TRS-80 4-p repair help

[Zion]

Hello, this is my first time posting so a little bit of information about myself and the project,I am 19 and love the '80s, I have collected a lot of various game systems over the years but recently have been using my Commodore 64 more.
I also have a Fiero.
I recently got this Model 4P from my great uncle who bought it new in the year it came out, once I brought it home I plugged it in and the floppy drive's lights flickered and then I got a puff of white smoke, My first encounter with RIFA capacitors ;-(

I bought the needed replacements and replaced the caps on the PSU, tried it again and this time a capacitor on the floppy drive shorted out.

I then disconnected the floppy drive that shorted and ran the other drive to the first one's cable, the drive would spin but I scratched the disc, thus ruining my only boot disc.

I then reached out to Ira and got a new set of discs, but didn't try them until I also bought (after searching on eBay and overpaying a good bit) a Tandon 50 drive, the same one as I have!
I plugged it into the bay expecting it to work but it still doesn't seek, the head doesn't move,It used to say floppy drive not ready, now all it will say is "Seek Error".Any help on this would be appreciated.
this occured over about 6 months
this is the original B/W monochrome screen release.
I can solder and do basic eletronic repairs but I am weak on diagnosing as of now

 

[gary245]

You will need to clean the head on the floppy drives.
I also recommend buying some clean/blank media to test the drive with. Not to read/write or boot. Just to spin in the drive to verify it's no longer scratching the media before you put a real boot disk into it.
Also do not turn the computer on or off with a diskette in the drive. it may erase the disk under where the head is sitting.
you can carefully/manually move the head away from track 0.
When you turn the computer on with no diskette in the drive it should prompt for a diskette.
If it does insert the diskette & close the door. The head should seek to track 0. and try to boot.
A video of all this (drive/diskette/screen) might be helpful.

 

[Tuc]

Hi,

Never heard of the Fiero computer, unless you're talking about the car. If you are talking about the car, NICE!

While getting another drive and trying is great, you unfortunately added more unknown into the equation. I'm actually finishing up a 4P for a customer who wanted both drives to be the originals. That wasn't fun. Fortunately, he also wanted an internal FreHD.

If you can afford to do it, I'd try to buy a Gotek. While you may not want it as a permanent fixture in the machine, it rules out any sort of alignment, head, disk media, etc issues. You don't have to go for the ultra fancy OLED/rotary knob version, the basic digital display works fine too. At least you know at that point the machine can boot. Then you can start diagnosing the physical disk issues. If not, you need some way to prove that the disk drive itself is working properly.

Does the drive seem like it WANTS to move, or anything? Does it make ANY noises? It just might need a little movement to "break" it's grip, or make sure all the control wires are intact. I purchased a drive where one of the stepper pins had been broken off which wasn't clear in the pictures.

I'll watch this, hope to help getting it running!

(DISCLAIMER: I own a Vintage Computer parts and refurb shop. I have a vested interest in selling people Goteks, Gotek extender cables, Gotek to Card Edge connectors, FreHD's, 4P Modem Slot Carrier boards for FreHD, etc. But, I'm a hobbyist too and like to see people get machines running)

Tuc

 

[ldkraemer]

Zion,
I'm not familiar with the Tandon TM-50's. Will you post the location of the Cap
that blew on the TM-50, so I can locate it on the Schematic.

Thanks.


Larry

 

[Zion]

You will need to clean the head on the floppy drives.
I also recommend buying some clean/blank media to test the drive with. Not to read/write or boot. Just to spin in the drive to verify it's no longer scratching the media before you put a real boot disk into it.
Also do not turn the computer on or off with a diskette in the drive. it may erase the disk under where the head is sitting.
you can carefully/manually move the head away from track 0.
When you turn the computer on with no diskette in the drive it should prompt for a diskette.
If it does insert the diskette & close the door. The head should seek to track 0. and try to boot.
A video of all this (drive/diskette/screen) might be helpful.

I have some old floppies that i will use to test so I don't ruin another disc. Thanks for the heads-up about data loss if it's turned on with a disc in. When I turn it on, both of the drives start to spin with no discs in, and it says seek error on the screen. I will take a video this weekend of it. Thank you very much

 

[Zion]

Hi,

Never heard of the Fiero computer, unless you're talking about the car. If you are talking about the car, NICE!

While getting another drive and trying is great, you unfortunately added more unknown into the equation. I'm actually finishing up a 4P for a customer who wanted both drives to be the originals. That wasn't fun. Fortunately, he also wanted an internal FreHD.

If you can afford to do it, I'd try to buy a Gotek. While you may not want it as a permanent fixture in the machine, it rules out any sort of alignment, head, disk media, etc issues. You don't have to go for the ultra fancy OLED/rotary knob version, the basic digital display works fine too. At least you know at that point the machine can boot. Then you can start diagnosing the physical disk issues. If not, you need some way to prove that the disk drive itself is working properly.

