51J4 Curious Meter

[nq5t]

I picked up a 51J4 (3 filters, good SSB/AGC mods in place, 4:1 tuning, 1/2 KHz end to end on the PTO -- how could I complain!) that works really well.  Didn't notice until I'd been staring at it for a while that the meter is a replacement -- with an "S" meter scale and a 0-400ma current scale. Probably from some unknown Collins transceiver (?).  The meter bezel is also a very good black, rather than the usual reddish (in the light) tint.  The S-meter scale is quite accurate.  I haven't put a cal'd generator on it yet, but it matches my K3 readings on comparable signals quite well.

I thought I might look for an original replacement, but I think I'll just leave this one in there.  Wonder if anyone recognizes the source?

[nq5t]

I think I've answered my own question.  I realized the scale is from a KWM-2.  Wrong housing, but it's probably either an original or substitute round housing with a KWM-2 scale mounted in it.

So that done, I'm off to reverse engineer the SSB/AGC mod to see who's version it is.  It's very smooth with no audible overshoot.  There's occasionally a very slight "tick" on attack (not terribly objectionable), so the attack time might need some attention ...

And just touch up the alignment in general before buttoning it up and adding it into the operating position.

[ka4koe]

On mine, I figured the 50 DB mark was around S9, assuming 6 dB per S unit, around mid scale. Need to get it on the bench and calibrate it. Not sure if I am going to mod it for SSB or not as its used only for AM.

[Rob K2CU]

You should also check the performance of the crystal filter. It can be quite effective as a notch filter.

You will want to check that whatecer your S meter reads, it is pretty close across the various bands. If S9 or normally 50dB is 50 or 100uV then is it the dame on 10 as it is on 75 for example.

Mine has the Bill Orr SSB product detector mod. Basically, a 6BE6 is substituted for the BFO tube and reconfigured as an convertor from 500Kc to audio. works fairly well, and better than original for SSB/CW. You may want to mod the filter unit to take HV DC off the filters. I used a 2.5mH RFC and .001 coupling cap to shunt feed DC to the plate and get the DC off the filter input. It also reduces the crash when changing filters. A separate 1Meg on grid of seocnd tube instead of using filter secondaries gets rid of crash too.

[nq5t]

Thanks.   I haven't had time to puzzle through the SSB mod or check on other things, such as getting HV off the mech filters.  It's quite possible the latter is done, but it's on the list if it isn't.  It's pretty clear that someone did take some time with the radio, so we'll see.  The other 51J4 and couple of R388's I've had needed a lot of work - fortunately, unless something blows up, this one doesn't.

One other project to finish up before I can clear the bench to dig into it.