Tweet
Hello to the Music Electronic Forum.
Let the show begin...
Ok, I bought a used amp with hum and I can't find the source.
I figured I’d see if maybe you guys might be able to help locate what I'm missing.
So, here’s some info and what I’ve done so far.
The amp is an old Univox U1011.
From using my scope it looks like a 120hz hum.
So, the hum.
1. With both volumes down turning the bass up brings in the hum.
2. With the bass turned down turning up the channel one and/or two volumes produces no hum.
3. With the bass turn up, and the hum humming, turning the channel one volume knob from 0-10 changes the hum from a lower pitched hum to a higher pitched hum. Note that the channel two volume is at zero.
4. With the bass turn up, and the hum humming, turning the channel two volume knob from 0-10 does nothing to the hum until you hit 10 and then the hum goes away.
5. With the bass up, channel two volume at 10, turning the channel one volume brings hum back slowly and at 10 gets pretty loud again. Funny thing. To me, this hum sounds likes it’s in between the lower and higher hum spoken of earlier (#3).
6. With the bass up, channel one volume at 10, turning the channel two volume has the same effect as in #4, only with the volume control switched.
So, what have I tried so far?
If you remove V3 (the 2nd gain stage) the hum goes away.
If you remove the phase inverter the hum goes away.
All tubes swapped out – no change. The tubes I swapped out for the power tubes were a used set and probably not a great matched quad, however, with that said the tube swaps made no change.
(holding off on buying a new quad of 6l6gc’s for the moment)
I’ve checked the board, pots, etc for bad ground connections, etc – no change.
I’ve moved wires around, etc – no change.
I’ve even effectively removed the reverb and tremolo out of circuit by removing the tubes and disconnecting wires from the board, etc – no change.
I disconnected the rectifier diodes and substituted another rectifier set up – no change.
The stock amp is essentially using a bus ground (via the circuit board) starting at the filter caps and terminating at the input jacks.
I figured I’d try isolating the totem pole stack and power tube grounds from the board, leaving only the three 22uf caps going thru the board ground – no change.
Which reminds me, this amp used two 2x47uf caps for the totem pole stack and one 3x20uf cap.
They’ve all been replaced.
I’ve double checked these for wiring issues, etc, and all seems fine.
Actually, all the electrolytic caps have been replaced.
Now, in order to isolate the bias supply circuit from the buss ground on the circuit board I’d have to cut the ground trace on the board.
Essentially I'd just cut the ground trace in one spot and provide a ground for that section.
To reverse it I'd just use a short piece of wire or flat copper and solder it over the broke trace, so no biggy really.
(holding off on doing that for now)
The switching/shunt input jacks and grounding have been double checked.
I’ve put the chassis top/cover on – no change.
My other amps have no hum issues plugged into the same receptacle and sitting in the same location.
I cleaned the pots, checked their values, etc – all seems fine.
And, I may have missed something I did and forgot to mention.
Oh, the power transformer has a small amount of mechanical buzz.
Both the PT and OT run without getting overly hot. I’ve also checked all the transformer windings to make sure there weren’t any shorts, etc.
I suppose I could have got a bad 'new' filter cap, I haven't tried subbing in a different caps to double check the new caps yet.
Hope this wasn’t too long winded, but I figured I might as well try to answer some questions I might get ahead of time.
I've attache a schematic: Univox_U-1011_LEAD_AMP.pdf
Let the show begin...
Ok, I bought a used amp with hum and I can't find the source.
I figured I’d see if maybe you guys might be able to help locate what I'm missing.
So, here’s some info and what I’ve done so far.
The amp is an old Univox U1011.
From using my scope it looks like a 120hz hum.
So, the hum.
1. With both volumes down turning the bass up brings in the hum.
2. With the bass turned down turning up the channel one and/or two volumes produces no hum.
3. With the bass turn up, and the hum humming, turning the channel one volume knob from 0-10 changes the hum from a lower pitched hum to a higher pitched hum. Note that the channel two volume is at zero.
4. With the bass turn up, and the hum humming, turning the channel two volume knob from 0-10 does nothing to the hum until you hit 10 and then the hum goes away.
5. With the bass up, channel two volume at 10, turning the channel one volume brings hum back slowly and at 10 gets pretty loud again. Funny thing. To me, this hum sounds likes it’s in between the lower and higher hum spoken of earlier (#3).
6. With the bass up, channel one volume at 10, turning the channel two volume has the same effect as in #4, only with the volume control switched.
So, what have I tried so far?
If you remove V3 (the 2nd gain stage) the hum goes away.
If you remove the phase inverter the hum goes away.
All tubes swapped out – no change. The tubes I swapped out for the power tubes were a used set and probably not a great matched quad, however, with that said the tube swaps made no change.
(holding off on buying a new quad of 6l6gc’s for the moment)
I’ve checked the board, pots, etc for bad ground connections, etc – no change.
I’ve moved wires around, etc – no change.
I’ve even effectively removed the reverb and tremolo out of circuit by removing the tubes and disconnecting wires from the board, etc – no change.
I disconnected the rectifier diodes and substituted another rectifier set up – no change.
The stock amp is essentially using a bus ground (via the circuit board) starting at the filter caps and terminating at the input jacks.
I figured I’d try isolating the totem pole stack and power tube grounds from the board, leaving only the three 22uf caps going thru the board ground – no change.
Which reminds me, this amp used two 2x47uf caps for the totem pole stack and one 3x20uf cap.
They’ve all been replaced.
I’ve double checked these for wiring issues, etc, and all seems fine.
Actually, all the electrolytic caps have been replaced.
Now, in order to isolate the bias supply circuit from the buss ground on the circuit board I’d have to cut the ground trace on the board.
Essentially I'd just cut the ground trace in one spot and provide a ground for that section.
To reverse it I'd just use a short piece of wire or flat copper and solder it over the broke trace, so no biggy really.
(holding off on doing that for now)
The switching/shunt input jacks and grounding have been double checked.
I’ve put the chassis top/cover on – no change.
My other amps have no hum issues plugged into the same receptacle and sitting in the same location.
I cleaned the pots, checked their values, etc – all seems fine.
And, I may have missed something I did and forgot to mention.
Oh, the power transformer has a small amount of mechanical buzz.
Both the PT and OT run without getting overly hot. I’ve also checked all the transformer windings to make sure there weren’t any shorts, etc.
I suppose I could have got a bad 'new' filter cap, I haven't tried subbing in a different caps to double check the new caps yet.
Hope this wasn’t too long winded, but I figured I might as well try to answer some questions I might get ahead of time.
I've attache a schematic: Univox_U-1011_LEAD_AMP.pdf
Comment