philco: Chrysler; C-1908 (C1908)

Good evening all,

I am restoring this car radio, and I encounter a strange problem. The second IF transformer resonates at approx 650kHz, instead of the 455 kHz.
I opened the transformer, and I discovered a series resistor, marked 25kOhm; but measured 35.8 kOhm.

But i suspect the trimming capacitors (mica). But I wonder how it can detune the circuits so much, from 455 kHz to approx 650 kHz.
Who can help?

 

Best regards

 

Attachments:

• IF transformer in housing (75 KB)
• View with the ceramic mica trimmers (85 KB)
• View with series resistor (86 KB)

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If the total capacity ís reduced to ca 50%, the circuit will detune by that amount You mentioned.
(650 divided by 455 and squared).

The trimmer caps may have lost internally contact or the conducting layer has deteriorated.
 That is - sad to say- quite often the case.

 

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Just a thought - was the resonant frequency measured with the transformer disconnected? 

Sometimes there is a loading C on the secondary and, of course, there are the wiring and output/input capacitances of the previous and next stages. 

This could account for some of the shift, but as Konrad says, most likely the trimmers!

Roy

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Hello Roy,

 

All measurelments have been done with the IF transformer(s) in circuit.
In the meantime I opened the trimming capacitors and I found out that the secondary part of the capacitor is quite unusual for me. I made an exploded view drawing and  a circuit diagram drawing of that part of the set (see attachments)

 

This shows that the detection resistor (25 kOhm) and the detection capacitor are inside the IF transformer.

The question I have is how to correct (replace, clean, or what else?...) this fault?

 

Best regards

 

Georges

 

Attachments:

• Philco 1908 a (48 KB)
• Philco 1908 b (24 KB)

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Hi Georges,

This is a compression trimmer.

It relies on the springyness of the metal to pull it apart and the screw tension to pull the plates together.

It can be cleaned and should be - either an ultrasonic bath or very careful rubbing with a very fine

"flour paper" abrasive paper/cloth.  It is probable that if the metal has slightly corroded this will inrease the spacing and hence reduce the capacitance.  Try this and if this does not succeed, you could always add a parallel capacitace of small value.   My guess is that careful cleaning will solve your problems.  Be careful not to break or loose the micas!!

Good luck - let us know what happens,

Roy

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