I've recently picked up one of each of the above models. They are in generally good cosmetic condition, but all three are non-starters. No power, no response to buttons, no LCD display (on the tuner models). I've seen other threads suggesting that this is not uncommon on this model series, but I never saw a resolution. I already have good experience in Walkman repair / restoration, but the WM-70x series are new to me. Before I dive headlong into troubleshooting and electrical diagnostics, does anyone happen to know if there is a common / typical fault with these models that makes them play dead? I've only opened the F701C so far. Confirmed that power is at least getting to the main board, but current draw is practically zero (less than 1mA). It doesn't appear to contain any electrolytic caps whatsoever, so that rules out the usual prime suspect! Any pointers appreciated!
Hi! Did you see this post by Valentin? https://stereo2go.com/forums/thread...orders-common-problems.7535/page-2#post-59825
Thanks. No, I hadn't seen that, but nothing in there that would explain the complete lack of response from all three of these units. They don't respond to the remote control either, so I'm pretty sure it's not simply bad contacts in the logic control buttons. Here's the inside of the F701C. Some evidence of slight corrosion from battery leakage, but I've checked continuity of a few of the worst-looking PCB traces and they all seem intact so far. I'll keep looking!
If you ever find a solution to this, do let me know. So far, I have encountered this issue only with several F707, and it still has me stumped. I have been able to trace it to the DC-DC converter (the copper box in the lower right). Have a look at the voltages going to and from the converter when idle, when pressing a button, and when under load (the flex cable). Mine is missing the 3,5 and 2,4V rails. When pressing a button, the 3,5V rail is wildly fluctuating up to 2,6V or so, but never reaching 3,5V. Very rarely, if you keep pressing a button, it will eventually turn on, but other functions will remain missing. So far I have been unable to figure out which part on the folded converter board has failed (as I presume). I am considering replacing several key components by trial-and-error. I suspect some transistors, but I am really not sure. Also have a look here: https://stereo2go.com/forums/threads/sony-wm-f707-restoration-dc-dc-converter-issue.9687/#post-78035
@Fulltimehuman, thanks, yes your thread was one of the ones I'd seen when searching for info about this problem. I hoped you'd found the solution in the meantime! I have done some further dismantling of the F701C. Discovered a flex PCB that I hadn't originally spotted, tightly folded and tucked in around the battery compartment, linking the motor and DC-DC to the main board. I thought it was Game Over when I saw the state of this - it's ravaged from battery leakage... (photos below are after cleaning, it looked even worse to begin with). But amazingly it turns out that all nine traces are still intact, and there are no shorts - so I've cleaned it thoroughly, coated it in lacquer*, re-assembled everything and re-checked continuity - but still no life from the unit. I've looked at the voltages on the DC-DC outputs and they are 1.5V and about 1.3V. They don't respond to any button presses. I can see from a quick glance at the schematic in the service manual that DDC_CTL will enable the converter when pulled low. So I have tried shorting this line to ground, but there's still no movement of the output rail voltages. This seems to confirm the DC-DC converter is at fault. I will have a go at opening this next. This is turning into a bit of a mission already... * my wife's nail polish
@nutronic, how did you figure out that the CTL line needs to be pulled low to turn on the DCDC-converter? Is Q801 doing the pulling? I have been wondering what this line does for a while, but that would be logical. (Surprise, I'm just a hobbyist!). I'll have to check with mine to see if there's corresponding behavior. (I also have an F701c that's still powering on. I was unaware that it also has a DCDC-converter that produces three output rails. Maybe I'll switch this one in or check for the proper behavior on this one. I have it on the bench and should be able to do this quickly.) So, are you missing both the 2.4V and 3.5V lines? That your ICs would obviously having issues. The DCDC-converter is fairly easy to open – easier than you might think. (It's just clamped-shut, not welded.) On the F707 the box contains a flexible board that is folded up multiple times (disregard the red circle): In the picture are three layers of the PCB that get folded on top of each other and then stuffed into the copper box. I tested all the components in the box, but – possibly due to incompetence of the tester – my results were inconclusive. Two diodes may be faulty (O.L. in both directions) and one transistor might also be faulty. I am planning to replace these and possibly other components, but I still have to order the parts. This might take a while. That's where I am at the moment. I shelved the project until the parts arrive, as I am working on other devices in the meantime. However, the F707 is a true beauty and I'd love to get it working again.
