This is a Trio TS-940S

which has developed intermittent deafness

 This rig is very heavy as it includes a substantial power supply transformer. I powered it up and checked its sensitivity using my Wavetek signal generator.

It was rapidly clear that there was a problem because to produce a reading of S9 plus 40dB on the meter required 1 volt input at any frequency and operating the ATT switch which sets 0, 10, 20 and 30dB of attenuation resulted in odd results. Sometimes the lower level of attenuation gave more attenuation then higher settings. I downloaded the service manual and found that the RF module carries a pair of relays to set attenuation. One relay gives 10dB and the second 20dB and the pair 30dB.


 Relays are the main weakness in circuit boards used for lifts and escalators so a poor relay is not surprising. The chief weakness is poor contact resistance due to low wetting current and low wetting current is definitely to be found in the front end of a receiver. I disconnected a score of cables and umpteen screws and extracted the RF circuit board. On it are three small Panasonic relays type DS1-M DC12V. I measured the normally closed contacts and the two used for setting attenuation level had varying resistance. You can get a similar relay type DS1E-M-DC12 for around £5 each but I tried cleaning the contacts as the original relays have detachable covers. After reassembly the attenuation settings were reasonable but not perfect. Probably after some use this will improve as relay contacts of this type are self-cleaning.

I tried the set later and it was deaf again, so out with the board and in with three new relays. That cured the problem...


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