Does the drive seem like it WANTS to move, or anything? Does it make ANY noises? It just might need a little movement to "break" it's grip, or make sure all the control wires are intact. I purchased a drive where one of the stepper pins had been broken off which wasn't clear in the pictures.

I'll watch this, hope to help getting it running!

(DISCLAIMER: I own a Vintage Computer parts and refurb shop. I have a vested interest in selling people Goteks, Gotek extender cables, Gotek to Card Edge connectors, FreHD's, 4P Modem Slot Carrier boards for FreHD, etc. But, I'm a hobbyist too and like to see people get machines running)

Tuc

Thanks TUC

Yes, I have the car! And I love it, it is my daily

I was hoping to get a fully working drive to get things going, but that didn't go as planned. I might want to get a freHD set up for a 4P from you. I already have a GoTec, but it is not set up, and I need a GoTec to edge/Shugart connector adapter, Do you know a place I could get one? ;-)

Drive does not want to move; it doesn't even try. Whether the drives are connected or not, it just says seek error

I am very grateful for any and all help I can get

You sound like a person I need to buy stuff from; do you have a store online?

 

[Zion]

Zion,
I'm not familiar with the Tandon TM-50's. Will you post the location of the Cap
that blew on the TM-50, so I can locate it on the Schematic.

Thanks.


Larry

I will, as soon as I get done with my work day tonight

 

[Istarian]

I have some old floppies that i will use to test so I don't ruin another disc. Thanks for the heads-up about data loss if it's turned on with a disc in. When I turn it on, both of the drives start to spin with no discs in, and it says seek error on the screen. I will take a video this weekend of it. Thank you very much

FWIW this is a good use for old disks that no longer reliably retain data written to them and/or have known bad sectors.

Just make sure to mark or label them so you don't mix them up with good/working disks.

 

[gary245]

Zion,
I'm not familiar with the Tandon TM-50's. Will you post the location of the Cap
that blew on the TM-50, so I can locate it on the Schematic.

Thanks.


Larry

I do not have a 4P open, but I have a pair of TM50s (pictures attached for reference). Not sure which caps tend to need replacement, but there are (2) logic bds. main/digital on one side (2 tantalum), and motor control(6 electrolytic) on the other. Zion mentioned the cap loaded down the power supply, so I would suspect a tantalum.

 

[Zion]

Here are some photos that I was able to take, all the board photos are from the board with the shorted cap.

 

[Zion]

Here are the links to the testing videos on my channel, They wouldn't upload here due to size. Number one is of the drive when it boots, the other is of the screen

 

[Istarian]

@Zion Depending on the file format of your original videos, transcoding them to a format with better compression might help get the size down.

E.g. AVI files are virtually uncompressed and even a couple minutes of video can be a huge file. Converting them to MP4 can shrink that down considerably using modern video compression technology.

Recording at a lower resolution can also make a big difference here, because every additional pixel adds several bytes/frame to the total. Plain old "HD" (720p) is more than adequate in most cases.

 

[gary245]

Lots to comment on. The board pictures are (2) different boards, what I call digital (Tandon read/write) & motors.
The digital bd tends to have (2) tantalum caps C6 & C7 (4.7uf and 35v) Most likely one of these.
The motor bd tends to have (2) tantalum caps C24 & C15
Tantalums tend to short power supplies. You could start by desoldering one leg of each & see if it no longer shorts the power supply.
Drive video the diskette is being inserted the correct way, WP notch down. Can't tell what the screen is doing or if the drive is selected.
The error message on the screen is odd. with no disk in the drive it should be prompting to insert a boot disk. it should not be trying to seek yet.
Suspect both drives have DS0 & DS1 jumpered and the drive ribbon cable is missing pins at the drive ribbon connectors that allows the 1st connector to only work for drive 0, and the one at the end of the cable be drive 1. (Tandy's M1, M3, M4 & CoCo favorite method for drive select, the drives are interchangeable, the cable manages it)
Should be a hard to read service manual for the TM50 attached. (nevermind too large)
Link below, scroll down to download the PDF.

https://dn760009.eu.archive.org/0/items/Tandon_TM-50_Service_Manual_1983_Tandon/Tandon_TM-50_Service_Manual_1983_Tandon.pdf

I'm used to seeing this screen when no diskette is inserted and the drive door is open.

Try disconnecting both drives from the computer & see what message you get on the screen.

Side note I agree with Tuc , a gotek (floppy drive emulator) is a very nice replacement for a floppy drive (Replace the floppy diskette with a USB memory stick, and it can hold multiple boot diskette images, scroll & select the one you want)

 

[ldkraemer]

Zion,
C5 & C6 destroyed, and in looking at the closeup photo it looks to me like CR14 is suspect.
It looks to me like it's too black on that end. It should be easy enough to lift one end
and check with a meter to make sure it isn't shorted. If it's shorted, make sure you check
U4.

I would also like you to double check the Drive Select Jumpers and the Terminator. The Photo's
you posted doesn't show a terminator inserted. It should be inserted in the last drive on the chain.
If you use one Drive it should have the Terminator.

