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Cant Set Bias On Concept 11.0

Tala Mink

Tala Mink

Active Member
I recently repaired a concept 11.0 with a very badly blown channel.
I had to replace almost every transistor + some resistors and while it does somewhat work there is a large problem with the bias circuit of the amplifier.

The bias pot is extremely touchy at certain points and can jump to the hundreds of millivolts with barely any movement, I can adjust the bias while the unit is on but there is very noticeable distortion on the top of the output no matter what setting it is at

If the bias is above 0 before the unit is powered on, when I do turn it on my bulb will stay lit until I adjust the bias to zero.

I'm 99% sure that this is a transistor issue because the problem is in both channels and the transistors are the only thing that I replaced in both.

Here's what I replaced the transistors with:

501&501 - 2SA872 - didn't change (the offset is adjustable and stable)
503&504 - 2SD668 - KSC3503DS
505 - 2sb649 - didn't change
506 - 2SD400 - KSC2383OTA
507 - 2SB669 - MJE15030
508 - 2SB649 - MJE15031
509&511 - 2SD675 - MJ21196
510&512 - 2SB655 - MJ21195
D503&D504 - VD1221 - 2 1N4148 in series

this isn't something I've encountered before and any help is appreciated

IMG_20230515_174154001.jpg IMG_20230515_182732503.jpg
 
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replace the pots. i do this as a regular process when refurbing/repairing a unit. just yesterday i replaced the trimpots in a kenwood after replacing the outputs because the bias was creeping upwards. it was rock solid after the pots were changed. may or may not help you but it rules them out.
 
I agree, most likely it is a transistor issue. Also check the Sabistor diode (usually installed on the heatsink) is measuring properly.

TR503A/TR504A: 2SC668 --> KSC2383YTA
TR506:2SD400 --> KSC2383YTA (KSC2383O will be ok; alternatively BD135 will also work)
TR505A: 2SB649A --> TTA004B,Q (or KSA1220AYS)
TR507A/508A: 2SD669A/2SB649A --> TTC004B,Q/TTA004B,Q (or KSC2690A/KSA1220A) - do not interchange

Schematic has some errors - double check PNP/NPN positions.
Your choice of outputs are OK

Hope this helps
 
Always always always replace the input pair. Even if things look good. KSA992.
Your drivers may be the issue, try KSC2073 and KSA940.
 
The original drivers have a fT of 110Mhz. The outputs used as per schematic are 2SB655B and 2SD675B as the outputs. These have fT of 10MHZ.
 
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p4spooky said:
TR507A/508A: 2SD669A/2SB649A --> TTC004B,Q/TTA004B,Q (or KSC2690A/KSA1220A)
Click to expand...

i had wondered about his choice for subs for these as i have used the ones that you listed in a radio shack receiver so i thought they might work considering the same company made concept but i wanted to see what other, more experienced people had to say.:thumbsup:
 
IMO, the Toshiba drivers rock from a sound quality perspective. I have used this in Sansui/Concept 6.5 that I am currently working on and they work great. Speed is a tad slow at 100MHZ compared to KSA1220A/KSC2690Y combo. KSA1220 is unavailable now unless you have a few lying around.

Yes, always looking forward to more experienced techs opinion on this.
 
p4spooky said:
IMO, the Toshiba drivers rock from a sound quality perspective. I have used this in Sansui/Concept 6.5 that I am currently working on and they work great. Speed is a tad slow at 100MHZ compared to KSA1220A/KSC2690Y combo. KSA1220 is unavailable now unless you have a few lying around.

Yes, always looking forward to more experienced techs opinion on this.
Click to expand...
i've got both combos. my 3.5 got onsemi and it sounds great. i haven't gone thru my 4.5 yet and it sounds dull and lifeless in comparison but it also needs fresh caps. i'll get to it some day.;)

gotta say, they used some badass glue, even acetone doesn't break it down without a LOT of work.
 
thanks for the input, I have a 9090 on my bench so I cant update on the concept until its done.

I did find a thread on a realistic 2100 restoration (the concept 11.0 and realistic 2100 have almost the exact same amp section) and 2SD668 was replaced with KSC2690 other than that they used similar transistors to what I used.

I have the Toshiba drivers and a pair of KSC2690's in stock and will put those in next time I look at the amp.

If none of that works I still have the original transistors from the working channel and will replace them one by one which should narrow down which replacement is the troublemaker.
realistic 2100.PNG conc.PNG
 
ivandezande said:
Always always always replace the input pair. Even if things look good. KSA992.
Your drivers may be the issue, try KSC2073 and KSA940.
Click to expand...

Sorry to poke in, is that for every project I'm doing or just the Project 11 on my fixer pile?
 
Its looking like the Toshiba drivers fixed it, sine waves don't have visible distortion and the amp can power up normally with the bias set, I did also replace the differential pair which may also have been the fix but it is pretty unlikely, the offset should be much more stable with 992's in place though.

I'm not sure if its normal but the amp gets pretty toasty, I put a 4 amp fast blow in it and, decided to run it at 10 watts into 8 ohms for 10 minutes but cut the test after 7 because I couldn't hold my hand to the heatsinks, I don't think I have worked on a class AB receiver that runs that hot and if someone has a concept 11.0 I would like to know if that amount of heat is normal.

It does put out a clean 110 watts so I'm assuming that its working normally, I think I might just up the size of the heatsinks on TR503&504 and add a small heatsink to TR505 because those three seem to get quite hot (503&504 were idling around 110F after being on for 40 minutes).
 
Glad to hear the new transistors did the trick. However, the heat is NOT normal. What is your bias setting on idle?
 
I just checked it and set it to 15mV, it was a little low, around 10.
It's still pretty warm at idle though.
 
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Bias seems OK but temps should be slightly above room temp on idle

Please check the following:
1. Base resistors for all 8 outputs - should be 4.7R
2. Output sockets - the transistor have a snug fit without loose connections (B,E pins)
3. Bias transistor replaced with KSC2690 (or KSC2383)
4. Check for any high frequency oscillations during idle or load
5. Turn bias pots down (may be below 2mV) and check temps.
 
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