Wot dem gentlemines said.
I'm not real thrilled by that soldering either. Dirty wave solder bath I'd guess.
The two joints arrowed are typical of the sort of cracks that can occur with larger components (or external sockets) where the joins can get strained and then crack;
The joints on the main filter caps don't appear to be cracked, but they don't look too darn hot either.
The scarring around C75/76 look like a bit of rough rework to me, perhaps somebody already replaced these at some point (but I suspect they were on the wrong tram), or maybe just a rough operator post-solder wave fit.
No, I don't think that's a short either, but check as
Enzo advises.
So what's going on with your voltage readings?
The DC readings seem okay but the AC readings on the same points are way off.
The reason is that most less expensive meters cannot read AC accurately when there is also DC present, they see the DC as a much higher AC voltage.
The way to measure AC ripple on a DC rail with such a meter is to use a small cap, say 0.1uF, in series with the meter probe. This blocks the DC but passes just the AC to the meter.
However, given;
42 VDC / 92 VAC across both C71 and C72
... I don't think there is a crack on either cap, this is what I'd expect if the PSU was okay, and that the problem lies elsewhere.
A snippit of theory;
I was also unaware that there should only be DC flowing through the caps.
Ah, no, caps are
insulators so DC doesn't flow
through them.
These main filter caps also used to be called "reservoir" caps because they act as storage pools for electrons. Every half-cycle of the mains the rectifier dumps a bucketload of electrons into the cap and its voltage rises a bit (
Q = CV). The cap provides a steady outflow of electrons to the output stage (
Q = It). Therefore there will always be some voltage ripple during this charge/discharge process, depending on the size of the "reservoir", bigger cap = less ripple (but it never gets to zero ripple no matter how big the caps are).
(this is either half of your power supply. The dotted line shows the output if the cap is disconnected, pretty serious hum)
These charge/discharge cycles do produce an AC current through the cap we call
ripple current which works out to be the same as the average DC output current. If you want to investigate the physics of how an AC current can flow through an insulator look for
capacitor displacement current.
Very loud hum doesn't automatically mean that there is something wrong with the power supply. This can also be caused by the output stage drawing excessive current due to a fault such as a blown transistor. This is why we all said "no" when you asked;
would it be safe to say that replacing all four caps would solve the issue?
Unless it turns out to be something simple such as a dud solder joint (which are fairly common) the output stage has many more components and has to be a serious suspect.
{Meanwhile, in a parallel Universe; if you had followed the advice of "someone" you would now be down two large caps, two small caps, four heavy diodes, and the time to fit them, none of which you can ethically charge to the client because there was nothing wrong with them - dead loss. And
you've still got the fault!}
(
Movin' right along...) So we've got -1.19V on the output now as our main (and only) symptom.
{did I say I wasn't mad about this output stage?
}
This circuit contains a number of fusible resistors marked "Fu<value>". In the pre-driver
R87, R88, R89, R90. in the driver
R102, R104, R106. These would be the next obvious and easy thing to check.
You can try measuring the resistance of each
in situ and should get reasonable results, but it would be more useful to us if you could
make up a list of the voltages at either end of each of these resistors. That will give us a scatter of voltages across the output stage which may help to paint the picture.
Test conditions; all controls minimum, speaker disconnected.
Recent Posts