@Fulltimehuman, DDC_CTL drives the base of Q801 through a current-limiting resistor R801, switching the Battery+ supply to part of the DDC circuit. Being a PNP device the base needs to be pulled negative with respect to the emitter to turn it on. It's a transistor on the control board (Q710 in the F701C / Q706 in the F707) which pulls the line low when the unit wants to switch on the DC-DC Converter. Yes, I am missing the 2.4V and 3.5V lines. Thanks for the tip on opening the DDC. I've got this open now. The folded PCB is really cool - amazing how much circuitry they managed to squeeze into that tiny box! Sadly though I think my F701C is irreparable. The transformer T801 is toast. Open-circuit on pins 4 and 6 (the bottom two pins in the photo above). There are no specs given in the SM for this part, so I don't fancy my chances finding a replacement. Would be interested to know if you have continuity between those pins on your failed unit? When I get some more time I will start taking a look at my F707 and R707 as well. Hope it isn't the same fault on all three, and that I can get at least one of them working!
Having said that... I thought I'd try dissecting the transformer to find out how it's failed. There is an outer sleeve (made of ferrite I think), which popped off with a bit of leverage, exposing the windings. I pulled the wire off one the pins that was showing open-circuit, and unwound the coil until I found the break. It looks like the windings have shorted and overheated. But then I wondered if it might be repairable? I have some inductor wire of about the right gauge, so I measured out the same length as the coil I'd removed, and re-wound the coil (which worked out as 17 turns) and soldered it back onto the terminals. Glued the ferrite sleeve back on, reassembled everything, and - happy days - it now powers up!! The radio section now works, and the motor spins up - haven't tried the tape section yet, the mechanism needs some attention. This gives me some hope for the other two units now. I'm going to try and get the rest of the F701C working first...
Great news! After reading your post, I was going to suggest you do exactly that. 17 windings seems manageable. Unfortunately, this does not seem to be the issue with mine. I have continuity through both of the lower two and upper three pins, but no continuity across – as it should be. However, since I see fluctuating voltages on the scope, maybe mine is in the process of failing and also needs to be reworked? I may have to open up mine, too. I was always guessing it might be an overheating issue. What's the pink wire for in your picture? (Edit: Oh, it's the ground to shield, right?) Did you manage to test the converter like this out of the full circuit? While you have it open, would you mind quickly beeping out some components as comparison for me? It would be great if you could give me some values for D804, D801 and Q803 and Q801. I am not competent enough to make absolutely sure I can test them properly here.
I'd already reassembled the DDC before my last post, sorry. Considering that you have at least some activity on the output rails I'd suggest it's very unlikely that any of the semiconductors have failed. But the transformer perhaps? I took a close look at the wire I removed from the transformer in mine. A couple of turns from one end it changes abruptly from light copper coloured to dark and brittle, so only a short section of it has overheated. The only possible explanation is insulation breakdown and a short to the opposite winding. I guess it's conceivable that if the wire in these transformers is prone to breakdown of the insulation with age, then perhaps you have a shorted turn within one of the coils (rather than between the two). This would severely impair the DDC performance but would be very difficult to detect from simple resistance measurements. Since you said you also have a F701C, swapping the DDC would be the easiest way to rule this out. To clarify, the 17 turns was only for one winding (the one without a centre-tap), I didn't touch the other. To confuse matters there is a mistake in the F701C service manual schematic, which shows the centre-tap on the wrong winding. It is correct in the F707 service manual. Also to answer your other question, yes I did test it out of circuit, with 1.5V supplied to the input and DDC_CTL grounded. I got about 3.5V and 2.6V from the outputs (with no load). The green LED on the board was faintly pulsating.* My F701C is now working, after finding and fixing a whole catalogue of other faults - but that's another story! I'll update this thread again as and when I get around to looking at my other WM-x707 units... * (For info: the green LED in this circuit looks like it's being used as a crude voltage reference, taking advantage of its well-defined turn-on voltage which is probably around 2V - like a zener diode, but operating in forward bias instead of reverse bias. The pulsating I noticed was probably due to the regulation control loop being unstable with no load on the converter output. This is not uncommon for switch-mode converters running at very low load currents.)
Thanks for the hint! I hooked up my converter to the scope tonight and took it for a spin. Unfortunately, I do not seem to be any wiser–yet. It turns out my converter is apparently working. However, while the 2.5V line produces roughly the correct 2.5V, it shows sharp peaks of up to 3.1V every 700ms or so, apparently coinciding with the LED's pulses. This seems to be the pulsating rail I saw earlier. The 3.5V line in fact produces a fairly stable 4.4V–which seems too high, though. If this isn't normal under no load, I guess almost a volt more could have caused some damage. Not sure if I should proceed with taking this converter apart. I may have to put it all back together and look for the root cause elsewhere.
