Parasound HCA-3500 Modification Thread

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#41
Thanks for the update on your restore! I bought this amp for parts or repair locally and found the R channel had a few blown outputs and an emitter and base resistor damaged, and the hFE was all over the place on both the R and L channel 32 outputs. I have found and purchased new old stock matched outputs for both channels and now need to start disassembling and testing components to look for a reason the R channel blew the outputs. I figured I can clear out all of the redundant bypass caps and simplify the design in accordance with the Big Sky mods while I am checking the amp out for faulty components up stream of the outputs. Luckily my Mosfet drivers checked out fine, so at least they survived. I have another post on DIYAudio of this saga I plan to update in a month or so when I get around to starting this restore.
 

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#42
Yea. I similarly got mine from a friend where one channel was bad.

Traded my friend a dac for it and was going to part it out. But this website convinced me to keep it and try to fix it.

My problem was this resistor. Cheap fix.
 

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#43
I have a follow up question for Big Sky Audio and anyone else with advice on eliminating the dynamic bias, so thought it would be good to ask it in this thread. On Parasound’s website they have a service doc in the “Education Pack” to improve reliability that says to remove dynamic bias by replacing C83 and TVR2 with jumpers to remove them from the circuit. The Big Sky Mod says to remove R94 and R140 to eliminate dynamic bias.

Is there one or the other method that is better? Also are any jumpers needed in place of the removed R94 and R140 resistors for the Big Sky mod? Seems R94 must have a jumper, but maybe not R140. I have included the schematic with Big Sky mod in pink and the edu. packet mod in orange.
 

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#44
bigskyaudio said:
Removing R94 (100k) will cut audio signal to dynamic bias input. Removing R140 (10k) will isolate TVR3 and Q35 optoisolator out of circuit.

Slightly adjust TVR2 if TVR1 will not give sufficient range to reach ~20mV (+/-2mV).

After you complete your mods, be sure to double check all parts and connection points, then power up the upgraded channel slowly with a variac and monitor current draw. Watch bias with a dvm and make sure to not exceed +50% of high bias (20mV). At continuous high bias, the amp will run quite hot so do not place it in an enclosed rack without plenty of forced-air cooling.
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This answers my question on whether any jumpers are needed for removing the dynamic bias. If anyone has any info on that service mod from the Parasound website, let me know. I plan to just follow the directions from the Big Sky Mod to remove the dynamic bias for my restore of the HCA-3500.
 
#46
Dneu2011 said:
Of course right after I ordered the IXYS diode bridges, I get an email from Diotec with details on purchasing their FDB3506P/T that fits the PSU board.

They had a deal in place for purchasing more, so I went ahead and purchased 12 pieces from them. I will post some kits for anyone interested in modifying their HCA-3500.

I will see if Mouser will take back the diodes I purchased from them.

Any help on the above questions I asked about the AC Polarity and the transformer wires being shortened?
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Do u still have these?
 
#47
I have two pairs, but they are in my F4’s. I’m open to selling them but they are installed with some of @rhthatcher Quasimodo boards.

I would be open to selling them but you would need to either remove the Quasimodo boards or just remove the resistors and caps and you probably could install them with the boards still there. Just adjust your standoffs a bit.
 
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