Re: 7A26 Ch 1 gain low

Colin Herbert
 

Hi,
Yes, it does occur at other volts/div settings. I was quoting what I did to quantify the low gain in some way. The frequency was the "standard amplitude" 1kHz square-wave from my PG506.

As it happens, I have two 7A26 plug-ins (I like them) and swapping them around between the two vertical bays of my 7623A mainframe shows that the problem lies with the 7A26, not the mainframe or one of the bays. The "good" 7A26 shows an excess gain of about one half of a major division on both channels.

I was thinking, from a cursory inspection, that the ICs were soldered in, but I think this may not be the case on closer viewing. From what I can see of the markings on the ICs, the first three numbers differ, but then the next group of six numbers are "0078 10" followed by another group of two numbers and a letter. My service manual lists three types of ICs, the earlier ones were "SEL" for "selected" I am guessing. The later ones were designated 155-0078-10 and aren't "SEL". The 155-0078-10" are available on eBay. I might just buy a couple.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of David C. Partridge
Sent: 11 July 2019 11:47
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

OK I'll make some guesses.

1. Have you checked that the output level from the attenuators is correct (same both channels)?

2. Does the problem occur at other volts/div settings?

3. What frequency signal are you testing with? Is the problem also present at DC?

4. You may be reduced to comparing signal levels on each channel through the various amplifier stage until you find one with low output. If you find one initially suspect any surrounding gain setting components, but equally could try swapping the ICs between channels.

HtH
David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 11 July 2019 11:03
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7A26 Ch 1 gain low

Doesn't anyone have any comments?

Colin.
-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 09 July 2019 13:20
To: TekScopes@groups.io
Subject: [TekScopes] 7A26 Ch 1 gain low

I have a 7A26 dual trace vertical amplifier with a Ch 1 gain problem. When trying to set the gain, I cannot set it correctly, as the GAIN adjust goes to fully clockwise but the trace on the CRT is a mm or so low. This is with the VOLTS/DIV at 10mV and a 20mV square wave from my PG506. The VOLTS/DIV VAR control won't go beyond this either. Ch 2 works fine, with about one small division of excess available using both the GAIN adjust and the VOLTS/DIV VAR. Anyone have any ideas?
TIA, Colin.

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