17-04-2012, 06:39 PM
(17-04-2012, 04:52 PM)Joe Wrote: I've now had a better look at the set.
There are only two electrolytics not on the audio board: C13, specified at 40mfd and C21 - 300mfd. Neither of these were healthy in terms of ESR and C13 also leaked like a tap. In practise C13 was 10mfd and C21 350md, the latter I replaced with a 470mfd.
C21 failing O/C is a fairly common occurrence, and can cause the set to squeal and whistle like mad. C13 is the AGC capacitor, and this causes audio distortion when O/C... I'm sure that changing these will have helped slightly, even if you can't hear any change just yet - and if not, at least they're done now...
Quote:These changes made no difference so I tried playing the set through my bench amp and it was fine. So, despite my earlier investigations the fault must be on the audio board. This certainly makes sense as there are far more electrolytic capacitors thereon and if he two I've tested are at the end of their days odds are at least some of these are too.
On these boards, I've found the wire-wound pre-set to be 100% unreliable - although this same component is used on a number of other sets and I've not had them fail. But this normally causes bad crossover distortion, and is worth checking out. The normal failure mode is to fail O/C.
Depending on the vintage, electrolytic capacitors are definitely in the frame, although I don't recall having to change too many of these.
The germanium transistors can fail. One of the output pair going low-gain seems quite common, and also they can develop tin whiskers. For the purposes of testing, the types aren't all that critical...
When working well, I quite like these. But I'm probably repeating myself, so will shut up now
