Need some elp troubleshooting Millett DCPP Engineer's amp.

wianieq

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Hello everyone,

I just finished wiring up the engineer's amp board with the recommended Edcor outputs. While trying to make the bias and balance adjustments I discovered an issue. I had checked the b+ before trying to adjust and it was okay. I could not get the left channel current test points to rise above around .355mv. I swapped the tubes around but the problem stayed in the same place. I'm getting 328v b+. On the left channel I get 298v on PLT1 but PLT2 reads negative 300mv. The plate of both 6cb6 tubes is around 155v. I'm using 6GV5 tubes and have the chimney caps connecting the plate voltage.

My own crude knowledge made me think that I got a bad output transformer. They were newly bought. I was hoping that someone may have an idea of what might cause the symptom of one side of a pp output to basically have no voltage that could arise from the circuitry, if they could point it out to me. My next step in crude troubleshooting would be to swap the outputs and see if it follows. Or, please let me know if there is a better way to test the output transformers.

I have attached the schematic.

Thanks for any thoughts on this.
 

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You should probably take ohm readings of the right output transformer and compare them to the left output transformer. Fully unloaded (no output tubes) of course.
 
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I'd start by measuring the voltages on each pin of the output tubes, and compare good side to bad side. In other words measure:

heater voltage between the two pins (AC voltage, between the two heater pins)
plate voltage (DC voltage, with respect to ground)
screen (grid #2) voltage (DC voltage, with respect to ground)
grid #1 voltage (DC voltage, with respect to ground)
cathode voltage (DC voltage, with respect to ground)

If the suppressor grid (grid #3) is not internally connected to the cathode, measure its voltage as well.

That will illuminate the issue with the "bad" side. Measure these voltages right at the tube pins, not at any other tap points.
 
I would start by completely discharging the power supply and doing an ohms check on that output transformer.
 
Ack. I unconnected the red B+ leads and blue and brown plate leads since while connected I couldn't get them to stop jumping around all over the place. Blue to red on both transformers then read 198ohms. Red to brown on the right was 173ohms and the left transformer read 0.F. So its looking like a bad transformer. Not the result I wanted because Edcor's wait time was 8 weeks to get these.
 
Ack. I unconnected the red B+ leads and blue and brown plate leads since while connected I couldn't get them to stop jumping around all over the place. Blue to red on both transformers then read 198ohms. Red to brown on the right was 173ohms and the left transformer read 0.F. So its looking like a bad transformer. Not the result I wanted because Edcor's wait time was 8 weeks to get these.
Are you sure that OPT is completely disconnected from the power supply? It would be extremely unusual that Edcor would send you a defective OPT. I am sure it went thru several checks before shipping. Maybe the B+ or plate leads is grounded somewhere? It could be that the plate lead for the bad side is grounded or has been pinched during mounting, something like that.
 
If your reading are referenced to the chassis ground then the plate lead that reads 0 ohms is grounded either on the chassis or within the OPT to the transformer to chassis.
 
Your tests still have not ruled out the OPT. If their was an open winding in the OPT , which is the usual failure, you should read "infinite" ohms.
 
Are you sure that OPT is completely disconnected from the power supply? It would be extremely unusual that Edcor would send you a defective OPT. I am sure it went thru several checks before shipping. Maybe the B+ or plate leads is grounded somewhere? It could be that the plate lead for the bad side is grounded or has been pinched during mounting, something like that.
That was what I measured with the transformer unconnected entirely. It is on its way back to Edcor in any case, they are sending a replacement. I hope that is the issue.
 
while connected I couldn't get them to stop jumping around all over the place
As a side note, I ran into this "jumping around" readings a few times and realized that if you short the output leads together, then, a DMM will read the resistance just fine. I think its the fact that the transformer is not loaded, so reflections of the AC used to measure resistance becomes unstable. Try it.
 
That was what I measured with the transformer unconnected entirely. It is on its way back to Edcor in any case, they are sending a replacement. I hope that is the issue.
Usually the defect will read as an "open" circuit. Almost all bad OPT that i have tested , probably over 20, have had this problem. There were a couple that read "0". These had been grounded to the transformer frame by the wire lead being pinched by the cover when that was fastened onto the frame. This caused a break in the insulation which shorted to ground causing the 0 reading. One was in a chifi amp where the lead was not carefully routed thru the hole in the chassis to the inside. the one lead looped under the transformer and when the trans was tightened the one lead shorted to the chassis.
 
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