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Model 4 upper drive problem

miniman82

Member
Joined
Dec 23, 2021
Messages
16
Greetings,

I got a model 4 from a neighbor and got it running (blown power supply caps replaced, brother was able to make me a TRSDOS 6 disk on his XT), but there’s one last issue I can’t seem to figure out.

The upper drive (1) refuses to read disks.

Now here’s the strange thing- if I swap drives, the upper drive works flawlessly in the drive 0 position.

The drive cable has good continuity, as does the ribbon cable connecting the floppy controller to the motherboard. Is it possible something failed in the controller?
 

mdh

Experienced Member
Joined
Dec 9, 2021
Messages
84
Location
North East Ohio
Maybe a bad connection? As I recall the TRS-80 used drive selection in an odd way that made them pull pins on the edge connectors, and have a twist in the cable (at least for model I) for drives 2 and 3. I'll have to see if I can find the pinout, but I'm sure google will find it...
 

ldkraemer

Veteran Member
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Mar 14, 2013
Messages
2,096
Location
Chaffee, MO
The Model 4 does not use a twisted cable. It is a standard flat cable. The bottom Drive is DS0, and the top drive is DS1
in a DS{0..1} configuration. If you have External Floppy's they are the same settings as the Internal for Drive Select.
ie. They are also set for DS0 & DS1, but because the hardware has STEP*E & STEP*I (External & Internal) they appear
as External Drives 2 &3.

The last Physical floppy on the cable typically has the Terminator (150 OHM Pullup Resistors), but the Terminator Resistors are already
installed as shown in the Schematic.

It's likely you have a cable that has a broken conductor in the 34 Pin cable. You could try a replacement cable as they aren't
expensive. If you move (flex) the cable a bit to the upper floppy drive it may work for a bit. You should be able to OHM
the conductors and locate the faulty conductor.

Larry

Click image for larger version  Name:	Model4-Floppy.png Views:	0 Size:	223.0 KB ID:	1236891
 
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ldkraemer

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Messages
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There is one more thing you will need to check. Carefully raise the Top of the case, WATCHING the CRT Neck as you raise the top to free it from the
base. Lay the CRT on it's left side, to the left of the base. Look at the Power Supplies. If any of them is a TANDY supply with a row of Pins for the
LOAD side of the Supply, remove the Power Supply. Use a Magnifying Glass to look at each solder joint of every Pin. If you see any circles
around any Pin, ALL pins need to be re-soldered. I use Kester 63/37 Solder that is .050 in Diameter (make sure it's Rosin Core Solder) and reflow
the solder around each pin. (lack of Power +5 or +12 to Drive 1 could be your problem)

Astec Power Supplies do NOT have this Problem.

Re-install power supply and test Drive 1. Then reassemble Top of case on base.

Larry
 

miniman82

Member
Joined
Dec 23, 2021
Messages
16
Both my supplies are Astec, and the drive has good voltage to it.

Just to clarify the drive does seek, but it seems no data is retrieved. After the first seek attempt the head makes a bunch of clacking sounds, like it’s trying to go too far or something. I thought I fixed that as I discovered a switch (I believe it’s called the track 0 switch) was faulty, so I replaced it with the write protect switch which was good but same symptoms persist. I jumpered out the write protect switch, but even if I hadn’t I would think it should still read normally.

Anyway, it feels to me like either the floppy controller has some issue or the cable is somehow bad. I don’t see how though, as that’s the first thing I suspected but it ohms out fine.
 

ldkraemer

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Messages
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Chaffee, MO
The first thing to try is to remove the Logic Board of the Floppy (unplug the Read Head(s)) and swing the Logic
Board out of the way and carefully and gently clean the Head(s) with a cotton swab dipped in Alcohol. You want to
remove any brown oxide residue that is on the Head(s). While I am doing that I also clean the Carriage Rods
that the Head Assembly rides on with another Cotton Swab dipped in Alcohol. Then, when the Alcohol is dry
I lubricate the Carriage Rods with about three drops of Dri-Slide across the length of the Rods. Then slowly
and gently move the Head Assembly full travel in both directions.

Dri-Slide is a Motorcycle Cable Lubricant that does not attract dirt. A local Motorcycle shop should have a small
can that will be a lifetime supply. It's also available on the Internet.

What is the Manufacture and Model of the Floppy Drive that does not read data? Is it a Tandon TM-100-??.

How about running some Diagnostics on the bad drive? Do you have access to someone that has a DOS Computer or
early Windows 98 or 98SE that can run Imagedisk Ver 1.18 (by Dave Dunfield) in DOS Mode to create a Diagnostics
Boot Floppy?

Or perhaps you are running a Linux Distro that can write a floppy using the dd command? (Debian Linux still supports
Floppy(s) but it needs to be installed to access the /etc/mediaprm file of CP/M formats that are supported.)

