200S Sunn

[g1]

Measure AC volts at points A,B, and C of your drawing.

[Valvehead]

The meter just jumps all around when measuring AC at those points....cant get a reading

[x-pro]

This measurement is best made with an arrow voltmeter or a "closed input" oscilloscope.
Oscilloscope with "closed input" - AC voltage measurement mode!!!

[Helmholtz]

A decent DMM should work.
Can you try a different meter?

500+VDC might be too much for many scopes and a 10x probe.

[Valvehead]

My DMM is rated for 600vdc , i can get a digital scope with a 10x , but i will need help with how to set it

[Helmholtz]

My DMM is rated for 600vdc

That doesn't mean much.
The meter needs to able to read a low ACV on top of a high DCV in AC mode.
Some cheaper DMMs don't block DCV in ACV mode. Those won't work here.

i can get a digital scope with a 10x , but i will need help with how to set it

Seems I wasn't clear enough.
With a scope you should use a 100x probe as a 10x probe doesn't attenuate DCV.

[Valvehead]

Ok , i have a china $35 mini scope here with a 10x probe. can i try this? If i damage it , no worries. I was looking at getting a Hantek pc based scope soon.

This one has 3 adjustments -
V/DIV 5mv - 20v
SEC/DIV = 0.1ms - 50ms
Trigger = auto/ norm / Sing

sorry - im new to O scopes !

[Greg Robinson]

Addendum: It's possible to add a bleeder resistor across the last B+ filter cap in the preamp to limit the Voltage along the filter string. You'll have to experiment with the value, or do the math.

Another option is to place zener diodes across the node B cap. Assuming a 450V cap in that position, I'd use a string totalling 440V. If the raw unloaded B+ is 560V, lets allow for mains variation and add another 10% margin, and calculate from 616VDC. That means we'd need to drop 176V across the choke and 15k 2W dropper resistor. The resistance of the choke should be negligible, most will be dropped across the 15k. That gives us 11.73mA and just over 2W dissipation. That ~12mA shouldn't be a problem for the choke. Might be advisable to change the 2W 15k resistor for a 3W one, but this would protect the preamp filter caps.
The 11.73mA at 440V means just over 5W dissipation in the zeners, so 3x150V 3W zeners in series should be just about perfect, and land close to 440V.
This method would avoid changing operating voltages like adding a bleeder across the node C filter cap would do.

With a scope you should use a 100x probe as a 10x probe doesn't attenuate DCV.

Depends on the scope model, many don't attenuate DC in AC coupled mode as the 1Meg is AC coupled from the input, but other scopes DC couple the 1Meg even in AC coupled mode. Both my Tektronix 2465B and Rigol DS1052E attenuate DC when in AC coupled mode. This can be confirmed by switching into AC coupled mode and measuring resistance with a DMM connected to the input. Both my scopes read 1M when in AC coupled mode.
If a scope reads OL in AC coupled mode though, it will not attenuate DC in that mode. Very important to know your equipment well when doing high voltage measurements!

[Valvehead]

Bear with me here.....lots of testing tonight..
if I pull V1 , the hum doesn't change....if I pull the wire off the wiper of the Treble pot ....no hum !!

So... I went and pulled every wire and component from the circuit .. from the Treble pot , and before it and back to V1.. 1 at a time . 1 leg or wire at a time...all of it between v1 ..the switches..and to Treble . All grounds re checked .

The hum is still there !! If I pull the wire off the Treble wiper --- no hum !!

Am I completely wrong...and certifiably insane.........could it be the treble pot itself ???????

[loudthud]

The wiper of the Treble pot goes to pin 8 of the 6AN8. Check that there is a 1 Meg resistor from pin 8 to ground. Is there an DC Voltage on that wire from the Treble pot ?

[glebert]

Question that may be a bit of a flyer, but perhaps helpful: If you probe between ground at different points in the circuit, do you easily get very low resistance (less than an ohm) or do you find that you have to really push the probes down hard into the metal? I am working on a Sunn 100S that has a nasty 120Hz buzz, in a slighty different location, that reminds me of a ground loop. I have to push probe tips down very hard to get continuity between the grounds, and am wondering if this metal chassis has an oxidation layer that essentially creates ground loops. I was curious if you see something similar on yours. To be clear, not trying to inject my problem here, and we should keep the focus on the OP's 200S.

[loudthud]

There can be issues with terminal strips that are riveted to the chassis. If there is a ground connection relying on one of those rivets, get out your big soldering iron and solder them to the chassis. Also squirt some De-Oxit on the input jacks and tighten the nuts very tight.

[Valvehead]

The wiper of the Treble pot goes to pin 8 of the 6AN8. Check that there is a 1 Meg resistor from pin 8 to ground. Is there an DC Voltage on that wire from the Treble pot ?

all good there, no dc

[Valvehead]

Question that may be a bit of a flyer, but perhaps helpful: If you probe between ground at different points in the circuit, do you easily get very low resistance (less than an ohm) or do you find that you have to really push the probes down hard into the metal? I am working on a Sunn 100S that has a nasty 120Hz buzz, in a slighty different location, that reminds me of a ground loop. I have to push probe tips down very hard to get continuity between the grounds, and am wondering if this metal chassis has an oxidation layer that essentially creates ground loops. I was curious if you see something similar on yours. To be clear, not trying to inject my problem here, and we should keep the focus on the OP's 200S.

i suppose yes...in some spots

But anything going to chassis ground I cleaned with a dremel

[Valvehead]

There can be issues with terminal strips that are riveted to the chassis. If there is a ground connection relying on one of those rivets, get out your big soldering iron and solder them to the chassis. Also squirt some De-Oxit on the input jacks and tighten the nuts very tight.

There are 2 rivet grounds. I cleaned up the metal around the brackets and soldered to chassis . also cleaned the ac ground with a dremel. Same with input jacks and they are tight