Replacing Fuse Wire in Baldwin C3

[Gahoo]

Hey all,

Working on a Baldwin C3 solid state head, apparently it was blowing fuses and someone kept feeding it more

Couldn't find a schematic for the C3 but I did find one for the C2 and the output stages are similar enough, if not exactly the same.



When I opened it up I found that 5 of the 6 "fuse wires" have all blown open. I'm wondering if I should suspect the associated transistors went as well. Also wondering what the best practice is as far as replacing fuse wire. Not sure what the current rating is since the schematic just suggests it's a .47 Ohm piece of wire.

Anyone work on one of these or one like it?

Thanks!

[Enzo]

Yes, of course check the transistors, check all of them, not just the ones with blown resistors. The only path for current through the resistors comes through the transistors. As to the resistors, I would likely just grab some 5 watt 0.47 ohm resistors and not look back. Looks like the transistors are germanium. If they are blown, you might consider converting the amp to silicon.

[52 Bill]

The "fuse wire" acts as a ballast just like the 0R47 (0.47 ohm) that you see in typical modern power amp designs, so you can replace them with 3-5 watt resistors. The usual reason that they go open, is that the associated transistor is shorted, so be sure to test them before proceeding.

A note about the schematic, the center section is the push button tone control. The schematic is missing about half of the circuit. I guess that Baldwin didn't want people to copy the circuit.

[Gahoo]

Great, thanks fellas!

[g1]

If someone were really picky about keeping it original, ni-chrome wire is often used like this and will have a specified resistance per inch.

[Enzo]

Yep, I still have like 98% of a roll of nichrome wire I bought close to 60 years ago. You will also sometimes see light bulbs used in that position. In fact I have some dual filament bulbs mean as a pair of ballasts. No idea what their ratings were.

[Gahoo]

I think I'm gonna replace the Ge transistors with silicon, can someone advise me on an appropriate replacement transistor? (Ideally TO-3 ) And how best to re-bias for the silicon?

Thanks!

[nickb]

I think I'm gonna replace the Ge transistors with silicon, can someone advise me on an appropriate replacement transistor? (Ideally TO-3 ) And how best to re-bias for the silicon?

Thanks!

Practically speaking, you really don't have much of a choice but to use silicon PNP.

Ge only has a 0.3V Vbe drop but silicon is 0.7V. This bias is required to prevent excessive crossover distortion. Currently this voltage is developed across the DC resistance of the driver transformer secondaries. So something needs to change to get the extra drop. The easiest way is just to increase the current through it. The problem with that idea is that it is already quite substantial at about 100mA so increasing to over 200mA will be problematic. It might be better to add 3.3 ohm resistors in series with the yellow and green leads of the winding to boost the voltage.

PS: I should have said, you will additionally have to adjust the other bias resistors ( 820 ohm & 1k) to set the idle currents through the power transistors to something lowish to say 25mA each should be in the ballpark.

[Gahoo]

Thanks nickb,

was gonna go with MJ15004 for the replacements, unless anyone has a better suggestion?