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PostPosted: 13 Sep 2015, 00:11 
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Joined: 08 Aug 2009, 03:11
Posts: 2208
Location: Chilliwack, BC
Hi Folks,

I have a real stumper here. One that now with 35 years of repair experience has me scratching my head :confused:

The QR-6500 is a quad amp with four independent preamps and power amps, but a fully common supply. The stereo is made with two amp modules, front and rear (swappable).

Problem: Right rear channel distorted and unstable (noisy). Separating the PRE/MAIN made me conclude this was purely a power amplifier issue.

So I begin testing transistors, OK. Some swollen caps, so I recap all four channels. Problem persists.

Another interesting problem is the clipping symmetry control (VR801) does practically nothing on the bad channel, but the channel biases up perfect and holds 30mA bias rock-stable. The other channels have 10V on one leg and 35 or so on the other. The bad channel has 12 on one and 15 on the other. OK, transistor hfe is out of whack, but passes the diode tests.

I replace all the transistors.

Same thing. The Vcc on the bottom 2SC1111 (C793 in the schematic) isn't a nice steady 30V either, it fluctuates between 15 and 20V.

Hrmmmm, a resistor has become a varistor (seen this in Sansui and HH Scott). I replace all resistors and the two trimmer pots.

Problem persists. :censored:

Maybe the socket is bad? I swap madules with the front board and it works perfect.

So every part on the bard has been replaced. Solder joints taken care of in the process. Hairline cracks in the traces? I jumper all traces with wire.

Problem persists. :bawling:

I put the rear board in the front module slot... same problem, just on front now. So it is that half of the board! And it now has new everything :confused:

Just the voltages are off after the 1uF cap from the input stage (yep, thought I had a bum new Silmic-II, so I replaced the caps again, all of them, TWICE!). Thank heavans for a well stocked shop!

I'm at wits end here. Any advice?

Cheers!


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-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 28 Oct 2015, 23:28 
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Joined: 08 Aug 2009, 03:11
Posts: 2208
Location: Chilliwack, BC
Solved!

Several problems led up to one cause. The most unusual one was this circuit will not work with certain types of audio caps. Nichicon FW and FG, no! KW, yes! Elna Silmic, yes!

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-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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