Speaker issue

[Sterling Spurrell]

C-36 170acv
c-37 -122acv
c-38-228acv

[Alistair Thomson]

These values are definitely off. It’s probably the way your meter measures DC and AC components of a signal, rather than something fundamentally wrong with the PSU. It would be good to put a scope across each of these capacitors to see exactly what the PSU is producing, but I suppose you don’t have one! Where are you located?

[Sterling Spurrell]

No scope and I live in newfoundland. How much are voltages off because I have been using this meter for awhile and always seemed to be close on other radios.

[Alistair Thomson]

Regarding the DC volts, I’d expect that the DC voltage at the top of C36 would be higher than that at C37, which would then be higher than that at C38. Otherwise, where does the additional DC come from?

Regarding the AC ripple voltages, I’d be horrified if the ripple at the top of C37 was any more than a volt or so. Millivolts would be more likely. Your AC voltages make no sense at all.

Especially with new caps.

[Sterling Spurrell]

Is it possible they are connected wrong? When you say top , do you mean connection point of positive lead? If you think they are wrong could you confirm connection points. Thanks.

[Alistair Thomson]

It looks from the schematic that all three capacitors are in the same housing. There should be four terminals: the three positive capacitors and the fourth negative common, which looks as though it might be connected to an outer can – but maybe not: it may not be a can at all, but an insulating sleeve.

So you would measure the voltages from each individual capacitor connection to the common terminal.

[Sterling Spurrell]

Ok if chassis ground is the common terminal that is how I measured them. The capacitor has 3 inner terminals in the center and 3 locking rings on the outside and the can was attached to chassis. I assumed that the three looking rings were common.

[Alistair Thomson]

Actually, looking at my (pretty blurry) copy of the schematic, I see that C38 doesn’t appear to be part of the PSU supply chain, but is simply a smoothing capacitor for the two screen grids in the RF and converter stages. Nevertheless the voltages you quoted are definitely wrong, especially the AC measurements.

But if you look at the schematic you’ll see that the chassis is not connected to DC ground – the DC ground , which you need to connect the -ve meter probe to, is indicated by a “comb” icon, and the chassis is indicated by an upside-down Xmas tree icon. The most easily identified point in the DC ground line is probably the battery negative terminal, which should have a yellow wire. At least, it was yellow in 1948! 🙂

If you make these voltage measurements again, using that point as the negative, you’ll probably get much more sensible values.

[Sterling Spurrell]

The schematic can be found on nostalgia air schematics

[Sterling Spurrell]

The acv voltage still measures the same from yellow wire b-.

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