EB5AGV's Workbench: Rohde & Schwarz NV 1/156 VHF MULTI-COUPLER repair

A classic tubed multi-coupler

NV 1/156

This page is devoted to the repair of my Rohde & Schwarz NV 1/156 VHF MULTI-COUPLER.

Tube lineup is: EC80, E180F, 5x EF802, 85A2. It covers from 100 to 156MHz.

What is a MULTI-COUPLER?. Well, basically it is a distributing amplifier (one input, several outputs) with (generally) unity gain. So I can use just one antenna for several receivers. My intended use is to monitor VHF frequencies.


Unit as received

Sadly, I got the unit with some transport damage.

A broken meter glass:

NV 1/156

A bent handle:

NV 1/156

A damaged something (don't know yet what it is!):

NV 1/156

This is the label of the mystery part:

UPDATE: I have got several e-mails (thanks!) regarding this part. It is a double selenium rectifier: 2x 400 V / 100 mA

NV 1/156

Internally, it was dirty but complete:

NV 1/156

Here you have detailed views of the tubes in the amplifier section:

NV 1/156

And the ones in the distributing section:

NV 1/156


Repair works

First thing I needed to do was to try to repair the meter. I got it out of the unit and found it had also its casing broken:

NV 1/156

First, I made a replacement glass out of a 1mm thick Lexan plastic, as it can be cut with scissors and it is very convenient. Here you have the piece still with the protecting plastic layers:

NV 1/156

I glued the broken part, put the new glass and reassembled the unit. Here you have the results (note the 12.66 datecode):

NV 1/156

NV 1/156

The meter is back to life:

NV 1/156

I removed all tubes to (carefully) clean them:

NV 1/156

I glued also the mysterious thing and added some rubber band below it to prevent further breakings:

UPDATE: I have got several e-mails (thanks!) regarding this part. It is a double selenium rectifier: 2x 400 V / 100 mA

NV 1/156

This is the aspect of the refurbished meter on its place:

NV 1/156

And this is a general view of the top side, once tubes were back:

NV 1/156

The bottom of the unit was very clean and shows some plated areas, plenty of high quality components:

NV 1/156

I gave the unit a try, after measuring resistance in critical areas (power capacitors and such). Here you can see some glowing tubes:

NV 1/156

NV 1/156

First checks suggest unit works, with very close level in all outputs, but it is a bit deaf.

Here you have a view of the unit working with my R&S CMD-84 as signal generator and my Yaesu FT-212RH as the test receiver:

NV 1/156

Using the front panel meter, which has 8 measuring positions, I have found that tubes 2 to 7 are OK but tube 1, first amplifier (an EC80) is soft. Of course, I have none in my stock. So I need one... any taker ;-)?


To be continued...


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