Biasing and offset info in att image.
Read it CAREFULLY as its a bit different to most amplifiers I have worked on.
Basics.
Let the amp run for at least a few hours.
Turn it off for 5-10 minutes.
Turn it on.
Wait 2 minutes.
Adjust bias to 12mv.
Adjust offset for 0.0 (< 20mv)
Do not readjust further.
I followed this to the letter.
But I also then let it run for several more hours and went through this same procedure again just to make sure.
I didn't need to. It was exactly right.
.....................
A friend gave me his KAV-300i to repair.
Fault was 'one channel was fuzzy irrespective of input or speaker'.
Examination revealed main filter caps were bulging.
In the middle of the amplifier board, under the input board, I found several other parts heat damaged.
I also found an opto coupler was faulty. This is directly in the biasing circuit. (U12 U15 - cny17-2)
As well as doing a complete re-cap, The following were heat affect and replaced.
Resistors R43 R44
Diodes D39 D40
Film caps C43 C45
Electrolytics C62 C63 C78 C79 C44 C48 C21 C46
Transistors Q25 Q26
Read it CAREFULLY as its a bit different to most amplifiers I have worked on.
Basics.
Let the amp run for at least a few hours.
Turn it off for 5-10 minutes.
Turn it on.
Wait 2 minutes.
Adjust bias to 12mv.
Adjust offset for 0.0 (< 20mv)
Do not readjust further.
I followed this to the letter.
But I also then let it run for several more hours and went through this same procedure again just to make sure.
I didn't need to. It was exactly right.
.....................
A friend gave me his KAV-300i to repair.
Fault was 'one channel was fuzzy irrespective of input or speaker'.
Examination revealed main filter caps were bulging.
In the middle of the amplifier board, under the input board, I found several other parts heat damaged.
I also found an opto coupler was faulty. This is directly in the biasing circuit. (U12 U15 - cny17-2)
As well as doing a complete re-cap, The following were heat affect and replaced.
Resistors R43 R44
Diodes D39 D40
Film caps C43 C45
Electrolytics C62 C63 C78 C79 C44 C48 C21 C46
Transistors Q25 Q26
Attachments
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Not a problem.
When I was looking I saw many requests, and zero useful info.
Just trying to give back to a community I like to think of as home
Edit.
pic 1+2 heat affected area.
Different angles of the same area.
The resistors are 6.19kohm ~1.5w, they measured 6.36ish, replaced with 6.2k 2w panasonics raised up a few mm for heat dissipation.
pic 3 after repairs.
Also replaced the two diodes and transistors purely so i had fresh full length legs to enable me to also raise them off the board.
When I was looking I saw many requests, and zero useful info.
Just trying to give back to a community I like to think of as home
Edit.
pic 1+2 heat affected area.
Different angles of the same area.
The resistors are 6.19kohm ~1.5w, they measured 6.36ish, replaced with 6.2k 2w panasonics raised up a few mm for heat dissipation.
pic 3 after repairs.
Also replaced the two diodes and transistors purely so i had fresh full length legs to enable me to also raise them off the board.
Attachments
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I'm the same way, but what I usually receive for service has no hope of looking factory ever again.
One of my teachers told me to make it look like no one has been there. Something I took to heart. I did a complete rebuild of a McIntosh pair of MC-40 amplifiers. The fellow who ended up with them honestly thought they were factory (even those the parts were types they didn't have back then). That was a nice compliment.
Your work really sets you apart from the average technician. I hope you are very well regarded in your neck of the woods!
-Chris
One of my teachers told me to make it look like no one has been there. Something I took to heart. I did a complete rebuild of a McIntosh pair of MC-40 amplifiers. The fellow who ended up with them honestly thought they were factory (even those the parts were types they didn't have back then). That was a nice compliment.
Your work really sets you apart from the average technician. I hope you are very well regarded in your neck of the woods!
-Chris
Ta.
Sounds like my work ethic.
