• Please review our updated Terms and Rules here

Model 4 (serial 000018) scrambled screen, capacitor fail or something different?

You can try to run just with bank 0 of the memory. Will be limited to the base 64k. Remove the 8 ram chips on the right hand side U85-U92. Normally those chips are not accessed unless you run a Memory disk or a program called Double Duty a task switcher. See if that helps. Also make sure that all the pins are in the holes of the socketed chips. Especially the 40 pin chips like the Z80a and the Video controller chip. I have had one or 2 pins not go into the holes cause they were bent inward and caused me a lot of headaches. Till I pulled them again and found the pins bent. So something to double check. Lot's of luck Whizzi!

And just FYI I had the exact same problem on a Gate Array model 4 with that same bad display. Had a floating ground on a 74LS02 chip which I corrected and 2 Gate chips went bad. The PLL (Phase Lock Loop) controller gate chip and the address decoder gate chip. Replaced those and all worked like a charm.

Also if you need the service manual/Technical manual look here. It is in alphabetical order. Look for Model 4 Technical Reference Manual and be sure it is the 1983 version. You may need 7-zip to open the archive. Hope this helps.

http://www.classiccmp.org/cpmarchives/trs80/Library/Manuals/Hardware/
 
Last edited:
and there was a bang, a lot of smoke (big RIFA exploded), another bang and again lot's of smoke (second rifa exploded), but the Tandy is working!

I reseated the video and Z80 again, but no chance. I triple checked that all pins on both IC's were connected, they were all just fine. I picked up my multimeter and started to measure and as I expected, the mainboard doesn't get any power. So, disconnected everything and reconnected all cables .. bang, machine turned on (well, it was reading the FDD) and with some tinkering on the knobs (man, I love tinkering on knobs, those big knobs.. hmmm :p ) and pressing reset+break I got the Cass? question.. Yay!



But the 'pffft' in the video was just a warning signal, about 10 seconds after this picture it went 'BANG' and there was the white smoke.. man, that's a lot of smoke coming out of that one.

About 2 minutes later, the second one went with the same theatrical drama while I was DJ-ing some floppy discs in the hope the thing would boot. But no luck, I guess the FDD's needs a big cleaning. But yay, it's doing something..



One thing I found odd (but it could be me), why is this saying Model III basic while it's clearly a Model 4? Was the Model 4 basic not finished or something? :p
 
Yep always something simple. Reseat connectors. Should have been the 1st thing to try see if you have power to the board. I had guessed you had already done that.
My bad.

Well basically it is a Model III! Just enhanced to be able to boot with a full RAM system. So any DOS 6.x such as LSDOS or TANDY DOS 6.x will kick the Level III ROM out of the memory space it normally occupies. That way it can run CP/M etc. That is if I remember correctly!.

Glad it is working.
 
You can't even imagine how happy I am that it's all normal and apart from the psi, working. The new RIFAs are on it's way so all left is cleaning the heads of the FDD. Perhaps I'll replace them with a double density, we'll see what comes across my path. I guess the reseat of the ICs did the trick with the garbage on the screen. So, yay!

Thanks for all the help, without you I wouldn't know where to start.
 
Your welcome from all of us here! Anytime.

Also about the FDD's not only clean the heads but make sure the drive carrier carriage is moving smoothly. No binding
 
poor poor filter


Computer is still working and the smell has pretty much left my room :p

For now I'll put the thing back together until I have the new RIFAs, as soon as these are in I'll make them work again (I've done that multiple times already, not particular a M4, but the drives I have (and had) with the Model 1).
 
Last edited:
Your welcome from all of us here! Anytime.

Also about the FDD's not only clean the heads but make sure the drive carrier carriage is moving smoothly. No binding

Also check that both the spindle hub (the metal part turned by a motor) and the clamping thingy (the plastic piece that clamps the disk against the spindle hub) spin freely. I've encountered stuck bearings on old floppy drives. So far, I've been able to fix all of them by disassembling them to remove the ball bearings, then working in a little bit of light oil and spinning them by hand until they turn easily.
 
Just returned from holiday, but just before I left:


So yeh, it's fully working again. Even the bottom disk drive works, the top one doesn't seem to do anything but that could be a connection issue or the drive could be dead. In the next few weeks I'm gonna look into that and replace the drive if needed. This machine will get the FreHD or the USB-reader when it's done :)

Again, thanks for all the help.
 
Back
Top