ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Good evening,
I posted recently about another of these nice machines which had a different tape playback problem.
My problem with this one is a loss of voltage in the tape section - which requires a supply of +/-30 VDC, +/- 15 VDC, 30 VAC, and a regulated 5.7 VDC.
It is this last voltage (5.7 VDC) that drops to 1.4 V when the tape section is switched on, so either there is a supply failure or it is getting pulled down. All other voltages are correct, as indeed is the 5.7 VDC when the tape section is switched off.
Assuming a regulator board failure, I replaced 5D4 bridge rectifier and checked TR3 (though I am aware it could still fail under load). There is no sign of burning on the board and the fuse is intact.
This didn't bring an improvement, so I am now working on the assumption that something has failed upstream and is pulling the voltage down. Again, there are no signs of burnt traces or solder bridges.
It seemed sensible to snip the 5.7v supply to the tape deck itself to rule out a failure of the photo Darlington detector or pull-in switch. This brought no improvement so the problem is downstream of that.
So, I suspect the function switch board (25) on which there are not so many avenues for failure. I have replaced TR6 which controls the recording lamp. Again no difference.
My next step will be to disconnect the driver chip for the VU LEDs in case that is pulling the supply down but if anyone has any suggestions, I'd be really grateful for guidance, as I'm running out of ideas.
Or should I revisit the regulator board and replace TR3?
Cheers,
Simon
And the board with the tape function switches, VU meter and Autostop:
I should have added that the first thing I did before applying power to this unit was replace all the caps in the regulator board.
A (partly) shorted lamp would not be likely, so the problem must be somewhere else.Is 5TR3 good?Is the 10,2V from 5D4 reasonably correct when the tapedeck is powered on?Is the 15V alive and good when the tapedeck is powered on?What is the voltage across 5D6, when the tapedeck is powered on?
Martin
Martin,
Thanks for pointing me back down the tracks.
A quick check showed that the AC supply into the bridge rectifier was way off. A further check showed a probable path to ground through defective insulation on one of the transformer output conductors. I suspect this unit has spent some time in a damp shed.
Clearing this fault has restored normal voltage. Nice to see an illuminated operator panel tape counter and normal-ish function (slack belts). (I will need some of your fresh belts for this as soon as this importing-into-a-third-country business has settled down).
I have rebuilt the pull in switch board on this following a high energy short in this location; more on that later.