need the voltages on the screen and anode of the PCL84 - pins 6 and 9 respectively to see if the video signal is being crushed or clipped.

The anode of the PY801 is OK so that means that the HT is correct - that's where it comes from.
Dont worry too much about the PL36 unless you are experiencing lack of width or foldover. It's just a switch.
Change caps one at a time and test!

Focus Diode 10th May 2007 8:46 am

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
You havn't mentioned checking the EF183, EF184 and PCL84 valves by substitution. I would try this before getting too deeply into it. Do the contrast controls work ok? R82 (4M7) between the slider of the contrast control and system switch may have gone high in value.

The advice from Steve and Mike is sound.

I used to have a 19" KB VC52 model. The pictures were superb on both 405 and 625.


Zelandeth 10th May 2007 6:46 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
Okay, so let's see what I just checked.

C59 is behaving itself according to the meter...though I will probably change it anyway. It'll have to wait for a bit though as I don't have a suitable replacement handy.

230V line (measured at the C134/C141 junction, as it's the easiest bit of it to find, is actually sitting rather high, at 258V (mains tapping needing adjusting?).

PCL84 pin 6: 178V, pin 9: 224V. I can't really comment on them, because I don't know what they SHOULD be. The PCL84 is actually new (though the original one actually gives 193V on pin 6, and 239 on pin 9).

The PCL84's the only one of these I have a spare for (at present...maybe time I should correct that), though I seem to have ended up with a couple of spare PL36/30P19's.

D6...or what at least I think is D6 is reading as o/c on the meter's diode tester. What would be a suitable replacement for this?

The contrast control doesn't actually have any effect on anything, though there is voltage at the control. Finding R82 however is is proving somewhat hard to find. What I thought was the correct resistor is actually measuring 4.24K...which seems a little odd. The component doesn't look to be a replacement either.

I can't find R82 in ledgible form anywhere on the layout diagram I have...which has been scanned at some point in monochrome as a GIF, and is rather "chunky." Does anyone have a component layout diagram of the top and bottom of the chassis that they could scan for me in a slightly more usable format?

Well, at least one likely duff component found! Now for the rest.

Steve_P 10th May 2007 7:23 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
There are 2 contrast controls, one for 625 and one for 405. They are switched in by S8 - System Switch. Find this and clean it again, and thorougly.

From here, the next port of call is R82. There are volts on these controls because they hang off a voltage rail.

Fault could well be here. Worth checking. Follow the wiper leads from the pots and they'll go to S8. Out of S8 the next part is R82.


Steve P

Zelandeth 10th May 2007 8:05 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
D'OH! Having consulted the schematic does indeed. For some reason I had decided that the wiper from the control went through R82 THEN to the switch...

Will finish me dinner than take another look.

Zelandeth 10th May 2007 9:03 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
...and now things start to get interesting.

Turns out that R82 has what appears to be a 700K resistor wired in parallel with it...That ain't on the diagram!

So it would appear that the Phantom Twiddler did more than stick one RS capacitor in, and nick my EHT rectifier valve...

...Edit ten minutes later.

And now we've lost all trace of the EHT...I thought it sounded quieter when I took those measurements earlier. Lovely.

Zelandeth 10th May 2007 10:46 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
Edit Edit. Found the reason for that, it's cooked R141 (PL36 grid feed resistor)...again. Second time it's done that. No visible signs of distress, it's just gone O/C. Finger there points to C127 drawing excessive current methinks.

Have ordered some more capacitors and yet another replacement for R141 (um..five of them this rate, I'll need them too!), and will no doubt post an update with me going ...? once they're replaced and it's behaving even more strangely. Until then though, there's not a whole lot that I can do.

Steve_P 10th May 2007 11:12 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
Put a higher rating resistor in. It'll not blow, hopefully.


Steve P

Zelandeth 11th May 2007 9:20 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
2 Attachment(s)
Well, with R141 replaced (and still getting worryingly warm), we have EHT and a picture again.

Having gone "Hmmm..." and investigated the purpose of the mysterious parallel resistor to the contrast controls by the highly technical method of snipping one of its leads, has led to having a working contrast control again...and I swear a brighter picture.

