Pye P75

[Techman]

This certainly looks like it's turning out to be one of the more informative threads on this model radio with some interesting thoughts and information being posted, much better than 'swap out all the Hunts' etc. with no more to be said. Making me think that I should dig my example out sometime and revisit the innards.

I grew up with a P75. My parents would have bought it soon after they married. I have many early memories of my mother trying to listen to the Home Service on medium wave in the evening (we had no TV) and struggling against the gradually increasing interference from what I think was Radio Berlin. Eventually the two stations would be at nearly the same signal strength, the only let up being the occasional fading of the German station. My mother would try to slightly tune the BBC station off frequency which would sometimes lessen the pickup of the other station. I remember how she used to go on about that 'new fangled VHF' that others were getting that had no interference at all. If only they'd waited for a couple or three years before buying the P75, they could have had that wonderful VHF FM.

As I got older and started to mess around with other radios (I wasn't allowed to delve into the Pye) I found that the Pye was rather lacking in performance, particularly comparing it to an HMV that my grandmother had given me that was broken and I managed to repair. The Pye P75 is actually quite a good looking radio, in my opinion, my only dislike is the two control knobs on the cabinet sides. It looks purposeful from the front and seems to shout out "look at me, I'm a radio!" and is what you'd expect a typical valve radio from the time to look like - or is it just me that thinks that?

Shame about the heat from the dial lamps causing damage to the dial backing, letting direct light to shine through the front. Pye seemed to not do very well with radio scale material, just look at the FenMan which is perspex and the markings like to drop off.

[SteveCG]

Well, thank you all for your thoughts.

The reason I was content with just shunting the capacitors with ones of appropriate value and whose impedance at the IF was not in doubt was that in other work I found that systems just on the edge of instability would show a perceptiple change in their outcome if a temporary change was made to the 'critical' stage of the system. Translating this systems-speak gives: "if adding, say, a small capacitor across a now-defective origional capacitor alters the frequency and/or intensity of the whistle somewhat then you may have located the culprit".

I am left wondering about the EBC41 as touching a x10 scope probe (which is connected to my scope) on the anode of the AVC diode causes more of a change than I would expect given the probe's loading of a few pF and 10M Ohm.

I still have my thinking cap on - and I agree with the thoughts about the cheese-paired decoupling!!

[Neil Breward]

The EBC41's AVC diode anode will have the full IF signal on it so touching it with a scope probe will certainly have an impact, faulty or not!
However ... it could be the case that some of the IF is actually reaching the triode anode. Maybe wiring a bypass cap. of, say 100-200pF from triode anode to chassis maught solve your instability problem.

cheers,
Neil

[SteveCG]

I'll try your suggestion Neil and report back.

[SteveCG]

Well, I tried Neil's bypass capacitor suggestion but it made no improvement. I tuned in to a moderately weak station, slightly off-tune so that I could hear the hetrodyne whistle. Then I tried first a 100 pF silver mica and then a 470 pF silver mica from the EBC41's anode to chassis - no difference whatsoever.

I have decided that enough bench time and space has been expended on this set. So I've screwed it all back together and put it to one side.

Reading over the thoughts here of others I would not be surprised if the set was only ever marginally stable, and the passage of the years has drifted the values of all the components enough to give the issue the set has. If I tune into a strong signal then not only do I get a large negative AVC/AGC voltage but I don't hear any of the heterodyning as the RF and IF stage gain will have been greatly reduced - no doubt reducing the feedback loop gain to below one.

Thanks again to all who have contributed to this post.

Oh, by the way - when I got the set somebody had extended the Pye's mains flex by wrapping some thin flex on the end held in place with sticky-tape...

[paulsherwin]

I refer you back to #7

[ms660]

As I read the posts, something is oscillating at or close to the IF frequency, the receiver is still fitted with it's original decoupling capacitors, the OP has an oscilloscope?

EDIT: Post crossed.

Lawrence.

[SteveCG]

Paul,

I marked well your suggestion, but sadly my slow recovery from a stroke means I have a backlog of things to do - and not much energy to do them - so I've decided on a priority list for them.

Addenda - yes, Lawrence, I have been using a 'scope looking for IF freq signals where they should not be and they are not across the decouplers.

[ms660]

With most IF amp stages if you take the effectiveness of the screen grid away your basically left with the gubbins for a tuned anode tuned grid oscillator, especially if the two tuned circuits are tuned to slightly different frequencies.

Lawrence.

[SteveCG]

Lawrence,

Well understood. When I first tried debugging this set - about 15 months ago now - my first point of call was the 2 uF screen grid decoupler.

[Neil Breward]

Originally Posted by SteveCG

Well, I tried Neil's bypass capacitor suggestion but it made no improvement. I tuned in to a moderately weak station, slightly off-tune so that I could hear the hetrodyne whistle. Then I tried first a 100 pF silver mica and then a 470 pF silver mica from the EBC41's anode to chassis - no difference whatsoever.

Oh well! It was worth a try - many, if not most, sets do have a cap. in this position.

When you get the urge to have another go at it, try splitting the decoupling to the IF and FC screen grids as previously suggested.

Best wishes, Neil

[Radio Tech]

I agree with Nick, no good shunting any capacitor, all hunts waxies simply need replacement and that goes for all electrolytic ones that way your certain that nothing bad can happen to the set, in general these P75 sets are good when they are working. You can obtain all the caps you need from Jellyfish Audio.

Good luck with it

Ken

[clay shooter]

Just make sure you have the right model, there is a pye 75 and a pye 75A, both physically similar externally, but slightly different in the circuit diagrams

[SteveCG]

clay_shooter, it is definitely a P75 (now packed away until the next time).

[crusher19860138]

My P75 is definately a great sounding set. The cabinet is a bit scruffy but Pye certainly built very well indeed!