B&W TV Pye B18T Project

The first model of set I took apart when I was a kid! I managed to drop the tube but for some reason it didn't go bang, the neck just fell off! Maybe that's why it had been scrapped? 

I will follow this thread with interest - fingers crossed for the tube being OK! 👍 

I have now done further work on this set including replacing the audio output valve grid coupling capacitor. I tested the Sprague branded coupling capacitor to the grid of the line output valve which measured 16 MO leakage on the 1000v range on the Megger, so have left this in place. The frame oscillator transformer looks to have been replaced with another part of the same type, as the ends of snipped leads were present at all four points where this is connected. Strangely, the insulation on two of the wires to the existing transformer had been damaged with some strands of wire exposed, so I have put heat-shrink tubing over these.

Before powering up I was unable to get a reading across the mains lead to ascertain whether the heater chain had continuity. In the end I traced this to bad contacts on both sides of the HT fuse holder, which had a small 2A fuse in it. I cleaned up the contacts with a fibreglass pen and replaced the fuse with a 1A item of the correct size, which then gave continuity. 

I initially tried powering it up with both the variac and 100w lamp limiter in circuit, but the HT didn’t get beyond 125v. Removing the lamp limiter resulted in a line whistle being heard when the HT reached 180v and then intermittent flashes of an out of focus raster on the screen! The cause of the intermittent image turned out to be the brightness control slider making poor contact.

The image is also shaded on one corner although the scan coils are right up against the flare of the CRT. There is also severe ion burn in the phosphor, although I’m hoping that this might become less pronounced the more the tube wakes up. Adjustment of the timebase controls has enabled a full size raster to be displayed.

The raster remains out of focus regardless of adjustment of the focus magnet and is at full brightness with the brightness control not having any effect. The 2uF electrolytic cap that couples the video signal to the cathode of the CRT has visibly leaked in the past and tests bad on my ESR meter, so I will be replace this next, try and clean the contact on the brightness pot and then apply a signal.

That’s looking promising! My TV22 has an ion burn like that, not too bad with a moving picture on it. I wonder if your focus problem could be down to you taking apart the focus assembly? I did the same on the PYE LV30 that I got going a little while back, and I couldn’t get it to focus properly after that, I ended up having to get another focus assembly that hadn’t been apart to make it work properly. Hopefully all it needs is a bit of adjustment!

Regards,

 Lloyd 

My Tv22 also has a burn in the centre but it was much less noticeable when I got the EHT up a bit. My set was suffering from low HT which made everything suffer.  

@Lloyd that did cross my mind as the manufacturer's service data is very clear that the focus magnet shouldn't be dismantled, but it seems a lot better following further work.

@slidertogrid I’m now getting 252v HT (measured on a DMM) so this appears satisfactory and I think the picture has improved now I'm plugging the set directly into the mains rather than powering via the variac.

Last night I dismantled the brightness control so I could detach the sliding contact to clean it properly. I also replaced the video signal electrolytic cap mentioned above. I then applied a signal which showed the set to be tuned to channel 1.

Following more adjustment of the picture controls, the test card displays surprisingly well and in focus when the contrast and brightness are set to minimum as shown in the first photo (with a few sync issues).  The second photo shows the image with the brightness fully advanced, and the third photo shows what happens when both the contrast and brightness are fully advanced. The original EY51 EHT rectifier is still in there at the moment, so I’ll replace that next.

I am still making progress. I replaced the EY51 last week which resulted in improved focus. I am slowly working my way through replacing the wax caps and found that replacing C27 which couples the anode of V8B interlace filter to the grid of V9 frame osc resulted in hold being achieved at the mid-point of the hold control rather than at one end, and also increased the width.

One issue I’d had since first powering up was that the audio sounded off-tune with quite a bit of noise and the contrast control was making the picture bright when turned up with the picture going slightly negative at full contrast.

I noticed last night that V2 hadn’t clipped back into its socket properly from after I’d cleaned the pins, so after I’d got it in properly the noise suddenly disappeared during warm-up and the picture went much darker and lost hold. After readjusting the frame hold I found that the contrast control is now actually changing the contrast rather than making the picture excessively bright and the sound seems to be on tune, so there must have been a bad contact on one of the pins of V2. I only have a few more caps to replace in the timebase sections now, so I will investigate the sync issue next…

I've now hit a bit of a problem, which I will hopefully be able to resolve soon. I replaced C57 (ref from Trader sheet 946/T5) which is between a resistor and the line amplitude control. This looked to have been overheating and tested as nearly being short circuit.  This involved a bit of disturbance of components in the vicinity including temporary removal of the line oscillator transformer to gain access.