Larry
 

ldkraemer

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You better check the Floppy's Schematic for the differences in the Write Protect Switch and the Track 0 Switch. The Schematic I looked at
for a Tandon TM-100 Series shows the WP Switch and the TRK0 Switch as being a Switch with NO and NC contacts and they are wired
different. The NC and NO contacts refer to the Shelf state of the Contacts. If you pick up a Switch off the shelf, the NC contacts are CLOSED
and the NO contacts are OPEN. Operation of the switch changes the states of both sets of contacts.

It's possible you could have the logic state of the switches reversed, according to the attached schematics.

Larry

Tandon-TRK0-SW.png
Tandon-WP-SW.png
 

miniman82

Member
Joined
Dec 23, 2021
Messages
16
Kraemer,

Both drives were cleaned already, that’s usually first step maintenance for old drives before I even try a disk in it.

I wired the switch correctly, they are identical so it was simple to get the contacts wired properly. What I saw agrees with the schematic picture you posted.

I don’t know who made the upper drive, the stickers fell off. The bottom one is Texas Peripherals, the top one looks slightly different in front (the LED is larger), but there are no other identifying marks and it has the same logic board so I have no idea.

Unfortunately I don’t have another computer here I can use to troubleshoot, the only way I was able to even get a TRSDOS boot disk was begging my brother to use a special program on his XT to write images to disks I sent him, then he mailed them back. I do have a working COCO drive here, but the model 4 doesn’t seem to like it. Which is odd now that I think of it, since the controller from the COCO worked with both model 4 drives- that’s how I originally found out they were good before I got a working boot disk.
 

ldkraemer

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Can you find a local person that can write a *.IMD file to a formatted Floppy? I can email you the
Diagnostic file. If you have a Desktop (Windows or Linux) that supports the 5.25" Floppy, you can
install an Emulator (there are three good Emulators that will write the Image to a real floppy) and
write the floppy yourself. Those are two good options. If there is a local HAM Operators Club,
likely some of those folks also have the capability to write Floppy's.

Emulator in order of my preference
1. TRS80GP By George Phillips http://48k.ca/trs80gp.html
2. TRS80 by David Keil http://cpmarchives.classiccmp.org/trs80/mirrors/www.discover-net.net/~dmkeil/
3. XTRS by Tim Mann https://www.tim-mann.org/xtrs.html

Larry
 

ldkraemer

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The first test I'd do is a RPM test of the problem drive with a floppy inserted. It needs to be spinning at
300 RPM plus/minus a couple RPM.

Larry
 

Torch

Experienced Member
Joined
Sep 7, 2018
Messages
74
Location
Indiana
Drive 1 doesn't work in upper drive one position. You put it in place of drive 0 at bottom position and it does work. Correct?
What happens when you put the original drive 0 in drive 1 position? If it doesn't work there, I don't see the point in screwing around with the drives, outside of checking for a Drive Select jumper conflict.

For fun, have you tried to run a drive configured as 1 off the external connector?
 

miniman82

Member
Joined
Dec 23, 2021
Messages
16
The first test I'd do is a RPM test of the problem drive with a floppy inserted. It needs to be spinning at
300 RPM plus/minus a couple RPM.

Larry

Like torch said, I don’t see a lot of point in looking at the drives. They both work, it’s the drive POSITION (1) that’s giving trouble. By process of elimination it has to be farther back, like the cable or controller.

I don’t have anyone nearby who can write images, I got the boot disk mailed to me from Wisconsin. I suppose I could bother my brother for another favor, but his old XT has already done so much to further the cause lol.

This thing is fighting me, I was so close to buying a FreHD in the beginning but I’m determined to do this the right way- with media!
 

clh333

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Feb 22, 2015
Messages
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Location
Cleveland, OH, USA
I don't have an answer to your problem but I do have a Model III, a Model 4 NGA, a model 4 GA and a Model 4P all of which have drive problems, three of them with the top drive. I believe the answer to the problem lies with the controller board; some component that commonly fails.

-CH-
 

ldkraemer

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How about posting a closeup Photo of the Logic board of Drive 1 (Top Drive) so we
can see the switch settings/Jumpers and the Terminator?

Is the Terminator (Pullup Resistor Pack) Installed or Missing on the Logic Board?

Do you have the Green GROUND wire attached to each Floppy Drive?

If you want to try a replacement cable contact Jay - Send me an email to jaynewirth@verizon.net

I'd like to get the voltage readings on each of the EVEN Pins on the Edge Connector, with the
Drive 0 (lower Drive) cable removed from the drive. Use the attached photo of the Edge Connector
and measure the Voltage on each Pin with the Black meter Lead on the COMMON (at the Power
Supply that is powering the Floppy Drives)

There should be +5.0 VDC on each of these Edge Connector Pins with the Model 4 powered up and Idle.

