CA-800 Bias Problems After DOA Rebuild

ekimezeerf

New Member
Hey everyone, long time lurker, first time posting.

A friend brought me a dead CA-800, would not power on and many signs of heat stress on main amp and power boards. Our thought was the PO has dumped a bottle of cologne on it due to it's aroma and the goop present everywhere inside.

I've worked on the main cap board, power supply, main amp boards, and mode switch - deep cleaned, changed all electrolytic capacitors, signal + power transistors, fuse resistors, micro switches, trim pots, diodes...

I've cleaned every switch, every pot, pre out- main switch, checked all grounds, re-soldered cracked joints on rear panel, re-worked all wires with signs of fatigue, checked all resistors I thought relevant...

I've read almost every post on AK that even mentions a CA-800 or CA-1000 and learned so much more than I ever wanted to know about this amp. But, it came alive! So thank you to everyone who ever took the time to leave their experience and knowledge here. I'm definitely standing on your shoulders.

Now, here's where I'm completely stuck. It's sounds terrible at any volume past ~ 10-15%. Distortion in the bass is most obvious at higher volumes but the treble is no better. When attempting to adjust the main amp boards (specifically the Class A/B Bias) in class B mode the lowest I can set the bias is 0.085V on both boards (but as high as 0.120V)

In Class A mode, the highest I can set the bias is 0.130V (and as low as 0.080V)

Differential voltage is right at 15V and the Mid-Point Potential is right at 0.0V. The +/- 50V supply is there and in Class B Mode I have +/- 45v and in Class A Mode i have +/- 20v (which is too high right?)

The Bias problem is the same on both boards which made me think there was a problem with the power supply or maybe the mode switch. I've exhausted myself going over both, even took the mode switch completely apart and cleaned the rails and contacts inside.

I have many more details and voltages to report if someone is interested in giving my a hand up. I'll include a few photos here of the before/after type. Maybe I've made a simple mistake, overlooked something, or maybe its a dying transformer...any help would be great!View media item 15301View media item 15302View media item 15303View media item 15305View media item 15304
 
Hmm..On the main amp boards there are 4 terminals on the top edge. A,B,C & S. Make sure they are all going to there proper destination on the mode switch board ( class A switching circuit). For right now don't even mess with class A bias adjustment trimmer until class AB is working properly. Set all 4 bias adjustment trimmers to there min. ie fully CCW I believe.
 
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On the A,B,C and S terminals you can verify the proper switching of the mode switch by ohming out the wiring and switch in both Class A and Normal switch position.
 
For testing the Ohms verifying the switch is functional, in NORMAL mode I should have something like this:
Normal:
B-C <1 ohm
S-C ~470 ohm
A-C >5k ohm

Class A:
A-C <1 ohm
S-C <1 ohm
B-C >5 kohm

Or do I have that reversed?
 
Thinking maybe the >5kohm are a little high, trimmers should be set to minimum, measuring path between pins A and B, expect <5k
From another thread
Mode------Pins--------L------------R-------
NORMAL............BC.....0.4 ohms.....0.4 ohms
..................BS.....469 ohms....472 ohms
..................CS.....469 ohms....471 ohms
..................AB....1.52k ohms...2.38k ohms
..................AC....1.52k ohms...2.38k ohms
..................AS....1.99k ohms...2.85k ohms

Class A...........BC.....1.52k ohms...2.38k ohms
..................BS.....1.52k ohms...2.38k ohms
..................CS.....0.3 ohms......0.2 ohms
..................AB....1.52k ohms....2.38k ohms
..................AC....0.3 ohms.......0.3k ohms
..................AS....0.3 ohms.......0.3 ohms

TR604 been replaced?
 
Here's the readings I got this weekend. I'll have to go back tonight and follow mbz's list of readings as they are more comprehensive:

Mode------Pins------L-------R--------
Normal---BC----0.9ohms--0.9ohms
SC----480ohms--484ohms
AC----8.06K ohms---8.1K ohms


Class A
AC----0.9ohms---0.9ohms
SC----0.9ohms---1.3ohms
BC----8.6K ohms---8.1k ohms


Seems like the 8.6-8.1K ohms readings are pretty high. I'll have to double check that the trimmers were set to their minimum (which I swear they were) and follow the rest of the above listed measurements.

As for TR604, Yes it was replaced with KSC2690 AYS
 
DSC04192.JPG You do know that those SONY TO-202 transistors ( EBC) are a different pin out than the KSC2690AYS (ECB)

.
 
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I don't see a problem with the A-C/B-C measurements. It just means the VRs were not at minimum.
My concern was that one of the VRs had become intermittent/open, clearly new VRs installed.
Need to double check any transistor replaced. TR605 on the heat sink catches some people out.
You should use KSC1815 or KSC1845, pinout ecb, pin closest to the VRs (top pin) is base. Pix
suggests correct orientation.
 
View attachment 1209747 You do know that those SONY TO-202 transistors ( EBC) are a different pin out than the KSC2690AYS (ECB)

.

I did note the pin-out difference and installed them accordingly. And TR605 was replaced with a KSC1815. It was turned 180 degrees from original transistor orientation, so the pin-out should be correct. It may be difficult to see in my photo (because of the heat shrink I used) but TR604 and TR608 are installed just like the photo above.
IMG_20180611_063720537.jpg
 
I also wanted to note that on the Right Amp board, when the unit is powered up in Class B mode, There is definite ringing (oscillation?) noise coming from the bi-polar 47uF capacitor that sits between TR604 and TR601.

If I touch it with an insulated screw driver, the bias drops 1-5 mv and the ringing gets much quieter. :dunno:
 
Maybe D607, R626, R627 out of spec? But both channels affected...
D603,4 failed short?
b/c/e voltages to GND for TR604,5,8,9,610,611 may be needed (or whatever
you have already taken). Please don't probe between transistor legs.
The 47uf cap...
 
Did the 2 fuses on the main amp board blow, they look new, would point to outputs/drivers/... maybe going intermittent
 
Did the 2 fuses on the main amp board blow, they look new, would point to outputs/drivers/... maybe going intermittent

Yeah...because both boards are affected/behaving the same way I'm really lost.

I'll go back and check D603,4 and start a list of the voltages for the transistors. (may take me a few days to gather and post).

As for the fuses, F602 on the Left Amp Board was blown when I first got the unit. I don't recall if the same was true for the Right Amp Board

TR610,11 were replaced with MJ21196 and MJ21195 (both sets). The original TR611 (2SA663) on one side was definitely shorted and failed.
 
Just checked some photos of the rebuilt amp boards...

R626 & R627 are supposed to be 4.7 ohm according to my schematic. Can I get a sanity check on that?

From my photos I can clearly see that I installed 4.7k ohm resistors in both location on both boards :no:

Time to go back and check all values of all the things I've replaced...sigh
 
Just checked some photos of the rebuilt amp boards...

R626 & R627 are supposed to be 4.7 ohm according to my schematic. Can I get a sanity check on that?

From my photos I can clearly see that I installed 4.7k ohm resistors in both location on both boards :no:

Time to go back and check all values of all the things I've replaced...sigh
Explains why it doesn't bias. My board here has 4.7Ω 1 watt.
 
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