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-   -   ITT VC51/1 Gremlin Hunting - Stage 1. (https://www.vintage-radio.net/forum/showthread.php?t=16810)

Hunts smoothing bomb 13th May 2007 7:32 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
 
OK,

I would say that the fading is definately down to the thermistor, my guess is that the wrong bulbs are fitted therefore more current is flowing through the thermistor and heating it which lowers it's resistance and then more current flows and eventually the lamps go out.
The other suspect could be that the mains volts tappings have been set to low causing too much current flow again heating the thermistor and leading to the extinguishing of the lamps.

Cheers
Lee

Zelandeth 13th May 2007 8:59 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
 
Thanks for the responses, guys.

Will get one of the bulbs out to see what they're rated at (though brightness wise they look about what I would expect), after that will take a look at the thermistor. Given the slow, smooth (amost intentional looking!) way they fade out over a period of five to ten seconds, I tend to think that it must be that at fault - a dodgy connection would be rather more erratic I would have thought.

I don't have the guide, but someone did send me scans of the relevant pages, so I've got the diagrams and such (though no doubt such books will become a necessary purchase at some point...I've already discovered that this is an addictive hobby).

Zelandeth 17th May 2007 8:28 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
 
Been fiddling around with this set some more.

Replaced quite a few of the caps now (which is proving somewhat fiddly due to the replacements being somewhat larger than expected (if/once the set is running to an acceptable level they may get re-replaced in some cases with caps of a slightly more sensible voltage rating). This was due to RS being out of stock of the closest equivialant values, so these being significantly overrated in terms of voltages. By orders of magnitude in some cases!

I haven't actually re-taken any of the voltage readings yet, but can state that the set is running rather a lot better in terms of stability. The horizontal and vertical hold controls generally had to be juggled every time the set was switched on in the past to get the picture to lock, and now and again when tuning into stations - I've not had to do that once today.

One thing I have noticed is that the contrast seems to "flicker" quite a lot (the control isn't the culpret), the black level seems to be pretty constant, but the brightness of light areas of the screen varies a lot from moment to moment. Not sure if this is symptomatic of a tube fault, or if it's just something like a duff cap I've not found yet.

I have noted that the line whistle seems to be rather quieter, and that the PL36 seems to be running a lot cooler now - though I'd not bet money on that being true.

Panel lamps are going to require a little more digging than I had hoped, as all the bulbs are 8v .3A examples, as they should be. Is probably a duff thermistor, but I'll have to find it first.

Zelandeth 17th May 2007 10:25 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
 
1 Attachment(s)
Okay, would appear that we've got a new symptom...one which is taking me a bit of time to figure out!

Partly because what I'm seeing on the set is contradicting what I'm seeing on the schematic.

The arrowed resistor in the picture attached was previously a 2.2K wirewound resistor (or rather two 1.1K's wired in series), clearly replacements - and rapidly turning from a wirewound resistor into a wire UN-wound resistor. Hence its replacement.

Now...according to the schematic, this bit of the circuit should be pretty simple...but this doesn't seem to match it.

I've identified D10, which according to this is in series with R165 (found), a fuse, and the dropper - then into the heater chain.

This resistor however is taken from the hot side of the fuse, and goes into pin 8 of the CRT. As far as I can tell...there's no evidence of this on the circuit diagram (That I can see anyway).

Though a little thought appears to have allowed me to figure it out. This is supplying additional volts to the CRT heater. This is NOT what one wants to find. Appears to be boosting the heater voltage from around 5.5 in standard form, to around 9V.

Um...so I guess this is where I start looking for another A47/26W CRT is it...

Blast.

Steve_P 17th May 2007 11:03 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
 
Looks to me like the Tube's been heater boosted. It was a common practice. So what do you do?

1: Put up with it.
2: Short Term - Lower the resisor to say 1.5k and boost it even more.
3. Advertise for a new tube.
4. Get the tube regunned.

Up to you now!

Cheers,

Steve P

Zelandeth 17th May 2007 11:56 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
 
Forgot to note that my symptom there was that my replacement resistor was confusing me by glowing cherry red when it shouldn't be according to what the old one was rated at. Will figure that one out another day!

1 isn't really an option, as the picture's well too dim to actually be watchable.

2...well...will experiment and see if I can get any improvements without ending up with an o/c heater.

3. Will be doing that.

4. Would LIKE to do that...but reckon that the cost would be prohibitive. Unless someone wants to prove me wrong!

Don't suppose that someone has a list of compatible CRT's do they?

Steve_P 18th May 2007 12:22 am

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
 
My Valve + CRT book says there aren't any! Does anyone else know any different?

Cheers,

Steve P

Studio263 18th May 2007 8:44 am

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
 
It used to be a popular bodge to run a big resisitor (usually 4K7 I think) from the mains input to the tube heater. This picked up the volts a little at that point and livened up the emission a bit. The set I remember is most clearly on was the Philips 210A, the last few in use seemed to have all had it done!
Obviously it is not a nice thing to do and not recommended, the real answer is another tube. An idea would be to remove the non-standard parts and repair the set as best you can with the pootube you have. Then when a good one comes along you can just drop it in.
Modifications like this to the heater circuit could affect the functioning of the bulbs, do they work normally once the resistor is removed?
I noticed another resistor across the dropper (the vertical white one). This looks as if it has been fitted to bridge out a defective section. Doing this is OK but it used to be considered wise to remove the wire completely from the original failled section just in case it joined up again when things get warm. Unlikely I know!

Zelandeth 18th May 2007 5:44 pm

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
 
That's basically the plan - get the set up and running well enough for now, then sit down and wait for a tube to appear...if it ever does.

The white wirewound resistor on the dropper is indeed bridging a defective section - what you can't see in that picture is that it's actually blown a stinking great hole in the windings around the back of the dropper - it ain't gonna be making any intermittant contacts! I will bear the pointer about removing the wire in mind though when I get to the "tidying up" stages.

Need to do something properly to tidy that bit of the set up anyway - there's wires, wires everywhere, one of the terminals on that replacement dropper resistor is perilously close to touching the heat shield, the automotive spade connectors are a mess...and the whole thing's full of cobwebs.

As for the panel lights, removing that resistor seems to make them stay running for longer, but they do still go out.

The really annoying thing is that the set seems to be working (panel lights and broken control shafts on the 405 fine tune and power/volume control aside) perfectly fine now!

Will finish tidying up this, put a wanted ad out for a new tube, then might attack the GEC. Not so sure though, as I've no way of feeding it a signal just now.

Just figures I'd pick a set with a tube without any readily available equivialants...

Focus Diode 19th May 2007 10:01 am

Re: ITT VC51/1 Gremlin Hunting - Stage 1.
 
I would've thought any 19" Rimbanded CRT would be sutiable, although spark protection would have to be introduced on CRTs ending with "R" or "S", eg: The A59-23WS as fitted in my 23" Ferguson 3655.

Tubes such as the AW49-91 would of course NOT be sutiable as they require protection from implosion.

Brian


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