Sony ICF-SW7600 Instability on switch-on

[Colin]

Not sure if this should be in the modern section but the set is over 20 years old so I'll post here and let the Mods decide .

I've just had a look at this Sony ICF-SW7600 for a friend. The set wasn't working at all with just 'hash' coming out of the speaker. These sets suffer from very poor electrolytics, and sure enough when I opened it up I found they were in a very bad state with electrolyte leaking onto the board in places.

These are all surface mount caps, but I've replaced them with miniature axial ones trying hard to keep the leads as short as possible. I replaced 24 caps as virtually all of them were poorly, and cleaned up the board where necessary. The three large blue ones were fine so they survived intact.

The radio now works fine on all bands and all is well ... except for one thing.

When switched on the radio seems to be unstable and unable to hold the pre-set frequency. This quickly turns into a motorboating oscillation, but disappears and completely settles down after about 20 to 30 seconds. It's perfect thereafter. It seems as if it needs to 'warm up' before it becomes stable.

I did nothing other than the re-cap but I'm worried that the slightly longer leads on the new caps may be causing this.

Has anyone come across this before or can anyone suggest a possible solution. The set is 100% better than it was and is perfectly useable but it'd be nice to sort it out properly before it goes home.

Thanks
Colin

[Nickthedentist]

Two things come to mind, Colin.

Have you thoroughly cleaned off all the leaked electrolyte?

And why are you so certain that the three big Rubycon caps are OK? I'd change those anyway just in case (or at least bridge them with new ones as a test)

Nick.

[G0PKH - Pete]

Just looking at the picture.
I notice that a quantity of glue is used around those electrolytics.

When I worked for Mitsubishi in the early 80's ISTR that the stuff caused us no end of problems, it has a tendency to go conductive ,
Might be worth replacing those caps and cleaning off that glue.

Pete

[Colin]

Originally Posted by Nickthedentist

Two things come to mind, Colin.

Have you thoroughly cleaned off all the leaked electrolyte?

And why are you so certain that the three big Rubycon caps are OK? I'd change those anyway just in case (or at least bridge them with new ones as a test)

Nick.

Thanks Nick, I used Isopropyl Alcohol to clean the board - I was pretty thorough but didn't check it in any way following the clean-up.

The three large Rubycon caps looked okay physically and had a very healthy ESR (0.1R or less) - I didn't check them for leakage or bridge them with a known good one. I was really pleased to find a few of the caps seemed okay! I'll change them anyway.

Pete - interesting tip regarding the 'glue'. Thanks. I'll remove it when I change the three caps. I'll test it to see if it has become conductive and post the outcome here.

Let's hope this gets it sorted

Regards
Colin

[Biggles]

You might actually have two seperate faults here. See if you can isolate the motorboating to the AF stage. The unstable display could be down to the oscillator. See if it happens on all bands etc.

Biggles.

[Colin]

Thanks for the advice everyone.

I replaced the last three caps this morning and cleaned off all traces of the glue as suggested. I'm pleased to report that the instability and motorboating is cured. The radio is fine now.

Should have replaced them when I had it in bits in the first place.

Rgds
Colin