Re: 7A26 Ch 1 gain low

Adrian
 

I would reinforce David's forth point about comparing channels.

I've fixed a handful of 7000 series 'ordinary' dual-channel amps (including a 26) that way, with the service manual open in front of you it is dead easy to apply the same signal to both channels and probe your way along the same points in each channel until you find the difference, in my experience that narrows it down to a few components really fast.

I may get shot for this, but in some ways it's often quicker (if not as instructive) to just do that than to sit down analyze the problem from 'first principles' It is true that an extender cable makes it physically much easier to do that than having to probe in the chassis with the side covers off though!

Adrian

On 7/11/2019 11:47 AM, David C. Partridge wrote:
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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