Well, everything is done. The amp is back together, and tomorrow I'll power up via variac/dbt and see if anything blows up. Then I'll work on dialing in dc offset and bias.
Possibly to everyone's dismay, I decided I'm going to let this amp go at some point in the near future, so kept everything stock, with exception of replacing the 4 electrolytic caps on each board, and the trimpots that were out of spec. This way, the next owner can decide if they are happy with it in stock form, or they can modify.
It sure seems like a lot of work to arrive where I am, but to be honest, as I was looking back at pic's and info on a EX-442 I had at one point, I realize that would be a better amp for me to keep, if I even decide I want this type of amp.
Let me make abundantly clear that you guys have made this process rewarding and helpful with an amp that someone told me had a short to ground and some bad Mosfets. I'm happy how it turned out and hope the amp is happy with my work. I thank you very much!
I'll report back with a final update on dialing the trimpots in, and measurements for posterity.
Possibly to everyone's dismay, I decided I'm going to let this amp go at some point in the near future, so kept everything stock, with exception of replacing the 4 electrolytic caps on each board, and the trimpots that were out of spec. This way, the next owner can decide if they are happy with it in stock form, or they can modify.
It sure seems like a lot of work to arrive where I am, but to be honest, as I was looking back at pic's and info on a EX-442 I had at one point, I realize that would be a better amp for me to keep, if I even decide I want this type of amp.
Let me make abundantly clear that you guys have made this process rewarding and helpful with an amp that someone told me had a short to ground and some bad Mosfets. I'm happy how it turned out and hope the amp is happy with my work. I thank you very much!
I'll report back with a final update on dialing the trimpots in, and measurements for posterity.
DC offset is under ±1mVDC.
Bias adustment ... I believe Bourns pots come preset in the middle-ish. As of 15 minutes of running the amp on the variac/dbt, heat is fine. In fact, it is not too warm, so I'm guessing the bias is lower than spec, which is better than too high. I love Bourns pots. Especially these 12 multiturn 3296P's ... and it was a good excuse to get a new trimmer tool from Bourns. Matchy, matchy.
Anyway, I need to get bias done. I'm not taking the amp back to the local guy, since that didn't work out so well. I'll report back once I get that done. I think I should ohm out the ps caps before pulling a fuse to do B&K's unique bias process.
Oh, KC ... the voltage across the ps caps is the same.
Bias adustment ... I believe Bourns pots come preset in the middle-ish. As of 15 minutes of running the amp on the variac/dbt, heat is fine. In fact, it is not too warm, so I'm guessing the bias is lower than spec, which is better than too high. I love Bourns pots. Especially these 12 multiturn 3296P's ... and it was a good excuse to get a new trimmer tool from Bourns. Matchy, matchy.
Anyway, I need to get bias done. I'm not taking the amp back to the local guy, since that didn't work out so well. I'll report back once I get that done. I think I should ohm out the ps caps before pulling a fuse to do B&K's unique bias process.
Oh, KC ... the voltage across the ps caps is the same.
Well, without speakers connected, and the amp connected to the dbt/variac, bulb goes dim and no problems. I just added test speakers and connected to preamp.
Plugging the banana plugs on the R channel, with NO power on, I got a thump to the speaker. The cord is unplugged. I had ohmed out the ps caps days ago, and just put the fuses back in the Neg rails. I got a tad spark on both.
After getting everything connected, and powering up on the variac, the bulb did not dim, so something is going on. I'm about to give up on this, which is okay, but a shame after all the help you guys have given me.
Any last thoughts on what I should check?
The only thing different from prior to recap is transformer wiring. I'm pretty sure that one loose connector came off when I pulled the ps board off the ps caps. That would make sense.
Plugging the banana plugs on the R channel, with NO power on, I got a thump to the speaker. The cord is unplugged. I had ohmed out the ps caps days ago, and just put the fuses back in the Neg rails. I got a tad spark on both.
After getting everything connected, and powering up on the variac, the bulb did not dim, so something is going on. I'm about to give up on this, which is okay, but a shame after all the help you guys have given me.
Any last thoughts on what I should check?
The only thing different from prior to recap is transformer wiring. I'm pretty sure that one loose connector came off when I pulled the ps board off the ps caps. That would make sense.
