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.
Using my Tek cross-reference manual, I've determined that the two
transistors are basic 2N3904 (current limit) and MJE340 (pass). Great, I've
got dozens of those in the drawer.
BUT ... The Op Amp is specified as a "741" selected for +-20V supply! In
fact, it's operated with a single supply with V- at ground and V+ at the
capacitor-filtered, rectified 25 VAC raw input. A quick calculation
reducing the AC voltage by 2 diode drops (full-wave bridge), gives roughly
at 34V DC. I'm sure that the 25V voltage level is for a nominal AC input of
(probably 110V) and I've got to expect that it could be as much as 10%
higher so that the Op Amp would see at least 37V power differential which
is, in any event, greater than the normal absolute limit for a commercial
grade 741 of +-18V. In the NS data book, they give the absolute supply
voltage limits of +-22V for the A/E/- grades and +-18V for the C grade. I
don't think I even have any better-than-commercial new 741s on the shelf.
Digi-Key only carries the C-grade! AFAICT, so does Mouser.
SO ... I'm stuck it seems with several alternatives:
1. Finding a supplier of the higher grade 741 Op Amp.
2. Trying to select from my own stock one or two ICs that meet the
criterion
3. Finding an acceptable substitute.
Even though I have literally dozens of RFE 741s in a box -- and I'm sure
several were probably better than commercial grade when originally used --
any testing for survival of the ICs with a 40V supply differential would
certainly be destructive and not exactly what I'd really like to spend my
time doing.
It's likely that I can find a modern "high voltage" Op Amp which meets or
beats 741 specifications in a "commercial" grade and quite probable, I
think, that it'll be cheaper than purchasing some new MIL-grade 741s.
I only have suspicions and assumptions at this point and I'm soliciting
recommendations on a good course of action for getting a replacement for the
Op Amp.
TIA
Norm
There may have been design changes in production. All I can tell you is
that in my manual, that power supply design is flawed on many levels.
It depends on the fact that the op-amp can't supply much current.
A faster/different op-amp may well oscillate.
First, are you sure it's the op-amp?
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.
Check what's written on each part.
Have you tried ordering one from TEK factory service?
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?
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'd worry about the op amp specs after you determine it's bad.
mike
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