BIG problems
- Thread starter Michael F
- Start date
One step forward, two steps back
Replaced TO3 mica`s.
Q11 collector to chassis 600K
Q12 collector to chassis 5K and increases
Those figures the same for L and R, before and after mica change.
Replaced out of spec Right channel out of spec R38, 39, picked up a .116 bias were there was none before. No change in bias on Left, no changes in L and R offsets.
Replaced TO3 mica`s.
Q11 collector to chassis 600K
Q12 collector to chassis 5K and increases
Those figures the same for L and R, before and after mica change.
Replaced out of spec Right channel out of spec R38, 39, picked up a .116 bias were there was none before. No change in bias on Left, no changes in L and R offsets.
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- Halfbiass...Electron Herder and Backass Woof
- Joined
- Jan 14, 2011
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- 74,939
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof
I carefully loaded the outputs following Joe`s suggestion today, one horizontal row at a time testing for collector to chassis shorts and bring it up on the DBT/Variac combo at every step.
For the first time in nearly 3 months, she sings, I cant believe it. Bias is still above spec at .525 on the Left and .475 on the Right channel with trim pot at the minimum position, that does not sit well with my OCD. Offset is nice at .05 and .09.
Both channels seem to be equal in volume and sound quality. I`m using a shitty center channel speaker as a load at the moment so I cant do a true 2 channel test yet but after running a half hour, heatsinks are still cool.
For the first time in nearly 3 months, she sings, I cant believe it. Bias is still above spec at .525 on the Left and .475 on the Right channel with trim pot at the minimum position, that does not sit well with my OCD. Offset is nice at .05 and .09.
Both channels seem to be equal in volume and sound quality. I`m using a shitty center channel speaker as a load at the moment so I cant do a true 2 channel test yet but after running a half hour, heatsinks are still cool.
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- Gillette, Wyo.
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Put it all back together and hooked up the (fused) JBL`s and all seems well. Cant really drive it hard at this late hour but I sure will tomorrow. I`ll just keep it running at a low level in the mean time. Simply ecstatic the amp is working again.
I`m curious as to what the consequences of too high, or for that matter too low a bias voltage would be?
HUGE thanks to Lee, Joe and others for guiding me through all of this.
Gotta go, Boz beckons
I`m curious as to what the consequences of too high, or for that matter too low a bias voltage would be?
HUGE thanks to Lee, Joe and others for guiding me through all of this.
Gotta go, Boz beckons
- Joined
- Jan 14, 2011
- Messages
- 74,939
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof
- Joined
- Jan 14, 2011
- Messages
- 74,939
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof
- Joined
- Jan 14, 2011
- Messages
- 74,939
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof
Post-mortem
I`m glad to report that the amp has been running flawlessly for 3 days now and it`s sounding just fine, no snaps, crackles, pops or other mystery noises, running cool too despite the higher than normal bias.
I haven't yet had a chance to do any critical listening so I`ll have to reserve further judgement but do hope to prior to the Rev C install which should happen sometime next week.
This has been one hell of an experience with a good outcome. I haven't used or applied this much electronics practice since High School (late `70s) but always wanted to, I guess I got my chance.
I`m hoping some folks got something useful out of this thread as there is a lot of good information contained within, at the very least some entertainment. It ought be of help to someone in the future.
Looks like BIG problems have been solved, thanks again guys.
I`m glad to report that the amp has been running flawlessly for 3 days now and it`s sounding just fine, no snaps, crackles, pops or other mystery noises, running cool too despite the higher than normal bias.
I haven't yet had a chance to do any critical listening so I`ll have to reserve further judgement but do hope to prior to the Rev C install which should happen sometime next week.
This has been one hell of an experience with a good outcome. I haven't used or applied this much electronics practice since High School (late `70s) but always wanted to, I guess I got my chance.
I`m hoping some folks got something useful out of this thread as there is a lot of good information contained within, at the very least some entertainment. It ought be of help to someone in the future.
Looks like BIG problems have been solved, thanks again guys.
Hey Mike
It just dawned on me to ask how you are measuring your bias? Directly across R38 with your plus and minus leads or are you measuring the topside of R38 with your positive lead and ground with your negative lead?
Glad all is working well for you and you are back to actually enjoying the amp.
It just dawned on me to ask how you are measuring your bias? Directly across R38 with your plus and minus leads or are you measuring the topside of R38 with your positive lead and ground with your negative lead?
Glad all is working well for you and you are back to actually enjoying the amp.
- Joined
- Jan 14, 2011
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- 74,939
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- Gillette, Wyo.
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- Halfbiass...Electron Herder and Backass Woof
Sorry to drag this up, but I have a tip that may help someone in the future. When I have a bias issue (DBT burning bright) I use a jumper with EZ Hooks between the base of Q7 and Q10 on the 14 and 20 boards. this is also the cathode of D10 and anode of D9. Effectively this shorts out the bias transistor and bias goes to zero. The advantage is no wires are lifted
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