Re: 7A26 Ch 1 gain low

Colin Herbert


I have done a bit more investigating on this and have found the following:

-Some of the voltages on the first CH1 IC (U1350) are different from those on a known good 7A26, so I am inclined to think the problem is associated with that circuit. Tek made some errors in Service Manual in that the polarities some DC voltages here are wrong (pins 2 & 3 are given as +0.8V but are actually -0.8V).
-Swapping the first ICs between CH1 and CH2 doesn't change anything.
-Putting the U1350 from CH1 into a known good 7A26 doesn't take the problem with the IC, so it is probably good.
-When the IC from the problematic CH1 was in place in the known good 7A26, the voltages around it were similar to when a known good IC was in place, again suggesting that there is nothing wrong with the IC.
-The +15V and -15V rails (where I have probed them) seem to be in spec.
-I have tried some of the initial calibration routines (with the correct equipment) and find odd behaviour (zero shift) when operating the BANDWIDTH and INVERT switches during some of this.
-I have also tried checking the waveforms at the various points shown in the Service Manual, but I am not too sure that Tektronix didn't make some mistakes here too, as I see large DC voltages which move the trace off the screen when using the stipulated DC input on the investigating scope.
I am at a bit of an impasse, partly because the circuits are differential and I don't understand them too well.

Any advice is appreciated.


-----Original Message-----
From: [] On Behalf Of David C. Partridge
Sent: 13 July 2019 15:40
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

The small ICs all over the 7A26 board are socketed. I would be inclined to suspect gain set components rather than the ICs themselves.


-----Original Message-----
From: [] On Behalf Of Colin Herbert via Groups.Io
Sent: 13 July 2019 12:05
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

I have purchased a number of the ICs - #155-0078-10 in order to do some swapping. I am still wondering if theses ICs are soldered in or not. Does anyone have experience of them?
TIA, Colin.

-----Original Message-----
From: [] On Behalf Of Raymond Domp Frank
Sent: 12 July 2019 18:35
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

On Fri, Jul 12, 2019 at 02:23 PM, Colin Herbert wrote:

However, on looking closer at the problematic 7A26, I think that the problem
is not only with Ch 1, as the Variable gain control on Ch 2 doesn't give as
much gain increase over the calibrated setting as do both channels on my
"good" 7A26. It is about one half of a major division on the "good" 7A26.
... Which is what I suggested earlier: "I think that's not enough, meaning that overall gain for *both* channels isn't enough. Usually, there's at least half a major division headroom".


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