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-   -   Pye VT4 - no EHT (https://www.vintage-radio.net/forum/showthread.php?t=164349)

Vectamart 29th Feb 2020 2:01 pm

Pye VT4 - no EHT
 
I have a few of these in my collection and decided earlier today to have another go at getting this particular one working. I am puzzled by the fact that I am able to draw a healthy arc from the input of the EY51 but there is bo DC output.

What puzzles me more however is that there is hardly any spark at the top caps of the PL and the PY. If I had a decent spark on the PL and the PY but not at the EY51, I would be pointing the finger of blame at the LOPT.

Any thoughts?

high_vacuum_house 29th Feb 2020 2:30 pm

Re: Pye VT4 - no EHT
 
Good afternoon,
I presume the EY51 heater is lit, the valve hasnít lost its emission or there is a failure in the EY51 heater winding on the LOPT. I would check all these things first as these would cause no DC output from the cathode of the EY51.

Christopher Capener

Vectamart 29th Feb 2020 2:54 pm

Re: Pye VT4 - no EHT
 
Quote:

Originally Posted by high_vacuum_house (Post 1221304)
Good afternoon,
I presume the EY51 heater is lit, the valve hasnít lost its emission or there is a failure in the EY51 heater winding on the LOPT. I would check all these things first as these would cause no DC output from the cathode of the EY51.

Christopher Capener

I did change the EY51 but cannot see any illumination from its heater. I am tempted to try substituting the EY51 with a solid state device such as a TV11 stick or similar. I am still baffled by the smallness of the sparks at the top caps of the PY and PL valves - or maybe this is normal for these very old sets? Perhaps I am too used to seeing the bigger sparks on later sets e.g. 1960s/70s.

TonyDuell 29th Feb 2020 3:11 pm

Re: Pye VT4 - no EHT
 
You could disconnect the EY51 heater wires (set turned off of course) and then

a) check the continuity of the heater winding (it'll show as a dead short on any normal ohmmeter, it's just a few turns round the LOPT core, I think).

b) power the EY51 filament from a 6V PSU or battery (again, set turned off) to see if it glows and how bright/visible it is

Techman 29th Feb 2020 3:34 pm

Re: Pye VT4 - no EHT
 
I agree, but un-solder and remove the EY51, or at least disconnect one side of its heater before checking the continuity of the few turns of heater winding on the transformer or trying to power the heater from a separate supply. If you power the heater with it connected to its supply winding, you'll short out and possibly damage your supply due to the very low resistance of the winding. You could check the continuity of the heater with a meter once it's disconnected, but this won't tell you if the valve actually has no emission due to ingress of air, where the heater will still have continuity, but will not glow.

From what you say about a poor spark from the tops of the PL and PY, it does sound a bit like somethings damping the whole output stage and this could be just a damp transformer - I'm sure you've read the instructional threads on how to deal with this. It could be a leaky boost capacitor or coupling capacitor to the PL. At worst it could be the LOPT has failed somewhere on its overwind, but hopefully not - dry it out first if you haven't already done so.

Heatercathodeshort 29th Feb 2020 7:09 pm

Re: Pye VT4 - no EHT
 
Agree there Techman
Two hot spots on the V4/VT4 chassis are the line output transformer and the scan coils in that order.
As suggested the boost capacitor and the line coupling capacitor to pin 2 of the PL81 must be replaced. The width inductor can go shorted turns dampening the LOPT. Disconnect it for a test. The chassis will work very well without it.
If you have a healthy spark at the anode of the EY51 [not shorting the test screwdriver blade to chassis] either the EY51 has an O/C heater or maybe the heater winding is faulty. You can wire in a 6v radio panel bulb as a test after checking the windings continuity of course.
It is possible the EY51 has an internal short but this would damp the anode spark.
Just a few things to try. John.

Sideband 29th Feb 2020 7:20 pm

Re: Pye VT4 - no EHT
 
Quote:

Originally Posted by Vectamart (Post 1221294)
I have a few of these in my collection and decided earlier today to have another go at getting this particular one working.

You don't say if the others in your collection are working. If at least one is, why not do a side by side comparison. You can check voltages and scope waveforms between the non-working and working sets.

I used to do that when I was in the trade and had an odd set and wasn't sure what was going on.

If you have a healthy spark at the input but no output, it suggests to me that there is plenty of line drive but the output is being damped. Try unplugging the EHT lead from the tube, insulate it and place it well out of the way of anything else and see if you then get a healthy spark from the cathode. Does the TV have an EHT smoothing cap (visconol) or does the CRT have an aquadag coating? If the former, it could be the visconol.

However John (HCS) has a good point about the scan coils. If the line scan coils are O/C this can cause the exact effect you have.

Heatercathodeshort 1st Mar 2020 10:39 am

Re: Pye VT4 - no EHT
 
The CRT is a Mullard MW36-24/44. It is aquadag coated and does not require a Visconol. The scan coils go S/C frame to line due to Cellophane insulation between the windings breaking down. Later trouble free coils use Polythene.
They can be disconnected for a test noting that a S/C in the screened wire may also cause problems.
These early rectangular Mullard tubes suffered from inter electrode shorts usually between the A1 and grid causing heavy EHT drain. They can be rewired as triodes but let's not go there until proven! John.

Vectamart 1st Mar 2020 11:41 am

Re: Pye VT4 - no EHT
 
The update is that I have substituted the EY51 with a BY187-01 and now have a raster. Brightness looks OK. There is a motorboating noise emanating from the speaker which various according to the frame speed. I have replaced the PY81 with a PY800/81 and the PL81 and the warm-up time has become much shorter, I read somewhere that the original PY81 valves took around 3 minutes to reach full emission.

The new PL82 made no improvement to the frame scan but I suspect that the cathode bypass cap probably need replacement. Now that I have a raster of sorts, I will go right round the set methodically one stage at a time.

Many thanks to all those who have posted comments so far.

FERNSEH 1st Mar 2020 11:50 am

Re: Pye VT4 - no EHT
 
Link to the electrically similar Pye CTM4 serviced in 2018:

https://www.vintage-radio.net/forum/...d.php?t=146572

DFWB.


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