Md-350 Tascam professional minidisc recorder power supply issue

[JayJay2019]

Hello

I have the above minidisc with a strange fault

It powers on fine and stays powered on as long as you don’t play or record a minidisc

If play is selected the motor on the unit whirs and it starts playing fine. After around 2-3 mins the display shuts off, the motor whirs (not necessarily in this order) and the unit powers down. It sometimes resets and comes back on (display turns back on).

The power supply seems a linear design with a few windings on the secondary. Pin 16/17 on secondary go through a couple of diodes (see attachment)—- and through a filter capacitor (C701)(10000uF 16v). When working the filter capacitor has around 14v DC on it. This then connects to two 5v regulators (7805a). When the unit powers down upon pressing play the capacitor voltage drops to around 9v and the 5v regulated supply drops to around 1.8v.

I have no way of testing the capacitor as I only have a digital MM. The other supplies 7v regulated and 33v don’t drop. Issue only seems to be present on this line.

I disconnected the cable going to the main PCB which in turn connects to the playing unit and display. The supply remains constant on the filter capacitor and the two 7805s.

I’ve replaced a few 470uF capacitors around that area with no luck.
I don’t know what’s dragging the power supply down on the filter capacitor.

I also noticed the service manual suggests the filter capacitor (c701) should be a 4700/16v but the fitted one is 10000/16v? As far as I know this has never been replaced or changed before and the unit has been working fine before.

Any pointers would be handy

Thanks in advance

Jay.

[R_J]

Re: Md-350 Tascam professional minidisc recorder power supply issue

There are two 5 volt regulators, [5VD & 5VM] do both outputs drop to 1.8v or just one? 9v input should still supply 5v output from the regulator. It could be that C701 is bad, bridge it with another 4700/16v, that should keep the voltage at around 14v if c701 (10000µf) is weak.

[redwire]

Re: Md-350 Tascam professional minidisc recorder power supply issue

If a voltage regulator overheats it can go into thermal shutdown. If there are heatsinks, I would make sure the thermal grease is not old or the screw loose etc. I have seen Sony let them run really hot (no heatsinks) and then they go weak from age, the IC wimps out.
Or if the line voltage is really high.

[JayJay2019]

Re: Md-350 Tascam professional minidisc recorder power supply issue

Hi R_J and redwire. Thanks for your reply and sorry for the late reply. I was awaiting the delivery of the filter capacitor C701. I replaced the capacitor to no avail as the voltage on the plus pin of it was still dropping causing both 7805a to drop/switch off

I checked the voltage drop on the rectifying diodes before the filter capacitor (which I had already checked previously) and it seemed a little odd and was fluctuating. I removed D701 and D702 from the circuit and tested them both good except D701s forwarding voltage drop was a little high.

I took a chance and replaced D701 and it seems that's got it working. It used to switch off after a 2-3mims but has been playing solidly for 8 tracks of a few mins each!

The voltage on the positive pin of the filter capacitor has also stabilised at around 9.3v

Thanks a lot for your input and help

Merry Xmas and a happy new year to everyone

Cheers

Jay.

[frica]

Re: Md-350 Tascam professional minidisc recorder power supply issue

JayJay2019 congratulations for your repair. One question, what was the measured value for the forward voltage drop (Vf)? Consulting a datasheet for a 1N4003 diode, I see a graph where for 10mA, Vf = 0.7V, and for 200 mA, Vf = 0.8V.

Datasheet: https://html.alldatasheet.es/html-pd.../1/1N4003.html