All was not good news however. I was suddenly becoming rather worried about the state of the the brightness control is advanced, things did after a point go rather silvery, with very little in the way of brightness being available. The first picture below showing what I was seeing.

However, after fiddling around a bit with the controls, I noticed that this seems to have improved somewhat with the set having been running for a little while (obviously given its current state of health, I can't really leave it running for a couple of hours to see if it gets any better!). I have noted that if you tune "past" a station a little, there's a tendency for the picture to blink out for a moment, then return in a much more watchable state, without any of the silvery appearance unless the brightness is turned up far further. AGC doing something odd, maybe? The second picture (Oh how I wish I had a test pattern generator...) shows what it's like now...despite the appearance, the second pic is actually quite a bit brighter than the first. Though I do seem to recall someone mentioning before that the image from these sets was a little dark anyway - I don't imagine that the oraglass front panel really helps, as it is a fairly dark colour for a contrast screen.

Just wondering is this something that's maybe down to the CRT having not been used in 20 or more years, and expect it to improve after a while, or is it likely to be a duff CRT? Don't know what sort of reliability record the CRT in this set has, pretty sure from memory that it's a Plessy tube, service data indicating an A47/11W or A47/26W. I've almost subconsciously scratched an EHT problem off as a possible cause for the pseudo low emmision CRT...but I guess that could be suspect too. One step at a time though. Once they arrive, I'll get all the caps to chassis replaced, and see where we stand. At least that will stop it from eating PL36 grid resistors. In theory.

And the panel lights still seem to go out after about ten minutes. Will have to investigate that later's low on the priority list though.

Ah, does make a lovely smell this old beast when it's running though!

Focus Diode 12th May 2007 5:56 am

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
R141 should be a 5 to 7W wirewound and will run quite hot, this is normal. If in doubt you could always make it fusible, thus if it is taking too much current it will spring open.

It is an unfortunate fact the manufacturers employed an ordinary CRT for these sets, which really has to be in first class condition as with the tinted screen this would show up problems long before a conventional directly viewed set would. My own KV 025/1 I had 25years ago sadly started to show signs of low emission. As it is your CRT is surprisingly good for a 40 year old set. I would avoid any attempts of boosting the CRT while the picture remains watchable.

I love the illuminated dial! I'm sure I've seen one of these sets years ago in domestic service. The intermittency could be something simple, such as if screw in bulbs are fitted they may be loose and may need a little tighten.


Mike Phelan 12th May 2007 8:37 am

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
Though I know it is difficult to see from here, your first picture definitely looks a bit "tubey". The highlights look grey and defocused.

It might be OK once you have put a few miles on it. The molecules in the cathode coating will have been asleep for a long time.

Hunts smoothing bomb 12th May 2007 7:17 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
Is there a getter in picture tubes?

Because I was thinking, over 20 plus years with no use a minute amount of air will inevitably enter the tube through the pinout seals. Perhaps this is why the tube looks silvery?
The getter will do its stuff with the heater lit and consume any air in the tube. (that is, if it has a getter)

Studio263 12th May 2007 8:14 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
This model was known as the "Deep Scene 19" and was one of the most striking and stylish sets of the year.

I had one a few years ago and it was lovely. What I do remember was that the system switch was an absolute pain, the contacts had lost their "springiness" and so all kinds of intemittent probelms were caused. The poor tube really struggles to punch the picture through the tinted screen and so both it and the LOPT EHT overwind are always suspect (The B&O MX2000 of the mid '80's, a really junky set in many ways, suffers from an identical problem). There are also a few high-value resistors in the width circuit that play up, use types suitable for high voltages (or several smaller values in series) if you need to replace them.

Moving onto the PL36 screen grid resistor, this used to be a question on the C&G television servicing course, why did it need such a high rating when simple maths suggested that a normal sized component would do?

The correct answer is that during the warm-up period odd things happen. Sets with solid state rectifiers (like yours) comes up with the HT straight away but the boost diode (PY801) takes ages to get going because of it's highly insulated cathode. Meanwhile the line output valve begins to conduct but as the boost diode is not ready there is little or no HT on it's anode. The current then of course flows to the next most positive thing, in this case the screen grid! It's a bit like when the O/P transformer goes open circuit in a radio set, though line output valves are designed to take it.

