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TRS-80 Model 4 drive trouble

Strange question, but was the system shipped with the cardboard piece in the floppy drive? I've seen times part of it somehow bonds with the head's surface due to moisture or whatever reason. Could it be possible this is what is happening to you? Could explain the strange behavior wreaking havok on floppies...
 
No cardboard protector in the drives; but they where promptly tested and both worked well with a copy of NEWDOS/80 that it was the first casualty when the system started to fail...

Until now I've replaced no less than six ICs, all of them are now socketed with no success. Also I've replaced some capacitors (mainly the electrolytic ones). It's starting to give me a good headache.
Incidentally I've found that the drives seems to behave the same as when the floppy cable is plugged upside down (all ground pins paired with the signals pins). Maybe there's some kind of short circuit in the board??

The main WD IC is labelled FD1793PL-02 and I'm not sure if I have spare pieces; but I think it's compatible with the MB8877A and IIRC I've at least one piece. It's already socketed; so if I find the replacement I can test it easily. I've always thought that the WD controllers were the hardest chip in the board :)

I'll keep trying...

Btw, at least the upgraded 4P works like a dream, I'll post some photos this weekend...
 
Thanks for spotting me out your site Druid6900, seems that you're an invaluable source of components :)

I've replaced the original FD1793PL-02 with a FDC1793B-02 predated from a CoCo disk controller and now a new weird behaviour has appeared...the computer doesn't boots or at least doesn't show anything on screen.
The drive seems to work as expected (it spins only some seconds and then stops); but the display remains black and doesn't really reads the boot disk.

I suposse that the original FD1793PL-02 has some kind of fault. The new FDC1793B-02 must be, incompatible?, faulty as well?, I'll do some more tests...
 
I'd check the connector from the main board to the video card and the PCB on the end of the CRT, both of which are easy to snag and pull off during disassembly/reassembly and both of which would result in no video.
 
I've swaped several times the 1793 and the behaviour is always the same. The old one, leaves the drive motor always spinning and the computer boots showing "Diskette?". The replacement, makes the drive head looks for track 0 as expected and the motor stops soon, sadly the computer doesn't boot and the display is pitch black.
Just to be sure, I've checked the onboard resistors and all of them are fine except one (270K) that raises slooowly its resistance until reaching more or less its expected value (I'm mostly a noob; so I don't have the slightest idea why it's doing that, a near capacitor maybe?).

@Druid6900: Can I get a quote of some ICs from your site?. I would be finally interested in a pair of FD1793's and WD9216's.
 
jltursan said:
@Druid6900: Can I get a quote of some ICs from your site?. I would be finally interested in a pair of FD1793's and WD9216's.
Click to expand...

Just go to the site (link below) and search on the parts you are interested in. There are 2 types of 1793s, the -02 and the B-02. According to the service manual, the Model 4s take the B-02s

Wait a week before ordering anything though as although it shows both Canada Post and FedEx shipping options, Canada Post is on strike (and will be legislated back to work shortly) and they are less expensive than FedEx.
 
Thanks to Druid6900, I've received some nice FDC spares and tested the still broken controller.....no luck, the last batch of symptoms remains: the drive seems to spin correctly; but hey, no screen display at all and nothing is loaded. No more "Diskette?" message. I'll try to check the tracks looking for damaged ones.
 
Not a problem, although I'm sorry to hear that it didn't solve your problem.

I know there is a controller alignment procedure for those FDCs but, in all my time with Radio Shack Computer Repair and in all the time since then, I've only had to do 2 alignments and both were the result of someone screwing with the adjustments.
 
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