Another TV22
 

Hi all,

Some of you may recall some time ago when I was asking some questions regarding the TV22 lopt, at the time I stripped it of its original black pitch, dried it out and coated in transformer varnish.

I have recently been working on said TV22 a little more, it's not been my main focus and it's very much shoved in a corner and brought out when I fancy doing a little more. It was evident that the set has been well and truly 'got at' in the past, some very questionable repairs were present and the two adjustable coils for Aerial and RF were screwed in so far that the iron cores were all the way out on the other end. One of these cores has snapped off, leaving only a small bit remaining so I'll have to see if I can still get a signal through with what I have left until I find some sort of replacement.

I have now got to a stage where the set powers up, most caps have been replaced, I have a fairly strong line whistle and the CRT has just been replaced with the MW22-17 I had spare.

The first thing to investigate is a complete lack of raster from the CRT, it looks to be caused by a lack of A1 voltage, CRT measurements (to chassis) are as follows:

Grid: 91.5v
Cathode:106v
A1:30v

EHT is a little low at 5.3kV, but I'm not too fussed for now until I start getting something on the screen. I need to find all of my service info which I've put somewhere and forgot, hopefully it will be quite straight forward from here on!

Re: Another TV22
 

The Metrosil or the 0.1uF decoupler (C28 on the trader sheet) would seem to be the likely cause.

Re: Another TV22
 

Thanks Graham,

I've checked the voltage before the metrosil and it looks as though the problem could be further down the line. R30, the 1.2k on the side of the chassis (I've currently got a 1.5k fitted) is also getting very hot, this was a fault that had happened before on the set as the paxolin board was burned badly and Andy Beer kindly supplied a replacement which I fitted. Voltage before the metrosil is jumping around between 268 & 274v. I've found some service info although it is for the early model using EF91s (mine is using EF80s) but most of the main deck should be the same I'm sure.

Re: Another TV22
 

R30 does tend to run warm.
Have you replaced C28 0.75uf? What service sheet are you referring to incidentally as not all use the component designations.
If its the R/C network I'm thinking about it's the odd Bush hum bucking circuit which is about as good as a chocolate teapot, I always remove it.
You have low A1 and it's OK...ish at the metrosil but what side? On the decoupler (C18 RTV volume 2 page 109) you should have 300v dependent the ohms per volt of your meter if its as low as you say its either the metrosil or the decoupler that's hidden behind the tag strip, other slim possibles are a leaky tag strip or a fault in the CRT.

Re: Another TV22
 

Hi Trevor
Thanks for your thoughts, I have replaced the cap but for now I will remove it so I can focus on the rest of the set and come back later, either the cap is no good or there is some odd fault going on. The voltage feeding the metrosil from the lopt primary is jumping around 268-274, whether this is a fault or caused by the load of my meter I don't know. After the metrosil /A1 around 30v.

I have just discovered the decoupler you mentioned, I replaced the other cap that fits under the board but missed this one completely! Hopefully this is S/C and causing the problem, I'll change it and see what improvements are made.

Cheers
Bren

Re: Another TV22
 

They usually go a bit leaky.
It's a high impedance circuit and even a small leak will kill the voltage.

Re: Another TV22
 

Well that's sorted it (somewhat). I now have 68v on A1, still much lower than it should be but I now have frame collapse on the screen so at least visually I have something to work with. It does look as though it is trying to scan, an occasional flicker as photographed appears every few seconds. I'll investigate more tomorrow but so far it's good news.

I have to say, I've done a couple of TV22s previously and this has been the most reluctant to get going. Do you ever get a feeling that a set is just all too happy being left alone in its retirement :-D

Re: Another TV22
 

Hi.
I'd replace the metrosil with a 2 2m resistor and a BA159 Diode with the cathode toward the CRT wire them in series with the resistor connected to the Diode anode.
One thought, when you check the A1 volts does the line disappear? If it does I would think your meter may have a low ohms per volt.

Re: Another TV22
 

Is your set equipped with the Mark 1 chassis? Easily identified by the large screening can around the line output stage.
If that is the case, the first anode voltage is rectified by a disc shaped metrosil device. This non-linear device rectifies the scanning waveform to produce a voltage which is 150 volts above the voltage at the cathode of the CRT, about 300 volts max.
I've found replacing the metrosil with a diode results in having too high A1 volts. A potential divider may required to adjust the A1 volts to the correct value, i.e. 300V.

DFWB.

