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Chip Amps Amplifiers based on integrated circuits

B&K CT600.1
B&K CT600.1
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Old 5th November 2017, 06:28 PM   #1
bullittstang is offline bullittstang  United States
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Default B&K CT600.1

I recently had a "pop, sizzle, smoke" incident with my B&K CT600.1 6-zone amplifier.
Upon opening the case - found 1 very crispy (and very burnt) LM3886.

So I tried to do some testing before I completely tear-it down and start replacing parts.
What I found was that both Zone 5 & 6 (zone 6 was the obvious dead LM3886) have 35vDC on the outputs. And after removing Zone-6, Zone 5 is running hot enough that the heatsink is getting warm. Not too hot to touch, but the chip is running much warmer than the other 10.
Checked all voltages against the schematic and they all check out.
I have attached the full schematic and with a thorough testing of what I could on the boards - it appears I have a few caps that are leaky and a pair of schottky diodes that are failed/failing.
All other channels check out with 1-2mV on the outputs and comparing all voltage, resistance and diodes with the faulty channels this is what I "think" I need to replace.

Items to replace:
2 - LM3886T (obviously)
D17 & D20 - schottky diodes
C17 & C18 - check as low capacitance (Zone 6 is 85uF and fluctuates, Zone 5 is 95uF and fluctuates)
I was thinking of replacing the BJT's (Q17, Q5 and Q11), since it will be torn apart - just to be safe.

Anything else I might be missing or should check before I order parts and move forward?

EDIT:
Not sure if a moderator can change the title. I got in a hurry and can't edit.

1-2 channels dead on B&K CT600.1
Attached Images
File Type: jpg B&K CT600 actual.JPG (817.7 KB, 114 views)
Attached Files
File Type: pdf CT600-3_Sch.pdf (419.7 KB, 35 views)

Last edited by bullittstang; 5th November 2017 at 09:26 PM. Reason: Title change request
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Old 5th November 2017, 10:33 PM   #2
tomchr is offline tomchr  Canada
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I'd look at R5, R6 (series output resistor) as well.

I wouldn't worry about the transistors. They're part of the mute circuit. If the remaining LM3886 works well enough to support mute, measure the voltage on pin 8 of the LM3886. You should see about -3 V under normal circumstances. If that works, the transistors are fine.

Tom
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Old 6th November 2017, 02:36 AM   #3
bullittstang is offline bullittstang  United States
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Thanks Tom - I know you are a guru with these LM3886 chips.
Quote:
Originally Posted by tomchr View Post
I'd look at R5, R6 (series output resistor) as well.
I checked them - they both are within spec at 2.24k

I also checked R7 R8, because the +8 and -8 are only giving me +6 -6, but they resistors are 2.26k on the schematic, but banding and measurement shows 1.5k, so I think there has been a few Rev. since my schematic was made.

Quote:
Originally Posted by tomchr View Post
I wouldn't worry about the transistors. They're part of the mute circuit. If the remaining LM3886 works well enough to support mute, measure the voltage on pin 8 of the LM3886. You should see about -3 V under normal circumstances. If that works, the transistors are fine.

Tom
I checked PIN 8 and for the "bad" channels I get -0.2-0.3mV and even at start-up or power-down it only goes to -1.2mV.
I checked 2 other accessible chips and they are at the same -0.2mV running, but they go from -3.2V down to -0.2mV in about 5 secs after power on. I think that is good, since the mute should be active then.

I also verified on the "fried” channel, C33, C34 appear to both be OL and the other faulty channel they are showing a much higher capacitance.
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Old 6th November 2017, 04:52 AM   #4
tomchr is offline tomchr  Canada
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R5, R6 should be 0.1 Ω, 5 W.

R7, R8 are in CH1, 2 which you say are working. Not sure why you're messing with them.

The voltage on the mute pin (pin 8) should be pulled low (-3 V) when the chip is supposed to be active (not muted). If Pin 8 sits at ground (0 V), the output is muted. If you get +35 V out under those conditions, the LM3886 is definitely dead.

C33, C34 are unlikely but not impossible to have fried. I'd replace them with 35 V or 50 V types if you can.

Are you measuring the component values with the parts in the circuit? If so, don't count on getting a good reading as you're measuring the component plus whatever is in parallel with it.

