Re: TDS 640 Noisy Vertical Signal at Fast Horizontal Sweep Rate


Dear Scott,

from my experience I can give you the following advice:

The TDS 640 has no FLASH ADC like the TDS 744 or equivalent scope, but
uses an analogue bucket-brigade device like memory (CCD), called the
FISO (Fast in Slow out). It is the rectangular PQFP Chip U850 in the
center of the 4 heatsinked chips (T&H). Under 10 usec/Div the CCD is
bypassing, above it is doing the sampling. I would start to look around
U850, it's power supply, and control signals. Maybe a corroded trace?
Worst case: U850 is dead.


TNX to Hakan again.

br and good luck,


I have a TSD 640 that I recently re-capped. It works fine, but Channel 4 appears to be dead. Looks like the input hybrid on that channel is dead.

I bought a TDS 644 on eBay that has a delaminated color LCD assembly. The manual says this has the same Acq board as the TDS 640. The Acq board is MOSTLY the same. It seems to have one extra RF socket, that nothing was plugged into. So I put it in my working 640. The scope passes POST and all channels work.

The problem is at higher sweep rates, I get lots of vertical noise.

For sweep of 10 usec and slower, it is PERFECT. Super clean waveforms. As soon as I go to 5 usec or faster, the vertical display turns to hash. It has at least 1 volt of high frequency noise on top of the wave forms.

If I turn the sweep way up I can see that the "hash" is probably 50 MHz or so rising asynchronously on my waveform.

Anyone seen this before, or have any ideas on where to start looking?

XERXES electronics GmbH
Dammgasse 75
A-9020 Klagenfurt

Tel.: +43-(0)463-890061-0
Fax.: +43-(0)463-890061-23
Mob.: +43 (0) 680 4405842

Firmenbuch : FN 307552h
UID : ATU64037018

Join to automatically receive all group messages.