No sound from any source in SONY JJ-700B

[tiv48]

The same used to happen with fusible resistors in tv sets the ones fed from the lopt they would look perfect and quite often no faults could be found that caused there demise .
Rob T

[Skywave]

Yes, Rob, I've 'been there' too. My theory for those types of failures is that an odd high-voltage spike occurred that exceeded the resistor's voltage rating - pop! immediate O/C resistor.

And as for "one of those inexplicable failures which occur from time to time", yes, I know what you mean: again, "been there". However, this is a somewhat different case to the example mentioned by Rob: everything here is at a much lower voltage. Of course, the good news is that you've found the fault and fixed it: hopefully. But what was the cause? There obviously was one. Is there a fault that is lying dormant and that will arise again - and cause the demise of that replacement resistor - or something else? (Yes, I've experienced that sort of trouble too.) You don't know if this will occur - and I'm not going to guess, either! But that's the rub with faults like this, isn't it? Perhaps the best thing to do now is to give the repaired item a substantial 'soak test': crossing fingers usually helps too!

Al. / Oct. 27, '13 //

[Old Sparky]

Further checks on the JJ700 reveal that the Radio is still silent on all bands. The supply voltage (+13v) is present, and there are reasonable DC readings around the I.F./detector I.C. , a type with which I'm not familiar. More work to be done to get this unit fully operational again!

[Skywave]

Your last post suggests that the cause of that failed 330 Ω resistor was due to something in the radio section that initially failed S/C, causing that resistor to go O/C, then that 'something' in the radio section subsequently going O/C: a failed transistor springs to mind, but there are other possibilities, of course.

Al. / Oct. 29, '13 //

[Old Sparky]

Went back to the JJ700 this afternoon, Al. The reason the radio section was silent was due to a failed 39 ohm 0.25 watt resistor in a +ve supply line going to the I.F./detector I.C., which incidentally is a SONY 'in-house' 24 pin D.I.L. device marked 168 2E 004. Replacing the resistor (R139) restored radio operation on all 3 wavebands.

[Refugee]

Well done.

[Skywave]

Excellent result! Your persistence and perseverance paid off. The only small issue now is why did both those resistors fail in the first place? Could it be that the PSU is 'playing up' and occasionally producing excessive d.c outputs? And if so, is there a pre-set Vout pot. in the PSU that has a dirty track? (Thinks: damp cellar.)

Just a few thoughts that might be helpful.

Al. / Oct. 30, '13 //

[Old Sparky]

No pre-set Vout pot present, Al. The PSU is a standard dual rail (+28/0/-28v) with bridge rectifier, and further r/c decoupled lines of +13 & -13v. One of the o/c resistors (on the PSU PCB) was in the -13v line, and the other, on the RF/IF PCB, further decouples a +13v line down to ca. 11v. No visible sign of ingress of damp, either. The unit is about 31 years old, so the two resistors could simply have failed due to age!

[Skywave]

I wonder if there are some electrolytic caps. in there that became a bit tired and leaky whilst the item was put away, stored and not used for that length of time? Then the sudden application of power caused a surge through the resistors and burnt them out, the caps. having attained most of the reforming before the Rs went O/C.
But the 'bottom line' is that you have now fixed it. We can only hope that it now 'stays fixed'.

Al. / Oct. 31, '13 //

[Joe]

That's what I was thinking, Al. If in doubt blame a capacitor!

- Joe