Hi Experts,
my Parasound HCA-1200mk2 (purchased 1994) shows the following behavior:
Turning on from cold state shows the normal behavior, first the red standby LED for a few seconds, then turning to green LED, relays click, then fine music.
After an hour or so, with music at low volume, the left channel disappears with a relay click, the power LED turns to red standby, the right channel still plays music for a few seconds, then after another click of the right channel's relay, also this channel is switched off. It seems to be a temperature issue. Both channels heat sinks have normal temperature, lukewarm, not more. In this state, with the standby LED red, here is no DC bias on the outputs (measured prior to the output relays, at the channels PCB where the 0.33 ohms are connected to the common output rails.) It also happens without input signal, hence no input bias. The bias voltage across the emitters 0.33 Ohms is 10mV +/- 2mV, regardless whether in active or standby. When switching off the HCA, after a pause of an hour or so, I can repeat the same thing.
The only modification/repair the HCA has seen:
Main power switch replaced 15 years ago.
New power caps 10 years ago. Replaced the 4 x 15,000uF 80V by 4 x 22,000uF 80V.
Overload LEDs never seen active.
Driving a pair of Infinity Kappa 8i with renewed crossover.
I have to mention that it is the European 230V version.
The power supply and transformer differs from the schematics.
I have two transformers instead of one.
There are fuses in the signal output path towards the relays, they are not shown in schematic.
I could not find the two fuses on the secondary side of the high power transformer outputs towards the rectifier D001, as shown in the schematic.
There is one fuse towards the low power rectifier D002.
My interpretation:
The main power supply is ok (power transistor collector rails have +/- 75V)
The channels PCBs are ok. No DC offset on input or output.
I think, the suspect is the lower power supply on the center PCB for the protection circuit, around C011, C012, C013, or the protection circuit itself.
Seems it enters standby without any need....?
Any advice or idea?
my Parasound HCA-1200mk2 (purchased 1994) shows the following behavior:
Turning on from cold state shows the normal behavior, first the red standby LED for a few seconds, then turning to green LED, relays click, then fine music.
After an hour or so, with music at low volume, the left channel disappears with a relay click, the power LED turns to red standby, the right channel still plays music for a few seconds, then after another click of the right channel's relay, also this channel is switched off. It seems to be a temperature issue. Both channels heat sinks have normal temperature, lukewarm, not more. In this state, with the standby LED red, here is no DC bias on the outputs (measured prior to the output relays, at the channels PCB where the 0.33 ohms are connected to the common output rails.) It also happens without input signal, hence no input bias. The bias voltage across the emitters 0.33 Ohms is 10mV +/- 2mV, regardless whether in active or standby. When switching off the HCA, after a pause of an hour or so, I can repeat the same thing.
The only modification/repair the HCA has seen:
Main power switch replaced 15 years ago.
New power caps 10 years ago. Replaced the 4 x 15,000uF 80V by 4 x 22,000uF 80V.
Overload LEDs never seen active.
Driving a pair of Infinity Kappa 8i with renewed crossover.
I have to mention that it is the European 230V version.
The power supply and transformer differs from the schematics.
I have two transformers instead of one.
There are fuses in the signal output path towards the relays, they are not shown in schematic.
I could not find the two fuses on the secondary side of the high power transformer outputs towards the rectifier D001, as shown in the schematic.
There is one fuse towards the low power rectifier D002.
My interpretation:
The main power supply is ok (power transistor collector rails have +/- 75V)
The channels PCBs are ok. No DC offset on input or output.
I think, the suspect is the lower power supply on the center PCB for the protection circuit, around C011, C012, C013, or the protection circuit itself.
Seems it enters standby without any need....?
Any advice or idea?
Last edited:
Usual standby problems on >20 year old amps is the timing capacitor in the cold start circuit. Usually electrolytic, usually leaks like a sieve over 15 years. Even Allen organs that "never need electrolytic caps" according to the servicemen, have this problem.
Don't have a diagram, not going look for one. Once an amp >15 years old has a problem, I change electrolytic caps 2 at a time until done. Sound check after every 2 caps, if the sound or performance gets worse, you made a mistake. Back up one step. Your most recent work is the new problem.
If you don't enjoy doing this job and don't want to repeat in 3 years, don't buy used, store shelf, aftermarket, or surplus caps. Buy caps from a distributor that will provide you with a service life rating. >8000 hours for small ones, >3000 hours for big ones can usually be found. Big distributors in Europe include RS, Mouser, Reichelt.
The timing circuit can be found by looking at the speaker disconnect relay, then tracing the circuit back to the transistor that drives it. The timing circuit will go to the base of that transistor, or rarely, op amp.
Don't have a diagram, not going look for one. Once an amp >15 years old has a problem, I change electrolytic caps 2 at a time until done. Sound check after every 2 caps, if the sound or performance gets worse, you made a mistake. Back up one step. Your most recent work is the new problem.
If you don't enjoy doing this job and don't want to repeat in 3 years, don't buy used, store shelf, aftermarket, or surplus caps. Buy caps from a distributor that will provide you with a service life rating. >8000 hours for small ones, >3000 hours for big ones can usually be found. Big distributors in Europe include RS, Mouser, Reichelt.
The timing circuit can be found by looking at the speaker disconnect relay, then tracing the circuit back to the transistor that drives it. The timing circuit will go to the base of that transistor, or rarely, op amp.
