ROLAND BOLT 30- 60 cycle hum...

[czech-one-2]

My electro harmonix 7591A tubes arrived and as suspected had no effect on the buzz. But they do sound good with more bass,kinda like a 6l6.

OC, Let me know if I should re-lift the 2 resistors and ground the cap to TP #5 again. Is there something else in this area of the circuit that you want me to look at?

I've heard of mods that are supposed to reduce hum, like artificial center taps. Does this apply to this amp?


Or 'star' grounding? I'm open to try anything.

Also, this youtube clip sounds alot like the 60 cycle hum in my amp. Mine matches an A played on the 5th fret and is a bit louder than this guys Fender Princeton. Unfortunately mine isn't caused by a tube :-(
https://www.youtube.com/watch?v=GrVtX0QGNls

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lift one end of C10 and C11 or you can pull them right out this will disconnect
the first stage of the pre and part of the switching circuit.. try amp - observe
Try and do it so the tracks don't get damaged

Oc, could you explain something to me? When I lifted C10 /C11 and the turned on the overdrive circuit the buzz completely dissapeared. I was excited that maybe you located the problem but maybe I'm missing something here...

[oc disorder]

Oc, could you explain something to me? When I lifted C10 /C11 and the turned on the overdrive circuit the buzz completely dissapeared. I was excited that maybe you located the problem but maybe I'm missing something here...

Hi again I mentioned that in post #69

I'm still thinking about that - by lifting those two coupling caps the first two fets were disconnected
as well as the two switches. The other two switches however would still be switching between the M.Vol (VR7)
and the volume (VR2). but... seeing its still there with C32 removed I find it hard to see how - as VR7 and VR2
are before C32 and disconnected !

Although the audio path was disconnected the only other physical connection from the front part of the circuit
was through the power supply to the last fet left standing . After you lift those resistors that will tell us a bit more.

"Terminal 11 on the power supply board has 3 wires
1] to terminal DC0.2A on the PT
2] to terminal # 11 on the preamp board
3] to TP #5 on the preamp board"

Do any of these actually go to the chassis?

3 inch square of shielding tape applied to the top of the cabinet.

Well there goes my next big idea, stealing a flat baking tray from the kitchen and laying it on the top!

I've heard of mods that are supposed to reduce hum, like artificial center taps. Does this apply to this amp?

As I see it we are trying to repair it I somehow doubt the Japanese technicians designed it with a hum in it.

Modifying it suggests there is something that is to be changed from the original design.

The talk about artificial ct's etc comes mainly from new builds ie prototypes and is part of the design process.

The tube section in your amp is fine !
and yes with the three resistors

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Ok lifted R78 / R60 and R58........Bzzzzzz :-(

[czech-one-2]

[QUOTE=oc disorder;360607]

"Terminal 11 on the power supply board has 3 wires
1] to terminal DC0.2A on the PT
2] to terminal # 11 on the preamp board
3] to TP #5 on the preamp board"

Question : Do any of these actually go to the chassis?

Answer: No

''As I see it we are trying to repair it I somehow doubt the Japanese technicians designed it with a hum in it.
Modifying it suggests there is something that is to be changed from the original design.''

I agree, I'm happy with the amp stock [without buzz of course]

[oc disorder]

Ok lifted R78 / R60 and R58........Bzzzzzz :-(

oh hmmm ... one I missed!!! point E is still connected.... pull out Q11 and R 57

[czech-one-2]

Ok, saturday I'm on it. Infact I'm free all day as the wife & kids will be away, so anything I can try post away!
Should I run a ground wire from that PCB pad terminal 11 to ground?

[oc disorder]

Should I run a ground wire from that PCB pad terminal 11 to ground?

Yes try that to chassis ground makes a lot of sense.

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Ok, lifted R57 and pulled Q11 , still buzzing.
tacked a ground wire from terminal 11 to the chassis, no change.

[oc disorder]

Well ... take a big breath here and have a look at this thinned out schematic "testing in progress2.jpg"



We will eventually get to "the standby mod" but that's a separate issue.

To the right is the main in.If we push a jack plug in there the hum goes.

So we know that little circuit whose output is on terminal 6 hums.

The first conclusion is the power supply.

We know you have replaced all the capacitors, checked the diodes so it would seem it's not that.

Is that 39K R66 (orange white orange) definitely connected to chassis ground ?....the same as the ground for the B1
power supply ?

EDIT: and is there only one ground to chassis on one of the front jack sockets? Ie the chassis only connects once?

Before swapping the FET or the C31 coupling capacitor try soldering a 1000uF 63v from a good ground
terminal 5? to B1 on the pre-amp board terminal 4.

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YEAH! Looks like I found the source of the buzzz!
I un-tied all the wires in the suedo-loom and started to seperate them. When I moved the orange wire from the power board terminal #4 to the fuse farthest from the preamp board the buzz disappeared. It seems this wire doesn't like to be loomed up with the other wires. The confusing thing is that the amp didnt buzz when plugged into the main in, and this wire is attached to the power supply board?
Could someone enlighten me as to what this wires function is?
Well, this cost me a new set of power tubes [needed a spare pair anyway] and a whole set of electrolytic caps [ 30 + years old so I'll consider that preventative maintenance].
It was a great educational tool at the same time as I will go back over this thread and try to figure out how OC seperated the various components on the schematic to isolate each section of the preamp.
Thanks everybody for your patience and time, this is an awesome websight and you all treated an electrical noob with a great deal of patience and a willingness to explain things in a basic,understandable way.
OC, if you ever come to Prague look me up and I'll show you around and treat you to some of the finest pivo [beer] on the planet!

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Oh and OC, I'd still like to do the standby mod!

[oc disorder]

YEAH! Looks like I found the source of the buzzz!

Great ! And that was all your own work !

Could someone enlighten me as to what this wires function is?

Its the ac input to the high voltage circuit for the tube section.

It's a connection from the transformer (via the fuse in export models) to the bridge rectifier , smoothing caps
and standby switch.

It's probably 200 volts ac or something similar and this must have induced (in the same way wires in a transformer couple)
into the supply lines to the pre-amp.

The power amp does not use any of those supply lines and is nowhere near as sensitive as the pre-amp board , so
when you plug into the "amp in" it doesn't hear the noise in the pre-amp supply !

So it seems your meter was possibly not far off the mark...

Just for fun re-measure the AC on the DC supply now you have moved the high voltage supply wire.

Can re assemble the pre-amp.

Oh and OC, I'd still like to do the standby mod!

The next bit involves the power board. You may have to unsolder some
short wires to turn it over.
I have to prepare a pic or two

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Well, looks like I'm celebrating too early...
I re-assembled the whole amp and its buzzing, but not quite as bad, so it helped. Still pretty annoying though. I could almost live with it as you don't hear it when playing but I was hoping to get it quieter. What about grounding this wire on one end? Probably a bad [or hazardous] idea?
The orange #3 terminal wire also affected the buzz when moved.

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It's a connection from the transformer (via the fuse in export models) to the bridge rectifier , smoothing caps
and standby switch.It's probably 200 volts ac or something similar and this must have induced (in the same way wires in a transformer couple)
into the supply lines to the pre-amp.

Question, so this wire doesnt absolutely need to go to the fuse? It seems that the shortest path from the transformer to the board would be a good idea?