JBL SA660 from 1969 - horse manure smell

never

Burnt shellac from a power transformer has that smell.

familiar

I should have said old horse-manure rathe rthan fresh

To work on this amp i'll have to make a frame to link the 2 sections together once the casing is removed. Unfortunately I think you may be right, do you reckon the bang could have occured inside the transformer eg primary to secondary short ?

familiar

I've now removed the side dished covers to the transformer. The smell is negligible inside it and nothing charred or smoke stained inside that I can see, but the two 110 coils measure 4.4ohm and 3.5 ohm . The failure occured a week after converting the 2 parallel for the USA to both in series for UK which is another reason for suspecting the transformer. But the mains fuse (correctly rated for uK) is still ok. I'll have to carefully power up via a variac I think

Genreally speaking the two winding should have *exactly* the same impedance. There is a difference between DC resistance and impedance but I'm of the opinion that the 25% difference is indicating a possible shorted winding.

A variac is probably the best solution. If you have an AC ampmeter, you could try to measure current.

Suggestion:

Can you disconnect the transformer from the circuit?

If you can, try this:

Set your variac to 120 volts.

Apply the 120 volts to the 4.4 ohm primary winding.

Measure the 2.5 ohm primary winding's voltage. It should be equal to the 120 on the other winding...

Do this with the transformer out of the circuit, however. And carefully monitor the voltage into the 4.4 ohm primary.

SZBEh.45000$ snipped-for-privacy@bignews8.bellsouth.net,

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Yes the "T" circuit requires a series of 5 diodes to set the central biasing for the cascade. Its a bit on hold at the moment because the owner says the same problem of going bang occured when it was in the States, repaired there, but he retained the replaced bits which suggests other than mains transformer. Waiting for him to get back to me to describe these bits. On retesting the primaries , excluding fuse and switch paths they were 3.3R and 3.7R which is not too removed from proper sorts of values.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I've now some history on this amp now. When it was "repaired" in the states the two 10 ohm resistors were replaced in the Zobel shunt on each output as severely burnt. This would suggest ultrasonic oscillation but there are no amendments to the amp, just the 2 Rs replaced. The reported pops were probably the two 2 terminal thermal? circuit breakers in line with each speaker going open circuit, resetting themselves later. Glass bulb devices , anyone any idea what sort or rating these are, not specified in the schematics, just listed as JBL part number 13176. I've now powered up to 150V , wired as 240V mains and all is settled with

75V on each primary and +-31V on the first power rails rather than +-51V , full mains . Proportionate voltages on each of the other 4 power rails per each channel on this "T" amp. Cold testing of all major components showed nothing untoward except the smell apparently strongest at the transformer The smell may be from where it was stored and nothing wrong with the transformer. First , before winding the mains up more, is to determine where best to put some hf oscillation suppression.

The two 10R replacements are fine this time but the charring on the originals and smoke damage on the adjascent large wire wounds mean the oscillation was on both channels. There is certainly a lack of any non-electrolytic caps on any of the power rails so add some in there for a start, any other ideas. ? Anyone know aof a JBL owners group ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on