Rob Sherwood's Comments

From: "Sherwood Engineering, Inc." <> Subject: Drake mods / info

Dear Thom,

I just read through most of your 60 pages of Drake info. It was interesting to see that much detail in one place. I will digest the rest of it later. If you are interested, I am noting some errors in your information that you may wish to correct in your manuscript. This info is from Rob Sherwood, NC0B, president of Sherwood Engineering, Inc. My company was started in 1974 with our first after market Drake accessory.

Howard Sartori is an excellent engineer, and his published articles delt with his solid tubes, and some mixer mods. He also was a dealer for me, Sherwood Engineering, for about 8 years. He did not, however, make any crystal filters. We sold the 16-pole filter you refer to, not Howard, W5DA. There were actually several iterations of the filter. The originals were 2.1 kHz wide, with shape factor of about 1.35, but were later pulled down to 1.8 to 1.9 kHz, and finally some 1.6 kHz were manufactured. My dealers at the height were Howard Sartori W5DA, Tim Duffy K3LR, Doc Sheller near Columbus Ohio, and Ingoimpex in Germany. Any of these people resold my filters / kits. (Sartori acutally never bought my kits, as he made his own without pc boards.)

The first IF filters in the C-Line is actually 1000 ohms, not 500 ohm. The transmitter had 500 ohm filters.

The BFO bleed seen as you turn the passband tuning is actually the third mixer signal on the same frequency as the fist IF. This 5645 kHz signal gets back into the first IF, and gets phase shifted in and out of phase as you electrically tune the 2nd IF crystal filter with the dualinjected 50 kHz BFO.

The real purpose of T-7C tap was to reduce the noise bandwidth of the third mixer in the 500 and 250 Hz positions. The higher Q in the tapped mode narrowed the bandwidth of this stage to about 1 kHz. You are correct on the cable difference, the white audio cable being much higher C than the later RG-174. There was extra loss in the narrow CW positions until this error was corrected by Drake somewhere in the 23,000 s.n. range.

Origianlly the filter insertion loss spec for the C-Line was 9 dB +1 -2 dB. (i.e. 8 db to 11 dB insertion loss) There were pads in the wider filters to bring them up to the loss of the narrow ones. Heath Dynamics and Piezo Technology followed this spec, but Network Sciences under Howard Fulk did not. Howard later sold N.S. to Darryl Kemper and his sons. I do not know if the tail-end Japanese filters were loss equalized or not.

The LM-383 audio amp, again, was a Sherwood design, not Howard Sartori. Howard, did however, design in the TL-442 for Drake mods, an IC that we used for many years in our PD-4 product detector mod kit. It was a great chip for its day, though overshadowed now by the NE-602A. The solid tubes were totally Sartori designs, which we resold for some years.

When Howard discontinued his business, Sherwood invented the MIX-4 solid state 3rd mixer replacement. It ran on 12 volts, however, not 150 as did the Sartori solid tubes. Howard later had to quit using the solid tube moniker, as it turned out to be trademarked by some other company.

The power supply that bootstraps the 1/2 wave rectified filament DC into the center tap of the 12 volt suppy is needed on old 16K to 18K radios. Those 12 volt supplys do not perform well with the 7812 and LM-383 as there is not enough input/output differential for the 7812. While only a small percentage of the hundreds of C-lines we have modified over the years need that boost, it is occasionally necessary. For that matter the set that is here now for mods had to have that mod added to it.

The later vintage mixer tube that replaced the 6HS6 is a 6EJ7, not a 6EH7. While Sartori's modified 3rd mixer injection was interesting, I feel the only real solution is our MIX-4 mod. It goes back to seperate inputs for signal and LO, runs FB on 500 MV diode-limited 5645 kHz LO injection, and doesn't go noisy (crackly) micorphonic. The hash that gets in via the power line into the floating grid is in deed awful; a terrible design on Drake's part. Light dimmers, acquarium heaters, motors, etc. really get into the 50 kHz floating grid. A good shielded isolation xfmr usually killed the crud, as did a ferro-resonant voltage regulating xfmr, but the real answer was to go back to T-6 feeding right into a tunted signal port that was a short circuit at 50 kHz.

The best answer to the buzz in power supply / audio was our PS-4 (like your 7812) and our AMP-4. It was not in the ground loop field as was the stock amp. The changed filter cap return grounding did help, but getting the amp right behind the headphone jack out of the chassis currents was the answer.

Yes the LM-383 can oscillate, but we have sold over 500 AMP-4s and over 300 SE-3 phase-locked detectors / amplfiers that use the same chip, and none of them oscillate with our compensation circuit. Without the right compensation, you are correct in saying that the chip can turn into a oscillator drawing upwards of 2 amps!

So that's it. Hope you do not take all this info as an insult. It is simply information from being in the Drake mod biz for 23 years.

Check out our WEB page It is new, and is still being built. My E-Mail address is

Sherwood also originated the 600 Hz first IF cw mod, the CF-600/6. It is still available today. Unfortunately we did not develop a good PC board/relay switching kit until Fox Tango came out with their GUF-2. I was engineer for KOA radio at the time, and Sherwood Engineering was a sideline business until 1987.

Our first product was the CF-125/8 2nd IF crystal filter. Ours was typically 115 Hz by 270 Hz, with a 8 dB insertion loss. The Japanese copies were 135 Hz, later 150 Hz, and near the end as wide as 175 Hz at -6 dB.

Enough trivia.


Rob Sherwood

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