I'm going to do some more sleuthing later ... but I couldn't wait for an initial check.
With the output boards UNplugged, the ps caps measure 86vdc. Good. With the output boards plugged in, the ps caps measure about 54vdc ... when I attach probes across ps caps, the DMM first measures 71vdc, then drops to about 54vdc. So something on output boards.
If I have just the L channel output board connected, I get 64vdc across the ps caps.
I should have time later to see what I can find ... but it would appear it's something on the output boards. I'll check solder points again to see if I missed one. Other than that, I triple checked the 3 caps per board for correct polarity, and of course the np cap on each board, no worry there. The Bourns 3296 trimpots is the only other change.
With the output boards UNplugged, the ps caps measure 86vdc. Good. With the output boards plugged in, the ps caps measure about 54vdc ... when I attach probes across ps caps, the DMM first measures 71vdc, then drops to about 54vdc. So something on output boards.
If I have just the L channel output board connected, I get 64vdc across the ps caps.
I should have time later to see what I can find ... but it would appear it's something on the output boards. I'll check solder points again to see if I missed one. Other than that, I triple checked the 3 caps per board for correct polarity, and of course the np cap on each board, no worry there. The Bourns 3296 trimpots is the only other change.
UPDATE -- Sunday, Jan 27
I don't have the ability to edit or delete the previous two posts. But good news.
During a conversation with a guy on the other side of my state, he suggested I consider dialing down the bias on the Bourns trimpots for bias. I figured they came from the factory set midway.
Dialed the pots all the way down, and then did increased bias on the wipers in 180˚+ turns ... 8 per side. So that's about 4 complete turns CW.
Powered amp up on variac / dbt, and bulb dimmed. Success! So amp is fine. I'm back at square one, sort of, to dial in bias. I do have two options next time I head over to that side of the state, which could happen during the next two months. But great news since the hum is low, at an acceptable level, and my work has not been in vain.
At this point, there isn't really a reason for me to post a follow up, unless something unexpected happens.
Again, thanks for your patience and help!!
I don't have the ability to edit or delete the previous two posts. But good news.
During a conversation with a guy on the other side of my state, he suggested I consider dialing down the bias on the Bourns trimpots for bias. I figured they came from the factory set midway.
Dialed the pots all the way down, and then did increased bias on the wipers in 180˚+ turns ... 8 per side. So that's about 4 complete turns CW.
Powered amp up on variac / dbt, and bulb dimmed. Success! So amp is fine. I'm back at square one, sort of, to dial in bias. I do have two options next time I head over to that side of the state, which could happen during the next two months. But great news since the hum is low, at an acceptable level, and my work has not been in vain.
At this point, there isn't really a reason for me to post a follow up, unless something unexpected happens.
Again, thanks for your patience and help!!
Playing the Vandersteen 3A's from the B&K ST-202 Plus with unknown bias setting.
Okay, I hope this isn't a dangerous thing for me to do. I'm just tired of listening to music with headphones all day. My Parasound amps are awaiting a call from Tony at Parasound so I can sort an issue out there ...
What I will say is that I'm blown away, at least at the low volumes I'm listening to right now, that this amp sounds really good. I do get the mild thump at startup, but I know that either me, or the next owner can get the soft start & speaker protect kit ... I'm almost of a mind to do myself for the fun of it. But then, I'd feel like I should replace ps filter caps and before you know it, I'm upside down in this amp. Which is not a bad thing, IF I was going to keep it, which I don't rightly know. I might need to offer it up in a trade for some Magnepan speakers I'm thinking of ... decisions.
Okay, I hope this isn't a dangerous thing for me to do. I'm just tired of listening to music with headphones all day. My Parasound amps are awaiting a call from Tony at Parasound so I can sort an issue out there ...
What I will say is that I'm blown away, at least at the low volumes I'm listening to right now, that this amp sounds really good. I do get the mild thump at startup, but I know that either me, or the next owner can get the soft start & speaker protect kit ... I'm almost of a mind to do myself for the fun of it. But then, I'd feel like I should replace ps filter caps and before you know it, I'm upside down in this amp. Which is not a bad thing, IF I was going to keep it, which I don't rightly know. I might need to offer it up in a trade for some Magnepan speakers I'm thinking of ... decisions.
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