CJ said:
ok we got er fixed, turns out the regulator transistor 2N3904 does not like anything over 40 volts, this is a 1975 amp so we see 42.5 volts on the input rail which the transistor might see during startup , Fairchild 3904 series not he best,
Actually during start up the current source has to charge up the capacitor so I would not expect turn on transient higher,,, 3904 is between base of pass element at 25V and zener connected transistor so all of 15v,
unless trim pot turned to zero then it sees maybe full unregulated collector to base... BTW I still don't like that trimpot going full range (especially to 0V).
that 10K tends to keep the ref voltage stable while turning the pot to adj voltage by supplying more zener current,
yes, stiffens it up, but only 1.5mA not huge in context of a couple mA down 3904. Does give a sharp cut off when pot turned below reference V (but pot shouldn't be turned that low).
we have drawn in the 4.7 resistor on the 3392 base.
That should not be necessary if circuit working properly, but could protect the current limit transistor "if" PS output shorted to ground "and" pass element has failed hard on. May reduce repair cost/complexity (something Peavey thought about at least during my time there 10 years later).
we stuck a 100 volt part, MPSA06 (because that is what we had) for the reg transistor,
voltage from the pot rotation is still wide which could be fixed by using a lower hfe part, but things are clean and stable so what the heck, engineers always seem to go for too much control instead of too fine so if anything changes they do not need to switch pot values, (witness Fender bias pots)
transistor beta should not affect adjustment range. We're talking single digit mA at that circuit node. I still don't like the full range adjustment and no protection against pot going open wiper, a common failure for cheap trimpots. But adding three resistors may be undesirable in a circa 1970s design.
small note of interest, in one schemo the collector of the ref is left open, on the other schemo it is grounded, does not seem to matter,
does not matter. The base collector diode grounded would only matter after device has failed at which point it doesn't matter very much.
we are still puzzled as to where the current goes thru the 2N3904 as the rev bias ref probably draws very little current, maybe the current goes thru the BCjunction and thru the pot to ground?
no the reverse biased base emitter junction of 953 looks like a low impedance above zener voltage. Roughly 8 mA flowing into that node, and 3904 is sucking out all but the few mA the pass element needs to deliver power supply output. I am too lazy to look up beta of 430 but will speculate around 50x so several mA dumped through 3904 into the zener.
here is the voltage chart for a working regulator thanks for all the help!
we could have used a 7824 with a few diodes in the ground leg and held our breath but we are glad to have the whole Mary Ann in there like a stock amp.
A simple resistor divider from output to ground, connected to base of 3904, with a resistor added in series with adjustment pot wiper will reduce the adjustment range and make it less concerned about trimpot failure especially if fixed resistor divider is setting nominally correct voltage.
I stand corrected about the zener voltage. I expected 6.8V. I have never done that in any of my designs. Back in the day I messed around on the bench trying to use one as a noise generator, but IIRC I was not happy with it for that either. Note another silly discrete transistor trick is to instead of using just the reverse emitter to make the zener diode, connect backwards across collector-emitter. That places the forward collector base diode in series with the backwards base-emitter junction. This gives another roughly 0.5v drop and I have seen npn and pnp pairs of these used in clamps across mic inputs (low Z clamp at around 10V but high Z at mic levels).. I think the added collector base diode delivers a different temp-co than just the base-emitter, but I've never used one as a reference. It is can be useful to understand how backwards junctions behave when troubleshooting circuitry failure modes.
JR
