¿ªÔÆÌåÓý

ctrl + shift + ? for shortcuts
© 2025 Groups.io
  1. TekScopes
  2. Messages
Single Messages Topics Expanded Polls

Re: 7603 horizontal fault

 

The readout looks squished horizontally to me so I think the problem is in the
horizontal output amplifier.

I would not give up on U510 yet because the outputs are currents operating into
effectively zero ohms at the bases of Q539 and Q547 which operate as
transimpedance (current in and voltage out) amplifiers. If you measure the
voltages at pins 12 and 13 then they will look decidedly non-linear because of
diodes CR530 and CR533. If you measure the voltages at the bases of Q539 and
Q547, then there should be practically no change in voltage and I believe they
should each rest at about +14.3 volts.

If there *is* a voltage change at the base of Q539 or Q547, then there is
something wrong later in the circuit.

There are a couple of bias voltages which should be checked. The junction of
R573 and R574 should be about 113 volts. The junction of R570 and R571 should
be about -9.4 volts.

There could be a leaky bypass capacitor. I once found a piece of conductive
something stuck in an air variable trimmer which caused weird problems.

If you do want to find a replacement for U510, it is a 155-0022-00 which may be
the most common custom IC Tektronix produced.

On Sat, 13 Sep 2014 15:30:47 +0200, you wrote:

Hi all,

My trusty 7603 developed a fault yesterday while I was using it, and I
fear that a Tek special IC has expired.

Until yesterday morning it was working perfectly. I looked away to do
something else for a couple of minutes, and when I looked back the
display had shrunk horizontally. It now looks like this:



Oddly the readout seems to be compressed into the left side, but the
main trace is compressed into the middle. I've measured a few things
around the horizontal amplifier board. The power supplies are all OK and
ripple-free, and the resistor values seem plausible based on a few
in-circuit checks.

Switching off the readout makes no difference (other than the readout
disappearing). Pressing 'beam find' has the expected effect, and it's
not stuck on.

I had a look at the waveforms going in to and out of the horizontal
channel switch U510 (156-022) and, while the input (pins 2/15) looks right:



the output (pins 12/13) looks far from right:



Apologies for the lack of vertical scale on the photos: the output is at
400mV/div and the input was 100mV/div, I think.

The 'display limit command' line, which seems to bias the channel
switch, is firmly at -15V as the book says it should be. I tried
reseating U510 but that had no effect.

Has anyone seen a fault like this before? Should I bite the bullet and
find another 156-022?

Thanks
Chris
Join [email protected] to automatically receive all group messages.