Pye VT4

[Murphyv310]

Hi Terry.
I understand it wasn't meant as political, I just made the comment to let the mods know I have no intent of making it such.
I'm intrigued though by the last sentence??

[peter scott]

1st of April??

[Terry]

Before noon?

[Murphyv310]

Ah. Sorry my brain is in turmoil, too much happening here right now.

[ppppenguin]

Having got some light on the screen of the first set, yesterday I thought I'd take a look at the other. First check showed an open circuit heater chain. Flurry of cold resistance checks showed what I should have found with Mk 1 eyeball in seconds. Somebody had nicked the ECL80 sound output valve. I've got plenty in the stores but I borrowed the one from the other set. Heater chain was fine. Next reform the main smoother can using bench supply. Leakage dropped to a very acceptable 0.5mA for both sections together. Disconnected leads and went to do something else. Came back to resolder connections to the cap. Assumed it had discharged itself by then. It hadn't I was clearly not having a good day.

Bring up HT on variac, sound appears immediately, line whistle audible and adjustable to something like correct frequency. No video on CRT cathode. No trace of light yet buit no signs of distress either. LOP can still on the set so can't check for sparks yet.

As a sideline, there's an interesting effect as to how different meters read AC current. I've got the variac supplying HT via a 1N4007. My true RMS Fluke 87 is in series to check current and reading around 300mA. The variac's moving iron ammeter is showing not far short of 1A. The 2 meters obviously take a very different view of the ripple current. I know I should really take the reading after the reservoir cap but it's less convenient.

[ppppenguin]

Not even a hint of sparks at the top caps in the LOP. Plenty of drive to the PL81, at the correct frequency though not locked. Just under 100mA going to the LOP as measured with the LOP fuse pulled. Nothing showing any distress. The LOP screen grid resistor was replaced before my time with the set by somebody who wasn't taking any chances. It's 5W wirewound! 47R LOP cathode resistor is OK.

I'm puzzled. I should see some evidence of high voltages.

[Murphyv310]

Scan coils can be a problem on them. Check the boost cap C106. One turn shorted on the overwind will damp the line stage completely resulting in no sparks on the PL81 or EY51. Oh one other thing EY51's can collapse internally and that can damp the line stage.

[ppppenguin]

I can easily change the boost cap, disconnect the EY51 and scan coils. With care I can disconnect the LOPT and ring it to see if there are any shorted turns. I would have expected any of these failures to cause some signs of distress. Or am I wrong?

[Murphyv310]

Hi.
The V4/VT4 doesnt have the best actual LOPT, it can struggle to give a really bright picture as the EHT can dip. I have two LOPT's here with duff over winds and other than no sparks there is no signs of distress, you should even still hear a line whistle if your ears are better than mine!

[ppppenguin]

This set has definitely been got at. I went to replace the boost cap. Had to search for it as it's right over the other end of the chassis from the LOP with one end wired to the sound OPT to pick up HT. A bit odd as it was a 1uF part (which measured as a 47K resistor!) rather than the specified 0.5uF. It had clearly had its leads extended and I realised what had been done. The VT4 has a 1uF and 1M2 to decouple the CRT A1 supply. Some hard pressed tech had presumably "borrowed" that cap to replace a duff boost cap. Curiously the V4 uses 10M and 100n to decouple the A1 supply. Trader 1198 sheet with the VT4 addenda doesn't mention the change but I have both the V4 and VT4 Pye data downloaded from Jon's site. The only problem with those downloads is that they don't have a physical component layout. So I'm using both the Pye and Trader 1120 data.