Randall RH150 constant hum

[Ceredox]

Hellllo. A band left a broken head for dead in an old jam space so I decided to take it home and fix it. It's a Randall RH150. I believe it has a solid state preamp and a MOSFET/12ax7 driven power amp. The amp was clearly dropped pretty hard because I found that all of the output transistors had broken off of the board (the cracked case was also a give-away). Miraculously, just the legs broke so I was able to solder them back on. The amp now works and sounds good but it has a constant hum. It doesn't matter if a cable is plugged in or not and it's not volume or channel dependent. It's just always there. At first I thought it could be an output transformer then I realized it doesn't have one. I have read that it's possible that filter caps could cause this. I would have no problem just changing the caps but my budget is low and would rather not spend money on parts that aren't bad. What do you think? Has anyone had this issue before? Thanks!

[g1]

It's more likely that one of the filter caps has a broken connection due to the impact.

[Ceredox]

Not a bad theory. However, I checked them and the connections seem fine. I see no breaks or solder coming loose. I preferred some new caps and I'm going to try them out. The previous owner said the amp has "caught fire" like 3 times. Not sure how much he is exaggerating because I see zero sign of anything like that. If anything it's probably due to him spilling his beer in it haha. I'll let you know what I find.

Thanks

[The Dude]

It's not unlikely that it's the speakers that caught on fire from the amp putting out DC. So, don't hook up any speakers until you get it working correctly. As far as troubleshooting, you need to determine if the hum is AC or DC. Meter the speaker output of the amp and see if you get primarily AC or DC.

[g1]

The filter caps are often glued down. If this is the case, and there is a broken leg, you won't be able to see it unless you remove the caps. But on the other hand, being glued down would probably prevent any broken legs.
Like The Dude mentioned, check for any DC at the speaker. It will make hum, and can fry the speaker.

[Ceredox]

Great. When I get home, I will surely check for DC. I. Curious, though, what usually causes DC to come into the speaker like that? I've heard of this but never understood it.

[The Dude]

Typically amps are made up of 2 sets of output transistors. One is powered by a positive rail voltage and the other by a negative. Preamp/Driver circuitry is often much the same in that respect. If there is an "imbalance" caused by shorted or open components in the amplifier section, it will output DC. The importance of knowing whether the hum is primarily AC or DC is that it can tell us if we have a power supply filtering issue or a problem in the amplifier output section.

[52 Bill]

The amp now works and sounds good but it has a constant hum. It doesn't matter if a cable is plugged in or not and it's not volume or channel dependent. It's just always there.

Exactly how bad is this hum? If a filter cap has come unsoldered, I don't think that I would say the amp sounds good.

[Ceredox]

The hum isn't extremely loud but would surely be a problem especially if the cab was miked at all. It doesn't interfere with the amp tone except the fact that it makes some notes sound dissonant because it's at a constant certain pitch.

[Ceredox]

Typically amps are made up of 2 sets of output transistors. One is powered by a positive rail voltage and the other by a negative. Preamp/Driver circuitry is often much the same in that respect. If there is an "imbalance" caused by shorted or open components in the amplifier section, it will output DC. The importance of knowing whether the hum is primarily AC or DC is that it can tell us if we have a power supply filtering issue or a problem in the amplifier output section.

So I texted this and I'm getting no DC. Just about .4v AC. My meter also has an Hz setting and the signal is at exactly 60Hz. You think this could be a power supply issue then?

[Jazz P Bass]

400 millivolts/ac is pretty severe.

Have you tested the power supplies for undo ripple (which would indicate a duff cap or cap connection)

[J M Fahey]

If 60Hz only AC from the transformer or its return ground; ripple , even from an open cap (absolute worst case) would be 120Hz.

[Ceredox]

Unfortunately, I don't own an oscilloscope so testing ripple would be pretty tough. I can check the ground on the power transformer again. Otherwise, if the ground is ok, could the 60hz from the transformer being getting into the signal path?

[The Dude]

If you turn down all volume controls (gains, masters, etc.), does the hum go away?

[Ceredox]

No, it doesn't. I starts as soon as the amp turns on and nothing changes it.