Shhhhh i was never here
yes, even though i stopped doing repairs a few years ago, i still get people asking me to do things.
it does seem my reputation preceeds me.
dunno why I'm not perfect, I have my failures.
I was never a full on trained tech. more a hobbiest that spent the time learning by doing and reading and research.
Ya know, Back in my day we had these things called books.
None of this black art voodoo intarwebs..
Schematics were for those that were factory trained and had the right credentials within the industry.
Always had spare cans of smoke on the shelf
these days i might do the odd job for friends, but i have other things i want (need) to spend my time on
Sounds like my work ethic.
Shhhhh i was never here
yes, even though i stopped doing repairs a few years ago, i still get people asking me to do things.
it does seem my reputation preceeds me.
dunno why I'm not perfect, I have my failures.
I was never a full on trained tech. more a hobbiest that spent the time learning by doing and reading and research.
Ya know, Back in my day we had these things called books.
None of this black art voodoo intarwebs..
Schematics were for those that were factory trained and had the right credentials within the industry.
Always had spare cans of smoke on the shelf
these days i might do the odd job for friends, but i have other things i want (need) to spend my time on
Update.
I know Krell seem to polarise people into love or hate regarding their sound quality.
Whether you believe in break in or not.
Keep in mind the below comments are straight after every electrolytic had been replaced.
Using either panasonic fc or fm, and nichicon for the main filter caps.
For the record, I honestly believe if the horrid yellow input film caps were swapped for something better it would help dramaticly.
But it's very cramped for space in that area so options are limited.
Here's the owners thoughts after he got it home back into his system. (I'm paraphrasing here).
It sounds great, it has more energy now, can't wait for it to break in.
My thoughts.
Comparing it to a pass labs x150 / x2.5 combo. Unfair of me, but it's what I'm used to.
Initially it sounded harsh and very grainy on mid and treble, bass was ok but nothing special.
After a full day of running / break in it smoothed out and the mid / treble lost a lot of the harshness and grain.
I would expect (hope) that after a few weeks it will improve further.
After my brief time with the Krell, the pass combo has far deeper bass, extended highs, and overall a much smoother, cleaner sound.
Given break in time, I'm sure the Krell will improve.
Maybe one day we'll look at better input caps. I just know those nasty yellows are holding it back.
I know Krell seem to polarise people into love or hate regarding their sound quality.
Whether you believe in break in or not.
Keep in mind the below comments are straight after every electrolytic had been replaced.
Using either panasonic fc or fm, and nichicon for the main filter caps.
For the record, I honestly believe if the horrid yellow input film caps were swapped for something better it would help dramaticly.
But it's very cramped for space in that area so options are limited.
Here's the owners thoughts after he got it home back into his system. (I'm paraphrasing here).
It sounds great, it has more energy now, can't wait for it to break in.
My thoughts.
Comparing it to a pass labs x150 / x2.5 combo. Unfair of me, but it's what I'm used to.
Initially it sounded harsh and very grainy on mid and treble, bass was ok but nothing special.
After a full day of running / break in it smoothed out and the mid / treble lost a lot of the harshness and grain.
I would expect (hope) that after a few weeks it will improve further.
After my brief time with the Krell, the pass combo has far deeper bass, extended highs, and overall a much smoother, cleaner sound.
Given break in time, I'm sure the Krell will improve.
Maybe one day we'll look at better input caps. I just know those nasty yellows are holding it back.
Attachments
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Old N Cranky, you wouldnt by chance still have the part numbers to the caps that were replaced?
I just acquired a factory stock version 1 of the same amplifier and decided to replace all those abysmal RCA connectors with some nice Neutrik units, replaced the nearly as bad binding posts with Cardas CCGGS.
Although the amp still sounds very good, I noticed one of the filter caps bulging a little so some preventive maintenance would be a good idea. I may as well replace others while I`m in there.
I just acquired a factory stock version 1 of the same amplifier and decided to replace all those abysmal RCA connectors with some nice Neutrik units, replaced the nearly as bad binding posts with Cardas CCGGS.
