Re: 7A26 transient response 5 nS all atten

Colin Herbert
 

Hi,
I, too, have a 7A26 with no letter at the front of the serial-number (102925). The label with this serial-number on it also identifies the plug-in as having been manufactured in Tektronix, Guernsey, which is why there is no "B" or "H" in front of the serial-number.
My manual, though originally produced in Beaverton (as I think all may have been), has a page in the "Change Information" section at the back referring to changes to Guernsey-produced 7A26s from tentative S/N 100396 and 100777.
I don't know how much use this will be to you, but I also have a source of the ICs used in the 7A26 (unless they have all been sold). They don't usually fail, but swapping them between channels (they are plug-in but be careful of bending the legs) is a pretty simple way of verifying them.
There are also a couple of resistors, one in each channel, which are at about their wattage limit, get hot and sometimes change value because of this. They are R1327 and R1329 (8K2, 0.25W. Carbon comp.) and would benefit in being replaced with resistors of higher wattage.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jean-Paul
Sent: 17 January 2020 12:20
To: TekScopes@groups.io
Subject: [TekScopes] 7A26 transient response 5 nS all atten

Hello all: My workhorse 7A26 needed rework, in 7804 mainframe.
Cleaned the AC/DC/GR contacts on attn with the iso alcohol-paper strips, and exercised some scratchy controls.
Poor fast transient response on CH2, same dip and peak ~ 5 nS at ALL V/div.

Setup: Leo Bodnar pulser 40 pS>Mini Ckt HAT-15>>Mini Ckt HAT 20>>precision 50 Ohm thru>>7A26 input
Same result if 7A26 is in L or R side 7804.
Shots are: Affected 7A26, CH2, CH1, Good 7A26 Ch2 Ch1
http://crypto-museum.org/TEK/7A26/7A26.pdf

Since the aberration is constant on every V/DIV, the attenuator and 2X amp seem to be exonerated.

Before I dare probe the CH 2 amp circuit, a few questions please:

1/ Test points mentioned on schematic are NOT in PCB photos, only the parts numbers. Need a board layout with test pts.
2/ Using normal P6137 probe. placement of ground lead ?
3/ Precautions in probing around the custom TEK hybrid ICs, eg U2350, U2450?
4/ Serial: Manual has sch versions for SN B179999 < and B180000>.
My other 7A26s have this format SN, but this has no letter, 708058. I think its a late model, but see no IC date codes.
Readout PCB is 672-0051-13 Amplifier PCB is 670-2549-21 attenuators are PN200-1442-00

Hope to receive some tips from our TEK veterans!

Jon in Paris

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