I just finished rebuilding a Dyna ST400 - including replacing all
output transistors. After making the initial tests specified in the
Dyna manual, I went to set the bias current. Well, I instantly got a
small puff of smoke and heard a click. I immediately shut down the
amp. Here's what I found:

1) BURNED resistors on PC28 = R205, 213, 214 and 224.
2) Diode 205 looks pretty distorted
3) someone had replced the bias current trim pot on the board w/ a 500
ohm instead of 1k ohm unit.


(I double checked all resistors and capacitors I replaced on the Pc28
board in question. All are the correct value. The replacement
capacitors are correctly oriented and of the correct voltage rating.)

BUT - and here's the real BUT - the biggest problem is an error I
made!!

I had switched around the new output transistors. I installed the NPN
2N5630 where the PNP 2N6030's should have been and vica versa!!!

So, my questions a

A. If I swap the output transistors over to the way they should be AND
replace the burned resistors, do I have a chance that the channel will
work?? Or, have I destroyed the output transistors and/or other
components on the PC28 board??

Any help will be appreciated!!

Pete
Chicago

kafer77 wrote:

I just finished rebuilding a Dyna ST400 - including replacing all
output transistors. After making the initial tests specified in the
Dyna manual, I went to set the bias current. Well, I instantly got a
small puff of smoke and heard a click. I immediately shut down the
amp. Here's what I found:

1) BURNED resistors on PC28 = R205, 213, 214 and 224.
2) Diode 205 looks pretty distorted
3) someone had replced the bias current trim pot on the board w/ a 500
ohm instead of 1k ohm unit.


(I double checked all resistors and capacitors I replaced on the Pc28
board in question. All are the correct value. The replacement
capacitors are correctly oriented and of the correct voltage rating.)

BUT - and here's the real BUT - the biggest problem is an error I
made!!

I had switched around the new output transistors. I installed the NPN
2N5630 where the PNP 2N6030's should have been and vica versa!!!

So, my questions a

A. If I swap the output transistors over to the way they should be AND
replace the burned resistors, do I have a chance that the channel will
work?? Or, have I destroyed the output transistors and/or other
components on the PC28 board??

Any help will be appreciated!!

Pete
Chicago


=======================

Anything's possible. I'd expect the outputs to be slag after being
installed backwards. But even if the outputs still test good, I
wouldn't trust them. Before you order new ones, check all the
transistors...especially transistors Q212-215. There's a good chance
they fried, too, and you'll be ordering more stuff.

Check every single diode in the circuit while you're in there.

A burned R205 most likely means a shorted Q203 current source
transistor, and probable damage to the input transistors Q201 and Q202.
D205 is a zener diode in the base of Q203. Replace them all (see
paragraph below, about Q201/202).

A burned R213 and R214 probably means a shorted Q207, whose collector is
DC coupled to the driver transistors and to the Q204-Q205 differential
pair. The collector of Q207 generally runs at about +1 volt or so
(exact voltage set by the bias and offset adjustments) and if it
shorted, it would have gone to about 72 volts and damage to other
transistors is likely to have occurred before R224 burned and stopped
the flow of current from the positive rail.

Burned R224 is to be expected since current flows through R224 before
reaching R213, R214 and R205, and you already know those got overcurrent
since they're burned.

Note on Q201 and Q202: If yours is an early production ST-400, the ones
you probably have, tend to go bad or noisy. If one goes bad suddenly,
it takes the Class B portion of the amp circuit to one power supply rail
or the other, risking a blow-up. "Bad" ones have the printed part
number covering almost the entire flat surface of the transistor; later
"good" ones have the lettering much smaller. A good matched pair of
2N2222 is a workable substitute; I've replaced mine with Japanese 2SC
devices of some kind, because Japanese ones with the same production
number are remarkably well matched right off; for those inexpensive
devices, I generally order ten or twenty at a time and pair them off by
gain, for use in differential amplifier circuits such as the first stage
of the ST-400 power amp.

Replacement of the bias trimpot P202 with a LOWER resistance should not
have caused damage to the amplifier due to overcurrent, but it may make
it impossible to get enough idle current in the output stage to
eliminate notch distortion. But you need to ask, why was it replaced in
the first place? If it failed, it probably saw excess current, and if
that happened, you need to check R18 also, because it's in series with
P202 and if it ever goes open, it may go open at some future time, and
if it does, the amp will blow up from excess current in the output stage.

The preliminary resistance measurements in the Dyna manual are next to
useless after a failure has been repaired, since they assume a
functional PC-28. The preliminary voltage measurements are not much use
either; all they are is a check of the voltages on the power supply
rails, and the regulated voltage supply to the PC-29 board.

When working on amplifiers, particularly ones of the size and complexity
of the Dynaco ST-400, it is a virtual MUST to power up SLOWLY using a
variac fused at one amp or so, and a current monitor; while monitoring
the DC at the output terminal and seeing that it remains close to zero
(within a tenth to two-tenths of a volt or so, on an amp that has just
been repaired and which hasn't had its output offset adjusted out yet).
This is the procedure described in the Dynaco manual in the
"Servicing" section. At the very least, fuse the B+ rails with 1/2 amp
fuses, so they can blow and hopefully limit current to the channel,
sparing parts from damage if something lets go. The amp doesn't draw
more than about 200mA at idle, if it's working properly.