Although the amp still sounds very good, I noticed one of the filter caps bulging a little so some preventive maintenance would be a good idea. I may as well replace others while I`m in there.
Attachments
Hi Michael.
Exact part numbers no. Depends on where you buy them I guess
Front to back. What I used;
The four main caps Nichicon 6,800uF 80V 85deg 10mm snap in.
The ten behind those are Panasonic fc 330uF 100v 105deg.
Watch hight with top top board.
Also on this one theres a cap hung under the top board near the left connector.
I had to lay this over to miss the new fc's.
Four in the middle of the board Panasonic fr's 3,300uF 25v.
Two resistors, panasonic 6.2Kohm 2w - Replaced so I could raise them off the board for air flow.
Two zeners 18v 1w (1n4746) - Replaced so I could raise them off the board for air flow.
Two optos cny-17-2 - Bias, one was dead, cheap, do both, opto's go off with age.
Two panasonic fc 10uF 63v
Two tall ones panasonic fr 220uF 63v
Four at back of board panasonic fc 10uF 100V
Under main board four AXIALS Nichicon 47uF 100v - These are what was there, and the only ones I found that would fit. Super tight squeeze.
Few more caps on top board.
Overall nothing critical, I just used decent parts. Should last another ? 20 years.
Latest report from owner is the parts have settled down (broken in) and it has never sounded this good.
I guess if you wanted to use silmics, have at it.
If it was mine i'd also upgrade the four yellow nasties at the back of the top board. (inputs)
But you'll be fighting for space...
EDIT.
I'd suggest pulling yours apart, and make a list (with dimensions) of what is on your board.
Might be different to this one ???
Also, when you put the top board back in, be VERY careful of the two sets of joining pin/sockets.
Make absolutely sure they are pin for pin lined up and connected straight.
Exact part numbers no. Depends on where you buy them I guess
Front to back. What I used;
The four main caps Nichicon 6,800uF 80V 85deg 10mm snap in.
The ten behind those are Panasonic fc 330uF 100v 105deg.
Watch hight with top top board.
Also on this one theres a cap hung under the top board near the left connector.
I had to lay this over to miss the new fc's.
Four in the middle of the board Panasonic fr's 3,300uF 25v.
Two resistors, panasonic 6.2Kohm 2w - Replaced so I could raise them off the board for air flow.
Two zeners 18v 1w (1n4746) - Replaced so I could raise them off the board for air flow.
Two optos cny-17-2 - Bias, one was dead, cheap, do both, opto's go off with age.
Two panasonic fc 10uF 63v
Two tall ones panasonic fr 220uF 63v
Four at back of board panasonic fc 10uF 100V
Under main board four AXIALS Nichicon 47uF 100v - These are what was there, and the only ones I found that would fit. Super tight squeeze.
Few more caps on top board.
Overall nothing critical, I just used decent parts. Should last another ? 20 years.
Latest report from owner is the parts have settled down (broken in) and it has never sounded this good.
I guess if you wanted to use silmics, have at it.
If it was mine i'd also upgrade the four yellow nasties at the back of the top board. (inputs)
But you'll be fighting for space...
EDIT.
I'd suggest pulling yours apart, and make a list (with dimensions) of what is on your board.
Might be different to this one ???
Also, when you put the top board back in, be VERY careful of the two sets of joining pin/sockets.
Make absolutely sure they are pin for pin lined up and connected straight.
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Thank you kindly for the replies. Are you refering to the input/preamp or main amplifier board? I was unaware of any circuit differences between the 2 versions except for the relocation of the transformer and a different PC board for the primary windings.
I had a 3ooi with massive distorstions left, one halfwave was missing almost.
Bias adjust impossible, New opto's no effect. New caps no effect.
One of the driver trans 15030 had an invisible contact problem, emitter leg soldering lost contact to the mainboard,bad soldering in factory...
Shi...y bias pots with oxydation created also a problem...
Bias adjust impossible, New opto's no effect. New caps no effect.
