SA-7500 differential pair issues.

Eastham

More Class-A than ever!
Hi guy's. So I recently ran into a problem, my newly accuired SA-7500's input differentail pair decided to die the other day with a loud bzzzz in the left channel, the differential pair tested fine on my multimeter but seemed to be breaking down in circuit causing DC on the left channel. No problem I thought, I put in a pair of KSA992FBU's, all voltages are good and I was back to the races. Or so I thought...

About two days later the same thing happens, BZZZZZ *click*. Okay. Maybe one of the 992's was a dud, so I replaced them both again and again after about two days. BZZZZZZ click.

Now today I decided to rob a pair of 2SA970's out of a spare amp I have, thinking maybe the lower collector current rating of the 992's was my downfall. So I power it up with the 970's in it and it worked on my DBT, powered it directly off the mains all was good. Hooked up my 8ohm test load and it ran for a few minutes idle before dying yet again...

When it's working all my voltages on the transistors are fine and pretty much match the right channel and the schematic.

I've got some KSA1015's coming as I was told these would be a better sub for the 726's but I can't help but feel that these are going to pop too. Nothings oscillating and it's nothing in my system that's causing it.

I'm currently considering stealing a pair of 726's out of my SX-737 but if those blow I won't be able to use it.

Any insight and suggestions are more than welcome. Thanks.
 
I would suggest NOT stealing the packages from your 737, but find the real problem.

Incidentally, is the a 7500, or a 7500-II?
 
FWIW, a KSA1220AY may be a better choice for those pairs, as well as the KSA1013 (or even the 1015). Given the current carry capacity of the originals, and the fact that the circuit may be taxing the driver (2SC1451), it might be pushing the limits of the 992s. The same applies to the 1451. It may be better off as a KSC3503.
 
FWIW, a KSA1220AY may be a better choice for those pairs, as well as the KSA1013 (or even the 1015). Given the current carry capacity of the originals, and the fact that the circuit may be taxing the driver (2SC1451), it might be pushing the limits of the 992s. The same applies to the 1451. It may be better off as a KSC3503.

Darn, I used to have a bag full of 1220's. I lost it moving. I'll wait for the 1015's to come in and if they fail, I'll buy another bag of 1220's. I'll also grab some 3503's. Thanks.
 
Going by the values on the schematic, the collector current for the differential pair (Q1 & Q3) should be less than 0.5 mA each resulting in a power dissipation of roughly 15 mW. Neither of these results would be a problem for the KSA992 IMO.
 
have any resistors been changed ? or anything else for that matter ?
or a better question is what has been changed with what ?
 
have any resistors been changed ? or anything else for that matter ?
or a better question is what has been changed with what ?

Nope, everything looks to be original aside from the caps. The previous owner used Elna Slimic II's and Elna Cerafine's for the re-cap.
 
Going by the values on the schematic, the collector current for the differential pair (Q1 & Q3) should be less than 0.5 mA each resulting in a power dissipation of roughly 15 mW. Neither of these results would be a problem for the KSA992 IMO.

Agreed. It SHOULDN'T be a problem at all, thus Post # 2.
 
big question is did it work ok before it was touched ?

Yeah, I didn't even know the previous owner had re-capped it. No doubt he want to hide his... Handy work...

Was Q5 & Q6 (2sc1451) replaced?

They look original but someone seems to have filled the top of their heatsinks with thermal paste for some reason.

Agreed. It SHOULDN'T be a problem at all, thus Post # 2.

I'm going to start measuring components then, what's weird is that all the voltages seem fine.
 
Back
Top Bottom