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!
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!
Well, I wouldn't assume anything without testing, but the old electrolytic capacitor is a great first suspect.
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!
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!
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
Doc
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.
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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.
Thanks.
350 volt capacitors should work just fine.
Antique Electronic Supply has a good supply of capacitors.
Antique Electronic Supply has a good supply of capacitors.
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.
pictures of the unit
Here are the pictures. The whole thing is a piece of art!
Thanks!
Here are the pictures. The whole thing is a piece of art!
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Thanks!
That amplifier is in great shape!
The rectifier is a 6X4 (I think).
When you replace the electrolytic caps, make sure that the first cap after the rectifier is no higher in value than the original. It's probably 10 microfarads. That is the maximum that the 6X4 rectifier can safely handle.
The rectifier is a 6X4 (I think).
When you replace the electrolytic caps, make sure that the first cap after the rectifier is no higher in value than the original. It's probably 10 microfarads. That is the maximum that the 6X4 rectifier can safely handle.
DC variac
Thank you for all your help. So, could this be done as simply as:
Connect my adjustable DC power supply instead of the power transformer/rectifier to the leads from the amp chassis and slowly raise the voltage to 250V. Tubes removed would 125mA be enough (the limit of my adjustable DC power supply).
Thanks!
Thank you for all your help. So, could this be done as simply as:
Connect my adjustable DC power supply instead of the power transformer/rectifier to the leads from the amp chassis and slowly raise the voltage to 250V. Tubes removed would 125mA be enough (the limit of my adjustable DC power supply).
Thanks!
Put a resistor between your power supply and the amplifier. You want to keep the current very low. 125mA is too much current if you're trying to reform the electrolytic caps.
rectifier
The rectifier is EZ80/6V4. All the tubes are Motorola, made in Great Britain.
The rectifier is EZ80/6V4. All the tubes are Motorola, made in Great Britain.
resistor value
Would three 10k in series (1/2 W each) be OK? Again, I would not need to desolder anything from the amp chassis and will reform the four caps at once. Is that right? Just slowly raise the voltage that I read on my variable DC power supply. Right?
Thanks again!
Would three 10k in series (1/2 W each) be OK? Again, I would not need to desolder anything from the amp chassis and will reform the four caps at once. Is that right? Just slowly raise the voltage that I read on my variable DC power supply. Right?
Thanks again!
If your power supply has an ammeter, you can safely put the voltage from the power supply directly into the amplifier.
Keep a close watch on the current. As the current falls to a few milliamps, you can increase the voltage. Bring the voltage up slowly and watch the current.
I have restored many old vacuum tube amplifiers. In all cases, I have replaced the electrolytic capacitors. Even if you are able to re-form the caps, I would not trust them in the long term.
I just built a small amplifier which uses 6BM8 tubes. It sounds nice. About 3.5 watts per channel in class "A" mode. About 10 watts in push-pull.
Keep a close watch on the current. As the current falls to a few milliamps, you can increase the voltage. Bring the voltage up slowly and watch the current.
I have restored many old vacuum tube amplifiers. In all cases, I have replaced the electrolytic capacitors. Even if you are able to re-form the caps, I would not trust them in the long term.
I just built a small amplifier which uses 6BM8 tubes. It sounds nice. About 3.5 watts per channel in class "A" mode. About 10 watts in push-pull.
Given the age of that amp many would tell you that you would just be better off replacing that supply capacitor with a new one. I have not had much luck reforming capacitors that are already behaving badly, and even in the case of ones that reformed OK I found plenty of instances where the fix was short term. In some cases it may take out the rectifier or even the power transformer..
Getonemaker, I believe that you are the same person that posted on that other site. You have plenty of answers over there. One of the caps inside that multi-tap can capacitor is shorted and no amount of trying to reform it is going to help. With capacitors as cheap as they are today, just replace the entire can with four individual capacitors or a single can if you can find one.
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