Larry
 

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miniman82

Member
Joined
Dec 23, 2021
Messages
16
CH- sure seems that way. It’s got to be something attached to the drive, not the drive itself. Feels like an upstream failure to me, if that makes sense.

Larry, see the attached photo of the board. I don’t see a green ground anywhere, but the shielding has good continuity and it wraps around and is bonded by the mounting screws to the drives which accomplishes the same thing.

I’ll see about getting those readings later on today, it’s cooking time!
 

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miniman82

Member
Joined
Dec 23, 2021
Messages
16
Nevermind, went ahead and took the readings.

All pins have +5VDC except the following:

34, 6, 4,2: 0VDC
18: .45VDC

Hope that helps.
 

ldkraemer

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Interesting that Pin 18 isn't +5.0 VDC. Let's do more testing.
Assuming that you had removed Drive 0 (Lower Drive) Connector with Power OFF the Model 4 and
ONLY the Drive 1 Cable was attached to the Drive 1 Floppy when you did the Pin 18 test try this
test with Power OFF.

Unplug the Top Floppy Data Cable, and Plug the Lower Floppy Cable back on Drive 0. Now hold the
Break key depressed, and continue to keep it depressed while turning on the Power. When you see
characters on the display, release the Break key and tap the Enter key twice.

TEST #2
Put your Black meter lead on the Power Supply Common that powers the Floppy Drives. Now test the
Pins on the Top Edge Card Connector (Drive 1) and see if all the EVEN Pins are +5.0 VDC as the
previous attached Photo RS1.png. We are looking for Pins 18 and 20 to be +5.0 VDC.
What are your results? If Pin 18 is +.45 VDC (same as first test) then we need to look at the FDC's
PULLUP resistor R2 (150 OHM) tied to Pin 18 (Internal Connector).

TEST #3
With the Data cable disconnected from Drive 1 (Top Floppy Drive) Test Voltage on Edge Card of Floppy
for Pins 18 & 20. (should be 0 VDC) Use TP4 (COMMON - LOGICAL COMMON for the PCB).
Now check the Voltage on Pin 8 of U9 (Pin 18's LOW Voltage is on Pin 9) and Pin 8 of U9 should be +5.0 VDC.
Do the same test for Pin 10 of U9 (Pin 20's LOW Voltage is on Pin 11) and Pin 10 of U9 should be +5.0 VDC.
What do you get?

Insert a 1/8 Watt or 1/4 Watt 150 OHM resistor into RP1's Connector on Pin 1 and then the other end on Pin 2.
Now check the Voltage on Pin 8 of U9 (Pin 18's LOW Voltage is on Pin 9) and Pin 8 of U9 should be +5.0 VDC.
Do the same test for Pin 10 of U9 (Pin 20's LOW Voltage is on Pin 11) and Pin 10 of U9 should be +5.0 VDC.
What do you get?

Larry
 

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miniman82

Member
Joined
Dec 23, 2021
Messages
16
OK, I booted it into basic and this is the result-

Test 2 connector as follows:
18- 5
20- 5
Test 3 pins as follows:
2- 0
4- 0
6- 0
8- 0
10- 5
12- 5
14- 5
16- 0
18- 1.3
20- 1.3
22- 1.3
24- 1.2
26- 0
28- 0
30- 0
32- 5
34- 0

U9 pin 8- 5
U9 pin 10- 0

There's no way I'm going to be able to hold resistor legs into that connector without shorting something out, and I don't have a breakout board. Do the readings thus far tell you anything? Seems like some readings aren't what they should be.
 

ldkraemer

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OOPS, I skipped a step in the last part of the test.

Insert a 1/8 Watt or 1/4 Watt 150 OHM resistor into RP1's Connector on Pin 1 and then the other end on Pin 2.
Now check the Voltage on Pin 8 of U9 (Pin 18's HIGH Voltage is on Pin 9) and Pin 8 of U9 should be 0 VDC.

Move the end of the PULLUP resistor from Pin 2 to Pin 3. It is now associated the Signal on Pin 20 of the
Card Edge Connector.
Do the test for Pin 10 of U9 (Pin 20's HIGH Voltage is on Pin 11) and Pin 10 of U9 should be 0 VDC.
What do you get?


Larry
 

Torch

Experienced Member
Joined
Sep 7, 2018
Messages
74
Location
Indiana
Your photo shows a late model Texas Peripherals drive. There are no drive select jumpers like on the Tandon drives. Drive select is done with pins removed from the flat ribbon cable.
You mentioned the drive cable has good continuity in your original post. If you have a straight through cable using all wires, that's your problem.
Also, a terminating resistor pack is not used on internal drives. It is only used on the last EXTERNAL drive on the system.
 
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