[Hammarlund] SP-600 JX17 rewiring

[James A. (Andy) Moorer]

This is fairly common. It is a real pain. I've had to do this on several receivers.

Bare wires are bad, of course. If you have the time, they should all be replaced. Teflon is fine, but you want to be a bit carefull with the long, twisted pairs that go from T1 into the IF strip - you want to use wires with relatively thick insulation to keep the capacitance of the twisted pair down as much as possible. These two twisted pairs are clamped onto the sheet metal of the RF cage to keep the radiation and stray pickup down as much as possible without adding a lot of capacitance. The highest frequency there is 3.955 MHz which is still not very high.

I would advise against putting in shielded wire where there was none before. In the IF strip, everything is 455 kHz, which has a wavelength of something over 2000 feet. Five inches of wire will not radiate (or pick up) enough to make any difference, plus the shielding adds capacitance.

The BFO coils (L44) often go bad for some reason. I have had several radios go by with that problem. Not sure why.

Just my opinion,
Hope this helps.

James A. (Andy) Moorer
www.jamminpower.com

  ----- Original Message ----- 


  Has anyone had the problem where the cloth covered wiring running from the front panel, into the under-deck has cracked, revealing bare conductors?

  How about the lack of a bushing on the chassis penetrations from the front panel to the same area on the right and left? I have unlaced the harness in the area of the front panel and threaded heat-shrink tubing on each conductor. But now I see that a significant number of cloth covered wires in the under-chassis are brittle with cracks in the insulation.

  I have seriously considered replacing all of the wire on the under-chassis with teflon coated wire. If I go that far I would install shielded wiring for the inter-stage wiring and to the bandwidth switch assembly.

  The previous restorer replaced all of the capacitor using the "clip the leads, form a loop and loop in a new component" technique. The soldering technique is sloppy with many solder connections having the granulated look (insufficient heat or rosin, or soldering to a conformally coated connection). I prefer teflon tubing on all component leads.

  The radio works fine except for no BFO operation (I will figure that out). I plan on making several component value changes to improve audio linearity and frequency response.

  I have already redone the power supply section and knocked the ripple on the B+ line down to around 15 mVAC. Thank goodness the turret modules and small long, narrow sub-deck where the antenna connection attaches to has had it's caps changed.

  Any opinions or suggestions out there? Has anyone accomplished a lower deck rewire?

  Tisha

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