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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Simple High Performance DC Coupled Class A HPA with sub PPM THD
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Old Yesterday, 07:17 AM   #351
yourownbuzz is offline yourownbuzz
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Here's measurements on the schematic - jfet stages seems to be working - similar voltages on both channels, mosfet stage - left (distorted) channel has 0V at the gate of both mosfets, and across R137 & 138.

Would bad mosfet(s) on the left be the conclusion?

I've also ticked off components against BOM - all seems correct now, I used mostly the through hole WIMAs instead of SMT caps, and there were a few dodgy SMT joints that I touched up as well.
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Old Yesterday, 10:57 AM   #352
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
There are only a few ways for the G of V133 to be 0v: a solder bridge on G to ground plane on opposite side, or for MOSFET to be dead (open circuit). As R133 is connected to JFETs which appear to be working, there should be similar voltage to L-channel. Are you certain that those are IRF9610 (P-channel) installed on V133 and V134?
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Old Yesterday, 11:56 AM   #353
yourownbuzz is offline yourownbuzz
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I used FQP3P20 since the BOM said they were the lower distortion sub for the 9610...all 4 came from the same tube, will double check the part numbers are correct & check for shorts
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Old Yesterday, 12:59 PM   #354
avtech23 is offline avtech23  Australia
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Are you using polarised caps for c10007, c10027 etc?

If so, c10027 is reversed when looking at the silkscreen.
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Old Yesterday, 01:52 PM   #355
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Quote:
Are you using polarised caps for c10007, c10027
Looks like he using Wima film caps there, so not an issue.
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Old Yesterday, 02:56 PM   #356
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Make sure you did not install a BF862 JFET where there should be a BC857 PNP BJT. The BF862 are labeled "AW2" I think. Since both are SOT23, it is easy to mix them up.
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Old Today, 12:23 AM   #357
yourownbuzz is offline yourownbuzz
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All the BF862 are BF862s, and no shorts were visible/measured from G of MOSFETs to gnd
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Old Today, 02:21 AM   #358
xrk971 is offline xrk971  United States
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So there is something very different between your two channels. Check the bad channel vs the good channel part by part - make sure all resistors are the correct value.

You verified that all installed MOSFETs are indeed P channel? Sometimes they put an N channel in a P channel tube. And we take it out and use it without looking to see if it is the right part.
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Old Today, 03:03 AM   #359
yourownbuzz is offline yourownbuzz
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part numbers on the package match between all 4 of them, will check parts again, I did it yesterday, but admittedly that was at night while I was tired.
I guess if it all checked out I'll swap the 2 mosfets for the right channel and see
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Old Today, 06:06 AM   #360
yourownbuzz is offline yourownbuzz
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All the resistors are correct values (checked SMD parts based of case marking - didn't measure), MOSFETS on right channel were good - pulled them and tested on those parts testers, confirmed working & P channel.

Swapped them anyway (tested the replacements before soldering them in) - no difference. Touched up all of the right channel solder joints too, and still no difference.

Right MOSFETS are still cold to touch whereas left warm up.

Is there a possibility of a bad BF862 in the right? Or is continuity checking everything the next step before pointing fingers at transistors?

Last edited by yourownbuzz; Today at 06:14 AM.
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