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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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13th May 2019, 8:57 am | #1 |
Dekatron
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PYE P45 Unstable
This is driving me spare,I am not in the habit of being stuck but this time I am running out of ideas.
It seems a front end fault and having changed IF silver micas plus many out of spec reses etc ,there is lack of performance and to much whistling. There are times it seems sensitive to even tapping the tuning gang,though I see no drys etc. I think all suggestions on a postcard! Thank you in hope.
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13th May 2019, 9:00 am | #2 |
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Re: PYE P45 Unstable
Microphonic mixer or IF valve?
Grid decoupling caps? Ground tag not good? |
13th May 2019, 9:09 am | #3 |
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Re: PYE P45 Unstable
Thanks Sam,tried a new ECH but not the IF valve,if indeed I have one.It just does not seem like a micro phonic valve at all.
Will have a search through my stock.
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13th May 2019, 9:53 am | #4 |
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Re: PYE P45 Unstable
I don't think that it applies to your set but I have found that if one of the IF cores falls off the adjuster it causes very odd whistles as there are now 2 cores in the bottom and they couple the 2 IF windings too much turning the transformer into an oscillator.
Just a thought, disconnect the AF feed from the output valve, just to make sure its not that which is oscillating. I have had it where the output is oscillating and the HT line transfers it into the front end, gets into a positive feedback loop. |
13th May 2019, 10:13 am | #5 |
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Re: PYE P45 Unstable
It is certainly front end as there is a gram position on the selector switch and all goes quiet.
The IF cores on this cannot fall off unlike the DAC90,s.
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13th May 2019, 12:19 pm | #6 | |
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Re: PYE P45 Unstable
Quote:
The way to fix this kind of thing is to collect more diagnostic information. All you require is a reasonable scope and a x10 probe and clip the probe tip onto the plastic insulation of wiring in the circuit (This way it acts as a fraction of a pF gimmick capacitor and does not affect the properties of the circuit under test) and wind up the scope gain. Then you can find where the oscillation is originating possibly seen at the anode of the mixer stage or maybe the anode of the IF and possibly locate any feedback pathways around a stage or between stages sustaining it. Without doing this its a bit like flying blind and it could be unlikely that you could home in on the the stages/s at fault, or the cause for that. |
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13th May 2019, 2:25 pm | #7 |
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Re: PYE P45 Unstable
If they are fitted, have you cleaned/lubricated the tuning gang's rotor earthing contact springs/fingers?
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13th May 2019, 4:18 pm | #8 |
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Re: PYE P45 Unstable
Thank you Argus 25 and dazzlevision ,I will try some De-Oxit and see what gives.
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13th May 2019, 5:13 pm | #9 |
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Re: PYE P45 Unstable
Just a suggestion. I have an Irish built P45 which used to whistle between stations markedly. The fix was a new EBF89 in my case. Maybe you have a faulty EBF80? The original thread is here:
https://www.vintage-radio.net/forum/...d.php?t=117367
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Dom Less snakes...more ladders! |
13th May 2019, 6:05 pm | #10 |
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Re: PYE P45 Unstable
Thank you Dom,I have already ordered on this morning and it did seem slightly heat dependant the symptom.
Kicking in shortly after warm up.
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15th May 2019, 4:07 pm | #11 |
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Re: PYE P45 Unstable
Tried all ,except a valve which arrived today.It is like the band is full of unmodulated carriers ,if that makes sense.
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15th May 2019, 4:15 pm | #12 |
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Re: PYE P45 Unstable
If it's the receiver it could be the mixer or IF stage oscillating at the IF, and the oscillation is producing harmonics across the band, 'scoping as previously mentioned might reveal something.
Lawrence. |
15th May 2019, 4:19 pm | #13 |
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Re: PYE P45 Unstable
Yes,you and Argus are right ,it is indeed time to get the scope out though it has not been used for years!
Will just try the IF valve first,just in case. Thank you all.
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15th May 2019, 9:27 pm | #14 |
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Re: PYE P45 Unstable
I've had similar issues in the past.
Not familiar with this particular radio but if it's one of those where there's a common screen-grid feed-resistor/decoupling-capacitor shared betwen the frequency-changer and the IF-amp, the capacitor getting 'tired' can introduce some strange feedback which only reveals itself when the AGC-voltage is low/nonexistent (like between stations) - tune in a strong station and the AGC throttles-back the IF gain so the feedback disappears. Also, it's quite common for failed RF-filter-capacitors in the detector circuit, or the low-value one from the first-audio-stage anode to ground - to result in a significant amount of IF signal making its way to the audio-output stage grid, where it gets amplified and then fed to the HT-line through the 'tone-correction' capacitor across the primary of the output transformer. having several volts of IF signal superimposed on the HT line is not good - this is why I always like to see the 'tone-corrector' fitted between tha output-valve anode and earth, so it provides both tone-correction and shunting-away-IF-signals-harmlessly-to-earth. [Some may say that having it in this position risks killing the output-transformer if the tone-corrector goes short-circuit; I'd say that you should fit a reliable tone-corrector capacitor in the first place!] |