51J4 Adventures

Not terribly long ago I picked up a decent 51J4 on eBay, with 4:1 tuning and "some modifications" of unknown purpose. To my surprise, the radio worked reasonably well, and the mods turned out to be a 6BE6 product detector conversion of the BFO and Lankford fast attack slow release AGC mods from HSN.

Unfortunately, the modifications were not done particularly well. I don't tolerate slop well, and decided to figure out exactly what was in there and clean up the mess.

The product detector was a bit different from Lankford's, and I suspect it was perhaps the earlier Orr or Scherer update (?) or maybe just a generic. I heated up the Hakko 808 and ripped all of it out, then replaced everything with the HSN #26/28 mods following the physical layout given by Lankford. A check of the 0.1 and 20uF bathtub caps during the process showed that ALL of them had far too much leakage, so I also replaced them all and obtained a new plug-in filter can from Hayseed Hamfest. I also replaced four of the heterodyne crystals with new crystals from ICM -- it's nice to have zero actually be zero on all of the bands now. The worst two bands are only out about a Khz, and those aren't ham-band related ranges anyway. I also added an internal voltage doubler off the 6.3v filament line to run the muting relay.

I did end up having to tweak some of the values from the Lankford SSB mods. I had to modify the voltage divider network feeding IF to the product detector to better equalize AM and SSB audio volume levels. I also reduced the value of the resistor in series with the AGC diode from 22K to 2.2K to decrease the attack time a bit further to eliminate popping on strong signals, but have not observed (at least audibly) the AGC overshoot issues Lankford referred to when he picked the value of 22k. SSB is smooth and (now) pop free with no twiddling of the RF gain control required.

I have to say that the changes increase the utility of the 51J4 greatly. It works seamlessly on AM (with an SE-3 doing detection duties) and SSB with the updated modifications. I found one of the 9.4 Khz F-series mechanical filters that were made available through the Collins list a few years ago, and that improves AM substantially compared to the stock 6 Khz filter for times when interference is minimal.

Left to finish up is to modify the mechanical filter assembly to get B+ off the filter input windings, and a complete alignment touch-up. Although I've done it before, I've had various inputs on whether it's really necessary to remove B+ from B-series filters along the lines of what is typically done with the R-390A/75A4. I figure given their age and replacement prices, it can't hurt and is worth the time required. It also has the advantage of eliminating the nasty "pop" that occurs when switching filters.

There will be several other radios around here going into full-time retirement once this one becomes the main vintage station receiver paired with a Globe Champ 300 and CE-100V. And I'll be down from two rooms full of stuff to this 51J4, an SP-600 for band scanning, a GPR-90/GSB-1 for it's sheer good looks, and an SX-115 just because.

Grant NQ5T