Halli. SX28 a 66 year old fault - UK Vintage Radio Repair and Restoration Discussion Forum

ms660 · 12th Aug 2011, 7:25 pm

Hello, I hope this is of some use to some one.

Amongst other things, before I did a replacement C's & R's job on this receiver, the AGC was not working properly and the noise limiter hardly worked at all, after all the work I did everything seemed ok and although the noise limiter was working a lot better I had a problem in so far as when it was switched on but the pot still at minimum (ie limiter diodes not at the critical bias) the audio on BFO resolved SSB and AM broadcast was clipping slightly..enough to be noticable particularly on AM broadcast stations.
After spending most of the day faffing about with it I finally cracked the problem.
One half of the limiter diode (6H6) had been wired incorrectly and it would appear very much like it was done during the receivers production.
For those that have one of these receivers either now or in the future with similar symptoms (the SX28 with antenna trim on bands 1&2) it would be worth while checking it out as it could be a stock fault on a certain production run.

The details are as follows:

There is the inner of a screened lead that should run from the junction of R24, R25, R27 & C50 via one half of the noise limiter switch on the back of its pot to pin 8 (k1) of V10 (6H6) in this receiver it was connected to pin 5 (a1) There is a lead that should run from the junction of R70 & C53 to pin 5 (a1) of V10, in this receiver it was connected to pin 8 (k1)

Putting back the leads where they should be cured the audio clipping at the minimum limiter setting. I got the missis to press the ignighter on the gas stove while I checked the limiter action and it now seems spot on ie: spikes clipped before signal clips....magic.

Lawrence.

12th Aug 2011, 7:25 pm	#1
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Halli. SX28 a 66 year old fault Hello, I hope this is of some use to some one. Amongst other things, before I did a replacement C's & R's job on this receiver, the AGC was not working properly and the noise limiter hardly worked at all, after all the work I did everything seemed ok and although the noise limiter was working a lot better I had a problem in so far as when it was switched on but the pot still at minimum (ie limiter diodes not at the critical bias) the audio on BFO resolved SSB and AM broadcast was clipping slightly..enough to be noticable particularly on AM broadcast stations. After spending most of the day faffing about with it I finally cracked the problem. One half of the limiter diode (6H6) had been wired incorrectly and it would appear very much like it was done during the receivers production. For those that have one of these receivers either now or in the future with similar symptoms (the SX28 with antenna trim on bands 1&2) it would be worth while checking it out as it could be a stock fault on a certain production run. The details are as follows: There is the inner of a screened lead that should run from the junction of R24, R25, R27 & C50 via one half of the noise limiter switch on the back of its pot to pin 8 (k1) of V10 (6H6) in this receiver it was connected to pin 5 (a1) There is a lead that should run from the junction of R70 & C53 to pin 5 (a1) of V10, in this receiver it was connected to pin 8 (k1) Putting back the leads where they should be cured the audio clipping at the minimum limiter setting. I got the missis to press the ignighter on the gas stove while I checked the limiter action and it now seems spot on ie: spikes clipped before signal clips....magic. Lawrence.