Re: FG 504 function generator odd behaviour

Andy Warner

On the FG504, the -25V rail is referenced off the +25V one.
Check for bad tantalums on the -25V rail and also independently check the
pass transistor used by that rail, it is in the chassis, not the module.

I have to work on the sweep section of my FG504, which looks like it will
be a right pain. Anyone got any tips for working on the middle board in
this unit ?

On Sat, Jul 11, 2020 at 13:09 Harvey White <> wrote:

Generic steps:

turn scope off, wait 5 minutes or so, turn scope on. See if -25 is good
(something could have latched up).

with power off, check continuity of fuses or with power on, check for
very low voltage drop across the fuse. There must always be a voltage
drop because the fuse acts like a resistor.

Look at the bulk supply for the -25. Check against voltage readings on
the schematic. Go through regulator circuit to output and see what
doesn't match. Any transistor where the base-emitter voltage is the
wrong polarity indicates transistor is off. Any transistor (except a
darlington) where the B-E voltage exceeds 0.7 volts is likely bad.

Typical things to fail: pass transistor.

in the event of a short circuit, check the current sense resistor (the
low value one from the pass transistor to the output) and then check the
(frequently done like this) transistor which has the base/emitter
junction connected across the resistor. Too much current and the
transistor turns on, stealing base drive from the pass transistor, and
turning it off. Current limiting.

In many tek supplies, the - supply is a positive supply in topology,
with what would be the ground being the negative lead, and what you'd
think ought to be the positive output grounded. The voltage across the
pass transistor is the difference between the bulk supply and the
desired voltage.

Check also to see what the reference voltage is for the -25 volt
supply. It must be exact, otherwise the supply will be off.


On 7/11/2020 1:56 PM, Colin Herbert via wrote:
Oops! I have checked the voltages and while the +25V (+24.99), +15V
and -15V (-14.98) are just about bang on, the -25V is sitting at -3.58V.
really can't remember if this was the case when I checked previously,
but it
is now. I did manage to short something out with the DMM probes, so that
have given this problem, but sure as sure it needs to be fixed, but how?

-----Original Message-----
From: [] On Behalf Of Dave
Sent: 10 July 2020 18:13
Subject: Re: [TekScopes] FG 504 function generator odd behaviour

The first thing to do is measure the power rails to make sure that the
voltages are in spec and clean.


On Jul 10, 2020, at 13:04, Colin Herbert via
<> wrote:
I have an FG 504 which is showing some odd behaviour. The triangle-wave
shows asymmetry in that the negative peaks of the wave are rounded-off at
any frequency and the positive ones have a similar but lesser problem at
higher frequencies. This also seems to manifest itself as what looks like
cut-off on the negative peaks of the sine-wave and lesser cut-off on
positive peaks. While the triangle-waves are more-or-less symmetrical,
sine-waves are definitely not so, showing a nearly three-to-one voltage
ratio in favour of positive peaks. There is also noticeably overshoot on
square-waves at anything above 1kHz and only a DC offset at frequencies
below 80Hz.
I am totally ignorant of how this function generator produces
triangle-waves, but I do understand that the square-waves and sine-waves
derived from the triangle-wave. Does anyone have any help that they can
offer? I would be grateful to get this useful function-generator working


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