One of the driver trans 15030 had an invisible contact problem, emitter leg soldering lost contact to the mainboard,bad soldering in factory...
Shi...y bias pots with oxydation created also a problem...
Hi Groove T,
How old is that amplifier now? How about all the other "shi...y" parts in there like capacitors, thermal compound, signal transistors?
Here's news for you. I have an instrument that was over $10K new and is still worth $2K today (last time I looked). It was built with the best parts available and guess what? The trimmers are bad in that too. So are the front panel switches.
Technicians know to check things like that, and to replace them with the same purchased from authorized distributors. Once I have the time, I'll rebuild it using all new trimmer controls. Not from Ebay - just saying. This thing is just a voltage, millivolt and current source. The calibration is going to take a long, long time since I'm replacing all the trimmers. Trick, the new ones will be put into roughly the correct positions before I put them in. Hopefully that will reduce setup time. Do the same for your bias and offset controls.
-Chris
That's one thing I'm really tired of hearing.
How old is that amplifier now? How about all the other "shi...y" parts in there like capacitors, thermal compound, signal transistors?
Here's news for you. I have an instrument that was over $10K new and is still worth $2K today (last time I looked). It was built with the best parts available and guess what? The trimmers are bad in that too. So are the front panel switches.
Technicians know to check things like that, and to replace them with the same purchased from authorized distributors. Once I have the time, I'll rebuild it using all new trimmer controls. Not from Ebay - just saying. This thing is just a voltage, millivolt and current source. The calibration is going to take a long, long time since I'm replacing all the trimmers. Trick, the new ones will be put into roughly the correct positions before I put them in. Hopefully that will reduce setup time. Do the same for your bias and offset controls.
-Chris
Hi Chris
Don't worry.
I used the s-word more because i was fooled, had 2 errors in the bias-circuit and one of the regulators was gone due failing cap.
The real problem with this amp is no complete schematic and the construction makes it nearly impossible to measure something in the driverstage while it is powered.
Frank
Don't worry.
I used the s-word more because i was fooled, had 2 errors in the bias-circuit and one of the regulators was gone due failing cap.
The real problem with this amp is no complete schematic and the construction makes it nearly impossible to measure something in the driverstage while it is powered.
Frank
Hi Frank,
Cool. It's just popular for newbies to blame perfectly good components that have survived well beyond their expected lifetime. I touch of good cleaning fluid generally returns normal operation with these parts anyway. Multi-turn controls can't be cleaned of course, and these are the favoured replacement and are normally inferior to the single turn controls. You can get good ones, but those are a little more expensive which rules them out of consideration by most people.
Best, Chris
Cool. It's just popular for newbies to blame perfectly good components that have survived well beyond their expected lifetime. I touch of good cleaning fluid generally returns normal operation with these parts anyway. Multi-turn controls can't be cleaned of course, and these are the favoured replacement and are normally inferior to the single turn controls. You can get good ones, but those are a little more expensive which rules them out of consideration by most people.
Best, Chris
Hi Chris
For me its always worth the better quality, customers not really.
One of those trimmers cost me maybe 1 $, but 10 or more $ for delivery service, another 10 $ because minimal order amount is 25 $ or similar..
Last week they sent me 4 resistors at 5 cents each, were not on stock on main order, now the charge me again with 25 $ for after sales service or whatever.
So i think the parts price is not the real PITA.
Anyway, the KAV 300 is working fine now, next project seems to be a 30 year old pair of KMA 160 in need of full service for caps, trimmers, connectors and relays.
best
Frank
For me its always worth the better quality, customers not really.
One of those trimmers cost me maybe 1 $, but 10 or more $ for delivery service, another 10 $ because minimal order amount is 25 $ or similar..
Last week they sent me 4 resistors at 5 cents each, were not on stock on main order, now the charge me again with 25 $ for after sales service or whatever.
So i think the parts price is not the real PITA.
Anyway, the KAV 300 is working fine now, next project seems to be a 30 year old pair of KMA 160 in need of full service for caps, trimmers, connectors and relays.
best
Frank
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