Pye P45 Chassis

[Pye4T]

Thanks for your thoughts, Joe.

The replacement resistors are wirewound types rated at 3 watts. They are still testing good at near 500 ohms.

I am working from a different schematic to Geordie's example. His R16, C37, C38 and C39 are labelled R18, C32, C33 and C34 on my example (as per Jeffrey's link).

Thanks again.

[Pye4T]

Well, Joe - just for the hell of it, I went and checked my supplier's datasheet for those resistors. I ordered (and they say they sent) 3W types, but the measurements indicate they sent me 1W types!
You've got a good eye - better than my supplier's!

[ppppenguin]

Wirewound resistors can look very tatty but if they read the correct resistance then they are OK. Those capacitors will need replacing, but it should be done after you've powered the set via a lamp limiter.

I think I warned that the Trader sheets renumber the components compared to the original manufacturer data.

The fact that NOTHING happens means you first need to check the continuity fromt he mains plug through to the transformer. With the set switched on, the resistance between L and N should be a few 10s of ohms. If it's open circuit then you need to find that fault first. If the switch is faulty then just short out the bad section. If the voltage selector is faulty just short it out int he correct position. If the transformer primary is faulty then you are very unlucky.

[AdrianPH]

Looking at the trader sheet for the radio and particularly at the Voltage - current chart in it, the drop across R18 should be 55 Volts at 53 milliamps which is just over 2.8 Watts, so if you had two 500 Ohm 3 watt resistors then they should be OK. Also looking at the early pictures it looks as though the mains switch has been bypassed with shorted wires although soldering looks a bit strange.

As Jeffrey suggests check for continuity along the input and hope that the transformer is OK it should be around 38 Ohms. If the primary side reads OK then check the secondary for resistance between the rectifier anode pins and chassis, resistances should be around 285 and 305 Ohms. The heater winding should be around an Ohm. All without mains applied of course.

Adrian

[ppppenguin]

A small warning. The contacts in mains switches will sometimes show open circuit on a multimeter when they are actually OK. This is due to oxidation which is easily broken down by mains voltage but not by the low voltage from a meter.

[Pye4T]

Firstly, thank you Jeffrey & Adrian for the suggestions and information detailed above. This has been very helpful.

What I found:
I can confirm that the mains switch has been hard-wired by someone. On close inspection, one of the shorts was slightly detached (bad soldering), hence no power to the transformer.
Removing the shorts confirmed that the switch is broken. (The switch is part of the tone control but is on a separate wafer.)
I was getting intermittent results from the transformer primary. I found that the voltage selector was a little loose, so I expanded the prongs to make a tighter fit. I am now getting the predicted reading of 38 ohms.
I haven’t traced out the exact transformer secondaries, but I’m getting readings of 1, 293 and 302 ohms from three of the terminals, so I think it’s good.

I will have to source some proper resistors for R18 before proceeding (I will probably look for 5-10W types to be sure). I don’t know how I’ll get around the mains switch problem as its probably not replaceable. I could detach the switch wafer and have a closer look, but I don’t think it’ll help. I will probably hard-wire it again and see how the lamp limiter test goes.

Many thanks again!

I’ll be back!

[ppppenguin]

Don't worry too much about the mains switch for now. Leave it shorted out and switch off at the mains socket. When working on a set it's always a good idea to unplug it before doing anything. Most of us have found out the hard way that the switch in the set isn't as good as it ought to be. I like to see an air gap

Ultimately you have several options with a duff mains switch. You can put up with it (I've certainly done that before), try to find a complete replacement pot or transplant a switch from another pot.

As for R18, if the original part isn't open circuit just put it back. Otherwise a pair of 470R 5W (or more) parts will be fine. The values aren't critical. If you have any wirewound resistors to hand in the 100R to 1K range just put them in for now. They will be adequate to get some life from the set.

C43 and C44 will definitely need replacing. Even if slightly leaky these will put a positive voltage on the grid of V3. The result is that V3 will overheat and possibly cook the audio output transformer too.

I've noticed that the volume control, V15, is also the grid leak resistor for V3. This is not a good arrangement. Any intermittent contact in the wiper will cause excess current in V3. Also if V3 is passing any grid current (it shouldn't but it happens) the volume control action will be horribly crackly. Not a worry yet, but something to be tackled later.

[Pye4T]

One of the challenges of that trader sheet is the mixing of circuit information. I don't think the P45 has a C43 or C44 - they belong to the P45RG?

[ppppenguin]

I mean C43 and C44 as shown in this screencap. I can see from the note that the complicated switching refers to the radiogram version. In any case, if there's a capacitor that has one end near HT and the other to the grid of the output valve it will normally need to be replaced for the reasons I described. Sometimes known as "that capacitor".

[Geordie McBoyne]

Extract from the Pye P45 circuit diagram which appears in Volume 1 of the Malloy and Poole radio and television serving books.
The radiogram version of this chassis might employ V2 (EBF80) as the gram amplifier.

Geordie McBoyne.