AR88 Filterchoke L50/L49 and output trans.

[JanWra1]

Hi Guys

I have noted thick black sticky stuff - like the body coat under a car - dripping from the terminals of filter chocke L50. but not at L49
Are these 2 filter chokes oil filled?
Is the leak anything to worry abt?

The same question abt the output transformer TR2, i.e. is it oil filled?

Jan

[HamishBoxer]

The worry is if the stuff is PCB.Handle with and dispose carefully.

[PsychMan]

I think they're potted with a tar like substance, as far as I know no PCB oil - but don't take my word on it

Filter caps and bathtubs are the only locations of questionable Oil in my view.

Are they running hot?

[dave walsh]

You need to look up info on the OP Transformer which is notoriously prone to overload and burn out [especially if not loaded] regardless of the oil issue! Plenty of threads on this issue.

Dave W

[Station X]

I attempted a post mortem on a dead AR88 output transformer. It was filled with a pitch like substance.

[turretslug]

L49 and L50 have similar inductance and resistance but L50 carries the whole HT current whereas L49 is connected after the 6V6 output valve's anode feed and carries somewhat less. L50 will thus always run somewhat warmer, but if it has been running hot enough to significantly soften the pitch, then it suggests that too much current is flowing through it. The main suspect is leaky Micamold capacitorors and the first of these to be suspicious of are the two parallel 4.7nF output valve grid coupling caps, C118 and C122- these will cause the output valve to draw excess current if even slightly leaky. Most of the original Micamolds will be at least a little leaky and together can add up to a significant HT current increase. As the heating of the chokes is proportional to the square of the current, it can be seen that only a slight increase of HT current results in significant increase in temperature- this will also affect the mains transformer HT winding, too, making the whole receiver run un-necessarily hot and risking long-term damage. Ideally, the Micamolds (priority) and also the triple-section bathtubs need to be replaced as part of an overhaul to preserve other, more expensive components.

[JanWra1]

Thanks Turretslug for your instructive comments. C118 and C122 have already been replaced by a 10nF capacitor. As to the "bathtubs" will work on them next.

Jan