I had a bit if an issue with my TU-719 for a few days. I had unwisely so it seems, reconnected it to my AU-919 with the tuner powered on, I can only think I might have briefly shorted an RCA inner to ground? - having trouble understanding how that damaged the tuner. Afterwards only the dial lights were on and nothing else. 
So I get it on the bench and discover 2 x 315mA fuses for the common AC feed to the +12V, -12V and +5V are blown. I check for short circuits and can't find any, so I disconnect the respective PSU outputs as above and that stops the fuses blowing. I measure these PSU outputs and they are all in spec' so I reconnect the outputs of the supplies to the tuner one at a time, first -12V = ok, then +5v = ok then +12V - and we are back to fuses blowing again. I have the tuner on DBT during this testing to minimise any further damage but the lowest wattage bulb I have is 60watts - not much use for protecting a tuner as the current draw is so low.
I begin isolating various parts of the tuner fed with +12V and reconnecting one at a time - finally the culprit is detected as the F-3000 board which contains the MSM5540 chip, the display driver logic and so on. Looking at the schematic there is nothing directly connected to the +12V supply which can draw enough current to blow fuses.... except IC03, NJM4558 OP amp. I desolder pin 8 on the IC and even with +12v fed to the F-3000 board the fuses remain intact. So I order up a replacement 4558 (and a socket
), they arrived today, socket and IC installed and all is good once more.
Note to self:- 'don't muck about with inputs on powered up gear'.
I hope the above might be instructive for anyone experiencing a similar problem, divide and conquer, do one thing at a time, work methodically, and check things thoroughly between fault finding stages.
So I get it on the bench and discover 2 x 315mA fuses for the common AC feed to the +12V, -12V and +5V are blown. I check for short circuits and can't find any, so I disconnect the respective PSU outputs as above and that stops the fuses blowing. I measure these PSU outputs and they are all in spec' so I reconnect the outputs of the supplies to the tuner one at a time, first -12V = ok, then +5v = ok then +12V - and we are back to fuses blowing again. I have the tuner on DBT during this testing to minimise any further damage but the lowest wattage bulb I have is 60watts - not much use for protecting a tuner as the current draw is so low.
I begin isolating various parts of the tuner fed with +12V and reconnecting one at a time - finally the culprit is detected as the F-3000 board which contains the MSM5540 chip, the display driver logic and so on. Looking at the schematic there is nothing directly connected to the +12V supply which can draw enough current to blow fuses.... except IC03, NJM4558 OP amp. I desolder pin 8 on the IC and even with +12v fed to the F-3000 board the fuses remain intact. So I order up a replacement 4558 (and a socket
), they arrived today, socket and IC installed and all is good once more.Note to self:- 'don't muck about with inputs on powered up gear'.
I hope the above might be instructive for anyone experiencing a similar problem, divide and conquer, do one thing at a time, work methodically, and check things thoroughly between fault finding stages.
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