Testing Power Supply Caps

[audiomagnate]

Hi audiomagnate
1. I designed this Amplifier and many others while @ BGW. Looking @ your questions you may not have strong tech training or equipment. So let’s try some steps.
2. Do you have the manual?
Yes.
3. Have you modified the 620B for Mono mode or is it still Stereo?
Still stereo.
4. If Mono mode then the problem could be the driven channel (Left). Remove the Octal connector to the Right module. Turn on the amplifier and see if the “HUM” is present.
NA
5. If the amplifier is stock the PS cap or a broken wire maybe the problem.
That's what I thought when I started this thread. No wires are broken.
6. Make sure that no connections other than AC power and a speaker are connected.
You may want to add a series resistor to the speaker to test to reduce any large power hum to them during any testing.
That's how I tested it.
7. Can you measure the ± voltages on the Big caps?
Yes
Are they the same?
Don't know yet, will have to try that.
If they are the same, the caps are still good.
Duke

 

[audiomagnate]

My cheap capacitance tester is not capable of distinguishing the bad cap from the good one, so I simply replaced them both and all is good.

Audio1Man: I love these amps so much I bought 51 of them!

 

[indianajo]

To determine if rail caps are bad, I measure the max power out of the amp, on the speaker if still installed. Very cheap test. On 225 W log resistors if out of circuit. Drive amp with earphone jack of transistor radio, turn up until you hear clipping. Measure voltage out with analog VOM. P=(V^2)/Z where Z is the impedance of the speaker. Like 8 ohms for a 6.5 ohms speaker on the DVM.
So if a 100 W amp is putting out 2.5 watts, you are pretty sure the rail filter caps are bad. Very quick test on organs that don't get sent to the shop, the repairman goes out to the site.
If the amp sounds bad at very low volume, more likely a coupler cap. Although it could be a bad speaker driver. Lots of those in certain age organs of a famous brand. Bad drivers rattle if driven at rated power.
When I change one e-cap, I change them all.

 

[audiomagnate]

To determine if rail caps are bad, I measure the max power out of the amp, on the speaker if still installed. Very cheap test. On 225 W log resistors if out of circuit. Drive amp with earphone jack of transistor radio, turn up until you hear clipping. Measure voltage out with analog VOM. P=(V^2)/Z where Z is the impedance of the speaker. Like 8 ohms for a 6.5 ohms speaker on the DVM.
So if a 100 W amp is putting out 2.5 watts, you are pretty sure the rail filter caps are bad. Very quick test on organs that don't get sent to the shop, the repairman goes out to the site.
If the amp sounds bad at very low volume, more likely a coupler cap. Although it could be a bad speaker driver. Lots of those in certain age organs of a famous brand. Bad drivers rattle if driven at rated power.
When I change one e-cap, I change them all.

Before repair this amp was buzzing into my speakers at clipping levels with no input, so this method was not an option.