How much ripple is too much?

[AMPREPAIR]

I am repairing an old Solid State amp (Kustom 250-2) that has noticeable "Humm" from the speaker.
Does anyone have a "rule of thumb" for determining when filter capacitors should be replaced?
The voltage across the caps is 40VDC with 175mV of ripple.

[soundguruman]

That's not very much ripple, noticeable hum can be a leaky transistor, DC on the output, a bad op amp, a bad rectifier diode...
a broken ground inside the amp, loose ground screw...
or it could even be old filter caps.
or a ground loop from something you plug into it...

[Enzo]

That sounds fine to me too. There are many things that can cause hum.

[Tom Phillips]

Although your measurements do not indicate excessive ripple there is one way to determine if your hum is caused by ripple on the DC supply.
Clip additional filter capacitance in parallel with the installed filter cap. If the measured ripple is reduced but the hum you hear is unaffected then you know that the power supply ripple is not the primary source of the hum in your particular case.

Just to check. Is the "noticeable" hum a new development for the amp or has it always had the hum. Are you sure that it is worse than any other average Kustom 250-2? Also tell us if the hum level is affected by the settings of the volume and tone controls.

Cheers,
Tom

[soundguruman]

I am repairing an old Solid State amp (Kustom 250-2) that has noticeable "Humm" from the speaker.
Does anyone have a "rule of thumb" for determining when filter capacitors should be replaced?
The voltage across the caps is 40VDC with 175mV of ripple.

A lot of manufacturers do not get all the hum out to start with.
In a Kustom, the reverb switch may need to be plugged in and reverb OFF.
The reverb caused some hum, in some Kustom amps, and turning it off stops the hum.

[AMPREPAIR]

Thanks to everyone for the terrific suggestions!
A little background on this amp. It arrived on my bench in a non-working condition (no audio output). I discovered one output transistors had shorted and caused several resistors to fail. I replaced the burned resistors and all of the output transistors. When powered-up, the amp produced the hum mentioned at the beginning of this thread. The volume controls have no effect on the hum level. I will try all of the mentioned trouble-shooting techniques when time allows. The amount of hum may be normal, but it sure seems excessive to my ear.

[loudthud]

Frequently on an amp of this vintage (I didn't look for a schematic) there is an additional filter cap for the input transistors of the power amp.

[soundguruman]

Yeah, reverb return was always on, and the reverb knob controlled the send.
Until you plug in the foot-switch and ground the return...then it finally stops.
This is all kind of noise / hum / ringing / feedback in early Kustom amps.
back in those days, reverb was like, psychedelic.

[Chuck H]

reverb was like, psychedelic.

Lol!

Yupper. The reverb in these amps seemed VERY sensitive to me when I repaired one a few months ago. Someone had tried replacing the stock reverb cotrol pots with lower value parts to stop the ringing. Kustom even installed a physical pad mute for the spring tank. This may or may not have been because of the problem. Anyhoo...

After discovering a few cold solder joints I went about the task of re-flowing every pad contact. Didn't actually take that long and it fixed 50% of the amps problems. A couple of noisy op amps and one dual op amp chip later the amp was performing well. Most of the chips, op amps and discrete amplifiers in these things are obsolete parts. There are a couple that have modern cross references, some that are still available as NOS parts and others that are made of pure unobtanium. There's one dual gate FET in there (some kind of early MOSFET technology, I read) that you simply won't find specs or replacements for. There must be a way to retrofit SOMETHING. But I don't have the chops to figure it out. Good luck if that fails.

For those who aren't familiar with these amps... The filter caps look like a pair of coffee mugs with screw terminals on top. Mammoth! The ones in the amp I worked on were a different brand from stock, so I assumed they had been replaced. But I priced similar caps in case it came to replacing them. VERY expensive. Prohibitively so.

[R.G.]

A very big deal when working with hum is to distinguish between real hum, which is at the power line frequency, and rectifier ripple, which is at twice the power line frequency. This is not easy, as they are exactly an octave apart, so it takes some close listening or an oscilloscope.

That's a very low amount of ripple on your filter caps, so I would suspect hum in one of its incarnations.

[Chuck H]

Reflow-reflow-reflow...

[soundguruman]

Lol!

Yupper. The reverb in these amps seemed VERY sensitive to me when I repaired one a few months ago. Someone had tried replacing the stock reverb cotrol pots with lower value parts to stop the ringing. Kustom even installed a physical pad mute for the spring tank. This may or may not have been because of the problem. Anyhoo...

After discovering a few cold solder joints I went about the task of re-flowing every pad contact. Didn't actually take that long and it fixed 50% of the amps problems. A couple of noisy op amps and one dual op amp chip later the amp was performing well. Most of the chips, op amps and discrete amplifiers in these things are obsolete parts. There are a couple that have modern cross references, some that are still available as NOS parts and others that are made of pure unobtanium. There's one dual gate FET in there (some kind of early MOSFET technology, I read) that you simply won't find specs or replacements for. There must be a way to retrofit SOMETHING. But I don't have the chops to figure it out. Good luck if that fails.

For those who aren't familiar with these amps... The filter caps look like a pair of coffee mugs with screw terminals on top. Mammoth! The ones in the amp I worked on were a different brand from stock, so I assumed they had been replaced. But I priced similar caps in case it came to replacing them. VERY expensive. Prohibitively so.

Hmmmmmm,
the deal is not to replace the tank,
but to rewire so the reverb control is functioning on the RETURN not the SEND.
I have no idea why the engineers left the return wide open, with no control but the foot-switch.
But, any vibration is picked up by the tank and amplified with no way to shut it off, except the FS.

The reverb also picks up the power supply, and the hum is amplified into the audio path.
Until you shut it off with the FS.

The last thing is that if the power transistors are out of balance, you will be getting a 60 cps hum also...

This is funny, I once fixed one that was all oxidized, after several hours of dinking around, I got it working OK.
Then the customer later told me that the amp had been submerged, completely underwater.
I thought that it looked a little "corroded."
But after 2 years of drying out, it was fixed again. ha ha ha. That's funny.

[Chuck H]

I didn't know that it was wired that way. Even though I had one on the bench! The only problem with the reverb in the one I worked on was that high volume settings would cause the springs to feed back. I assumed it was an old, overly sensitive pan. They act like that. That IS a pretty poor way to do it.

Reflow-reflow-reflow...

[soundguruman]

Yes, the 60 cps mechanical vibration from the power transformer is actually shaking the chassis and the reverb pan a tiny bit.
This sets the springs in motion...the sound comes out of the speaker and shakes the amp more...and the ringing starts....by a circle of positive feedback...
Also, any external vibration, such as guitar, drums, bass.

You can see how much of a problem this would cause...until the foot switch is plugged in, and the reverb return is grounded.

[AMPREPAIR]

Ah ha!
I have discovered 3 of the 4 driver transistors on the amplifier PCB are shorted, partially shorted, or open; only one transistor checked good. Does anyone know an equivalent replacement for the RCA 38736 and 38737 transistors? I am not worried about the fancy built-on heat sinks with the original transistors as I can use slip-on heat sinks instead. While we are on transistor equivalents, does anyone know what will work for the TO-3 (PP3324) output transistors for this Kustom K250-2?



I would still like to learn a good "rule of thumb" for how much ripple is considered too much.

Thanks in advance.