ROLAND BOLT 30- 60 cycle hum...

[czech-one-2]

Ok back to the chase for fun lets jump to the other end and disconnect C32 right near terminal 6 in the centre of the board.

I'm really hoping it will fall silent with this one as there is only one FET Q8 running from the 48volt supply here.
The next "interruption point" is the main in jack and we know it's silent after that !
Keeping my fingers crossed

Before I lift C32, should I re-attach C10/C11 or leave them lifted?

[oc disorder]

I doubt you will have to lift them again but perhaps if you can without damage lightly tack solder them to the top of the tracks.
It doesn't really matter too much as at this point in time that part seems ok. I'll leave that for your discretion.

[czech-one-2]

ok, c32 lifted and buzz is still there, but seems [a little] lower in volume.

So, when I lifted c10/c11 and then clicked on VR1 the buzz disappeared. I was hoping that located the source of the buzz?

[oc disorder]

Have to do something else here to make sure. Need to connect that capacitor you removed .. (probably need a short piece of wire)
between the fet input ie the pad directly behind the fet and ground (terminal 5?). The back of the board is the easiest way to do it.
It's like a live wire picking noise up.

A couple of things I will mention -

it's a good idea to cover over the live points in the chassis mains 240 input
and back of fuseholder.. the high tension (HT) high voltage terminals. Just some insulating tape will be ok for that,
just while you are working on it.

Also if you tack a wire on the back of each pot and ground it it will save you having to remove and refit the board in the chassis
when checking hum. Maybe even wrapping a light wire around all the "threaded bushes" and grounding that.

[J M Fahey]

It is the diode that makes it an RF probe, not the cap. A cap should work. A cap is what we use to turn an amp into a signal tracer. It isn't any different from a coupling cap between stages in a tube amp.

Dear enzo: the diode is there, just inside the meter
The rotating scale selector simply adds a plain (1N4007) diode in series with the red terminal, and another contact multiplies DC meter sensitivity by roughly 2.5 .
So the internal series diode will charge the added input cap to peak AC voltage.
It is a very crude design, but since it works well to measure wall voltage and as a side extra transformer secondary voltages (what a homeowner needs when troubleshooting his home wiring, desk lamp or whatever) they use it.
Those meters are meant to be sold to electricians or homeowners at WalMart or Home Depot for $10.
An Electronics Tech should use a better one

On the contrary, an amp is AC coupled and the cap just insulates it from dangerous high DC volotage, a different function.
No rectification involved.

[czech-one-2]

Have to do something else here to make sure. Need to connect that capacitor you removed .. (probably need a short piece of wire)
between the fet input ie the pad directly behind the fet and ground (terminal 5?). The back of the board is the easiest way to do it.
It's like a live wire picking noise up.

A couple of things I will mention -

it's a good idea to cover over the live points in the chassis mains 240 input
and back of fuseholder.. the high tension (HT) high voltage terminals. Just some insulating tape will be ok for that,
just while you are working on it.

Also if you tack a wire on the back of each pot and ground it it will save you having to remove and refit the board in the chassis
when checking hum. Maybe even wrapping a light wire around all the "threaded bushes" and grounding that.

So you want me to put the cap back and connect a wire between one of its legs and the pad directly behind the fet and ground?
Not real clear here and I dont want to damage anything.

I'll go ahead and ground all the pots [ala guitar pot wiring]

[oc disorder]

Yes ground the pad by the FET via the capacitor not connecting it to the other pad.

[czech-one-2]

Yes ground the pad by the FET via the capacitor not connecting it to the other pad.

Ok, tacked a jumper wire on from the ground TP to c32, buzz is the same/ guitar inputs dont work.
Thanks again for your how-to diagrams!

[oc disorder]

Yes Can you see why they don't work ? Only the last fet is connected in the audio chain it passes through one to the other and I am trying to simplify the circuit to narrow down the noise.Once the cap is reconnected in the proper place the guitar input from the preceding circuit will be restored.

Ok seems to be heading back to the power supply ... we have 1 fet preamp !
Before I suggest removing it and borrowing one from earlier in the circuit as at the moment they are not being used..
I want to try completely isolating the power supply to just that fet.
Remove or lift one end of R78 1K5 the 30 volt supply to the reverb chip and the top two on the schematic R60 1K & R58 1k
If it stops , gradually add them back until you hear the noisy one .
Speaking of coupling caps did you replace C31 1uF 50 volts ?
Try replacing that .. for testing purposes an approximate value will do.
check the attachment

So, when I lifted c10/c11 and then clicked on VR1 the buzz disappeared. I was hoping that located the source of the buzz?

