PYE P 35

[Lindsay]

I have undertaken to try to get a friend's father's P 35 going again. My experience is very limited to making radios/amps and even an analogue computer in the 1950/60s.
The P 35 barely gets LW signal but has improved, now getting faint MW progs after replacement of the rectifier valve AZ 31 and the det. and amp valve EBC 33. There is still much crackling and buzzing but my next move will be to replace the condenser on the output valve. Can anyone help? I live in Selkirk in the Borders. Thanks.Lindsay

[Refugee]

Make sure the main power supply capacitor is not getting hot. It is usually a big can.
The g1 capacitor is a good start too.
I would go thru all the wax capacitors one at a time as they all get a bit leaky.
Some of the resistors may have drifted in value too. You can measure them with the resistance range on a modern meter.
I shot of switch cleaner on the wave change control will also go some way to getting rid of crackles.

[ppppenguin]

Lindsay, welcome to GVR. You'll find the help you need from our friendly crowd.

Do you have the schematic? The Trader index says it's 958 but that seems to be for the P19. The sets may be very similar but I don't know.

Repalce the output stage grid coupler capacitor ( I think that's what Refugee is calling the g1 cap) as a matter of urgency. It may save the output valve and transformer from a nasty fate. It's 0.005uF according to trader 958. A modern 4n7 (4700pF, 0..047uF) plastic cap rated at 250V or more is ideal. Most of us would use 400V or 630V parts simply to save keeping multiple voltage ratings to hand.

Refugee's other advice is all good. Use switch cleaner such as Servisol. Don't use WD40.

[Lindsay]

Make sure the main power supply capacitor is not getting hot. It is usually a big can.
The g1 capacitor is a good start too.
I would go thru all the wax capacitors one at a time as they all get a bit leaky.
Some of the resistors may have drifted in value too. You can measure them with the resistance range on a modern meter.
I shot of switch cleaner on the wave change control will also go some way to getting rid of crackles.

Thank you for such prompt advice. I have downloaded the service sheet and circuit diagram and will try to understand it. I will go through the various things and get hold of the switch cleaner to see if that helps. The g1 capacitor on my diagram is designated 'C40' and is .005 microfarads @ 1000V.m I'll get one and replace the one inside the P35. (I'm way over my head here but I will not give up yet)

[Lindsay]

Lindsay, welcome to GVR. You'll find the help you need from our friendly crowd.

Do you have the schematic?  The Trader index says it's 958 but that seems to be for the P19. The sets may be very similar but I don't know.

Repalce the output stage grid coupler capacitor ( I think that's what Refugee is calling the g1 cap)  as a matter of urgency. It may save the output valve and transformer from a nasty fate. It's 0.005uF according to trader 958. A modern 4n7 (4700pF, 0..047uF) plastic cap rated at 250V or more is ideal. Most of us would use 400V or 630V parts simply to save keeping multiple voltage ratings to hand.

Refugee's other advice is all good. Use switch cleaner such as Servisol. Don't use WD40.

Thank you for your advice and the welcome. I will get hold of the condenser and some servisol and see where I get to after that. This is quite a learning curve for me!!!

[ppppenguin]

Which service data are you using? The Trader sheets have different component numbering to manufacturer's data.

For that capacitor, 400V rating is ample. They probably only fitted a 1000V part because they had them to hand when the made the set. As I said before, the modern capacitor values are mainly in a 1/2.2/4.7 series. The exact value isn't critical in most places. So that 0.005uF (5nF, 5000pF) part can be replaced by a modern 4.7nF (often written 4n7). Most of the other wax capacitors will also need to be replaced bu that's the one that really matters at this stage.

The set would actually work with just about anything in that position from 2n2 (might give lack of bass) to 100n or more.

Reemember that all modern material calls these things "capacitors". Condenser is a lovely old word but you won't find it in any modern catalogues.

[Terry]

Some of the resistors may have drifted in value too. You can measure them with the resistance range on a modern meter.

It is easy to get confused when doing this, particularly when using a DVM that gives very precise readings. You are most likely to find 20% tolerance resistors in this set.

This makes it very easy to jump to the misleading conclusion that all of the resistors have changed in value when they may, in fact, have not changed since the day they were manufactured! Some will have changed, of course, but not necessarily outside their tolerance band.

Some years ago I drew this diagram to show the tolerance ranges involved for 20%, 10% and 5% resistor ranges which you might find useful. It also explains the apparent increases in the gap between successive values as you go up each decade - something which puzzled me as a schoolboy (mumble) years ago!

