Desoldering large IF box screens

[ronbryan]

With regard to post #16, I wanted to know the DC voltage across the stabilising capacitor C526 to compare it with the forward drop of the diodes fitted. The voltage is proportional to the strength of the FM carrier, and if it is low because a previous stage has low gain, the effect of unbalanced diodes is likely to be more significant.

Ron

[Boulevardier]

Well, I finally managed to remove the original AA112 diodes, and they tested OK as follows:

Diode 1 - Ir = 1.3 microamps, Vf = 244mV
Diode 2 - Ir = 4.3 microamps, Vf = 561mV

Out of interest, I tried loosely wiring in a couple of Schottky 1N5819s (off the same bandolier, so probably same batch - both had exactly same Vf of 246mV, and Ir's of 1.1 and 1.2 microamps), and the audio was just audible but extremely distorted. Maybe these weren't suitable replacements anyway, but I decided to stick with the AA112s. (At this point in procedures the tuning drive-cord decided it was time to snap - I hadn't touched it or been working anywhere near it, so just gratuitous malevolence). After nearly a day's time tracking down some cord and fitting it, I switched on - to silence. All the mechanical disturbance had broken a power connection (just a solder bridge) between the vertically mounted IF board and the main pcb (terrible construction method). The joint was unrepairable, so I had to drill a small hole in the main pcb and run a wire for a new connection.

This restored operation - still with the sibillance and now with no AM (something I've decided to revisit later, since I rarely use it).

Re the sibillance, I still wonder about the de-emphasis circuitry. Is this a likely cause? I can't isolate the components (see attached) - can anyone help?

Mike

[G6Tanuki]

If the capacitor in the de-emphasis circuit has failed-open it could deliver a high-boost effect.

[Argus25]

Originally Posted by Boulevardier

Well, I finally managed to remove the original AA112 diodes, and they tested OK as follows:

Diode 1 - Ir = 1.3 microamps, Vf = 244mV
Diode 2 - Ir = 4.3 microamps, Vf = 561mV

Out of interest, I tried loosely wiring in a couple of Schottky 1N5819s (off the same bandolier, so probably same batch - both had exactly same Vf of 246mV, and Ir's of 1.1 and 1.2 microamps), and the audio was just audible but extremely distorted. Maybe these weren't suitable replacements anyway, but I decided to stick with the AA112s.

Mike

Yes the 1N5819 would have too much capacitance I would guess. The AA112's are good for the task as would be the 1N5711's.

See if you can get your hands on, buy or borrow, a Philips PM5326 generator or similar and sweep the IF. That way you will be able to see what has gone wrong with the ratio detector. It is unlikely (but not impossible) the de-emphasis components have failed in a way that induces distortion.

[Boulevardier]

Thanks, perhaps sweeping will be the only way forward. I don't have an FM sweep generator.

Back to that question - just to eliminate the possiblility, can anyone identify the de-emphasis components for me? I'm not just getting sibillance, but also a lower level of middle audio frequencies than normal.

Mike

[Boulevardier]

Originally Posted by Boulevardier

Well, I finally managed to remove the original AA112 diodes, and they tested OK as follows:

Diode 1 - Ir = 1.3 microamps, Vf = 244mV
Diode 2 - Ir = 4.3 microamps, Vf = 561mV

Out of interest, I tried loosely wiring in a couple of Schottky 1N5819s (off the same bandolier, so probably same batch - both had exactly same Vf of 246mV, and Ir's of 1.1 and 1.2 microamps), and the audio was just audible but extremely distorted. Maybe these weren't suitable replacements anyway, but I decided to stick with the AA112s. (At this point in procedures the tuning drive-cord decided it was time to snap - I hadn't touched it or been working anywhere near it, so just gratuitous malevolence). After nearly a day's time tracking down some cord and fitting it, I switched on - to silence. All the mechanical disturbance had broken a power connection (just a solder bridge) between the vertically mounted IF board and the main pcb (terrible construction method). The joint was unrepairable, so I had to drill a small hole in the main pcb and run a wire for a new connection.

This restored operation - still with the sibillance and now with no AM (something I've decided to revisit later, since I rarely use it).

Re the sibillance, I still wonder about the de-emphasis circuitry. Is this a likely cause? I can't isolate the components (see attached) - can anyone help?

Mike

Just realised that my previous post (above) was made nonsense of by incorrect figures I gave for D1.

It should have read:
"D1 - Ir = 3.1 microamps, Vf = 581 mV
D2 - Ir = 4.3 microamps, Vf = 561 mV"

Sorry about that!

Mike

[Argus25]

Originally Posted by Boulevardier

can anyone identify the de-emphasis components for me?

Mike

I think its just the R-C LPF immediately on the output of the ratio detector. Though there is further LPF filtering in the amplifier, but that is not uncommon. These sorts of components rarely have any faults that could introduce non-linearity (distortion).

[Boulevardier]

Thanks Argus, I had also concluded that the de-emphasis components are R532 and C527 (that's ignoring the small effect of C528). However, these alone (1.8k and 4.7nF) would only give a time constant of (if I calculate right) 8.4 microseconds. So presumably we'd have to assume that the ratio detector has an additional output impedance of around 9-10k to give 50 microseconds. Is that plausible? I haven't got a clue how you'd calculate the output impedance of that RD circuit.

Mike