Tom
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Old 6th November 2017, 04:58 AM   #5
PRR is online now PRR  United States
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Quote:
Originally Posted by bullittstang View Post
Quote:
Originally Posted by tomchr View Post
I'd look at R5, R6 (series output resistor) as well....
....I checked them - they both are within spec at 2.24k....
"Series output resistors" are typically *small* Ohms; much less than 8r. Not 2K.

I see R5 R6 as 0.1 Ohm.
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Old 6th November 2017, 12:45 PM   #6
bullittstang is offline bullittstang  United States
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Quote:
Originally Posted by tomchr View Post
R5, R6 should be 0.1 Ω, 5 W.

R7, R8 are in CH1, 2 which you say are working. Not sure why you're messing with them.

The voltage on the mute pin (pin 8) should be pulled low (-3 V) when the chip is supposed to be active (not muted). If Pin 8 sits at ground (0 V), the output is muted. If you get +35 V out under those conditions, the LM3886 is definitely dead.

C33, C34 are unlikely but not impossible to have fried. I'd replace them with 35 V or 50 V types if you can.

Are you measuring the component values with the parts in the circuit? If so, don't count on getting a good reading as you're measuring the component plus whatever is in parallel with it.

Tom
Yes I checked R5,R6 in 4-channels, they are all measuring the same.
Sorry about that - got on wrong page (low-power supply) and measured those.

Makes sense, I plan on replacing both.

Thanks for that - I had 250V parts in my cart, 50V should be a little cheaper.

I am, but as much as I'm looking at the schematic I am also comparing to the other channels (and calculating the parallel resistance) to make sure they are similar.
Thanks Tom.

Quote:
Originally Posted by PRR View Post
"Series output resistors" are typically *small* Ohms; much less than 8r. Not 2K.

I see R5 R6 as 0.1 Ohm.
I see that now - and I did measure the right ones (and in spec), just confused and put the low-power board measurements.

Thanks for jumping in and highlighting the specific part(s).
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Old 11th November 2017, 03:36 AM   #7
bullittstang is offline bullittstang  United States
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Back for some more advice. How hard would it be to take out the zone inputs and just turn this into a 10-channel amp for DSP duty?
As I mentioned before - I'm new to reading diagrams, so the different ground/earth symbols has me confused and all the different connections going across multiple pages.

Trying to figure out what voltage should be going to the mute circuit (highlighted JP3 pin 5/6) and then how to wire an RCA off of the plugs, appears to be Pins 1/9. But when I go to JP4 and JP5 on the Zone board it appears to be different numbering. From there - I'm lost.
I attached what I think are the important parts of the schematic for your ease, but could be wrong.
Any assistance would be appreciated.
Attached Files
File Type: pdf Schematic.pdf (172.1 KB, 7 views)
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Old 12th November 2017, 02:12 PM   #8
bullittstang is offline bullittstang  United States
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Anybody want to weigh in on this?

- How can I eliminate the "zones" and connect the RCA's straight to the amplifier boards to simplify and use the amp as-it-sits?

Any input appreciated.
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Old 16th November 2017, 05:04 PM   #9
bullittstang is offline bullittstang  United States
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Default Converting 6-Zone LM3886 amp to multi-channel DSP amp

Bump - to try and get some possible answers to eliminating the "zones" and using this as a 10 channel amp for a DSP project?

1st page of attachment (I drew in different colors to assist w/routing):
This is the LM3886 and it's mute circuit. Any idea, how I can “tap” into the “1”, “9”, “5 & 6” pins to separate from the zones in this amp? I
get that Pin 1 & 9 are the positive input from the RCA, but I'm missing something, because I get no sound at the moment with them attached to RCA's.

Question(s) –
- what do I connect to pins 5 & 6? Should there be voltage going to the base of Q17 - if so what voltage? I can't see any voltage on those pins with my DMM when it's on, so I'm at a loss.
- do pins 1 & 9 go to center of RCA?
- do I need to ground all the other pins? Or leaving open is ok?


2nd page of attachment:
This is the connector from the LM3886 above (JP3) and it’s associated connector on the 6-zone board.
Attached Files
File Type: pdf CT600 _alt. Schematic.pdf (133.9 KB, 7 views)
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Old 18th November 2017, 12:20 PM   #10
bullittstang is offline bullittstang  United States
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Any help would be great, even if it's "you can't do that", or "it's not worth doing".

Figured I'd spend $$$ and time, repurposing what I have, than trying to fix the 2 burned channels, because it appears there is a problem in teh "zones" board now, so no channels are working at this time.
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