Blank disk problem with MDS-JB940

[Eeyore]

Hi all,

I have just bought a JB940 on ebay. The unit is able to play back minidiscs recorded with an RH1 fine, but unfortunately it suffers the not so uncommon, as I have learned, problem of blank disks. That is, the unit seems to record fine from an analogue source, the sound level bars move, tracks are being made and all that. But whenever I try to play a track just recorded, the only thing I hear is the drive moving forever. Ejecting the disc and inserting it again to the JB940 results in the display showing the "blank disc" message. It also is blank for the RH1.

Moreover, when I insert a disc with some tracks from the RH1 (not in HiMD, of course) into the JB940 and do any kind of editing with it, like changing names, recording another track, etc., the disc is completely blank after being ejected.

I am resolved to return it to the seller, but before doing so, I just wanted to make sure there is not any relatively easy way for me to restore recording capability. In particular, can somebody say, if that repair guide for blank disc problems with the MZ-N1 (a mobile unit, I suppose), which is sticky in this forum, does in any way apply to the JB940 deck?

Thanks very much to anyone trying to be helpful!

Regards, Peter

[sfbp]

I have three words for you: "Busted overwrite head".

Sorry.

You can try and diagnose it yourself but you will probably need a laser power meter. Alternatively come to some agreement with the vendor, keep the unit and use it or something else for spare parts to rebuild whichever is better.

[Eeyore]

I have three words for you: "Busted overwrite head".

Sorry.

You can try and diagnose it yourself but you will probably need a laser power meter. Alternatively come to some agreement with the vendor, keep the unit and use it or something else for spare parts to rebuild whichever is better.

Ok, thanks, Guru, for your answer. So which do you think is more likely: Is the overwrite head physically broken, or has it "just" suffered mechanical shock which distorted the positioning mechanism. I read somewhere on here that the head should be in contact (or as close as possible) with the disc. Maybe it's is "just" bent out of its way?

I did some research prior to buying this deck. As I had to decide between a Yamaha and a Sony one. It was said about the Yamaha, that it has the same laser drive as Sharp's which is prone to fried laser diodes. That's why I turned to Sony. And when I saw the high-end JB940 at a very competitive price the winning bid was quickly placed. Now it seems Sony units also have, albeit with the magnetic head, different problems.

So is this a very common problem. Is it even advisable then to by any used Sony deck, considering the threatening prospect of your recorder suddenly being demoted to a simple player?

Kind regards,

Peter

[sfbp]

For portables, at least, the commonest fix is to re-solder the ribbon cable where it broke.

However on decks, the real question is whether the problem is mechanical or electrical. Ie is it a matter of joining two wires or is something bent?

A positioning problem may be the result of something else. For example on the portables, the overwrite head can get locked into a place where the first inserted MD will quite reliably break it off, by a simple series of actions that include removing the battery when the drive is locked for writing. Sigh. I actually acquired a unit (NH700) that was this way, otherwise brand new.

I think the decks are highly reliable, but it could be some other problem has locked the overwrite head in the wrong place. Such as a microswitch that detects disk insertion being jammed or electrically fried.

The only one that ever died on me was the MDS-JE510 and it was notorious for exactly this fault. I think you;ve gotten unlucky.

[Eeyore]

Hi sfbp,

thanks for your attention. There is one minor detail, that I would like to mention, still.

I have already mentioned that during my test I had also used a partially recorded disc with the JB940, had done some editing and recording operations on that disc, and that upon ejecting and re-inserting it was blank. However, I noticed that before ejecting the disc, new tracks (created by the JB940) could indeed be played back immediately after recording (leaving rec by pushing stop and pushing play).

Does this give a clue as to what exactly could be wrong with the write head?

Thanks again, Peter

[sfbp]

Nothing different. As you have worked out, overwrite does not happen all that often.

In the situation you describe, the TOC simply has not been written. It's the overwrite head that helps to write the TOC. Up until you push eject, the TOC is in memory and the deck has enough information to read and write stuff on the disk without it (the TOC) getting written. Worth bearing in mind (power loss usually loses everything with no flush). Switching off the deck will flush it too.

[jim.hoggarth]

Peter

An interesting sequence of events. Every case of this fault in my experience is caused by a busted overwrite head. Busted, as in physically broken, ripped off, or bent. Now I have always attributed this to damage caused when the disc is inserted or removed, perhaps inserted incorrectly, or forced out with an 'implement' because the eject motor has stalled, or perhaps caused by a bent disc shutter catching the head on the way in/out, or the incorrect use of a head cleaning disc..

