Pye P75

[SteveCG]

This LW/MW/SW 1950's set was bought at a car boot. It looks to be untouched (especially the IF cores and the trimmer capacitors). Its main problem is low level IF self-oscillation, so tuning into a weak signal gives that characteristic heterodyne signal shift - more normally associated with IF image issues.

My question is this: Is this a 'stock' problem with this set?

I have the Trader information sheet and have already checked many obvious things. I've also changed the grid coupling capacitor to the EL41 audio output valve.

Any experiences from other P75 owners please?

[paulsherwin]

I suspect the decoupling caps have had it.

I don't have a P75 but it appears to be a bog standard B8A era superhet.

[HamishBoxer]

I have had screen IF decoupling causes various problems.Also the important thing you stated is "untouched" IF cores etc let me say that is the best way to leave them.

[SteveCG]

Let me say what I've checked so far:

1) The resistors are within 20% of nominal
2) I've shunted the electrolytics with known good electrolytics
3) I've shunted the Hunts and waxies with known good polyester waxies
4) I've checked the old Hunts and waxies are not short circuit.
5) I get the same problem on LW, MW and SW
6) the set's sensitivity seems to be what I'd expect.
7) Valve voltages are alright.
8) Valves are original and I've cleaned their pins.

HamishBoxer - yes I've little intention of touching the IF cores - the RF/LO trimmer capacitors still have intact paint seals.

[Nickthedentist]

Originally Posted by SteveCG

3) I've shunted the Hunts and waxies with known good polyester waxies
4) I've checked the old Hunts and waxies are not short circuit.

But what if they're doing what they do best, i.e. leaking?

[Techman]

You cannot check the Hunts type paper capacitors by shunting them with another capacitor.

You cannot check them for leakage by measuring for short circuits with a standard meter.

I'm not one for the unnecessary replacement of capacitors without good reason, but I would agree that on this occasion that faulty decoupling could well be the problem - and yes, the P75 does suffer from the fault you describe.

You are doing the right thing by not doing the all too usual blanket capacitor replacement that some folk do. The way you're approaching this set will teach you a lot more about radio and the circuits used.

Shunting the electrolytics is good to determine loss of capacity. Leakage of electrolytics cannot be checked in this way, but if they're on the HT line it will show up as warming, which if you don't notice quickly will result in them going bang.

[paulsherwin]

I do support blanket recapping in radios of this type, simply because all the Mouldseals and waxies will be bad. Not changing them just means you waste lots of time chasing strange faults which are likely to be down to bad caps anyway. There will always be differing opinions on this subject though.

[Techman]

The trouble is, Paul, that the person learns absolutely nothing about radio and traditional fault finding. I would agree in one way that if you just want a working radio and you have little interest in how it works, then yes, swap out the lot. However, I would pose the question that if the person is inexperienced and makes an error in either connection or component value, how will they be able to 'fault find' their error without understanding how the circuits work?

[ms660]

If you've ever worked on a Hallicrafters SX28 RF section then there is only one practical option...change the lot in one go before firing up.

It's not difficult to change the lot in one go and avoid any mistakes, eyes/drawing/schematic/notes/check etc.

Lawrence.

[Techman]

Yes, but we're talking about a Hallicrafters, not a Pye P75. The Hallicrafters 'could' be regarded as an interesting set, whereas, dare I say it, the P75 could be regarded as a very plentiful and not very interesting radio, so having yet another one working having donated nothing to the wealth of knowledge seems a little pointless, to me anyway.

I should probably add that I do actually own a P75 myself.

[SteveCG]

Alright folks, I'll go away and put my thinking cap on !

[Techman]

If Steve carefully replaces one at a time, he can report back on which one, or ones, caused the fault - then everyone is a winner!

Edit: Unless it's a dodgy valve - unlikely, but not impossible - or even something else.

[ms660]

The circuit is not a very good design so far as I can see, a single electrolytic to decouple the mixer and IF screen grids and the HT feed to the audio amplifier.

Lawrence.

[julie_m]

What Techman says -- replace one capacitor at a time, and test the set between each one and the next. That way, if you miswire anything, it will at least be obvious where the mistake is. You will also gain a useful insight into which circuit positions make the greatest difference.

[HamishBoxer]

Correct!

[Neil Breward]

Originally Posted by ms660

The circuit is not a very good design so far as I can see, a single electrolytic to decouple the mixer and IF screen grids and the HT feed to the audio amplifier.

The P75 was nothing special. its true, but when sorted out they sound good because PYE put a little effort into the audio quality with this model, with a fair degree of negative feedback around the AF stages.

The real weak spots are, as you say, that single electrolytic decoupling the AF amplifier anode supply and the screen grids of the IF and frequency-changer and the lack of cathode components for the EF41 and ECH42. Both examples of cost-cutting and poor practice.

I suspect PYE's designers were under orders to claw back some of the cost of the extra negative feedback components. Daft penny-pinching.

On a set I had I retained the electrolytic decoupling the AF stage (replaced with a new one) but added an extra 15k resistor and decoupling cap (0.1 mfd) to feed the EF41 and ECH42 screen grids. The addition of two cheap components made a considerable improvement!

The first component I would replace would be that decoupling electrolytic. I suspect it has dried up and lost capacitance. Something like a 10mfd would be OK, perhaps paralleled by a 0.1mfd for improved RF performance if you dont want to do the modification.

Good luck!

[G6Tanuki]

I've done a couple of P75s in the past and agree with the suggestion of splitting the FC/IF and AF decoupling: on mine I even fitted separate resistors/capacitors for the FC and IF screen-grids, which helped reduce a degree of 'pulling' of the LO as the AGC operated.

IF 'regeneration' can also be caused by failure of the decoupling capacitors around the detector: the typical triode-and-pentode audio stage valves often have significant gain at IF and it only needs a schmierchen of 'IF' to leak through to the grid of the triode and before you know it you've got tens of volts of IF appearing at the output-stage anode too!

[Neil Breward]

Yes - for that reason many sets have an RF bypass capacitor of a few hundred pF from the AF triode anode to chassis. It's absent on the P75. Another example of penny-pinching on this model?

[Hybrid tellies]

I would tend to go with Pauls advice, #7, and change out all those Hunts and Waxie capacitors. If they are not causing trouble now they soon will do.
I would change out the capacitors in blocks starting with those around the front end then work your way through to the detector and AF stages that way you it will be easier to trace any mistakes you make plus its more educational and will give you more of an idea where the fault lies.
Have fun and let us know how you get on.

[Nickthedentist]

Originally Posted by SteveCG

Any experiences from other P75 owners please?

I can confirm that these simple sets are very good performers with a very nice sound quality indeed when working properly.

See what Paul Stenning has to say here: https://www.vintage-radio.com/manufa...-sets/pye.html