[Murphyv310]

Hi.
Welcome to the Forum, you'll get sound advice form the people here.
It's always an idea not to run up a set after years of not being used. I often say to people would you start a car that has been sitting for 30 or more years without checks etc.
Inevitably these sets will as has been said need a few capacitors replaced. Just plugging in a radio to mains is fraught with possible problems, the main smoothing capacitors need a slow awakening so one of the things you should make is a lamp limiter if you dont have a variac, this reduces the current available to run the set as the "Bulb" is in series with the mains to the set, you can use different wattage bulbs to vary the current available to the set. A filament or Halogen bulb has to be used and wattages from 15, 25, 40, 60 & 100 can be used to gradually bring up the set. When it comes to resistors valve sets are very forgiving with values that have gone high and I'd concentrate on coupling and de-coupling capacitors initially. As has been said the grid capacitor to the output valve should be an automatic replacement and the smoothing capacitors not subject to sudden full voltage, the fact you have replaced the AZ31 makes me think the set is drawing more current than it should, even a tired AZ31 will normally work fine in a radio, the AZ31 is what is known as a directly heated rectifier, a good enough valve but they pass current very quickly and if running with full mains the smoothing capacitors charge up very fast with no load from the other cold valves, another good reason to make a lamp limiter or use a variac.
Take your time don't rush and enjoy the restoration.

[Lindsay]

Many thanks to Murphyv310, Ppppenguin and Refugee for all your helpful comments. 
Firstly I downloaded a manufacturer's service sheet from a site named 'Stenning'. The circuit diagram was included and the numbers for components are the ones in that bundle. That is what I have been working from.
  i had a good look at the innards yesterday 
and discovered a resistor which had become disconnected. The lovely ladies at Pye in 1951 had trimmed the ends of it so that, after a lot of checking, there was only one place it could have come off, so I soldered it back on. The solder had changed in appearance from normal shiny to a rough crystalline surface so I reckoned that time had got at it.
  Lo and behold, I got acceptable reception on MW and even Radio Zagreb at 5 and 9 on the 49 metre band!
  I'm afraid I broke the rules and have had the set on and off several times before I read Murphyv310's warning re stirring sleeping capacitors.
  I think the reception would improve a lot with an adequate end fed aerial and even an earth but I still have not been able to make much headway on the LW band although it does faintly get radio 4. The rest of the band is a load of very loud buzzing which got me banished from the kitchen!
  The detached resistor is designated R12 in the diagram and connects from the +ve end of C32 (2uf, 450 v.) to a terminal on the V3 (diode/triode EBC33; det. and A.F.amplifier). The curious thing is that there was already one resistor of the same value between those two points (red, red, yellow) which I took to mean 16k ohms. Thus two of the same value were originally connected in parallel. The circuit diagram says it should be 220,000 ohms so something doesn't add up/
  Iintend to replace the capacitor on the output valve which is a 50 mu F at 12v. Does this sound in the right order of things? Have I got it right - it connects to the grid in parallel with a resistor 150 meg.
  Finally, nothing is heating up abnormally - particularly the capacitors in cans. I have taken several pics.
  Thanks Guys!

[ppppenguin]

The buzz on LW may well be interference from other electrical equipment. Including LED and fluorescent lighting.

" Iintend to replace the capacitor on the output valve which is a 50 mu F at 12v. Does this sound in the right order of things? Have I got it right - it connects to the grid in parallel with a resistor 150 meg."

This doesn't make sense. There's a 150 OHM (not megohm) resistor from the CATHODE of the output valve to chassis. This is decoupled by a 50V electrolytic capacitor. All very normal. The cap may be OK but I'd replace it anyway. A modern part will be 47uF though if you used 100uF or even 220uF it wouldn't matter. Voltage rating can be anything more than 12V. 47uF 25V parts are very common. The critical capacitor is the the 0.005uF grid coupler that we mentioned in earlier posts. That one MUST be replaced as any leakage will put positive bias on the grid of the output valve. This in turn causes excess current in that valve which can kill both the valve and the output transformer.

" The detached resistor is designated R12 in the diagram and connects from the +ve end of C32 (2uf, 450 v.) to a terminal on the V3 (diode/triode EBC33; det. and A.F.amplifier). The curious thing is that there was already one resistor of the same value between those two points (red, red, yellow) which I took to mean 16k ohms. Thus two of the same value were originally connected in parallel. The circuit diagram says it should be 220,000 ohms so something doesn't add up/"

There should be a 220K resaistor from decoupled HT (the +ve end of that capacitor) to the anode on the EBC33 triode section. Red Red Yellow = 2 2 and 4 zeros. In other words 220K. I don't see how you got 16K from that colour combination. 16K would be brown/blue/orange.

I get the impression that somebody has been messing around in this part of the set before you. Possibly replacing R12 but not bothering to remove the old one.