28th Oct 2012, 8:01 pm | #1 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
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A Pye P75.
Normally, I don't get involved with 'domestic' radios - my preference being much more for military/commercial 'communications' stuff - but at an August-bank-holiday car-boot-sale, a rather sad radio grabbed my attention.
A Pye P75. The poor thing had been painted all over with white 'vinyl matt', but looking at the back revealed an unmolested rear card with no signs of it having been disturbed for decades. The seller didn't know if it worked. I offered him £1, on the basis that even if it didn't work there would be an output-transformer, a mains-transformer, a 2x500pF variable capacitor and probably a couple of decent valves. £1 having changed hands I dragged it home and coaxed the dust-encrusted chassis out of its paint-defiled carapace. All valves present-and-correct, though the output EL41 looked to have had a hard life (the inside of the glass being silvered in the areas where the 'holes' in the anodes were). True enough, 'that capacitor' was leaky, so a suitable 600V-working replacement went in. The capacitor across the output transformer also proved to be a bit 'soft' so was removed. I generally dislike these 'treble-cut' capacitors. In went a NOS EL41 and the chassis found its way to 150V from a variac. All went well and after 20 minutes we were up to full mains-voltage without problems. OK, the volume-control was distinctly scratchy... and things still seemed a bit 'deaf' so i did an alignment which improved matters a bit - but still not spectacular. Perhaps I need a new ECH42? Or maybe I'm expecting too much, comparing a 4-valve-plus-rectifier radio to my usual shack AR88D? The case was stripped of its horrid white overcoating using 'nitromors' and wire-wool; then treated with a couple of coats of Danish Oil to bring back a discreet but deep shine. The fabric over the speaker-baffle was utterly beyond redemption - and to be honest I don't really like fabric, so it got stripped off.. So I got some expanded-mesh aluminium and sprayed it deep-bronze. A thin sheet of brown felt spray-glued to the speaker baffle-board stops the mesh rattling. Personally, I think the result looks rather good, even if it's a distinctly lacklustre receiver inside! |
28th Oct 2012, 8:28 pm | #2 |
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Re: A Pye P75.
Nice cabinet job.
I would replace all the waxies and Mouldseals, not only 'that capacitor'. Consumer grade caps like these are much less durable than the tropicalised types you'll find in comms receivers. 4 valve superhets all have very similar sensitivity when working properly. They were only intended to receive reasonably strong broadcast transmissions and will indeed be much less sensitive than anything with a tuned and amplified RF stage. You may well find that the original EL41 is fine. The internal silvering is probably from the initial getter firing rather than a sign of heavy use. I've had them in a shocking state with blackened internals and burnt lettering and they still perform perfectly well. Just check the operating conditions to ensure nothing is amiss. |
28th Oct 2012, 9:51 pm | #3 |
Dekatron
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Re: A Pye P75.
Yes indeed. You only need a small leakage in an AGC decoupler to affect the sensitivity. Usually there is a common screen grid decoupler as well. Being a domestic set it is common for the screen grid of the frequency changer and the screen grid of the I.F amp to share a common feed resistor and decoupler. If the decoupler is leaky,(and/or the feed resistor has gone high) you can imagine how this will affect the overall gain.
Rich.
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29th Oct 2012, 11:03 am | #4 |
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Re: A Pye P75.
Sounds like I need to do some more capacitor-changes. Yes the P75 does have the "one decoupler for both the screen-grids of the FC and first-IF" money-saving circuit so this will be my first target (along with a check for the high-ness or otherwise of the feed resistor). AGC-decoupling capacitors will get the replace-on-sight treatment at the same time as I replace the 'tired' volume-control.
The original UL41 is definitely "tired" - volume was significantly improved by fitment of a new one. From the waxy-drips under the mains transformer I suspect it may have been run for some time with the UL41 pulling rather more current than it should courtesy of 'that capacitor'. |
29th Oct 2012, 11:59 am | #5 |
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Re: A Pye P75.
Sorry, I forgot that this radio uses a UL41 not an EL41. You can easily check for internal leaks by checking for +ve grid volts.
You can probably fix the original volume control. You will need to dismantle it and clean the track and wiper with WD40 and a cotton bud, and maybe also adjust the wiper. It really isn't worth trying to identify individual bad wax caps. You might as well do the lot as they'll all be bad. Most of the values aren't at all critical. |
29th Oct 2012, 12:06 pm | #6 | |
Dekatron
Join Date: Apr 2012
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Re: A Pye P75.
I made a mistake - yes the P75 uses E-series valves throughout - it's got a conventional mains transformer and 6.3V valves, not a nasty series heater-string. The EL41 was definitely past its best-by date.
Given that to dismantle the volume control to try and clean it, and to do this it's going to have to be removed from the chassis, it's easier/quicker to fit a new one (which I happen to have) and know it's good rather than spend half an hour removing/cleaning/refitting it and finding it's still scratchy! Like you say, some serious re-capping is definitely in order: I have a nice supply of new 0.05 and 0.1uF 400VW polyesters just waiting to be deployed... Quote:
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29th Oct 2012, 12:12 pm | #7 |
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Re: A Pye P75.
A nice job!
Just a comment regarding the capacitor across the output transformer primary; it may have been included in the design to prevent HF instability in the output stage and not simply a ‘tone’ treble cut. Some sets show this instability on LW reception at highish ‘volume’ levels and can tax the output valve. MM |
29th Oct 2012, 12:14 pm | #8 |
Octode
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Re: A Pye P75.
Very nice job. Good performers these sets. I picked up two of these from the local tip last week, complete but cabinets will require work. Nice jobs for these dark nights, Malc.
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29th Oct 2012, 12:26 pm | #9 |
Octode
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Re: A Pye P75.
I had one of these and was always happy with its performance. It's EL41 had had its pip knocked off and it seemed quite happy with an EL84.
As well as the waxies I had to change the 2μF electrolytic (C16 on the Trader sheet) as it oscillatored like a sod. A 0.1μF across it provided a temporary fix until I got the correct replacement. - Joe |
29th Oct 2012, 12:47 pm | #10 | |
Dekatron
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Re: A Pye P75.
Yes I've come across this problem in the past - a typical output-valve has quite a bit of gain at the 2nd and 3rd harmonics of the IF and can often cause "burbles" at certain frequencies. Perhaps I'll put something back there, but a bit smaller than the original? While treble-cut might have been a good idea 50 years ago when broadcast-band reception was invariably accompanied by heterodyne whistles, these days there are so few broadcasters using MW that you can afford to broaden the bandwidth a bit in the search for better audio quality.
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