Completely Out of My Depth - Modulus 50w 69 Build

[Marcomel79]

Could the 220k resistor (in blue) be faulty, although it measures correctly with the low voltage from the DMM?

I thought about it as well. If you have a spare 220K resistor i would take that out and replace it

 

[Helmholtz]

Do the positive ends of the bias caps measure ~zero resistance to ground/chassis?

 

[BlueX]

Do the positive ends of the bias caps measure ~zero resistance to ground/chassis?

This is a valid question: Is the bias circuit grounded at all?

Looking at the picture in post #181, I can't see if the black wire is soldered to any lug (or where it is connected at the other end, of course)

 

[NickKUK]

I went through a calculated the voltages back from the DC current across there 15K resistor - in short they don't marry up. Even when taking when given a rough finger in the air averaging on the meter.

My thinking was from that - the DC current flow is 0.4mA which is quite low but not unheard of. However when you caculate back on the schematic you're about 110V short.. a higher current would resolve this drop 'gap'.

I would be reasonably comfortable with the 352V measurement if there was a low current. That's only 10% above the transformer, and both averaging high (can be 10% high or 60% low - which seems unlikely!) and transformer tolerances (can be ±%) leaves the other option which logically would be a low loading of current or higher wall voltage.

It's either that or the CT on the PT secondary is badly grounded but that would cause issues on the rest of B+..

 

[Pete Farrington]

Ill open up my jmp later today and do a comparison. Maybe @Pete Farrington can give us his opinion?

Could be interesting to see what resistance you measure between 4 and 5, when you turn the bias pot both full clockwise and full counterclockwise.

No voltage, drained caps!

As already mentioned, -160 VDC bias voltage is not normal.

View attachment 159041

Do the positive ends of the bias caps measure ~zero resistance to ground/chassis?

Exactly, my guess is a connection between point 5 and the circuit common chassis is missing.

 

[Marcomel79]

Dont worry man, well get your amp up and running! But i bet youre gonna pace yourself on the next build

 

[kingfredward]

Oh wow!!! Thanks for all you responses!

Certainly got a load of home work to do. Currently on holiday so trying my best to answer things from afar

Wait I just realised does it need to go to ground of the chassis first and then the filter cap?
If so I havent done that that means in theory I have to re run a wire from ground of the trim pot -> to ground where the presence is also grounded from there to the 1st filter cap and that should solve the issue?





Honestly you guys are the biggest legends ever can’t thank you enough for helping me out here

 

[Helmholtz]

Please measure as advised.
BTW, the wiper of the bias trim pot doesn't look well soldered.

 

[Trouble Free]

Wait I just realised does it need to go to ground of the chassis first and then the filter cap?

Yes you need to go to ground from the bias pot, if you ran the wire directly to the filter cap It's not going to work.

 

[NickKUK]

Can you check where this black wires goes and to which grounding lug it goes to? (see red arrows below).
Those grounding lugs should have a shakeproof washer (the sharp teeth cut into the surface past any surface treatment):

 

[Trouble Free]

Can you check where this black wires goes and to which grounding lug it goes to? (see red arrows below).
Those grounding lugs should have a shakeproof washer (the sharp teeth cut into the surface past any surface treatment):

Looks like he didn't connect it to a grounding lug, He just ran the black wire from bias pot directly to filter cap.

 

[Marcomel79]

Can you check where this black wires goes and to which grounding lug it goes to? (see red arrows below).
Those grounding lugs should have a shakeproof washer (the sharp teeth cut into the surface past any surface treatment):

View attachment 159073

the output jacks are going to the presence pot, and from the presence pot he ran a wire to the PI filter cap. All without being connected to ground it seems....