optonica 3636 noise problem

[sdoom]

Hi all,

I have a Sharp Optonica 3636 amp that has some big noise problems on both channels. As the amp allows to bypass (RCA jacks on the backside) the pre-amp / tone control (those work fine) I could determine both main amps have the noise problem. Occasionally the protect curcuit is activated, too. (The protect curcuit only turns on /off a relay and connects/disconnects the speaker terminals)

As the amp also uses two seperate power supplies for each amplifier section I thought it would be something both amps share and doublechecked all wire connections and ground connections.

Power supplies are rated at +/- 39V . The true-RMS voltage meter reads +/-38,5V . No spikes or noise in the power supplies.

Traced some voltage measurements in both channels ( have some reference values out off the schematics) and that resulted that the left channel output signal has a "jumping" -1 to -3 V DC offset, the right channel has a jumping +2 to +6V DC offset, if the DC offset is too big the protection curcuit is beeing activated. Disconnected the feed signal from the right channel to the protect curcuit and then the protection is not hit. So I assume the left channel DC offset is not big enough to activate the protection. Still -2V is -2V too much. Additionally to the DC offset there are spikes and noise on the output signal. (BTW music is still beeing amplified but the popping noises are very loud)

I then checked voltage readings inside the left channel and found R417 and R435 way out off tolerance (those are fusible resistors) and replaced those. Still only little change in popping sounds and DC offset , DC offset got a bit better but is still there. Also replaced all caps as some seemed to be dried out and turned out to be "just inside tolerance" values.

Checked the Zener diode voltage and that reads 14,7 volts , clear of spikes. What I now wonder is that the complete 14,7 volts can be measured at R411. Pin3 of Q401 has -0.2V potential (measured to GND).

All transistors (including the "IC 401" which is basically a complementary set of power transistors) are fine with the diode tester. But i know of some amps where the transistors still give fine readings with the diode tester but lack dc-current gain. Could that be the problem ? On pin1 of Q401 i already have a -0.4V to -0.5V offset, although K401 connected to GND and R401 also shows only GND potential.


I found another topic here on DIYAudio where those small 5 pin transistors caused popping noises. Or any idea what else to check to get that baby running again ?

[nigelwright7557]

I have come across the low hfe problem before after I blew an amp.

I took all the transistors out so it was easier to check all resistors.

I checked a MPSA42 on the multimeter and it had a hfe of 1 !
Replaced it and hey presto working fine again.

Noise on both channels could be an earthing problem.

[sdoom]

That was also my first thought. All connections inside the amp (even the 230V wires....crazy Japs ) are made with "wire-wrapping" .

I already resoldered both amp circuit boards and all wire connections to make sure that´s not the problem. As both resistors were burned I assume the amp has been fired a lot in the past and that more than only the two resistors have gone bad.


is it possible to replace the 2SA798 (at least for testing) with a pair of BC557 transistors ?

[sdoom]

is it possible for testing to replace the 2SA798 with a pair of BC557 transistors ?

[sdoom]

update : amp is up and running again on both channels distortion free Will make sure the amp can be powerd all the way up with a sine generator, too and hope nothing goes up in smoke.


It was both, the 2SA798 and the 2SC1628 transistors in both channels. The diode tester values seem to be fine but as I assumed earlier the current amplification is not ok anymore. So for anyone trying to repair a unit with these transistor types be sure to double check these little troublemakers . the 2SA798 was 2€ and the 2SC1628 was 50 cents each, so not really an expensive repair.

PS: The Optonica 3636 seems to be one of the better far east choices back in the 80´s. Little basic noise is audible but sound is great and not like out of a can

Thanks guys for advice.