Now when I switch on I appear to have a short to chassis somewhere in the heater chain, as the CRT heater does not light up first as it did before, but the PZ30 rectifier glows too brightly and the other valves in the line and sound output stages light next. I haven't left it switched on beyond this point for obvious reasons.

I am suspecting one of three causes: a) I've made a mistake or caused a short by moving something, b) there could be a heater-cathode short in the PZ30 or c) one of the decoupling caps between the heater chain and chassis has gone short-circuit.

I have re-examined my work on C57 several times but can't see any issues. I have a spare used PZ30 which I repaired as it had a bent loose pin, but the heater in this one lights brightly too. I have tested C63, C64, C65 and C66 between heater chain and chassis with the adjacent valves removed to break the heater chain and all test normally on a low voltage range on the Megger. These are not brown Hunts caps, but are white ones as shown in the attached photo.  Do these have a similarly high failure rate compared with Hunts?

It may be that one is breaking down when power is applied but not under test. I will continue investigating, but if anyone has any ideas or thoughts I would be very interested to hear them!

Thanks

I found the cause of my issue referred to above, which was a mistake I'd made. I always try to avoid leaving anything unsoldered before I finish any session of work, as it's easy to forget what you did after a few days. I had to wait for some new caps to arrive in the post after having removed the line osc transformer and when I came back to it I completely forgot that I'd had to unsolder a wire from a valve base to enable the transformer to be removed. This wire then ended up tucked away so I didn't see it and of course it was the heater connection to V10 which had ended up making contact with the chassis. After reconnecting this, normal operation resumed. 

I continued work on tracing the sync issue, R28 in the grid circuit of the sync separator valve V7 had gone rather high at 4.4MO as opposed to 2.7MO, so this was replaced but made no difference. I then turned my attention to the electrolytic C23 shown on the snip of the circuit below, which decouples V6 anode to chassis and was showing signs of past leakage. Replacing this completely cured the sync issue which is a great improvement.

The main problem I have now is that the audio sounds off-tune with background hum and a bit of distortion. I still have to replace a few wax capacitors in the sound output stage and there is the cathode bypass electrolytic on the sound output valve to replace too, so hopefully this will improve matters and it's not due an alignment issue, as it does sound how my other 405 line sets sound when slightly detuned.

I also suspect there is scope for a bit of improvement in the frame time-base, as the best picture I can get at the moment is just slightly stretched at the top with the frame amplitude control at maximum.

@lsmith That is looking good now. The ion burn seems to much less noticeable. I see it as all part of the set history and shows that it has had a life. Your tube has better emission than the one in my TV22, I have to draw the curtains!  

Having spent most of the wet weekend working on this set I've now finished replacing all the wax and lower value electrolytic caps. I ended up concentrating on the RF strip before replacing the remaining electrolytics and Spragues in the timebase, as the distorted sound problem was bugging me.

Replacement of all the wax caps in the RF strip didn’t really make any difference and similarly using a 12dB attenuator in case my signal was too strong also didn’t help. I eventually traced the problem to R56 (2.2M) situated between V13B noise limiter cathode and chassis, which measured open circuit. Upon removal one of the leads from this resistor was loose and be moved back and forth so had come adrift inside the component. Replacing this completely removed the distortion.

Also, replacing C31 between the grid of the frame output valve and chassis resulted in much increased frame scan range and after replacing the remaining Spragues in the line output stage the picture is no longer slightly shifted to the left. Flyback lines are visible on some scenes, but I understand that this isn’t that unusual on a set of this design.

I had the set running for around an hour last night and I had to adjust the focus on a few occasions, so I hope the LOPT is going hold up.

I’m going to work on the cabinet next. There is a chunk out of the veneer above the screen, but I scrapped a ‘30s Pye T/Q battery radio that was rather rotten earlier this year and retained some of the veneer which is looks to be a perfect match, so I will use this to replace the missing part.

Outstanding work Laurence, the test card definition is incredible, all the frequency gratings look resolved. Just shows these sets were capable of really fantastic pictures. It reminds me of the LV20 I repaired 13-years ago, when I first launched the Vrat forum and started on TVs (scary how time passes) that tube had ion burn.

Thanks for sharing your work, there are very few members left now upholding the Black & White TV section, we all look forward to your next project. 👍

@lsmith That’s really good, 405 line sets could produce really excellent pictures like this example. 👍

Thank you for the kind words 👍 

In all, 31 capacitors were replaced along with 5 resistors and the EHT rectifier.

I'll post some more pics once I've finished with the cabinet and got it all back together.

That’s come up really nicely! Great work! I do think PYE sets give a very good picture, that LV30 that I got going is pretty good too.