[Collins] Help on troubleshoodint a 51S-1

[Dr. Gerald N. Johnson]

On Wed, 2007-09-12 at 18:05 -0600, DW Holtman wrote:
> Hello,
> 
> I recently acquired a 51S-1. I spent a good deal of  time cleaning the complete receiver and treating the contacts, switches etc. I have a problem now that I was wondering if anyone had seen before. 
> 
> AM works great. USB and LSB have a lot of distortion. Signals on the air can be heard, but it is raspy and very difficult to understand. Looking at the schematic, there is only the Product Detector and SSB/CW Pre-Amp that takes a different path from the AM signal. 
> 
It may be that its of the old school design, where one runs the audio
gain up for SSB/CW and backs off on the RF gain. Where the AGC doesn't
work well on SSB/CW because of the BFO signal reaching the AGC detector.

> I ordered the C162 and some 1N34A diodes. Has anyone had problems with old Germanium Diodes breaking down? The Cap could have dried up. I ordered all of the electrolytic caps in the receiver from Digi-Key.
> 
Germanium diodes were never great for leakage or stability.
Electrolytics don't age well, especially when not powered but they don't
last forever when powered. If there are any molded oil paper capacitors,
they were probably bad when installed and more leaky now, bad news on
AGC circuits.

> What are the thoughts on replacing the old rectifier diodes with something newer? This is the first receiver that I have worked on for a couple of years with a solid state product detector. I'm not going to modify it, but I bet a product detector/mixer from Mini Circuits in the TUF-1 family would improve the performance. 
> 
Impedance matching would be the problem. Germanium diodes in place of
6AL5 circuits look like fairly high impedance compared to the MCL mixers
that would have about 50 ohms impedance on all ports. It probably could
be matched with transformer links or emitter followers but its a major
redesign.

> Any suggestions or ideas on similar problems with this receiver would be greatly appreciated. Also any suggestions on alignment. I have looked at the manual and it is long and involved.
> 
> Best,
> DW Holtman
> WB7SSN


-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer