Norm Dresner wrote:
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"mike" wrote in message ...
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 oscillation was a caution about using a different op-amp type.
; the output shows pretty much the same ripple as
the input power.
OK, now I think I understand what you're talking about.
The main 20V supply is controlled by U420.
The supply you're talking about is the 20V reference supply.
Your first post said,
"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"
This (and the next) post says that the supplies are all high by
35%...not zero????
Don't know what to make of that discrepancy.
First thing I'd do is measure the voltage across R411 to make sure the
op-amp is sourcing current. Shorted CB junction in Q410 will cause your
symptom. Again, all this is based on my schematic, which may be
different from yours depending on serial number.
For a general solution, you don't have a lot of choices about what to
do. Get the high voltage op-amp. (see discussion of swapping opamps
around further down)
If you're willing to accept restricted operating parameters, you can
put a LM317 in the +supply of the op-amp.
The generator won't work at low line voltage any more, but that may
be an acceptable tradeoff.
FYI, the FG501 probably doesn't work at low line worst case mainframe
load conditions anyway. It was designed before people started worrying
about
such things. Worst case condition is extremely rare.
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.
What do you mean by regulated? If the op-amp input voltages are not
equal, the output cannot be "regulated"???
You will blow up tantalum caps if they haven't already.
The sine shaper is VERY sensitive to power supply voltage...just to name
one.
Check what's written on each part.
It's clearly different than the other "741" Op Amps in the circuit.
According to my manual, ALL the opamps were switched to -06 starting
with B010204. Maybe yours got replaced with a low-voltage unit and
failed again. If that's the case, put U40 in the U410 slot and put a
low-voltage amp in the u40 slot.
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
--
Return address is VALID.
500MHz Tek DSOscilloscope TDS520
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
http://www.geocities.com/SiliconValley/Monitor/4710/
--
Return address is VALID.
500MHz Tek DSOscilloscope TDS520
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
http://www.geocities.com/SiliconValley/Monitor/4710/