QRX7500A Big Job, some faults.....

It's a good thing you live where you do, or you'd be getting all my stuff to restore.. as always, an amazing job! Do these smaller units suffer from the same broken solder joint issues as the rest of them?

I'd be happy to get all your stuff to restore, I quite like sitting in my lab working on this stuff. Years of rock and roll, touring, big PA systems and such has taken its toll, and I like the quieter side of life these days...

No the 7500A doesn't have any of those pesky double sided boards with pass-throughs, there's a lot of discrete wiring on rotary switches and such...
Very well built unit. As I mentioned somewhere else, I might just buy one of these of I ever see one for sale..
 
And while I am waiting for the main caps to turn up, I decided to finish up as much as possible on the QRX3000.
The 3000 is also a nicely put together unit too.
Whopping great 15 watts per channel from a quasi cap coupled design.
There were issues with one of the amp channels, when I put it all back together I began to go through and set the bias.
This all went very well until I got to the Left front channel. NO bias. So I chased my tail around for a couple of hours head scratching, it was definitely on the driver/amp board, but i just couldn't find it.
ALL transistors measured good. ALL resistors were good, caps all in the right way, I thought I was going mad...
THEN, I measured point to point through the output transistor and this was telling me the transistor was OPEN.
So I immediately thought broken track, I could not find a broken track!!
I had the amplifier idling with the meter on the emitter resistor looking at the NO bias current, and I just happened to press one of output transistors and it all burst into life, so to cut a long story short, one of the legs of the transistor had become detached from the internals of the transistor, the leg was loose.
Of course these TO218 transistors are not something I have lying around, everything I had in TO220 (essentially the same) was too fast, these little things are 4mhz, everything I have lying around is 30-100mhz...
BUT I had some Sony 2SD313 lying around which were taken out of a working regulated power supply, they actually fit the bill perfectly so I used them.
Then I aligned the tuner, which obviously had never ever been touched, so not a lot was needed, just a little dial calibration and RF tuning calibration.
Interesting that this particular tuner has a 67khz SCA sub carrier decoder!! Something which was used in the 60's and early 70's....pretty cool to see that.
So this one is all ready for its new Main Cap now.

My policy has changed regarding main caps, I have seen two leaking main caps in the last few units i have done, time is marching on and I think every living electro cap needs replacing now, they're all time bombs in my opinion.

Some pictures...

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Thanks Casey,
Well, I moved back onto the 7500A yesterday to finish that up, I did mange to get these Nichicon Gold Tunes in there. I thought it wasn't going to happen at first, but engineered a solution by discretely elongating the holes in the chassis, and drilled some new holes for the mounts. Might not be the way some of the purists would do it, but these caps are really very good and worth installing if you can 10,000µf 80V is what I used..
This amp puts out 50watts per channel now. Those supplies are not sagging at all, quite impressive really...
Now I just have to finish up a niggly little fault with the rear/left pre-amp and its out the door back to the owner...
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Thanks guys appreciate the responses , just chasing down one last fault, something is breaking down in the Left Rear preamp somewhere, I think it might be in the EQ circuit, seems to be the only thing ALL inputs run through...so hopefully that's where it is, I am hopeful.
Man I can't believe what a difference those caps made to the power output, quite astonishing actually, I checked the frequency response of the amplifier, and I also think there might be some sort of filtering going on somewhere which is choking the low end, I'll see if I can find what might be causing that if its not too time consuming, I have already spent too much time on this one...
 
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