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My Model II - it begins...

S

ScutBoy

Experienced Member
Joined
Jan 9, 2014
Messages
302
Location
Northfield, MN USA
I was able to steal an hour or so and check out my new-to-me Model II.

Opened the case, found everything I expected to be there - 4 boards in the cage, everything cabled up, no obvious bad caps or components on a visual of the power supply.

I pulled each card and gave the chips a quick reset. Made sure the four cards were seated correctly and fully back in the cage.

Now, being foolish _and_ impatient, I plugged in the keyboard and the power, and hit the switch.

Case fan and floppy motor started. The red LED on the front did not come on.

With the brightness up, there is raster on the screen, but no indication of any output.

Hitting the reset switch did nothing.

No combination of keys on the keyboard does anything. None of the keys with LEDs light when pressed.

When I power off the machine, the floppy drive light flashes briefly. I never put any floppies in the drive. Oh- I have the external drive cabinet - I tried both with it connected and not connected. The doc talked about having some kind of terminator on the disk port if you didn't have the expansion, so I made sure to try it with the expansion chassis powered up and connected.

I looked briefly through the Tech Ref troubleshooting section, but nothing there made any difference.

Seems at this point, I should confirm that I'm getting good voltages from the PS, but I need to figure out good places to measure.

I'm guessing the failure of the power LED may be significant; I did make sure its connector and the reset switch connectors were making good contact at the boards.

Any wisdom from the collective on what to check/what to try next? I knew this guy would be a challenge - now I will begin to find out how much of a challenge :)

TIA for any advice!
 
@ScutBoy.

Welcome to the growing group of big Tandy users.

Sad to hear you have problems with your new model II.
I have some easy things to check before you modify things, you better don't.

Remove the boards.
Check all the voltages on the power connector on the bus board.
If not, check the fuses on the power unit.
The motor on the floppy drive and the cooling fan are running on 110 volt, so they should always work.
Check the connectors on the bus board for damaged contacts.
Insert the boards in following order from right to left.
CPU, Floppy interface, Video, Memory.
Check if the boards are fully seated.
Connect the two connectors on the video board and pay attention to the slots on the connectors.
Power on.
The light on the front should be on now. If not check the led and the power on the led.
If this doesn't help swap the video board with a know good board.
The floppy should be on and the led on the drive should be on. If not check the cables and the orientation of the connectors.
The display should ask to insert the disk.

Don't forget to mount the bar in the right way to fix the boards to prevent short circuit between the boards.
First start the computer without the keyboard cable connected.
Sometimes there is a short circuit in the cable or the connectors, which would blow the 5 volt fuse.

I hope this helps.
 
Last edited:
If you are getting a screen raster then it sounds like 12VDC is present, but most of the computer runs on 5V so check your fuses on the PSU as Hans says and look for missing voltages. If memory serves the Model II uses 5V, 12V and -12V.

Ian.
 
Ian, your memory is almost completely correct. You're just missing one voltage that the II-series uses....

TRS-Ian said:
I...If memory serves the Model II uses 5V, 12V and -12V.
Click to expand...
And +24V, only needed for the floppy disk drives, as I recall.

One caution is that the power supply used in the II has a pretty large minimum current rating in comparison to many, with the _5V needing a minimum load of 2.15A, the +12 needing 1.25A, and the -12 needing 0.05A (+24V had no minimum load).

The Model II Technical Reference is quite good, and available in many archives.
 
For checking voltages, I usually test on the open card slots with all cards installed. This gives you an accurate voltage reading under full load.

Ground @ pins 3 & 4
12V @ pins 5 & 6
5V @ pins 9 & 10

Let us know what voltages you see.
 
Measured at the card cage, with all the cards in, the 12V shows 11.65, and the 5V shows .5

A visual inspection of the (dusty) power supply doesn't show any obviously failed components. The three fuses I saw didn't appear to be blown, but I can't check better unless I pull the PS out of the machine - which I'm guessing I'll be doing sooner or later. I'm sure there should be some cap replacement if nothing else.
 
Try removing cards one at a time to see if you can narrow down the 5v issue. Could be a short on one of the boards. Also try with no cards.
 
Remember to pull keyboard too, just in case you didnt.
 
JonB said:
I'm thinking it's not a short pski, otherwise he'd be reading 0v on the 5v line. He said it was .5v.
Click to expand...

Sure, understood it’s not a direct short. But a properly working Model II should have 5.0 v at pins 9&10. Removing cards one by one will quickly narrow down the issue.
 
Following this thread. I got my hands on 2 complete Model IIs and with the drive bays (filled). They haven’t been turned on for years and I’m going to repair these to a working museum-state condition. :)
 
pski said:
Sure, understood it’s not a direct short. But a properly working Model II should have 5.0 v at pins 9&10. Removing cards one by one will quickly narrow down the issue.
Click to expand...

Aye, Pete. We'll see if he needs any dummy loads to get the PSU working right. On my Superbrain I used a 12v car bulb..

@whizzi - welcome to the Big Tandy / Sore Back club!
 
JonB said:
Aye, Pete. We'll see if he needs any dummy loads to get the PSU working right. On my Superbrain I used a 12v car bulb..

@whizzi - welcome to the Big Tandy / Sore Back club!
Click to expand...

Thanks :)

As far as I know, the HomeComputerMuseum now has the largest and most diverse collection Tandy computers in a museum of Europe.
 
pski said:
Try removing cards one at a time to see if you can narrow down the 5v issue. Could be a short on one of the boards. Also try with no cards.
Click to expand...

It's going to be a bit before I get back to it. Busy three days, and then off to Florida for a little winter vacation :)
 
JonB said:
Hmm. Depends if you consider the UK in or out of Europe. The CCH in Cambridge has an enormous collection!

http://www.computinghistory.org.uk/

Although if it's TRS-80s in particular, they may not have as much as your museum.
Click to expand...

I kinda lost count, but the list from the top of my head:
Around 7 Model Is (and 3 Expansion Interfaces)
2x Model II (+2x disk cabinet full)
2x Model III
2x Model 4
2x Model 4P
1x Model 16B
2 CoCo 1s
6 CoCo 2s
2 CoCo 3s (NTSC)
1x MC10 (PAL, Boxed)
2x Tandy 100
2x Tandy 200
PC1, PC2, PC6
Tandy 1000HD, 2000, 1000EX, 4000SX and 1400HD

So, if they have more, they're not part of Europe :p (just kidding).
 
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