But, here's the thing: you reckoned that tracks were laid down as they could be subsequently replayed, but the TOC was not written just prior to ejecting (hence the blank disc). Which leads me to believe the OWH was functioning perfectly while the disc was already in, and failed with that disc still in the drive. Hence, not attributable to physical damage. So the only conclusions I can come to is that the head is NOT damaged as is normally the case, but is probably one of these:

i) the write head cable failing and going open circuit, perhaps when at the home track where the TOC is written (I have never known this to happen yet - the write head is not the same design as in the portables)

ii) the write head coil has burned out due to a fault in the driver transistors

iii) a piece of 'crud' has come adrift from the disc and stuck to the underside of the write head.

iv) it's all a government conspiracy

In all cases, it's worth opening the machine up just to have a look. If the overwrite head is physically busted it may be obvious to you. If you are careful you may be able to clean the underside of the head with a Q-Tip soaked in methylated spirits. BUT BE VERY CAREFUL NOT TO BEND THE HEAD UPWARDS OR LATERALLY. And if there's a radio bug in there, you know case iv applies.

Jim

PS - Stephen, on different subject, the NH700. At least 50% of the ones I have seen in the workshop have the overwrite head bent backwards by 180 degrees, obviously cause by catching on an inserted disc. In theory a disc can't be inserted when the OWH is loaded, ie down, as the lid lock lever is also in place (although I admit it is not very strong on the NH700). So in practice the battery must be re-inserted to release the lid lock, and lift the head back out of harms way. I am mystified as to how customers manage to ram discs in when the head is loaded, but it is obviously happening. Perhaps once I see more of these in for repair, I will ultimately find out what is going on. Oddly I have never seen the problem on the N10, N910, NH600 or NH900, which have the same basic chassis design.

[sfbp]

(this is just about the NH700) Agree - but I actually managed to trick an RH910 into the locked position too, after opening it (first removing battery) when TOC writing would not complete.

The real no-no is to get it into the "locked" position and then try to put it back together. Been there, done that.

But the one that was busted was busted before I ever got it, I never actually busted one.

[trott3r]

I have exactly the same problem with my jb940 which i bought new

and eventually it doesnt keep what you record on ejection.

I just use it as a playback machine nowadays.

Its the only full size deck that i have had problems with.

Used a jb920 the 940 and currently a 980 alongside the portable

HiMD mzrh1

Martin N

[jim.hoggarth]

(this is just about the NH700) Agree - but I actually managed to trick an RH910 into the locked position too, after opening it (first removing battery) when TOC writing would not complete.

The real no-no is to get it into the "locked" position and then try to put it back together. Been there, done that.

But the one that was busted was busted before I ever got it, I never actually busted one.

Interestingly, I have just opened an RH710 I have had in stock for several months (years?) which I had not looked at yet. I can quite plainly see the write head bent back through 180 degrees, as discussed. I can also see that the lid lock lever is in its locked position, and there are subtle marks around the edge of the lid where an implement such as a knife has been used to pry it open and presumably retrieve a disc. The machine itself is dead. I assume it failed during record, the owner could not open by the eject button, so took a tool to open it. Shame that they tried to ram another disc in after it, as now it has two faults not just one!

Jim

[Eeyore]

Dear Jim,

not expecting any further development of this thread (back in January, when I last posted), by chance I rediscovered it yesterday and saw your message! Thanks!

Today, I took the lid off and cleaned both the lens and the overwrite head (gosh, the mechanics of it are so delicate) with isopropyl alcohol. The OWH does not seem to be physically affected. I watched it doing its thing after inserting a disc. It lowers down with no gap between it and the disc surface, at least by human standards. I took great care in cleaning the OWH applying as little force as possible. Sadly, to no effect.

Can someone elaborate on the causes for a busted OWH. The word "busted" is not very precise, is it.

Isn't it just a miniaturized version of those heads found in MC-recorders.

Anyways, I have not returned it to the vendor (I hope he is happy now), at least I can use it as a player. Yesterday I found an OWH on ebay (from Germany, where I am too): http://cgi.ebay.de/ws/eBayISAPI.dll?ViewItem&item=150588320289

The vendor says it is new and still in its original packaging. He also says it would fit the JB940. Can somebody confirm this or say otherwise, please? How sensible is it to go and try to replace the OWH myself? Wouldn't I need some testing equipment to do some adjustments in the service menu, or is that usually not necessary.

Thanks, Thanks, Thanks!

Peter

[sfbp]

Looks way too expensive to me. Jim can probably help.