Finally figured out the power connections for my BC-348 Q...have it running. Brought it up on a variac after checking the condition of the filter and bypass capacitors in the set first.

Works good. But, can't switch out the BFO insertion signal. That said, all I get are AM broadcast stations mixed with an annoying squeal.

Checked the CW switch. It is ok. Ended up temporarily severing the "red" wire feeding the VT 233 screen from the BFO assembly. That took away the insertion sound, therefore the squeals. Now everything seems to work fine on the AM bands.

Not sure what's going on with the BFO assembly. Maybe one of those micas need replacing.

Again, CW switch checks out fine on the ohm meter. Cutting the "red" wires cures what ails.

Any idea beyond replacing the micas on the BFO assembly that I need to do? I plan to do a wholesale replacement of all the capacitors, by the way. Thanks. N5NJR Reggie.

Reggie:

How heavily modified is this receiver? In the '50s, hams made a lot of modifications on a lot of these sets. Some of the work was well-done; some was not. The first thing to consider is checking your wiring against a wiring diagram to see if any changes were made to the BFO circuit. You might also double-check the switch and its connections to the BFO circuit.

You can get high-quality downloads here: http://www.jamminpower.com/main/bc348.html According to that site, the BC-348Q differs from many of the other BC-348 models. Therefore, you will need the manual that covers the Q model to get an accurate schematic and wiring diagram.

Does the Q model have Micramolds? If so, they all need to go. I'm working on a variant of the BC-342 and every Micramold in that set was leaking badly. It's a wonder it worked at all with all those bad caps in it.


Joe Connor

1- The Q does not use Micamolds. Some tubular papers marked "Fast" may be almost as leaky, however.

2- What is meant by "AM bands?" The BC-348 does not cover the broadcast band, other than a small bit above 1500 kHz.

You will have to trace why the BFO screen is getting power when the switch is in the OFF position.

1-I've traced most of the circuits and find them to be exactly like the schematic, with the exception of a couple of 6.3 filament changes made in the power conversion.
2-Will trace why "screen" is getting voltage when CW switch is off.
3-When I say "broadcast bands" I'm referring to the international shortwave bands in the 49 and 31 meter bands.

Will report on my efforts Monday night. Reggie N5NJR

Geoff:

Is there any reason why they didn't use the Micamolds for the Q model? I'm curious whether those caps were already known as a high failure item.

Joe Connor

I don't think anyone knew the Micamolds were going to be a failure issue that early, although the company did seem apologetic in their advertising as far as saying something like the capacitors were just "adequate" for the job.

J/N/Q sets were by Wells Gardner and they use a big oil filled can capacitor assembly under the dynamotor but are otherwise a cheaper design than all the others. The chassis is steel plate rather than the aluminum of the others, and the wiring is point to point rather than the neat boards and laced cabling you see in the others. The capacitors are primarily paper in oil tubulars which seem to have held up better over the years although I do see many of them showing excess leakage these days. I would hazard a guess that the Micamolds were actually more expensive at the time, but it's just a guess. Some civilian gear used the same Micamold capacitors into the 1950's, such as the Lambda modulation monitor and three or four other pieces of test gear I overhauled and then forgot about, so they were still being made even then.

An interesting thing about the Micamolds. If you measure them with a high impedance voltmeter, they show a small voltage! I have had some that read nearly a volt. I am guessing the wax had some acid content and they have mutated into little batteries. I was tempted to see if I could make a long-life bias battery with several of them.

Yeah, I had a resistor that did that: it read negative ohms on my Fluke.

The Wells-Gardner enginers must have looked at the 348 design prints, preparatory to bidding on a contract, and said "we could build these better for half the cost." So they got the contract to redesign them.

I have a Q, with an AC supply in place of the dynamotor but otherwise all original. I don't run it very often any more, but when I did, it worked fine.

Yes, some of us have to make due with a "cheapened down" BC-348 "Q" , and I'm almost embarrassed to mention I have an R-390...... "A"

""""""An interesting thing about the Micamolds. If you measure them with a high impedance voltmeter, they show a small voltage! I have had some that read nearly a volt. I am guessing the wax had some acid content and they have mutated into little batteries. I was tempted to see if I could make a long-life bias battery with several of them."""""

I bet it is because of dis-similar metals aka galvanic corrosion[?]
brass against zinc add a little moisture and voila! a battery! which creates corrosion
I think brass against aluminum might do the same thing