Products > Test Equipment
Tektronix 2235 repair thread
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TERRA Operative:
I found a little Deoxit went a long way on my unit.
A few of my switches were crunchy, but they came up like new after a bit of a clean.
bd139:
Good to hear. I’ll have a go with that (I have a can). The flickering is the only thing that is worrying me. If it’s the multiplier or transformer then I’m going to need another unit.
bd139:
Ok did some analysis on the flickering while it's open and safely connected to DC and I found that voltage at TP842 drops whenever it flickers. This is not on the driver side of things after hooking a scope to the collector of Q840, setting persistence to infinite and leaving it there for a few minutes so I assume the flickering is an HT issue. I will replace the known hooky HT resistors first and see if that solves it. If not, I will do all the capacitors. If not that I will cry into it and find a donor unit for multipler and transformer.

I will win if it kills me  :-DD
David Hess:

--- Quote from: floobydust on November 26, 2018, 10:50:54 pm ---I don't think the Tek pre-reg PSU is properly understood. It looks like a flyback converter but has a resonant winding/cap C908, I think for low EMI.
--- End quote ---

What I have been told is that C908 and the extra winding inject a counter current into the chassis cancelling radiated emissions and this is what the schematic shows with the separate power supply and chassis grounds.


--- Quote from: bd139 on November 26, 2018, 11:09:42 pm ---2235 targets 60Khz apparently. I was wondering if the frequency was related to loading. Higher loading, higher switch frequency.
--- End quote ---

But the 2235 and 2235A have the lowest power requirements which is why they have no fan.

I would take a very close look at the operating frequency.  The TL594 clock is 60kHz but since only one of the outputs is used, why isn't the output frequency 30kHz?
floobydust:

--- Quote from: David Hess on November 28, 2018, 04:40:07 am ---
--- Quote from: floobydust on November 26, 2018, 10:50:54 pm ---I don't think the Tek pre-reg PSU is properly understood. It looks like a flyback converter but has a resonant winding/cap C908, I think for low EMI.
--- End quote ---

What I have been told is that C908 and the extra winding inject a counter current into the chassis cancelling radiated emissions and this is what the schematic shows with the separate power supply and chassis grounds.

--- End quote ---
Thanks David for pointing out the two different grounds on C908. I'm not sure how T906 is wound but this extra winding surely is to lower EMI.
When I see care and attention like this, I wonder how using a 48VDC replacement SMPS can work without wrecking the scope's noise floor.
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