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Old 3rd Aug 2022, 3:12 pm   #1
kestrelmusic
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Default Tuning coil problem

I've been working on a Pye T7 and am much puzzled by alignment problems.

The relevant bit of the circuit is attached.

The IF (127Kc/s) is spot on.

At some stage, and by the look of the soldered joints, a long time ago, someone has removed the feed from the aerial socket to the aerial coupling coil L1, and inserted a lead from the socket to the top of L2/L3. Applying a signal to the top of L1 produces a very feeble output.

Applying a signal here gets a response. The LW tunes about right, but with a rather weak output. The MW tunes way wide of the mark, with an even weaker output. Adjusting C4, the trimmer for L2/L3 seems to have no effect at all.

Taking L2/L3 right out of circuit and applying the signal to the top of L4/L5 gets a much stronger signal on both bands, though the MW is still wide of the mark.

I'm assuming that it is a coil problem. However, the coils are contained in steel cans and it means taking quite a bit of the set apart to get at them.

Any thoughts on how to check it? The oscillator circuits seem to be running quite happily.
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Last edited by kestrelmusic; 3rd Aug 2022 at 3:28 pm.
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Old 3rd Aug 2022, 4:31 pm   #2
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Default Re: Tuning coil problem

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Originally Posted by kestrelmusic View Post
Applying a signal here gets a response.

Sorry, I meant "Applying a signal to the top of L2/L3"
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Old 3rd Aug 2022, 7:57 pm   #3
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Default Re: Tuning coil problem

Well the most obvious answer, which you've maybe tried already, is to measure the dc resistance of L1 using a high impedance Digital Ohm meter. The spec value is sometimes given in the service sheet. If the coil is o/c then that is most likely the cause of the fault, and perhaps why somebody soldered the aerial input directly to L2/L3. If L1 is s/c then possibly the trimmer C4 could have developed a short. In the end you will probably have to open the can to look for breaks in wires. Jerry
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Old 4th Aug 2022, 2:27 am   #4
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Default Re: Tuning coil problem

If you measure the DC resistance of an inductor, best to use an analogue meter. Digital meters sometimes used pulsed voltage, so may see reactance.

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Old 4th Aug 2022, 6:42 am   #5
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Default Re: Tuning coil problem

When you say MW tunes way wide of the mark, do you mean it's out of alignment with the dial scale? Have you followed the procedure on page 6 'Adjusting the Signal Frequency and Oscillator Circuits' ?
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Old 4th Aug 2022, 7:32 am   #6
kestrelmusic
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Default Re: Tuning coil problem

Quote:
Originally Posted by cathoderay57 View Post
Well the most obvious answer, which you've maybe tried already, is to measure the dc resistance of L1 using a high impedance Digital Ohm meter. The spec value is sometimes given in the service sheet. If the coil is o/c then that is most likely the cause of the fault, and perhaps why somebody soldered the aerial input directly to L2/L3. If L1 is s/c then possibly the trimmer C4 could have developed a short. In the end you will probably have to open the can to look for breaks in wires. Jerry

Sorry, I should have added that the DC resistances of all the coils are clost to the book values.
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Old 4th Aug 2022, 7:37 am   #7
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Default Re: Tuning coil problem

Quote:
Originally Posted by PJC58-Hythe View Post
When you say MW tunes way wide of the mark, do you mean it's out of alignment with the dial scale? Have you followed the procedure on page 6 'Adjusting the Signal Frequency and Oscillator Circuits' ?
Yes: on LW the dial positions are correct, but on MW they are way off beam.

And yes, I have followed the alignment procedures to the letter.

I am wondering if possibly one of the coils within the can has slipped out of position. I don't want to open up the can if it can be avoided because it means dismantling quite a bit of stuff on top of the chassis in order to get at the terminals on the side of the tuning gang, but I think I've exhausted any other possibilty. Alternatively, I could just re-route the aerial lead to the top of L4/L5...
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Old 4th Aug 2022, 8:45 am   #8
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Default Re: Tuning coil problem

Check the LW/MW oscillator switch contacts S7 and S8. Otherwise how do the V1 electrode voltages compare with the service data? Worth trying a replacement A80A/FC4. Finally you might have oxidised contacts on the trimmers. Unusually there don't seem to be any fixed capacitors in the oscillator circuits apart from the g1 coupler C17, but it might be worth trying a replacement for that one. Cheers, Jerry
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Old 4th Aug 2022, 10:23 am   #9
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Default Re: Tuning coil problem

Make sure the oscillator is above the signal frequency on MW.

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Old 4th Aug 2022, 11:05 am   #10
cathoderay57
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Default Re: Tuning coil problem

Has the wire to the top cap of the A80A/FC4 been replaced and, if so, with what? I recently acquired a Philips Monoknob using the same valve, and the grid circuit of the mixer wouldn't peak at all; like yours, the trimmer had no effect. The problem was caused by a previous restorer who had replaced the old screened cable to the FC4 top-cap with audio grade coax, which had too much capacitance. I replaced it with a pvc wire loosely slipped inside some quarter inch braid; the circuit then peaked perfectly. Jerry
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Old 11th Aug 2022, 6:10 am   #11
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Default Re: Tuning coil problem

Many thanks for the helpful replies. In the end I connected the aerial via a 240pF capacitor to the top of L4/L5. The output still feels slightly low but it tunes sharply enough and respondes well over both LW and MW ranges.

I sometimes wonder whether the elaborate band-pass circuits on these 1930s sets were needed then because there were then a lot more AM stations than there are now broadcasting on frequencies quite close to one another!
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Old 11th Aug 2022, 9:25 am   #12
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Default Re: Tuning coil problem

The double tuned RF front end was more to reduce the second channel breakthrough problem of the low IF. At 127 kHz, the 2nd channel is only 254kHz above the signal frequency and on MW that's in band all the way from 531 to 1348kHz.

The "sharpness" of the tuning is mostly down to the IF bandwidth which can be quite narrow at 127kHz.

So, yes, to reduce interference from the (many) other stations in the band but not because they're close together as such.
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