Pioneer SX-750 Static Burst

I have a single voltage SX-750 which exhibits random static burst in the L channel.

The History - The unit was a DOA purchase, shipped in safely, and was in generally poor condition with lights out, one "dead" channel, hum/buzz in the other, and various other maladies. The power supply had AC leaking through C16/C17 and various hot transistors and suspect solder joints. The treble control shaft was frozen and snapped off. The unit appears to have been serviced at some past time as evidenced by the soldered wire-wrap connections in several locations.

The Remedies - The PS was mostly rebuilt and voltages are within 3-5% of specs. The AC component in the supplies is reduced to a minor ripple on the Tektronics and there is no hum present in the audio output. The treble control was replaced with an OEM take-out and all other controls were cleaned. The speaker relay contacts have been burnished, function and speaker switches cleaned. Once warmed, idle currents are set and stable at 29.5/29.8 mV L/R with no apparent excursions during the static episodes.

The Symptoms - The audible static burst phenomenon occurs in all modes, through both the "A" and "B" left speaker outputs, and seems to occur most frequently at lower volume levels including at zero volume. No static occurs in the R channel - Ever. Switching from STEREO to MONO mode does not change the occasional burst and does not translate it to the R channel, leading me to believe the problem is after that switch.

The Questions - I expect the cause is in the left final amp stage but there are none of the usual suspects installed in that circuit. There IS a 2SC1438 in the power amp pre-driver section, but that is not on my list of problem devices. Is that also a commonly suspected device? Is it more likely that a cap in that section is failing?

The few 725s in the unit are in the phono preamps and the lone 725/1344 in the MIC EQ amp, but those are easily switched out of circuit. The tone amp is after the MONO switch but uses SIP amps (TA7136P1) instead of discretes. Are those also suspect?

As always, any advice is appreciated.

Does it still do it with the tone control(S6) OFF. When off it bypasses the tone controls . This is after the Stereo/Mono Switch (S4).

I've never heard of any problems with the TA7136P1's.

This isn't the "Yellow Submarine" is it?

Larry

Still occurs in the L channel with the TONE off (flat). It does it with no source input whatsoever, MONO, TONE, TAPE MON in, and every other mode. The power amp does have the dual 2SA798s, but those are also not on my "suspect" list.

No, this is a different unit - SX-750 #3, or the one with the discrete PHONO amp boards. The "Yellow Submarine" is cruising along just great, although it isn't so yellow any more.

Tone controls are passive anyways....

I would just swap TA7136PL's and wait.

Next would be swap 2sc1438, then 2sa798

are these the original bias trimpots?

Sirs,

The linear ICs are on the tone amp and it is assembled back into the unit for now. I am tempted to try the other options before jumping back to the tone amp again.

The trimpots are original, but they were cleaned and exercised stop-to-stop (power off, of course) numerous times before adjusting the idle current, and reacted without a hiccup during the setting procedure. I understand the concern but feel that is a less likely possibility since, as mentioned, the measured idle current doesn't even twitch a µV when the sharp pops occur.

Since it is probably easiest of all the options, I'm thinking I'll swap out the 2SC1438s on the main amp board. While I have that out, I'll inspect for suspicious joints, and may replace the caps in the signal path and refresh the heat sinks on the board. The component count for caps is so low and I likely have most if not all the devices inventoried.

I'm tempted to wait on the 2SA798s since they seem to be more difficult to source or match up discrete replacements, but will change them out if the other work does not prove fruitful.

Thanks for the suggestions. Results to be reported soon.

A soft static, random, like a distant storm picked up on AM.

Any funny pops or clicks when you work the controls?

Those symptoms sound like a bad cap.
I had a pair of solid electrolytic in a Sansui run me in circles once.

The symptom is a very pronounced, "I'm gonna blow your voice coil and let out the magic smoke..." type of pop. The "distant lightening" type noise is likely a poorly connected or random noisy junction somewhere on a chunk of doped silicon, in my book. This sounds like throwing a few volts of DC at the speaker terminals, or a "burst" of current which is quickly ramped up then cut off - As in square wave rise and cutoff. I'd liken it to a cap charging then discharging, but the timing is not as regular as I'd expect in a capacitance circuit. Then again, my synopsis is based on how a WORKING circuit would operate, and about anything can happen when charge blows holes through paper or cracked junction gaps open then close randomly.

I cannot generate any noise by using any of the controls, but they have already all been treated to some oleic acid and its carrier/solvent additives, then lightly lubricated with PAG oil reduced to a low viscosity.

I had a Fisher with a bad power pack do that.
Loud nasty crackle. It would work if you pressed on it just right.

IIRC, the other times I've had that nasty cracking was a bad ground.

UPDATE - Replaced C1/2, C7/8,C9/10, C13/14, Q3/4, resoldered the emitter resistors, power pins, and a few other dull joints, greased all the heat sink mounted devices, more importantly DIDN'T break off any STV leads, and reassembled the unit.

After a bit over an hour, there are no pops evident in either channel. While the problem was only heard in the L channel all the mating parts in the R channel were replaced just because I was there and for some semblance of balance. The KSC1845s seem to be happy in place of the 2SC1438s. Cross your fingers - I may be down to simply the cosmetics at this point.