I am trying to repair a WM-EX655. I have cleaned out the old belt goo as usual and replaced with a belt from FixYourAudio. When powering the walkman from external DC on my bench supply it performs superbly with normal current draw during playback - however when I first connect it to power (read: not when I first press play but just simply connecting active DC power) it draws 350-370mA @ 1.45v (0.5W) for about 2 seconds and then back to zero - once it does this its ready to respond to any button presses. No motor movement or anything mechanical is happening during this step - it is simply drawing power to the chips and for some unknown reason it is surging current for a brief moment. This startup surge of current draw is too high for the batteries to supply - gumstick won't work at all, and AA sometimes will work but playback is wobbly and weak. I looked at the schematic scanning for any capacitors on the DC input that might be going bad but I don't see anything that would be an obvious "first thing to check" - has anyone else experienced a similar issue or know what I should check? I will add, it behaves this way even with no belt installed (i.e. no mechanical resistance or anything) - it seems to be a logic or DC-DC power issue on startup only. The only physical damage to the unit is corroded gumstick negative terminal, but I have bypassed this with alligator clips to verify it is not related to the problem.
Something isn't clear here: you stated this current spike only happens for 2 seconds at startup and then current draw goes back to normal (normal means 55mA in playback according to manual). Given this, how is the playback "weak and wobbly" if the draw returns back to normal (and hence the AA is able to supply the necessary current) ? There is a DC-DC convertor (switched capacitor type) generating a 2.5V rail, but this is only powering LEDs (AVLS, Dolby, MB, Groove, BATT and reel photodetector). Meaning the unit will work without it (looping between FWD and REV because no reel rotation signal). You can remove Q502 to see if it makes a difference. How did you connect the alligator clips ? Have you checked there is no voltage drop (measured at the PCB) during this initial 2 seconds ? High resistance of terminals is a common problem on such units.
Valentin seems right on track. The DC-DC converters can have caps in there that can go bad or they can go bad anywhere in the deck- Electrolytic caps that are old can leak and instead of high charge and infinite impedance they will conduct. You describe a charge up condition but if the function is normal otherwise maybe that is how these are. I would say though that the oiling of the bearing of flywheel and reel tables have not been addressed in many years- this can add to the stress on the motor and higher current draw. Sometimes that negative terminal has other connections that you do not see that you do not address?
I was able to verify this current draw is normal for this model (tested another one that was working fine). I think the issue was that the walkman wasn't able to draw enough current the way I had it clipped to the battery as a test, and the battery spring contacts were both too far gone from corrosion to work properly. I replaced them with cut springs from a pen and now its working fine powered by a gumstick battery.
