Completely Out of My Depth - Modulus 50w 69 Build

[kingfredward]

Sadly its always been connected so can’t say that’s the issue there is continuity too. any other suggestions potentially?

Ok, its not clear from the picture, but is the center lug of the trimpot connected any of the other two lugs of the trimpot? If not, connect it to the lug thats connected to ground. Do that and measure again the max negative voltage at either pin5 of the power tubes

 

[Marcomel79]

Sadly its always been connected so can’t say that’s the issue there is continuity too. any other suggestions potentially?
View attachment 158993

can you read the value of the pot? Is that 1Meg? It should be 25K...

 

[kingfredward]

can you read the value of the pot? Is that 1Meg? It should be 25K...

23.2k apparently so I guess not that

 

[Marcomel79]

23.2k apparently so I guess not that

We'll pick it up tomorrow, its midnight here

 

[Trouble Free]

without the power tubes check the negative voltage swing on pin 5. You said it maxed out at -162v (that's a lot) what is the minimum you can read when you move the pot the other way?

 

[kingfredward]

We'll pick it up tomorrow, its midnight here

sure thing thanks for your help get some rest

The minimum was around -157v wound out and wound in it was -164v. It wasn’t a huge range

 

[Trouble Free]

Don't bother putting the power tubes in until you figure out why so much negative voltage. Maybe you have the wrong dropping resistor? maybe it's bypassed? Bad ground?

 

[NickKUK]

Agreed, it’s almost like the bias is referenced to the bridge rectification ins such a way thay the bright gives a reference of -100V and then the bias then drops below by a further 50-60V resulting in -153 to -167V. That could be AC or DC and the AC could be driven through a large cap(s)..

Can you take a photo of the rectification, both the bridge, the bias rectification and the caps being used (ie where they connect to the filter caps).

 

[Marcomel79]

sure thing thanks for your help get some rest

The minimum was around -157v wound out and wound in it was -164v. It wasn’t a huge range

Definitely something is not right there.. can you measure the value of these resistors in ohm?



Also can you measure the voltages at these four points? Point 1 and 2 will be AC, point 3 and 4 will be DC..

 

[kingfredward]

Thanks here are the restults

Resistance
1- 222k (Upper left)
2- 14.4k (bottom)
3-55.6k (right)

all of which marry up to the wiriing diagram




Here are the voltages:

1. 353.3V
2. 257V
3. -169 V
4. -163 V

 

[PelliX]

1. 353.3V
2. 257V
3. -169 V
4. -163 V

1 & 2 are VAC and 3, 4 are VDC, I presume?

 

[kingfredward]

1 & 2 are VAC and 3, 4 are VDC, I presume?

CORRECT

 

[Marcomel79]

CORRECT

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

 

[BlueX]

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.

 

[kingfredward]

Thanks for your responses

Resistance fully anti clockwise is : 78.6k OHMS
Fully clockwise it is: 55.5k OHMS

Does this mean anything? Guess it makes sense as the pot is 25k OHMS

 

[BlueX]

Thanks for your responses

Resistance fully anti clockwise is : 78.6k OHMS
Fully clockwise it is: 55.5k OHMS

Does this mean anything? Guess it makes sense as the pot is 25k OHMS

With a 56k resistor and 25k pot in series it should be from 56k to 81k, so looks normal. Still no explanation for the -160V bias voltage, unfortunately.

Ref. to post #190: On my '69 JMP50 (1986) I measure 305 VAC as your "1" (353V), but only -41 VDC on pin5 (V4/5), with the same bias arrangement.

 

[BlueX]

Is there a picture showing how you connected the probes when you measure about -160 VDC bias voltage?

 

[BlueX]

Another thought (once my brain got started):

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

There are three 220k resistors on the board (1 in blue, and 2 in red). Unless you got an extra 220k, you could switch places. Replace the one in the blue with one from the red circle, and put the one from the blue circle in the empty spot in the red circle (hope you follow). Measure bias voltage again.

 

[paul-e-mann]

@kingfredward have you looked at Rob Robinette startup guide and troubleshooting guide, these were life savers for me when I built my amp. Also do you have a spare set of tubes, just cuz tubes light up don't mean they work, I had a bad rectifier tube and power tube in a brand new amp kit, once I figured this out my amp was up and running, but it was a process to get there! Robinette steps will help you isolate where the problem is.

 

[PelliX]

@kingfredward have you looked at Rob Robinette startup guide and troubleshooting guide, these were life savers for me when I built my amp. Also do you have a spare set of tubes, just cuz tubes light up don't mean they work, I had a bad rectifier tube and power tube in a brand new amp kit, once I figured this out my amp was up and running, but it was a process to get there! Robinette steps will help you isolate where the problem is.

Solid advice, yet other valves are not going to magically drop the bias voltage by 100V or so. I'm wondering if there's any AC on there, too.