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AU-888 Refurb: Driver Board Issues

David Weaver

David Weaver

AK Subscriber
Subscriber
I recently acquired another 888 that was in pretty rough shape. Seems like it may have been stored in a damp basement or barn and has some minor rust and corrosion. After cleaning all mechanical connections and replacing all fuses, I fired it up on my DBT. Initially it failed but I localized the problem to the left driver board (F1282). I replaced the caps and pots and was able to set the balance and bias current with it passing the dim bulb test. I then connected speakers and a source only to find the left channel was cracking in and out badly. The right channel sounded great. I swapped the driver boards and the problem followed the board. After swapping them back, I did the old, pointer-finger-temperature-probe on the transistors and all felt fine until I felt TR807. It was screaming hot, like burn the skin off your finger hot. I sprayed the part with canned air but the results were the same. I also inspected the back of the board and found two questionable areas that may have been flux corrosion that potentially caused a short. (I've since cleaned them up)
I know from reading on this forum that the "flying saucer" transistors are problematic and can be replaced with KSC2383 and KSA1013's. What I don't know is do they have to be gain matched? Also are there any other components I should replace while I'm rebuilding the board?
Any advice would be appreciated!

Left_F1282-1.jpg Left_F1282-2.jpg
 
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Thanks!
I was just rooting through my stock and I have a bunch of KSA992's with hfe~450 left over from my 555 project.
What would account for TR807 really cooking or is that normal?
 
It's possible that a faulty/leaky TR1 is causing the crackling noise issue AND the TR807 heat/bias issue. Replace the differential input pair and see if the problems are resolved.
 
I replace the 495's with 992's and it biased nicely, 30ma. (thermal 2sc984 OK?)
~0 dc offset (I would assume the main caps are ok)
TR807 is still cookin' and sound is slight and snap-crackle-pop.
 

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Seems that TR807 in the left channel is indeed faulty. Is the TR807 on the right channel running much cooler?
 
Oddly enough, TR807 on the right driver board was swapped out for a 2N3053.
And yes, it runs cool.
I'm hoping I don't have issues with the 2SC680 or 2SA566 (TR815,817)
 

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Vbe multiplier by-pass capacitor mod: Install a small film capacitor (e.g., 1uF) across the emitter and collector of TR809; capacitor can be install on component side of F-1282.
Baxandall Diode mod: Replace R847 with a 220ohm 0.5W resistor and install a diode (e.g., 1N4148) across the resistor; diode cathode is towards emitter of TR817 and it can be installed on solder side of F-1282.

F-1282 mods.jpg
 
Cool!
I have everything except the resistors in my "stockpile." Can I use the WIMA's @ 1uf 50V? The 450's are for my other 555a.
I think I'm going to implement this once I know the unit is stable. No sense adding variables.
I'll report back on my progress.
Thanks for your help!
 

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Yes, those Wima 1uF MKS capacitors are perfect for the Vbe multiplier by-pass capacitor mod; the Panasonic MKPs would be too large for the board installation.
 
I finished my second AU-888!
When I bought this amp it was in pretty rough shape. It had buckled and delaminating veneer, rusty hardware and bottom cover and corroded and fragile contacts. Apparently it was in a damp storage unit for over 20 years!
With help from Leestereo, I was able to sort out the left channel issues. After recapping, I rebuilt the driver boards and applied the Vbe multiplier caps as well as Baxandall Diode modification.
She sounds so sweet! It also feels so good to have saved this unit from an uncertain demise. I'm sure she will outlast me and hopefully continue to perform and be treasured.

Original:
1_LR.jpg

Aux connection prior to repair:
AuxL Signal Connection.jpg

Recap and rebuild:
IMG_4276Lowres.jpg

Finished unit:
AU888_LR.jpg
 
Such a nice save - inside out !

Any tricks to share on the cabinet?. Gorgeous looking.
.
Does the baxandall and vbe mod change sound (compared to your first 888) ?
 
stereofun said:
Such a nice save - inside out !

Any tricks to share on the cabinet?. Gorgeous looking.
.
Does the baxandall and vbe mod change sound (compared to your first 888) ?
Click to expand...


Thanks! I'm pretty proud of how she turned out!
The cabinet was in pretty bad shape; only a few small chips but the exposure to moisture made the veneer warp almost to the point of cracking. It also had pretty bad water stains.
I used a syringe to inject some watered down wood glue under a few areas at the edges of the case, covered them with wax paper and clamped till dry.
Then I used Howard Walnut Restore-a-Finish liberally with 320 wet dry sandpaper. The wood was so dry and brittle it needed some moisture to relax. Howards actually "melts" the finish and then redeposits it.
My intention was to saturate the veneer to the point the raised sections would flatten out and also re-adhere. It seemed to work pretty well after performing the aforementioned about 3 times.
Here is a pic after the Howards treatment:

888case1.jpg

However, the finish still looked a little blotchy and there were some dents that I could not steam out, so I decided to hand sand the entire case. (I had to heat the stickers and pull them off)
I dry sanded with 220 to get through the remaining finish and wiped the case with mineral spirits. Then I used thinned down Minwax Antique Oil Finish and 320 Wet dry sandpaper.
This creates a slurry that fills the cracks and also that slurry was used to fill the missing chips as well. Then I let it dry for a day then repeated the process two more times.
Once I was happy with the surface, I lightly sanded with 400 wet dry, wiped with mineral spirits and then applied 4 coats of Minwax Antique Oil Finish.
(Oil finish, IMO is fool proof and provides the best grain "pop". You literally cannot screw up an oil finish on non-stained wood!)

Here is what I used:

shmoots.JPG

Hope that helps!
 
Almost forgot!!
I think, in comparison to my first 888, the Vbe multiplier and Baxandall Diode modifications (and the rebuild of the drivers) provided quite a bit more clarity to the sound reproduction.
The bass seems to be slightly punchier and that may be a product of the 2.2 WIMA's on the drivers, however it is not offensive.
I still need to give her some more burn-in time, but I think all the modifications has made her sing beautifully!
I intend to swap out my first 888 in my system with this unit and then tune up the first by cleaning and checking bias.
Then place both in my rack with my QS-1 in between.
I'll update with a pic or two!
 
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