In your case I would suspect that either the PY801 is a bit "lazy" (does the set take an absolute age to come on?) or the PL36 has some air in it (excessive blue glow?), that's why the resistors get cooked up and don't last very long. I like to see a 5W component in this position, that should be OK if the set is working properly.

Zelandeth 12th May 2007 9:17 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
Yep, CRT's do have a getter in them in the same way that other valves do, between that and the no doubt rather lazy cathode after not having been used for so many years - will just have to see how things do. The "tubey" appearance did seem to be decreasing somewhat as the set was run while I was testing things.

The set DOES take an age to actually start up, so the PY801 (can't remember if I have a spare or not) might be suspect. The PL36 (or rather 30P19) that was in the set was gassy (glowing a lovely pale blue colour), and has since been replaced with a new component. The EHT rectifier (DY802 if I remember rightly) was also replaced because it looked really rather well cooked.

The panel lamps are likely to be something simple like a dodgy thermistor, as they work fine for about the first ten minutes, before slowly fading out. I'll probably investigate that when I have the chassis out to deal with the broken volume control, as I *think* the suspect component is in the bundle of wires down in the front left corner of the chassis. The resistor is indicated as 2K2, 5W on the datasheet, which is what the replacement I've ordered is. Will see how things go with that once the capacitor it connects to is replaced (it does look somewhat worse for wear).

SO FAR, the width appears to be fine (I assume that the picture starting out in a tiny little box, then slowly expanding to fill the screen is normal - as I'm sure I've seen other sets do that).

The one symptom that I'm somewhat puzzled about, is that when the set is first switched on - it warms up (eventually), works fine for about a minute, then the picture fades out, behaviour of the picture suggesting the EHT dropping, before after 15-20 seconds, returning to normal again. Is this just a quirk of the set, or is it a suggestion of some other problem?

Really looking forward to actually watching something on this set now! Got a bit of work to do yet though.

Hunts smoothing bomb 12th May 2007 9:38 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
You don't reckon the panel lamps are supposed to fade out after a while do you?

I was just wondering if the set is designed so you turn it on and you have time for it to warm up, faff about tuning to find something to watch and by the time you take your seat the dial lamps are fading out?

I would of thought with the dial lamps on all the time that it would be distracting as the set is tinted so picture would be less bright.


Zelandeth 12th May 2007 10:34 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
Lee, that's a good question! If I had an instruction manual I'd be able to answer that question I guess...but I don't.

The circuit diagram may have the answer, but it's unfortunately in the flat, while I'm out at my parents house at the moment.

Um...anybody got an instruction manual for a KV024?

Studio263 13th May 2007 12:28 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
The lamps should stay on, they did in mine!

If I were you I would look at the switching first, a different lamp comes on when you switch to 405 VHF so we are back to the system switch again. Either that or the cheap bulbholders so beloved of all British setmakers...

I can't remember exactly how the lamps were powered but I'm pretty sure they were in series with the valve heaters and shunted by a resistor to keep the current sensible. It is just possible that the resistor is playing up, it should be easy to check. It's worth checking the heater conditions in general, slow warm-up and a tube that takes a while to give it's best both suggest insufficient heater current (or just a worn out old set, as it usually is for me!)

Studio263 13th May 2007 6:09 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
Right, here's the answer to the light bulb questions.

They are included in the heater circuit, the break is made between the PCL84 video O/P valve and the EF184 final IF stage. It is clear that bulbs should remain on all the time if everything is working correctly.

the bulbs should be 8V 0.3A types (two in series per standard) and are shunted by a VA1015 thermistor so that the set can keep working if a bulb fails. Switching is done by the dreaded system switch, the section concerned being S7, bang in the middle (almost...). It should have a brown, a brown and grey and a grey wire going to it. Fetch me the stiff wire, it's single-standard time! (only joking)

There are plenty of suspects here, wrong bulbs, defective system switch etc. If you have the R+TV S "red books" then the one to look in is 1968 > 1969, this covers the VC51 range in general. The VC51/1 is the version that went in the "Deep Scene" models with the inclusion of the bulbs being the main (and possibly only) difference. The full range of chassis variants is given in the following year's edition.

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