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Muddler 2nd January 2012 02:04 AM

Parasound HCA-2200 problem - need some help
 
3 Attachment(s)
I have a Parasound HCA-2200 (1st version) that has one channel (right) that makes a very LOUD sound on power up. Apologies up front for my lack of knowledge; I am a novice with electronics. The following info may give a clue to someone who knows what there doing. Some of this info maybe be relevant and some extraneous.

Amp comes out of standby fine, seems to play fine after its initial noise (low volume/test speakers).

On power up I connected a MM to each channel and set it to record max voltage:
L: 7V DC and 2V AC (quiet channel)
R: 49V DC and 29V AC (noise maker)

Amp has two stacked toroidal transformers. Behind them are four big capacitors. Following measurements taken at top of cap screw posts.

LF: 74V (left front)
LB: -74V
RF: 60V (obviously not what it should be)
RB: -74V

DC offset (no input or load connected)
L: 5mV
R: 6mV

Bias current across the emitter resistors (I think thatís what there called) seems about equal for both channels ranging from 18mV to 22mV (I checked about 12 so that were accessible).

Any insights at all to get me pointed in the right direction would be greatly appreciated.

I have included a couple of pictures. The close-ups are of the two small boards mounted on each side. They seem be browned (baked?) a bit in certain areas or maybe cap leaks? I am guessing these are part of the power supply, but again remember I am new to this so take my info with a grain of salt.

Adv<thanks>ance

john curl 2nd January 2012 02:51 AM

It would appear that the protection circuit is not working properly, for one thing.

diy didi 2nd January 2012 04:43 AM

Check for leaky caps, incl the filter caps.

Muddler 2nd January 2012 07:59 PM

1 Attachment(s)
Not sure where the protection circuit is, I am assuming somewhere close to the outputs and relays?

The big caps don't seem to be leaking or bulging. The smaller caps on the side boards aren't bulging but maybe the brown burnt look to the side boards is from leaking? Just don't know. I am not sure which are the filter caps. Would this cause the B+ rail voltage issue?

Again, I know it's painful trying to help the electronically challenged (caps are lil soda cans, relays are lil black boxes, etc..). I have attached a schematic, is it possible ID the parts I should check from that? (some nostalgia for John)

diy didi 3rd January 2012 01:57 PM

Looking at the schematic, try adjusting tvr101/102 for zero offset voltage. Mark the position with a sharpie, use some contact cleaner, return to original position, turn on. let it sit for 30min and with no load, connect DMM and adjust for 0V.

Muddler 2nd March 2012 07:43 PM

Parasound HCA-2200
 
Update - One of the big filter caps in the power supply had given up the ghost. Not bursting or leaking or physically damaged; I guess just dried up. (about 20 years old I imagine). I replaced those big four along with the other caps on the side boards (see pic). Six on each side, I think these are part of the power supply also. These may have leaked but I can't be sure, maybe just crusty old glue on the board. Bottom line - I now have 75V +,- on each side. No more noise or high voltage on power up, no more hum or buzz - very quiet.

diy didi you are correct pots tvr101, tvr102 are for setting DC offset, as are TVR201 and TVR202 (per Tony @ Parasound). Why there are two for each channel is beyond my understanding. I tried tweaking these to zero out the DC offset, which I assume at 6mV & 7mV per channel isn't too bad to start with, but didn't have any luck. DC offset did seem to bounce around a bit as I trimmed them but it always returned to its original value. However, each of these pots seems to alter the bias with about the same precision as turning the bias pots TVR301 and TVR401 (I was using 2 MM to monitor both bias and offset). Not sure if this indicative of another issue.

As John C. noted the protection circuitry is probably not working either. The relays both seem to click on right away on power-up. If I recall correctly, my other Parasound amps would enter standby (red light) for a second or two before the relays opened and went green. I do remember, before I replaced the bad caps in this amp, that the relays on occasion would "chatter" on power-up / multiple on/off clicks? I really wish I could find a service manual; I tinker around for fun but don't really have enough understanding/experience to allow me to test / diagnose these issues with just a schematic.

elektroj 3rd March 2012 01:02 AM

OK, I'll bite..
Quote:

Originally Posted by Muddler (Post 2930720)
diy didi you are correct pots tvr101, tvr102 are for setting DC offset, as are TVR201 and TVR202 (per Tony @ Parasound). Why there are two for each channel is beyond my understanding.

Actually, they're there to balance both differential amp stages. Adjust TVR101 for 0V between TP1 and TP2 and TVR 102 for 0V between TP3 and TP4.
Quote:

I tried tweaking these to zero out the DC offset, which I assume at 6mV & 7mV per channel isn't too bad to start with, but didn't have any luck. DC offset did seem to bounce around a bit as I trimmed them but it always returned to its original value.
:D
You won't be able to adjust DC offset and you don't have to! Your amp has DC servo (see IC102; NJM353)

Muddler 5th March 2012 08:02 PM

elektroj - Thanks for the info about the DC servo and the differential balancing. Did as you suggested and adjusted them as close to 0mV as i could get (within 4mV +,-). Adjusting TVR101 at TP1 and TP2 would alter the TVR102 reading at TP3 and TP4. Seems to be a constant delta between them (ie. adding 1 mV to one would change the other by 1 mV). I did the same for the other channel TVR201, TVR202 with TP5-8.

Does anyone have any thoughts about the relays opening immediately on power-up. Should there not be a delay. Is this indicative of the relay being bad or something signaling the relay to open? Any and all thoughts appreciated.

bigskyaudio 5th March 2012 09:16 PM

When first powered up, the red standby light should be on for about 3-5 seconds. Then the speaker protection relays will click on as the green normal light comes on. If this doesn't happen, you most likely have a fault in the protection circuit.

I can see heat damage to the driver board in your picture right by the XLR jacks resulting from heat dissipation from the 2W dropping resistors. Examine the pc board underneath for any cold or cracked solder joints in the area and make sure that none of the pc board traces have burned or cracked by the eyelets from the heat. The resistors are used in the protection circuit and for driving the LED lamps. I see heat damage in virtually every HCA-2200 including the series II. If you are replacing capacitors, change the electrolytics (NP and standard) in the protection circuit with new.

Muddler 7th March 2012 09:50 PM

Thanks bigskyaudio !!! Yes, the 2w resistors next to the XLRs have baked the board. I will do as you suggested and also replace the caps in the protection circuit. I'll swap out those resistors too, they measure fine but look "chalky"? from being cooked I guess.


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