TRS-80 Model 1 Monitor question

[Divarin]

Last weekend I picked up a TRS-80 Model 1 monitor (just the monitor) at the flea market. The seller said he had the computer at home and would bring it next weekend, but in the mean time I wanted to test out the monitor as much as possible.
It powers up and I get a raster, it doesn't appear to be vertically collapsed or anything as far as I can tell it's fine.
So I wanted to put a composite video signal into it and started looking for the pinout. The best info I could find is this page which shows the two pins I need to connect:


So realizing that I'm working with the other end of the plug (the monitor, not the computer) I need to mirror the orientation which gives me:


So I brought out my handy-dandy composite video generator (one of those Atari plug & play games that I keep around for just such a purpose) and first testing it on an LCD monitor to confirm the batteries are still good and it's still working (it is) I went about connecting the video out of that to the monitor.

I don't have any spare 5 pin din connectors at the moment so just for a test I used two clip leads, making very sure that each was only touching the correct pin and not the neighboring pin. However I couldn't see anything pop up on the screen.
I thought maybe the monitor just didn't like the output but I would expect to see some hint of something when I connect it, I didn't even see a flicker ... no indication on screen at all. The screen itself is working I can turn up the brightness and see the gray glow but other than that nothing.

Do I have the pin order wrong here? If these are the correct pins I guess the next step is to open up the monitor and check that there's no break in the video cable but thought worth checking that I'm even connecting it right before I do that.

Also, I read about the monitor's optical isolator circuit (what the 5v goes to) maybe the monitor is not able to display anything unless that is powered?

 

[Eudimorphodon]

So:

Per the pinout here, you'll see that the TRS-80's video connector also has a +5v output. The reason for that output is the TRS-80 monitor was built out of an extremely cheap television chassis that doesn't have an isolation transformer to protect signal ground from potentially carrying line current. To fix this the cheapest way possible Radio Shack added an optoisolator circuit to "air gap" the TV guts from the computer... and that circuit is powered by that 5v line. With that left disconnected the monitor is going to be blind to any incoming signals.

If you can provide regulated +5 volts to that circuit it should be able to "see" the incoming signal from a normal composite source. Don't get your hopes up with regard to quality. (Also, be sure not to wire it up backwards; if you melt that optoisolator you'll have to repair it before it'll work on a TRS-80 again.)

 

[wmcbrine]

Yes, you absolutely have to provide the 5V.

 

[Divarin]

Okay that makes sense. In that case I'll probably just wait until after this next weekend and hopefully be able to pick up the Model 1 from that seller. If, however he doesn't come through or someone else picks it up before I get there or we can't come together on a reasonable price or whatever I might be looking to build an adapter cable to re-purpose this monitor for something else. If that happens I'll need to send 5 volts to the optoisolator. Is the "Computer Ground" pin used for both composite video ground and the 5v ground?

 

[Drayson]

Hello there, I also have a TRS-80 video monitor. I recently found out how to use mine with other monitors, and how I did it was I simply connect the 5v input on the connector to the 5v output of an arduino, and inputting any composite (or s-video luma) signal will work. And yes, the ground pin is common with the 5v and video inputs, the remaining 2 pins in e connector are NC.

Here, I have slightly modified my connector and soldered a diode to the 5v input (for added protection):
Here are some pictures of things I have connected to the monitor:

 

[Divarin]

Okay I will hopefully be picking up the Model 1 computer tomorrow if that seller comes through for me but I could use some knowledge to help me negotiate a good price. I understand that the Model 1 had some different revisions, some with 4k of ram and some with 16k. Also there's earlier ones with Level 1 basic and later ones with Level 2 basic.

The basic rom I'm not too concerned with as it should be pretty simple to write the L2 basic rom to a chip and pop it in but I don't know how easy it would be to upgrade a 4k board to be 16k.

Without being able to hook it up and test it (it's an outdoor flea market with no power), and without being able to open it up and look at the internals, are there any clues that I can gather from looking at it? I know some of the later revisions have a numeric keypad so that's one thing and I think there might be some clues in the serial number or some other number(s) printed on the underside of the computer but I'm not sure what I should be looking for there.

2nd question, I was looking at the power supply situation. I don't know if the seller will have the PSU or not but it looks like it's pretty non-standard so if not I'll be finding myself trying to build my own (or source one). So if he doesn't have the PSU what are my options there? Just so I have some idea of how much I should deduct from the asking price.

 

[Eudimorphodon]

The basic rom I'm not too concerned with as it should be pretty simple to write the L2 basic rom to a chip and pop it in but I don't know how easy it would be to upgrade a 4k board to be 16k.

How easy the BASIC upgrade is depends on the board revision. The very last models are set up to take an eight and four K ROM in the two sockets on the board (12K total) with little hassle, the older ones needed a satellite board to hold three 4k ROMs with some flying wires to other spots on the motherboard. There are modern modifications to change the old boards for modern ROM setups, not a big deal, but be aware you’ll probably need to do some hacking if you get an older one.

