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Need some help with Model 1 repairs

VERAULT

VERAULT

Veteran Member
Joined
Jan 30, 2012
Messages
8,467
Location
Connecticut, USA
So I decided to get both my model 1s out of the closet and see how they are. Both having matching monitors. The first was fine when put away years ago. Took a couple adjustments and socket cleaning but my model 1 with level II BASIC and I think might be a lower case mod (Z5 is doubled up and wires are running to Z30 and z60 to a 3 way switch?). sliding the switch changes the characters into garbage characters. This unit also has the cassette mod. The machine boots and it shows 16KB. The lower half of the screen has garbage so bad VRAM I guess? Anyone have a good lead on the VRAM needed as I dont have any of this type in stock. Attached in a photo of the Service stickers on it. One isnt dated unfortunately.IMG_20211013_140724.jpg - model 1 service stickers

The other machjine is not so good. I got it from a separate seller than the monitor. IT came with a briefcase and foam cutouts for the model 1, power supply and cassette deck. It never worked. The monitor was shipped from just over in New York but the seller did not know how to ship a CRT and it arrived mangled. I repaired the broken wires and washed the grimy board. USing Epoxy I repaired the CRT standoffs in the from plastic. The monitor works now although its got a slight almost strobe quality to it. Will fix that another time.

The second model 1 Does NADA/ Just the power light comes on. Nothing on screen (either screen). This unit has the cassette mod and Original leve 1 BASIC. The power switch was dirty and wouldnt do its double action but using deoxit I got it working again. I guess I want to assume someone connected power to the video. Where should I begin on a black screen repair on a model 1?

Im a novic on model 1 repair so thanks in advance.

Thanks

Mick
 
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Mick,
Start with the Power Supplies on the Motherboard. Verify the +12 Volt is good, then check the +5 VDC Power supply. It should be
right at 5.00 VDC. The -5 VDC also needs to be close to -5.

This site gives some good troubleshooting tips.
http://ts-inc.dyndns.org/Diagnose Repair TRS80 TRS-80 Model 1.html

You can pull the ROM and test also remove the DIP Shunt at position Z3, and verify the results.

The 10,000 uF 16 Volt Cap probably needs to be replaced.

I don't know what Modifications your Model 1's have but Lowercase is a good one. I have an article about replacing the RAM
in the Model 1 with 64K Ram IC's as the newer RAM don't require -5 VDC. (Those old RAM IC's like to short to the -5VDC
internal to the RAM.) This allows you to remove any RAM in the Expansion Interface and all the RAM is located in the
Keyboard. You can check the temperature of the RAM IC in the Keyboard and tell a lot about what is going on.

One thing you need to always do is turn off AC power to the Power Brick(s), because they get Hot and go bad internally.
Don't let them sit and cook for weeks/months.

I've got some code for a EPROM that does some testing and runs the Babyroot Memory testing program in a loop.
That might help you with bad RAM, and what's going on in the Video RAM.

You can also hold down the BREAK key, and keep it depressed when turning on the POWER. When you see text on the display
release the BREAK Key, and depress the ENTER key three times.

Typing ?MEM and hitting the ENTER key will tell you the memory that exists in the Model 1.


Larry
 
Also Ill ask again, but does anyone know a good source for the Ram or Vram chips? Id like to buy some as I dont have any on hand.
 
It looks like the second sticker is a refusal to repair: "Seal broken, Level II Not Installed by Repair Dept." Or just notes maybe. 1st one says "16K, Bad LC Mod"?
 
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I thought the same as well. It does have RS level II basic installed so I am guessing the Tech just filled it in badly (or had his own way of writing noteS)
 
eswan said:
It looks like the second sticker is a refusal to repair: "Seal broken, Level II Not Installed by Repair Dept." Or just notes maybe. 1st one says "16K, Bad LC Mod"?
Click to expand...

I think the sticker says "16K, PAD LCmod". So the 16K upgrade, keypad, and lowercase mod were probably what was installed.
 