I have attached photo's of those two sections of the manual.


Larry

 

[gary245]

Once you get the caps fixed on the 2nd drive, try it in the 1st drive position on the drive cable ( if the head is clean)
(Embarrassed I missed the burn marks)

 

[ldkraemer]

Zion,
I forgot a couple of things I was going to tell you on the previous posting. I use floppy drives in various
Computers and I install a switch with 7 or 8 toggles versus the punch down devices. It's a lot
easier to toggle a dip switch that modify or change that punch out part.

Once you have C5 and C6 out check from the +12 VDC Pin to GND to make sure there isn't something else
also shorted, just to be 100% sure.


Larry

 

[Zion]

Lots to comment on. The board pictures are (2) different boards, what I call digital (Tandon read/write) & motors.
The digital bd tends to have (2) tantalum caps C6 & C7 (4.7uf and 35v) Most likely one of these.
The motor bd tends to have (2) tantalum caps C24 & C15
Tantalums tend to short power supplies. You could start by desoldering one leg of each & see if it no longer shorts the power supply.
Drive video the diskette is being inserted the correct way, WP notch down. Can't tell what the screen is doing or if the drive is selected.
The error message on the screen is odd. with no disk in the drive it should be prompting to insert a boot disk. it should not be trying to seek yet.
Suspect both drives have DS0 & DS1 jumpered and the drive ribbon cable is missing pins at the drive ribbon connectors that allows the 1st connector to only work for drive 0, and the one at the end of the cable be drive 1. (Tandy's M1, M3, M4 & CoCo favorite method for drive select, the drives are interchangeable, the cable manages it)
Should be a hard to read service manual for the TM50 attached. (nevermind too large)
Link below, scroll down to download the PDF.

https://dn760009.eu.archive.org/0/items/Tandon_TM-50_Service_Manual_1983_Tandon/Tandon_TM-50_Service_Manual_1983_Tandon.pdf

I'm used to seeing this screen when no diskette is inserted and the drive door is open.
View attachment 1327962
Try disconnecting both drives from the computer & see what message you get on the screen.

Side note I agree with Tuc , a gotek (floppy drive emulator) is a very nice replacement for a floppy drive (Replace the floppy diskette with a USB memory stick, and it can hold multiple boot diskette images, scroll & select the one you want)

Thank you, Gary. Correct, C6 is the one that blew, but I probably will have to replace a few others. I didn't have replacements on hand, so I ordered some more capacitors. They should come in mid next week. With both drives disconnected, it still shows the same error message. Ever since the cap on the Floppy drive shorted out, before that happened, it read the same message as your screen. I checked the jumper settings, and they look like they are all set to the same settings. I will attach photos in my next post. Could I see your settings?

 

[Zion]

Zion,
C5 & C6 destroyed, and in looking at the closeup photo it looks to me like CR14 is suspect.
It looks to me like it's too black on that end. It should be easy enough to lift one end
and check with a meter to make sure it isn't shorted. If it's shorted, make sure you check
U4.

I would also like you to double check the Drive Select Jumpers and the Terminator. The Photo's
you posted doesn't show a terminator inserted. It should be inserted in the last drive on the chain.
If you use one Drive it should have the Terminator.

I have attached photo's of those two sections of the manual.


Larry

Thank you, Larry. As soon as the new caps come in, I will be replacing them. I didn't notice CR14; I will check that one out. I will check on the terminator resistor. I am sad to say I only just learned about those!

 

[Zion]

Zion,
I forgot a couple of things I was going to tell you on the previous posting. I use floppy drives in various
Computers and I install a switch with 7 or 8 toggles versus the punch down devices. It's a lot
easier to toggle a dip switch that modify or change that punch out part.

Once you have C5 and C6 out check from the +12 VDC Pin to GND to make sure there isn't something else
also shorted, just to be 100% sure.


Larry

I will do that. That is a good idea with the toggle switches. I will see if I can do that while I am waiting for the Capasitors to come in.

Zion

 

[gary245]

Thank you, Gary. Correct, C6 is the one that blew, but I probably will have to replace a few others. I didn't have replacements on hand, so I ordered some more capacitors. They should come in mid next week. With both drives disconnected, it still shows the same error message. Ever since the cap on the Floppy drive shorted out, before that happened, it read the same message as your screen. I checked the jumper settings, and they look like they are all set to the same settings. I will attach photos in my next post. Could I see your settings?

I expect the dip shunt to be set the same. In TRS-80 Model 1, 3, 4 4P, 4D, and CoCo, Tandy did the drive select in the ribbon cable connected to the drives ( removed the pins for the other drive selects), making each cable connector unique. It's possible the seek error occurs when there is a dead or missing floppy (I've not run through all possible scenarios, and I do not have a working 4P apart right now).
Neither drive in this setup has a terminator. I think the 4 series added termination/pullup at the main board (a little wrong, but simplifies things. This is an external pair of TM50s I had available).
You'll notice the cable connector is missing pins, that's how drive select is managed. (yours should be similar)