Re: Another TV22
 

Thanks both, I know these metrosils do give trouble as there is an abundance of threads and thoughts online as to their replacement. I do recall (I think) Andy Beer had created a small circuit to replace the metrosil, but I can't seem to find it anywhere. For now I have something on the screen so I can carry on work but this metrosil will have to be replaced. It is the Mk1 chassis, large screening can, EF80 RF deck.

I'm sure I checked the frame blocking transformer (dc resistance) and it measured okay, I'll re-check tonight and investigate the frame collapse. I did have some signal yesterday but I soon lost it again so some more work on the RF deck is required.

Re: Another TV22
 

Hi.
Agree that a potential divider "may" be required. My Frankenbush certainly needed a 4.7m returned to chassis with the circuit I described above, using the standard Bush LOPX I had around 320v on the meter but no doubt rose a little in real life. If the picture looks washed out it can mean the A1 volts are too high.

Re: Another TV22
 

Hi EF80TVVALVE,
I've attached a little circuit to replace the metrosil on a TV22 that used to be on my website (which I haven't had for a long time).

Cheers
Andy

Re: Another TV22
 

Hi Andy.
I'd not use the 1N4007 as a 10khz rectifier. I've tried them previously as a metrosil replacement and find they run warm as the recovery time is too slow, with about 10% loss due to this. In fact using a BA159 the voltage is higher because of less losses.

Just my thoughts as the diodes are only designed for low frequency rectification.

Re: Another TV22
 

Hi Trevor,
you are right though in that it is not the best choice, it's all I had at the time. Yes a BA159 or UF4007 would be better.

I tried it successfully in the past. but that might be because the diode feeds the 1.5meg resistor before the capacitor. If the diode was connected directly to the 0.1uF cap (and then the potential divider across the capacitor), then it could be more of a problem.

Cheers
Andy

Re: Another TV22
 

The UF4007 or 4008 are excellent choices Andy.
Just depends what you have in all honesty.

Re: Another TV22
 

Thanks for the advice everyone, I do have some 1N4007s so I'll try andy's circuit first and see how I get on with it, now I know exactly what the metrosil is doing I can have a play around there.

As for the frame collapse I'm getting a bit stuck so any pointers would be appreciated. I've checked the frame blocking transformer which measures okay as well as checking the circuit around V15 ECL80 (I'm using trader sheet 1003/T15), everything checks out fine so far. As I said earlier I'm getting a flash which looks to be a raster very well timed at every second or so, as if it is trying to scan.

I'm going to check some voltages and see if it tells me anything but so far the circuit checks out okay.

Re: Another TV22
 

What's a "metrosil". The other side of the pond is clueless.

Re: Another TV22
 

Metrosil is an early trade name for a VDR, Voltage Dependant Resistor.
Of course we are more used to using VDRs across a supply as a shunt regulator, a bit like a zener. However, when fed with a waveform with a narrow pulse (ie not a square or sine wave etc.) then it can be used as a low current series rectifier. This seems an odd use to us today, but then again I guess we would just use a silicon diode.
The rectified voltage is a function of the input voltage, the pulse frequency and width, the value of the smoothing capacitor, the load and the characteristic of the VDR. So in the TV22 application it helps turn a short pulse of about 1KV into a smoothed 300V. It also used in the same way in the Ferguson 941T of 1949, but I've not personally seen it anywhere else.

It is possible to use a modern VDR in place of the metrosil, but I have never got around to finding out which values work best, though I believe others have tried it.

Cheers
Andy

Re: Another TV22
 

Hi
This issue is detailed on pp. 19-20 of Hugo Holden's comprehensive report:
http://www.worldphaco.com/uploads/BUSH_TV22.pdf
I have tested his recommended VDR on my tV22 and the behaviour is excellent.
Helder

Re: Another TV22
 

Thanks everyone for the replies, I'll hopefully be looking into the metrosil this weekend. For now, before I do that I need to sort this frame collapse. With fresh eyes today I decided to just re-check all of the circuitry around V15 (ECL80) and the frame blocking transformer. I also checked the scan coils for their DC resistances and cleaned some valve bases to ensure good contact. I've also double checked values of components which are correct, a few resistors are a bit out but nothing that I'd think would cause this. I'm just going to re-check some voltages around the ECL80 now, last night I checked the triode side of the ECL80 and the anode voltage was around 200v, according to the data I have this should be around 100v but other than this I can't see anything else so far. I'll update with my findings soon.