Big John
pfm Member
Today I went to Malvern to see Mark the Ming (Mark Mainwaring White ) who is Ming Da UK. Having played with valves since he was a boy Mark also repairs any make of valve equipment. The reason for the trip was my Leak Troughline Stereo receiver had stopped working properly no sound & the magic eye (EM84) did not move when tuning. This Troughline stereo has an external Tim de Paravicini MPX decoder because the built in decoder is poor indeed it has been disconnected
It transpired that a copper wire inductor had moved causing it to short out, the trip had already been anticipated as the sensitivity of the tuner had been dropping off over time & the AFC & best reception were out of sync.
I had read on line that the FM discriminator T4 in the Troughline containing germanium OA79 diodes which could be to blame. initially they appeared to be made of unobtainium perseverance prevailed, I found some @ £8 plus postage some on fleabay that were used and out of spec according to quoted figures, I then saw an add for Littlediode of Malden Greater London on the bay two cost under £8 posted I asked if I could have a matched pair I therefore went to Mark prepared.
Mark found as I watched that both OA79 germanium diodes had drifted more than 25% off so had the associated resistors. A bit of soldering later then connecting the required oscilloscope and signal generator together with judicious tweaking of the 3 double ended potentiometers.
Mark checked all other caps & resistors
The end result was absolutely stunning to listen to.
below is a photo of the discriminator T4 from Keith Snook site showing diodes replaced
It transpired that a copper wire inductor had moved causing it to short out, the trip had already been anticipated as the sensitivity of the tuner had been dropping off over time & the AFC & best reception were out of sync.
I had read on line that the FM discriminator T4 in the Troughline containing germanium OA79 diodes which could be to blame. initially they appeared to be made of unobtainium perseverance prevailed, I found some @ £8 plus postage some on fleabay that were used and out of spec according to quoted figures, I then saw an add for Littlediode of Malden Greater London on the bay two cost under £8 posted I asked if I could have a matched pair I therefore went to Mark prepared.
Mark found as I watched that both OA79 germanium diodes had drifted more than 25% off so had the associated resistors. A bit of soldering later then connecting the required oscilloscope and signal generator together with judicious tweaking of the 3 double ended potentiometers.
Mark checked all other caps & resistors
The end result was absolutely stunning to listen to.
below is a photo of the discriminator T4 from Keith Snook site showing diodes replaced
