Old Motorola console tube amp, fuse problem

[getonemaker]

Hello All,

I bought one of those old Motorola console stereos for $27. Looks great inside, Golden Voice speakers, brass chassis, etc.

It has a separate power transformer / rectifier chassis. The fuse was soldered under the power transformer to connectors and was blown. I put in a fuse holder, put the fuse in and the power transformer and the rectifier works. I tested it with a multimeter. Also, a pilot lamp is on, uses the 6.3V filament power.

When I connected the amp chassis, the light came on for about 3 sec then died. The fuse was blown (1A , slow). I removed all the tubes from the amp section, put new fuse in. Again the light came on and died in about 3 sec.

Am I correct to assume from here that the problem is the large can electrolytic capacitor?

The point-to-point wiring looks great, there are no paper capacitors.
I love these old amps. A work of art.

Thanks!

[brucetassin]

A few pictures would be very helpful. It sounds like you are on the right track.

[HollowState]

Originally Posted by getonemaker

Am I correct to assume from here that the problem is the large can electrolytic capacitor?

Well, I wouldn't assume anything without testing, but the old electrolytic capacitor is a great first suspect.

[getonemaker]

I will take pictures later. Of course find the right capacitors would be difficult.
The values are: 10mf, 20mf, 50mf, 80mf all at 250V

There is a 250V, 10mf mini electrolytic capacitor for $0.27. Would it be a bad idea to buy 16 pieces of this and put 8 in paralell for the 80mf, 5 for the 50mf and so on?

Thanks again!

[thaumaturge]

Cap technology has come a long way since that amp was built. Could no doubt find a lot smaller parts for those same values that could be wired into available space. But what you propose should work.
Doc

[HollowState]

Originally Posted by getonemaker

There is a 250V, 10mf mini electrolytic capacitor for $0.27. Would it be a bad idea to buy 16 pieces of this and put 8 in paralell for the 80mf, 5 for the 50mf and so on?

Electrically this will function. but it's a schlock way of doing things. Please, don't be el-cheapo about fixing up a nice unit. Search eBay and other places for a more original style part. I would pay the extra cost and maybe get one here:
ELECTROLYTIC CAN CAPACITORS NOS 4 SECTION TWIST MOUNT | eBay

If you have a means of powering the unit slowly with a variac, you may well be able to reform the capacitor and use it as is.

[getonemaker]

Thank you for your advice. I did look and could not find anything close that would have the needed vales. However, on that ebay page I found something that had 20 50 80 mf capacitor but at the voltage of 350. I read somewhere that the capacitors are not performing well way under their voltage rating. Is that true?

Thanks.

[Frank Berry]

350 volt capacitors should work just fine.
Antique Electronic Supply has a good supply of capacitors.

[HollowState]

Originally Posted by getonemaker

I read somewhere that the capacitors are not performing well way under their voltage rating. Is that true?

Like Frank said, 350 volts will be fine. Remember, the exact values need not be duplicated. A little greater in value for both capacitance and voltage is perfectly ok. But one shouldn't go lower. Regarding that business about electrolytic capacitors not performing well at lower then rated voltage, that's basically untrue. Perhaps only in the extreme case of a 350V cap working at 10 volts might be an exception.

[getonemaker]

Here are the pictures. The whole thing is a piece of art!





















Thanks!