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 !

I'll have to retire soon it's still yesterday here !

[oc disorder]

Whoops missed one R48 and this power supply feed goes to the VR2 section which is the clean channel
and I think when you pressed v1 before it turned this off so worth checking.

[oc disorder]

Just before I hit the sack I presume the amp is properly grounded at the power inlet?
Check resistance from the chassis to the ground pin on the power plug.
You could try swapping the fet while its input is grounded for another the same from elsewhere
in the preamp.
The way the circuit is at the moment there are only a handful of components on the preamp board
not much to go wrong and the power supply.
You have checked the diodes in the power supply, replaced the capacitors, measured the DC output
checked the grounding but have measured wildly excessive AC which for the moment the opinion
seems to be - the meter is not measuring accurately.

Having exhausted that investigation I suppose now put every thing back.to how it was
and all I can think of to prove if it is the power supply is to hook 5 x 9 volt batteries together
and disconnect B1 at terminal 15 at the power supply and connect it to the positive terminal
and connect the negative of "the battery pack" to ground at the power supply.
45 volts will run those fets and that will give us some new info....

Hopefully somebody spotted something while I was obsessing !!

[52 Bill]

Hopefully somebody spotted something while I was obsessing !!

OC you have done a great job at leading this hunt, so I won't add to it to confuse things. Somewhere along the way, Czech-one-2 described this as a buzz that will change when he touches pot shafts. That would lead me to believe that it is ground related more than power supply.

A better description of the symptom will lead to a better direction in the cure.

[oc disorder]

Somewhere along the way, Czech-one-2 described this as a buzz that will change when he touches pot shafts. That would lead me to believe that it is ground related more than power supply.

"With the chassis removed, the buzzing is worse until I touch a pot shaft and ground it. But even with grounding its still there."

I am assuming this is just because the pot cases are not grounded and is additional hum to the existing we are trying to chase.

[52 Bill]

I am assuming this is just because the pot cases are not grounded and is additional hum to the existing we are trying to chase.

Are you sure? I agree with the ungrounded pot cases adding buzz, but he sez the buzzing is worse. That's why I think it would be helpful to get a very clear description of the noise. Is it hum or buzz?

[oc disorder]

So Near but alas so Far
Roland Bolt 30
Fet/transistor Pre-amp with cmos switch
Tube output stage with a 12AT7 phase inverter and 2 x 7591A output tubes.

The amp exhibits a pretty loud 60 cycle hum when powered on, with or without a guitar plugged into the input.
Plugging into the "main in" bypassing the pre-amp, the hum disappears.
The 'main in' is utterly silent from buzz. The buzz has nothing to do with volume/gain/eq tweeks.
The buzz is the same with the volume at zero or 10.
The hum is an annoying loud buzz that seems to shift audibly between 2 tones at 2 second intervals.
It had hum when I got it, got worse [more noticeable] when I put the Century in, a 102dB rated speaker.
I don't mind some hum, but this is real annoying hum.
The hum its not effected by any of the controls
Resistance from ground point on pre-amp board to chassis .3 ohm (point three of an ohm).
**With the chassis removed, the buzzing is worse until I touch a pot shaft and ground it.
But even with grounding its still there.
**I am assuming this is just because the pot cases are not grounded and is additional hum
to the existing we are trying to chase.

The power supply for the pre-amp comes from the same centre tapped transformer winding and bridge rectifier
that the -22 volt bias for the output tubes is derived from.
There are two series pass regulators the first Q1 is held at 47 volts the second Q11 (via a 120 ohm resistor and point E)
is held at 15 volts with a voltage divider R56 3k3 & R55 3k3 providing 7.5 volts.
The output of these was measured to be
47volts schematic= approx 48 volts measured
15v schem.= 13.4volts
7.5v schem = 6.7volts

There was excessive AC measured on all these DC test points but the considered opinion
points to the meter not being accurate for this.

30 volts DC is also derived from that 47 volt supply via a 1k5 resistor (R78) and a 30 volt zener for the reverb IC .

There is also a secondary plus and minus 7.5 volt supply for the spring reverb driver transistors
and this is derived from the filament or heater winding on the transformer.

This was temporarily disconnected but had no effect on the hum.

Then on my suggestion the coupling capacitor to the last FET on the pre-amp board
was disconnected and grounded .
This effectively removed all audio connection to the first stages of the pre-amp including the cmos switch but
with the DC supplies still connected to their respective places and there was still no difference !

Am awaiting the results from disconnecting those temporarily not needed supplies to the rest of the pre-amp
and changing the coupling capacitor and that last fet with one borrowed at random from the disabled part of the circuit.