As described in the Tek manual for the FG501, the +20V supply is the
reference supply for the remainder of the power supplies. AFAICT every
other supply is running "perfectly" proportional to the too-high voltage on
this line. I fully intend to swap out the Op Amp, but given that all of the
supplies are running "perfectly" at 1.35 x normal voltage, why isn't the
unit operational? I suppose it could be there's a resistor somewhere whose
value is chosen to be just below a diode-drop with the correct voltage and
with the higher voltage the base is running too high and always turned on --
or because some other component failed. Has anyone had a similar experience
with this type of unit?
TIA
Norm
"Norm Dresner" wrote in message
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"mike" wrote in message
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Norm Dresner wrote:
Background: I'm repairing a non-working Tek FG501 function generator
plug
in. So far I've determined that the voltage in the regulated +20V
circuit
is zero. I'm assuming -- for the moment -- that this is caused by a
failure
in the regulator section which consists of a reference diode, an Op
Amp
and
a transistor booster (with a second transistor for current limiting)
(and a
suitable number of passive components). All of this is very simple
circuitry and should be easy to debug.
First, are you sure it's the op-amp?
Well, I've measured the voltages at just about every point in that part of
the circuit. I can explain every one except that the non-inverting input
to
the Op Amp is 6.2 volts, the inverting input is ~8.2 and the output is
~27.2
volts. Since the voltages on the pass transistor Q410 are consistent with
a
normal diode drop from collector to base and a reasonably saturated
transistor between collector and emitter, I've ruled out failure of this
transistor. Similarly, the voltages on the current limiting transistor
Q415
also indicate a good unit. Based on the conclusions that the transistors
and the resistors in the circuit are okay, that leaves only the Op Amp
which
is clearly misbehaving. BTW, I've checked with a 'scope and the Op Amp is
definitely not oscillating; the output shows pretty much the same ripple
as
the input power.
If it is, can you swap it for another that's running on 20V and use the
commercial grade replacement at 20V? Even tho they're selected
differently, they may have in fact put the same part in all places.
I think what I might do is to lift the lower leg of the pass transistor's
emitter resistor (also the current limit sensing resistor) and inject +20V
from a known good supply at this point. If the unit still doesn't work,
I'd
be tempted to scrap it because there may be who-know-how-many consequent
failures in the remainder of the circuit.
What's not totally obvious to me from a very, very cursory inspection of
the
circuit diagram is what would not work with the higher "regulated"
voltage -- unless something else has failed from that higher voltage,
perhaps a tantalum capacitor as you've suggested -- though I don't "see"
anything that's broken or burned.
Check what's written on each part.
It's clearly different than the other "741" Op Amps in the circuit.
Have you tried ordering one from TEK factory service?
No, I've just started working on this unit.
Can I assume you've tried it in a different hole to rule out the
possibility of a mainframe transistor failure or failure in a solder
joint on one of the transformer windings?
Yes. I've also looked at and measured the AC input and its rectified and
filtered version and they look fine.
Since you didn't mention any of the other supplies being down, one might
rightly or wrongly assume that they're ok and the reference is working.
Look for shorted tantalum caps. Sometimes they short so bad they don't
even get warm.
I haven't even tried to measure the other voltages -- I think that they're
all set "proportional" to this one.
I'd worry about the op amp specs after you determine it's bad.
I'm pretty sure it's bad.
mike
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