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Sony STR AV-370 receiver: weird problem...

Hi all,

First of all my sincere admiration for this site and all its members!

Many many years ago I bought a Sony STR AV-370 receiver (new), which I eventually replaced. However, I want to re-use this receiver for my hobby room (model trains and electronics), but the receiver has a weird problem.

When powered up, all works fine. But when I put it in standby-mode, there is a relay which starts oscillating with approx. 1 Hz frequency. I need to take all power off and put the mains plug back in, and it works ok again. Long time ago, the receiver has been twice at a repair shop, worked fine for a couple of months afterwards but the problem re-appeared. The repair shop was pretty vague at that time and said something about a typical problem with an internal ground connection (???) Most funny thing is that when I give the unit a kick, the problem sometimes goes away for a couple of hours/days but always comes back.

Does anybody here ever came across a similar problem and was able to solve it? Any help/suggestions are welcome!!!

Greetings from Belgium, Paul



Re: Sony STR AV-370 receiver: weird problem...

Hello Paul:
I worked for an authorized Sony service agent in the early 1990s and saw many Sony receivers with similar problems. Some would simply go into protect mode for no apparent reason. Factory suggested repair was to add star washer beneath PCB where main ground trace connected to chassis. Additionally, it was instructed to secure all external bonding (ground) screws --- those connecting RCA sockets to chassis, etc. For it's age, there is a chance smaller electrolytic capacitors in standby circuits may need replacing. Leaving receiver disconnected from mains supply allows these 'ltyics to dry out faster. It is suggested to keep the unit connected to mains AC, unless unit will be unattended for a month or more. So far as excessive heating of transistor, I suggest cutting a small piece of aluminum 10 mm x 25 mm from a can, then wrapping it around body of device. Silicone heat-sink grease is not needed, but would be beneficial. Assure metal does not contact any surrounding components or wiring. Metal will conduct heat into the air, reducing temperature at the PCB surface.

Re: Sony STR AV-370 receiver: weird problem...

Hi Paul,

I do recall ground problems with Sony receivers of that vintage. The result was usually intermittent faults or actual humming in the audio.

The cure was to tighten the philips head screws that secure the rca connectors to the rear panel. Some of these were used as a ground. Also tighten all of the screws that secure pc boards in place inside the unit. If you can obtain some, put some star washers under the screw heads. For screw heads that only connect to the fiberglass pc board surface, you can put a star washer between the pc board and the chassis. The star washers provide a better way of electrically connecting the two surfaces. They look like sun bursts with jagged edges on the outside but have a hole in the middle that is the size of the screw.

As for the 1 second pulsing, it could be caused by a ground or due to the age of the unit it could be a capacitor failing in the standby power supply.

-bob

Re: Sony STR AV-370 receiver: weird problem...

Hi Bob,

many many thanks for your suggestions! I will install the star washers in the coming days... good tip! cross my fingers!
In case this does not solve the problem, I will check/replace the elco's in the standby psu.

I'll keep you guys informed on any progress/solution !

Once more, great community with great people !

Paul

Re: Sony STR AV-370 receiver: weird problem...

Hi Bob,
When I opened the unit, I noticed 3 wires which are not original, probably put there by the repair shop to solve the grounding problem. I tightened some screws and the problem seems to be gone... However, I noticed a discoloration of the pcb around Q704 which seems to be providing -26 Volts. It does indeed gets pretty hot after half a minute, with almost no power demanded from the power section. I'm gonna further look into this.
grtz,,, Paul

Re: Sony STR AV-370 receiver: weird problem...

Hi Paul,

Voltages like that were usually used to supply bias to the VFD (vacuum fluorescent display). So it will be hot regardless of the power output of the unit. It has lasted this long so the electrical design seems to be sound.

It is not unusual for consumer devices to suffer heat damage to the printed circuit boards. Some manufacturers were better at design than others. I think Sony's problem was that they produced way too many products and didn't put all of the R&D into each design before scrapping it and coming out with a new model.

Is there a way to elevate Q704 away from the board? Sometimes you can re-orient the part so that the heat is conducted away from the pc board. Also, if the unit is in a stack with other components, put some spacers above and below the unit to allow free air flow. Something like a couple of hockey pucks work well.

-bob

Re: Sony STR AV-370 receiver: weird problem...

Hi Bob,

thank you very much for your advice. At present, all is working fine after th trick with the screws, so I'm gonna leave the unit as it is and see if the problem re-occurs in time,
I like your idea of the hockey pucks... especially since my grandson is playing ice-hockey ..:-)
Paul