Sony Rcvr STR-DE675 random loud pops

Caps can (alas) go quite bad even if they don't swell. 10 years of steady use could be enough to cook the electrolyte, especially if they're located in a hot section of the chassis and weren't of terribly good quality in the first place.

This could indicate the presence of "leaky" electrolytic capacitors, possibly ones in a DC-blocking role in the signal path. They could occasionally "flash over" and conduct when you don't want them to. 'Lytics can become leaky with age, especially if they're being operated with a low DC bias across them (i.e. far below their WVDC rating).

Cracked or "cold" solder joints are (as you obviously know) another prime suspect. Your receiver is new enough that it may use lead-free solder, which is less flexible than leaded and more prone to cracking under stress. "Ring fractures" around component leads would be one thing to look for.

It would help you a great deal if you could determine whether the pops are specific to one channel or the other. If so, you know where to start looking. If not, that suggests common circuitry (e.g. the power supply, bias circuitry, etc.).

Since they're independent of the volume control setting, odds are that they're in the later portion of the preamp circuitry or in the amp, not in the input stages.

One of my favorite debugging tools, for faults like this, is a wooden chopstick. Hook the receiver up to test speakers, power it up, and then start tapping around the PC board with the chopstick. If tapping in a particular area of the board elicits a POP, or otherwise changes the behavior, there's a good chance that there's a bad solder junction somewhere within mechanical-shock distance of the point of tapping.

I had an audio amp which was prone to such popping, some years ago. The fault turned out to be at the driver transistors... they were metal-can types, which ran hot, and they'd been snugged down right to the PC board before being soldered... no "give" in the wire leads. Differential heating/cooling cycles eventually cracked the solder joints on the board.

Physically check the connecions to the main filter caps. The big ones.

** Try with only one speaker connected - then you will see if the pops are coming from just one or both channels. ** This usually means there is DC voltage on one of the speaker outputs causing the relays to open and bringing on the "protect light".

Finding the exact cause is a process of elimination and one that makes repair techs really earn their money. Typically it comes down to one of the small transistors in the power amp or a zener diode, bad solder joint or almost anything.

.... Phil

time for a recapping.... You have one that is shorting, it does not need to be swollen as you say, most likely a small package cap with a lot of F for its size.

Jamie

Wow - I was typing a big long reply and my mail program crashed, lost it all.

OK.

1. It would be helpful to know if the pop is on one channel or all. All channels would point to a power supply problem, such as perhaps bad solder at a fixed voltage regulator. Probably a good idea to resolder all your TO-220 style voltage regulators for good measure.

1. The base voltages should be around 1.1 volts or so. Although the service manual incorrectly drew the ouputs as bipolar, they are Darlingtons, the voltages shown in the schematic are correct. I have serviced Sony receivers of this general type for over 20 years, and this has virtually always been the case. If you really read 0.6 volts on the bases, you may need a new multimeter. The MUTE function of the driver IC's would have resulted in 0 volts at the base, and it's not a bias problem, so there you go.

2. The uPC 2581 driver ic's do run quite hot and dry out nearby caps. The IC's should be resoldered and any suspect caps replaced. The STK350-230 (IC501) is a VERY common troublemaker - although not often intermittent, usually just bad (at least by the time I see them).

Mark Z.

I will second Mark's comment here. Many a Sony has been "fixed" by resoldering the uPC 2581. I usually remove the solder and resolder. Very common problem.

Dan

Oh, very good. I wasn't clever enough to think of that! So far I have determined the pops are in the left main channel but not the right. Still trying to localize the area of greatest mechanical sensitivity - almost anywhere on the board generates a pop, and then it's 30-60 seconds or so until I can reset the "protect" mode by power-cycling. Working on lighter taps!

Thanks to everyone else for excellent ideas as well. I'll report back once I get a better idea of the location.

--
G. Paul Ziemba 
FreeBSD unix: 
 5:36PM  up 2 days,  5:06, 7 users, load averages: 0.73, 0.88, 0.85

I used a plastic rod to tap around the board to localize the problem. The two big capacitors were fine, but almost everywhere else wasn't. Connecting only one speaker at a time was a good idea - left main channel only.

There were multiple solder joint faults. The big cap in the 30v circuit, c819 (on the PCB layout but not on the schematic), the coil of the headphone relay near it, and the driver ICs (uPC 2581) all had unstable solder (not visible to the naked eye). Working through all of these required multiple disassembly and reassembly iterations (no easy-access panel from below, grr) but now the amp has been running for an hour without a pop. Thanks to all for the various suggestions - all were helpful in reaching a satisfactory conclusion.

--
G. Paul Ziemba 
FreeBSD unix: 
10:11PM  up 2 days,  9:41, 9 users, load averages: 0.74, 0.71, 0.74