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Old 12th May 2024, 11:21 am   #1
Martin G7MRV
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Default Restored Pye 15A died - mains transformer failure?

The pye 15A I have lovingly/frustratingly restored has died oddly.

I'm just wanting suggestions of what my be the issue based on the few symptoms I can give. Unfortunately I cannot do further testing just now as it's on display today.

Essentially, having been working perfectly, after moving it to the museum all I could get was hum. No stations, no response to signal generator or even a signal on the IF frequency. No response to an audio signal via the pick up input. A brief test at the time showed the top cap voltages fine. Today, shes totally dead. Not even the dial lamps working.

I cant take the chassis out at the moment, and we're just about to open so can't just now do any further above chassis checks. The fuses are fine.

Could this indicate a failure of the mains transformer? I thought perhaps the rectifier but then surely the dial lamps at least would be lit? The previous hum suggests to me something died slowly.

So really just wanting to know likely causes so when I can open her up I've got a better idea where to start!
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Old 12th May 2024, 11:46 am   #2
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Default Re: Restored Pye 15A died - mains transformer failure?

You could start by putting an Ohm-meter across the pins of the mains plug and see if the on-off switch works.

The resistance of the primary winding is usually in the range 30 to 50 Ohms.
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Old 12th May 2024, 3:15 pm   #3
cathoderay57
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Default Re: Restored Pye 15A died - mains transformer failure?

If the mains transformer primary has approx correct dc resistance then look for short circuits in the heater wiring and pilot lamps. Damaged insulation or a stray solder blob could be enough to cause the problem, initiated by moving the set. If the heater supply were s/c the transformer would certainly hum. best of luck. Jerry
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Old 12th May 2024, 6:04 pm   #4
Leon Crampin
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Default Re: Restored Pye 15A died - mains transformer failure?

On/off switches fitted to potentiometers are not immune to failure. As stated, an ohmmeter across the plug pins is a good first step. This can also reveal an intermittent or lazy switch without hammering the electronics under power.

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Old 12th May 2024, 6:21 pm   #5
Martin G7MRV
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Default Re: Restored Pye 15A died - mains transformer failure?

Thanks, I was so rushed earlier it simply never occured to me to measure the resistance across the plug pins!

Well, curiously, im now home and have the offending set on the bench, although not yet out of the cabinet. And.... the pilot lights are on! I can also see the heaters are glowing in all the valves. But... measuring the voltages on the top caps of each valve (except the rectifier of course which hasnt got one) looks odd.

Now heres where im somewhat unsure of terminology. The service sheet has screen grid voltages listed, is g1 the screen grid? and therefore the top caps? If so, the sheet says 100V for V1 ECH35 and V2 EF39, and 275V for V3 the EBL31. Im reading just over 1V on V1 and V2 and 23V on V3!

I need to get the knobs off and get the chassis out to check the anode voltages.

The transformer primary reads 20 ohm. If the HT is low for whatever reason, then that probably accounts for it being totally dead at the museum, where I suspect our mains is a little on the low side.

All the obvious capacitors have already been replaced so it shouldnt be one of those failed.
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Old 12th May 2024, 6:26 pm   #6
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Default Re: Restored Pye 15A died - mains transformer failure?

Grids number from the cathode towards the anode.
Normally:-
g1 = Control Grid.
g2 = Screen Grid.
g3 = Suppressor Grid which is often internally connected to the cathode.
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Old 12th May 2024, 6:48 pm   #7
Martin G7MRV
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Default Re: Restored Pye 15A died - mains transformer failure?

Thanks Graham, thats very useful

Well, I was just measuring the anode voltages, which all seem too high at 325V, when I noticed that one of the wires to the audio transformer primary has come off!!!
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Old 12th May 2024, 6:48 pm   #8
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Default Re: Restored Pye 15A died - mains transformer failure?

No the top caps are G1...the control grid....and you shouldn't have any positive voltage on the control grid! G2 is the screen grid and I think you'll find that's pin 4 on most common octals although the EBL31 has it on pin 6. If you are measuring 23V on the top cap of the EBL31, you have a serious problem. Double check that....the audio coupling capacitor isn't blocking high voltage DC in this circuit so the 23V (if that's correct) present on the top cap (G1) can only be due to leakage within the valve. Best thing is to check all the voltages on the valve pins against the circuit. If necessary, list them and post them here (list the voltages you measure against the circuit voltages).
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Old 12th May 2024, 6:49 pm   #9
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Default Re: Restored Pye 15A died - mains transformer failure?

That could mean that all the output valve's current is flowing via the screen which is very bad for the valve.

EDIT. Posts crossed. I was replying to post #7.
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Old 12th May 2024, 7:29 pm   #10
Martin G7MRV
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Default Re: Restored Pye 15A died - mains transformer failure?

Well now working again with the wire reconnected. Hopefully the brief period it was powered in the faulty state hasn't done too much harm
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Old 12th May 2024, 8:16 pm   #11
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Default Re: Restored Pye 15A died - mains transformer failure?

Valves are pretty forgiving as long as the power wasn't on for too long with the poor output valve having a red-hot screen grid! EBL31's are scarce and expensive so no point in reducing the life of an otherwise good one.
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Old 15th May 2024, 2:30 pm   #12
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Default Re: Restored Pye 15A died - mains transformer failure?

Re post 8. sorry but there should be a voltage on the top cap of the EBL31. The 15A does not have the usual coupling capacitor found in many designs.

The grid is connected via the volume control track and R9 to a tap in the cathode voltage potential divider formed by R11 and R12. The circuit shows a cathode current of 39.2mA and R12 as 470R so the voltage at the top cap should be around 18 volts, though on my 15A it measures 13 volts.
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