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BSS DPR-402 repair
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Old 1st May 2012   #1
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BSS DPR-402 repair
hello all, after reading these forums for some time I decided to register to ask for help on this specific matter, I have this BSS DPR-402 which has already been serviced three times and after little use it starts showing the same problem.

I have the schematics and I wonder if anyone can give me a clue to find and solve the issue. the unit has noisy ("scratchy") outputs and it shows signal on both the output meters.

I have been told something about the output being shorted when using the unit with unbalanced cables, so the output op-amp gets fried and the output is noisy.

I understand how the output can be shorted: since the BSS is pin3 hot, when using with single TRS insert console, the + gets shorted to ground.

BUT my unit has had the XLR wires swapped for "normal" polarity, PLUS my console has separate send/return jacks PLUS I always repatch the unit with power button switched off.

so where to look? my best guess is that some other component around the output op-amp is failing too and this leads to the chip that fries.

any help is appreciated, I have full schematics and a multimeter. I maybe could grab an oscilloscope if needed.

PS: I have another identical DPR-402 which is working fine but has a broken power switch (it does not latch anymore)... any source for the part? it is extremely difficult to get BSS parts here in Italy even through the distributor.
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Old 1st May 2012   #2
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The signal on the o/p meters may be unrelated. Have a look at the 14-pin DIP ribbon connector that goes from the meter board to the main board...re-seat it, clean it if necessary and see if the "signal" goes away.
I will look at the docs for the o/p issue when I get a minute.
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Old 1st May 2012   #3
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will certainly "do" the ribbon and report back, thanks for now.
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Old 1st May 2012   #4
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ok, the meters are now fine. as you said, it was ribbon's fault. I also found something for the o/p, will report back in a short time.
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Old 1st May 2012   #5
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I guess I found the problem, please see attached picture. R103 and R104 are 22 ohms while they should be 22 KILOohms. I am no guru but I guess that TR6 and TR7 don't like this low resistance and they fry.

I am replacing with correct resistors, replacing TR6/TR7 and IC10 too.
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Old 5th May 2012   #6
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the 402 seems to work fine now, BUT there is a small "hum" (seems power noise) in channel 1, I can hear it clearly in the headphones with the unit connected but with no audio passing through it. note that if I press the bypass switch, the hum goes away.

I tried swapping the small VCA option board between the two channels AND THE HUM SWAPS TOO, so I thought it may be a fault in the small board.

this has only two THAT 2155 VCAs, a BC546 transistor and a TL072 op-amp, plus some other passive components. I do not stock the THAT VCAs so I could only replace the transistor and the op-amp, and the hum IS STILL THERE.

I also tried to turn the trimmers on the board (marked them before) with no luck.

as soon as I can I will replace all the caps on the small board but then I'll have to stop.

any idea?
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Old 7th May 2012   #7
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the caps should arrive by tomorrow so I'll try and see if it fixes...

any other guess?
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