So I bought ten 2102 VRAM IC's from craiglist and I am starting to think most (or possibly all) are bad. Hold on before you guys possibly jump on the "fake chips on aliexpress bandwagon". I buy lots of old ram from aliexpress. And I check every single one. 4164 ram for instance I buy 60 to 80 IC's at a time. Sure some arrive bad sometimes and I always get a partial refund on those chips. I have never had "fake chips" these 2102 chips seem authentic, I think they are just bad.

Ok with that said. I have socketed 6 of the 7 Vram IC's (Z46, Z47, Z48, Z61, Z62, Z63). I didnt do that last one (Z45) last night because it looks like it is directly tied into the "lower case" mod. Here are photos top and bottom of the board. Circled Z45 top and bottom in yellow although I'm sure its obvious to most folks which IC Im referring to.

It looks like its just two 2102 Ic's piggybacked? I am guessing one or both could be the culprit of bad ram. I need to socket the IC and test a new one in it. Can someone better explain the piggyback mod?
IMG_20211119_111235.jpg - model 1 2102 mod pic1IMG_20211119_111242.jpg - model 1 2102 mod pic 2
IMG_20211119_111326.jpg - model 1 2102 mod pic3 underside
 
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Well I have tried all possible combinations of VRAM and no change. I purchased 9 more chips of 2102 RAM from Jameco, thanks Eudimorphodon.
So once all these 2102 ram chips die, does the Model 1 with it? There is no alternative right?
 
I wonder if it might be worth cooking up a simple Arduino sketch or similar to test the ICs, I’m a little concerned you might be barking up the wrong tree and the issue is elsewhere. Some photos of the symptoms might help shed light on the likelihood of it being bad RAM.

Re: the piggyback mod, the problem there is there are several different variations of non-radio-shack lowercase mods, and the exact details vary a little. The general common thread is the additional chip is there to add an eighth bit to the originally seven bit video memory so the machine can support enough characters for both lowercase and graphics. (There are 64 graphics characters, which doesn’t leave enough out of the 128 you get with seven bits for lowercase.) Where the devil is in the details is how the mod undoes the weird hackers that Radio Shack did to shuffle around the ASCII character set to work with the weird way they allocated their brain damaged video memory. (In retrospect it really comes off as stupid they went to so much effort to save about $5.) If you just remap everything so it’s a proper 8 bit video memory then Level II basic will display weird graphics characters instead of uppercase letters because Microsoft decided it use the fact that with the original arrangement writing zero through 31 to memory produces the same character as 64-95. Some third party lowercase mods don’t care and expect you to use the switch if you don’t have the driver installed, others do weird things to either translate writes *to* video memory into the “corrected” value, or to hijack reads headed to the character generator to accomplish the same thing.

The official Radio Shack mod worked around it by subbing a new character generator that has two sets of uppercase letters in it.
 
Verault,
Ian explains the method he uses in this VCF posting.

http://www.vcfed.org/forum/showthre...l-1-in-2016/page2&highlight=Model+1+Lowercase


I simplified it this way which has served me well since the 80s.

If your character generator (Z29) last 4 numbers are 3001 or 6670, then you do the mod Lowercase with Upper from Custom TRS-80

If your character generator last for numbers are 6673 or 6674, then you do the Radio Shack lowercase mod, from the same book.

All other LC mods are complicated or unnecessary in this day and age, and in fact when I buy systems with other mods I undo
them and set them up as listed, above.

For the record:
3001 : early systems, lowercase but no descenders, and a flying 'a' due to Motorola manufacturing fault (so they sold these
chips cheap to RS who figured they didn't need LC anyway)
6670 : mid-production systems, same as above but with no flying 'a'
6673 : late systems, custom part made especially for the M1 for Radio Shack and included with most late 79 and 1980 machines.
By this time RS was selling the LC modification "kit" for $99 but if the computer already had the 6673 then the 1-min job
was to cut the trace, install the stacked 2102s into the already fitted socket, and solder two wires. It probably took
longer for the soldering iron to warm up than to make the mod.


Basically, the Lowercase Mod is shown in the Following PNG file. Extra 2102 Z45A is placed on top of Z45 and one PCB trace is cut.
Two Wire Jumpers are added.

Model1_Lowercase_Mod.png.

Larry


Model-1_Lowercase_Mod.png
 
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