7A13 relay substitution report

Fabio Trevisan
 

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 (https://www.amplifier.cd/Test_Equipment/Tektronix/Tektronix_7000_series_amplifier/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
https://omronfs.omron.com/en_US/ecb/products/pdf/en-g5v_1.pdf
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: https://omronfs.omron.com/en_US/ecb/products/pdf/en-g6k.pdf
or
OMRON G6J-Y series: https://omronfs.omron.com/en_US/ecb/products/pdf/en-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).
http://www.mouser.com/ds/2/283/relay_ea2_eb2_e-528008.pdf
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:
https://groups.io/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










For the SPDTs

Join TekScopes@groups.io to automatically receive all group messages.