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-----Original Message-----
From Fabio Trevisan
Sent: Wednesday, October 17, 2018 9:00 AM
Subject: [TekScopes] 7A13 relay substitution report

Hello guys,
After I got that 7A13 some weeks ago I have been fighting with its
inconsistencies as I turn the different range knobs...
As anyone who just purchased something, for a couple of weeks I was still
in that denial phase where you believe you're going to track down the
problem to one or two major culprits and fix it nice and easy.
Denial phase over.. I came to understand how many different weird
behaviors can arise from high impedance attenuators, when their action is
switched by unreliable contacts.and, yet, to learn that a poor "make"
contact is the least of the problems that can happen, and the worse is
when some of the contacts fail to break contact completely (while even
being able to make on the other contact).
I came to understand how BAD those relays are (or should I say EVIL?) and
decided to just get rid of all of them, so I`m replacing them by modern
PCB signal relays... not exactly of the same kind that is mentioned here
(
mplifier/amplifier_7A11.htm) repair, linked on the 7A11 page of TekWiki.

Instead, I selected and am already installing some of the ones below:

For the SPDT of the X1/X10 gain circuit (K60 and K61) OMRON G5V-1 series

The up side of the OMRON PCB signal relays is they are characterized for
R.F. performance...while the majority of the regular PCB signal relays
don't even mention frequency response characteristics.
They can isolate 100MHz down to -45dB, which is the best figure you will
get from "normal" PCB signal relays.
Their down side however, is that they're "only" rated to 125VAC / 60VDC,
which inside the amplifier circuit is not a problem.

Note: There are signal relays meant for RF that ARE characterized up to
6GHz or so, which can go as low as -85dB @ 100MHz but those are more
expensive, and requires specific layout (TL) to meet the specs.

For the DPDTs inside the amplifier circuitry (K47, K48, K480 and K490):
OMRON G6K series:
g6k.pdf
or
OMRON G6J-Y series:
g6j.pdf
They share basically the same R.F. performance of the G5V-1 above and also
the shortcoming of the limited switching voltage.

For the input DPDTs (K6, K26, K7, K27) and also for the Vc input switching
SPDTs (K10 and K30) (i.e. all relays that are subject to the full input
voltage) NEC / TOKIN EA2 or EB2 series (EB2 is through hole, and EB2 is
meant for SMD, but after straightening out the leads, both are the same).

Note: these ones are a compromise... They were the only ones I could grab
that would withstand 250VAC / 220VDC, but they're not characterized
whatsoever for H.F. use (although it seems they perform acceptably in that
department).
Like these ones (without H.F. characterization) there are many brands and
models, including the AXICOM / TE connectivity referred on the 7A11 report
linked above.

If you don't care about the inputs being able to withstand 250VAC/220VDC
(which is as close as it will get to the 7A13 original specs), you can go
for the OMRONs G6K or G6J-Y.

See some pictures of the assemblies here:
/g/TekScopes/album?id=72382
Those are the ones that will replace the K7 / K27 (they are SPDT + SPST
N.C.)

Last comment...Attention must be paid, on a relay-by-relay basis, to the
polarity of the voltage being applied to each of the coils.
The Tek relays were not polarized (in most of the cases, A.F.A.I.K.), but
most of the modern mini relays ARE polarized and they won't engage if fed
in reverse.
For instance, the wiring to the coil of K7 need to be the opposite of K27
(both are the X10 input att). Also inverted between K10 and K30 (the Vc
input switching).

Rgrds,

Fabio