QX-949 Adventures

[QSilver]

Some time ago now, I picked up a saved QX-949 from a fellow member on audiokarma. When I got it, it seemed to be working pretty well except channel 4 (rear right) was dead. I figured it was likely a bad transistor.

Anyway, I got to looking at today. After a lot of trouble shooting and reworking the control amp, I found that it wasn't a bad transistor in the power amp, or the control amp.... it was a crushed signal cable between them! Cut out the bad bit and now we've got 4 channel audio!

I've started working on the power amps. Something that everyone has said that you need to pull off the wire wraps to get the board out. So far, I've managed not to by unplugging the output transistors from their sockets and then removing the heatsink this way to give me room.

A couple things struck me as odd though when I started getting in....

Both power amp boards look to be in original condition and no signs of resoldering etc. But.... the output transistors.... don't quite match...

The power amp nearest the front is out and it's transistors are all "Sanken" and 2SA745 and 2SC1403.

Then, the power amp to the rear has one pair of these Sanken devices, then it has a completly different pair of "Toshiba" 2SA679 and 2SC1079.

I know one set of amps is run harder in 2CH mode which IIRC requires some stronger output transistors? But still, which amp board and which transistors are original? I'm guessing its supposed to be at least one amp of the Toshiba devices?

 

[markthefixer]

The scats and parts lists of the QX-949
show 2SD370 (2SC1403) / 2SB530 (2SA745) on one half of the awh-027 board
and 2SC1079 (2SC1116) / 2SA679 (2SA747) on the other half.

Thus it is original in it's found state.

IIAC they also run on different raw DC supplies, B1 and B2 off of different transformer secondaries, rectifiers and caps.

Instead of having separate front amp board (L.R) and a rear amp board (L,R) schematics and parts list
they had ONE amp board LEFT (F,R) and one amp board RIGHT (F,R) with the SAME schematics and parts list.

 

[QSilver]

Thanks Mark, that makes sense. But, the board nearer to the front of the chassis doesn't have any Toshiba transistors. so I have 6 Sankens and 2 Toshibas. I would have thought that the two boosted channels would have the toshiba outputs so one on the front board and one on the back?

Just want to make doubly sure that its all ok.

I've photographed them lined up as they are in the amp.

 

[larryderouin]

Functionally the Toshiba 2sc1079 and 2sa679 are the same as the Sanken 2sc1116 and 2sa747. They may have run out of Toshiba's and had the Sanken's on hand. I've seen this in a QX-747 before.

 

[QSilver]

Ok, good to know. I'll put them back in now, finished re-capping and changing transistors on the amp boards. They didn't have any thermal compound on the outputs... should I put some on or is it safe to leave it. My gut tells me to put some on but wanted an opinion...

 

[blhagstrom]

I was wondering how that turned out for you.

I wonder if the cable got pinched in the crash.

That unit was rescued from a recycling center and they mistreated it.

The quad reel-to-reel that must have been it's partner was totally destroyed by the rough handling.
They literally throw the units into piles.

 

[markthefixer]

the manufacturer substituted clear glue to hold the mica insulator to the heat sink, and the transistor to the mica.

clean them (heatsink, mica, transistor) with acetone and use thermal compound.

 

[QSilver]

I was wondering how that turned out for you.

I wonder if the cable got pinched in the crash.

That unit was rescued from a recycling center and they mistreated it.

The quad reel-to-reel that must have been it's partner was totally destroyed by the rough handling.
They literally throw the units into piles.

Its going ok, the tuner was a little out so I carried out the alignment and now the tuner is pretty sensitive. I'm working on a LED conversion for the little level meter - super bright LED's as the 120mA bulbs are cooking the board and aren't that bright...

There's a few little bends and squashed cables, nothing major though... its a very lucky unit!

Haven't re-made a new case yet.. but its in the works....

the manufacturer substituted clear glue to hold the mica insulator to the heat sink, and the transistor to the mica.

clean them (heatsink, mica, transistor) with acetone and use thermal compound.

Will do! I've removed the heatsinks and cleaned them off. Though, its surprising how clean it is inside - no yellowing of thermal shielding around the cables etc...

 

[markthefixer]

That was probably more of a trophy unit for the original owner.

And it wasn't bottled up in a heat bubble when it was run.