38-7 dial switch stuck & possible bad field coil - Printable Version
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+--- Thread: 38-7 dial switch stuck & possible bad field coil (/showthread.php?tid=20161)
38-7 dial switch stuck & possible bad field coil - adamrobertg - 07-02-2019
I picked up a 38-7 table radio, and got around to taking a look at it while I take a break from another project. I found a couple things I didn't know what to make of so I thought here would be a good place to ask, though I haven't been on here in a long time.
When I bought the radio, I was told it 'lit up but made no sound'. The first thing I noticed was that the switch (really a knob with pull/push switch in it) on the cone-centric dial seems stuck in the 'out' position, which I believe mutes the sound. I've sprayed it with contact cleaner, penetrating oil, etc - and it just doesn't want to un-freeze. Any thoughts on how to free it up?
The field coil measures somewhere between 800k and 1.2meg. The schematic states it's an 1140ohm coil. What's going on? Wouldn't it measure open, if it were open? Also, the primary of the output transformer measures open. I guess either way I will need to find a speaker and output transformer if I want to get this working.
The radio had a 6K6 in it instead of a 6F6, and I don't believe that's an acceptable substitute - would that have destroyed the field coil or output transformer? Or would a shorted wax cap have done it?
RE: 38-7 dial switch stuck & possible bad field coil - morzh - 07-02-2019
Consider it open. Some deteriorated material form paper etc could provide for hundreds of Ks or Ms of resistance instead of fully open, but the wire is interrupted.
RE: 38-7 dial switch stuck & possible bad field coil - Radioroslyn - 07-02-2019
The 6k6 draws less current than the 6v6 so I wouldn't think that it would be the cause of the open fc. Maybe a shorted 2nd filter cap, wire rot, or something else down the line. Be careful w/an open fc there is no load on the hv supply so the voltage @ the 1st filter is going to be quite high and could cause it to short then the rectifier tube then the power transformer.
RE: 38-7 dial switch stuck & possible bad field coil - morzh - 07-02-2019
....and this is why we try to use 500V capacitors.
RE: 38-7 dial switch stuck & possible bad field coil - adamrobertg - 07-03-2019
Both filter caps were replaced at some point, but the originals left in circuit. Wouldn't surprise me if they shorted. Unless I find a similar speaker, I'm guessing I'll have to replace it with a PM speaker and a 1.2k resistor (I'm thinking 25w would be enough). The original speaker had a humbucking coil - should I just increase the 2nd filter capacitor value to compensate for that? I think it was an 8mfd originally.
RE: 38-7 dial switch stuck & possible bad field coil - morzh - 07-03-2019
The second cap probably won't eliminate the hum: it occurs due to the ripple current in the field coil used as the filter coil (if it is not then there is usually no humbucking coil in the speaker either) and there is always the ripple current through the field if used in the filter.
However making the humbucker is easy - put a few turns of wire around the field coil and connect in series with the voice coil. And if it makes it worse - reverse the polarity of the humbucker.
Folks here probably could tell you how many turns it needs to be. Regular PVC insulated wire will do.
RE: 38-7 dial switch stuck & possible bad field coil - adamrobertg - 07-04-2019
That doesn't sound too bad. And if I go with a PM speaker and resistor, I'm assuming the humbucking coil wouldn't be needed?
The other thing I noticed, is I measured the resistance of the power transformer, and noticed that each high voltage side measured higher than the schematic. It calls for 120 and 130ohms, I was getting 185 and 194. I tested it with a dim bulb tester and a 40-watt lamp was extremely dim, so I am thinking it's ok. Just strange readings.
RE: 38-7 dial switch stuck & possible bad field coil - Radioroslyn - 07-05-2019
> I'm assuming the humbucking coil wouldn't be needed?
Yes because it works off of the magnetizum provided be the field coil and your not using one.
> It calls for 120 and 130ohms, I was getting 185 and 194.
As long as the each of the windings (from center to center) are fairly close in resistance I wouldn't loose any sleep over it. If it were 20 ohms or more it may suspect.