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8080db F-2624 repair

bolero

bolero

Super Member
I'm looking at an 8080db that has been through at least 3 other repair hands. The good news is most electrolytics have been replaced.

The last tech " got rid of the protection circuit " but thankfully the board is still in there :) I just need to figure out how he bypassed it.

I pulled F-2624 and sure enough, the fusible resistors are all in there. A couple look to have been replaced: R34 with a carbon comp, and a higher watt one at R39. I will replace them all with metal film rated to original specs.

So I suspect that's why the protection was bypassed. I'll enable it again & use that to troubleshoot the amp, once I shotgun 2624. I'll also replace TR1-4 with 1845's as per Trnsfmr's advice here: ( RIP )

https://audiokarma.org/forums/index.php?threads/restoring-the-f2624.709267/

My question: there is a transistor replacement I see at TR5, labelled

MJE15033G
AFC12

TR5,TR6 are listed as 2SA899 in the SM. TR6 is labelled

TIP32C
G40DE VW
CHN421

and looks like it might be original?

Does anyone know about these, are they ok in there?

Thanks!

Pics below
 
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I can't upload images for some reason. But I found some clues in Trnsfmr's thread linked above, which probably explains exactly what happened :

"TR 5,6 these get hot and blow when the 180 ohm resistor R33,34, drifts up as high as 400+ ohms. This transistor amplifies the signal from the diff pair and sets initial bias levels in the input end of the amp. You can replace these with a KSA1220 or MJE350. The KSA1220 is supposed to be quieter and we use it, but I can't tell any difference."
 
Not my choice in that location, both are pnp medium power transistors not matched of course, a better choice would be KSA1220AYS.
Bypass the protection circuit supposedly? I would remove the tuner tray and take a close look at the protection board F-2657 and ascertain if the relay has been bypassed. Also that board has fusible resistors that need replacing but do not trust the schematic, they are not identified properly and will need a visual inspection. Finally, if the receiver goes into protection with any application of volume the 4 1N60P germanium diodes D5,6,7,8 will cause this (I replace with BAT41's) and also replace the protection fault transistors TR1 and TR2 2SC945's with KSC1845's as long as you have it apart.
 
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Some good news: I accidentally ordered an extra 12,000 uF 100v filter cap for my 999 rebuild. Looks like I can use it in the 8080db. For whatever reason nobody replaced the 2 big filter caps.

I need another though :)
 
Overundr1:
"Finally, if the receiver goes into protection with any application of volume the 4 1N60P germanium diodes D5,6,7,8 will cause this (I replace with BAT41's) and also replace the protection fault transistors TR1 and TR2 2SC945's with KSA1845's as long as you have it apart." =>shouldn't that be KSC1845?
 
bolero said:
Got pics working, so here they are for posterity.

I'll order new TR5,6 based on Jim's advice.

View attachment 2782877 View attachment 2782884 View attachment 2782885
Click to expand...
I still see fusable resistors, epoxy diodes, corrosive glue, zeners, and the old trimmers.

You have a lot more to do.
I would replace every single transistor there.
Input KSC1845, TTA004B/TTC004B for predrive, MJE15032/3G for driver, and KSC1815/KSA1015 for current limiting.
Bourns 3386H for trimmers.
 
Thanks!

I had noted the fusables, but that's a good list!

What's the story on the epoxy diodes? Are you talking about D03, D04?
 
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