The 4K-16K upgrade is well documented and easy, it just kind of sucks if the 4K is soldered down instead of in sockets. I think Radio Shack did it both ways.

2nd question, I was looking at the power supply situation. I don't know if the seller will have the PSU or not but it looks like it's pretty non-standard so if not I'll be finding myself trying to build my own (or source one). So if he doesn't have the PSU what are my options there?

It uses weird voltages out of a multi-tap transformer you can’t buy anymore, so it’s a pain in the neck. There are plans for a DIY replacement (and they’re sold on eBay sometimes) that uses two separate transformers, but just the parts were about $60 last I checked.

Honestly, the regulator circuitry inside the Model I could probably deal with a less exact replacement, but it’s still a pain given it needs both AC and DC at voltages you won’t find in normal wall warts.

I know some of the later revisions have a numeric keypad

Level I units usually didn’t have the keypad. Level II units sold that way or upgraded by Radio Shack usually did. There are plenty of Level II keypad-less M1’s out there, but they’re usually DIY upgrades of Level Is. (Or early? I seem to recall there being a TRS-80 newsletter from Tandy announcing the keypad was standard with level II in late ‘78 or ‘79.)

I am going to toss out a general warning here: Model Is are getting kind of crispy in their old ages. Whatever you pay, try not to be too disappointed if it needs some TLC to work well. Everything that goes wrong with them can be fixed, there’s no truly unobtainium parts, but… there’s a decent chance it’ll want some love. Maybe hint at that during negotiations.

 

[Divarin]

Thanks, that's all good to know. Yeah I expect it will need a little work, I'm trying to size up the time and money investment that I might have to put into it after acquiring it. I did see one of those modern PSU replacements for sale on eBay, looks like after shipping it's right around $100. I also looked into making my own but saw the same thing you mentioned that the parts are going to be at least $60 (two transformers at $30 a piece plus some other cheap components) so after you factor in all those components and the shipping on them and the time investment in putting it all together I might as well have just bought the $100 one fully assembled.

 

[Eudimorphodon]

The last Model I I bought was from an eBay listing that was obviously a storage room cleanout. The guy broke a TRS-80, expansion interface, and monitor into three lots and priced them at $200 each, which I wasn't going to pay. A couple weeks later he dropped the price of the "keyboard" to $100 and I went ahead and got it, thinking worst case it's spare parts for my other Model I. $100 for *just* a model I keyboard unit in unknown condition is probably about going rate, give of take 50 percent or so.

The keyboard didn't come with the PSU because it was inside the expansion interface. A week later the guy dropped the price of the EI to $100 so I bought that too, because obviously that was a heck of a deal for two power supplies. (I took the bet they were in there, and won.) I let him keep the monitor, hopefully someone gave it a good home.

FWIW, that Model I got did need a lot of TLC. I've got the system unit running... "fine", after replacing some passives in the power regulation (that's all on the motherboard in a Model I and it's pretty arcane compared to computers just a tiny bit more modern) and touching up bad solder joints from the DIY modifications it has. (Homemade lowercase mod, "true" reset switch, and switchable Level I and Level II BASICs.) The EI is a whole other ball of wax that I haven't gotten to yet because I've kind of been charmed by the crudity of running a cassette based system.

 

[Divarin]

Well that seller didn't show up this weekend and it's the last flea market of the season. I refuse to pay those outrageous ebay prices so for now I'm giving up on adding a model 1 to my collection. At least I am a lot more knowleable about them so if I know what to look out for, mostly that if I see one without a power supply that's a big deal unlike with a lot of other vintage computers.

 

[Eudimorphodon]

At least I am a lot more knowleable about them so if I know what to look out for, mostly that if I see one without a power supply that's a big deal unlike with a lot of other vintage computers.

I guess I’d call it more of a “moderate” deal; if you really didn’t want to cough up the money for the modern replacement I suspect you could Macgyver something cheaper. (I kind of wonder if you could use a doorbell transformer for the AC component, and there are universal laptop power bricks that can output around 19vdc.)

Do you have a ballpark for what you thought this guy was going to charge?

 

[Divarin]

Good question, I got the monitor for $40 and it came in a hardshell padded case and seems to have been really well cared for so I suspect I could probably have got the computer for around $100.
Yeah the DC voltages are easy, if nothing else I can find something that puts out a bit more voltage and use a buck converter to bring it down to what I want. The A/C can be tricky especially if it's a strange voltage not used by anything else.

 

[syzygy]

Good question, I got the monitor for $40 and it came in a hardshell padded case and seems to have been really well cared for so I suspect I could probably have got the computer for around $100.
Yeah the DC voltages are easy, if nothing else I can find something that puts out a bit more voltage and use a buck converter to bring it down to what I want. The A/C can be tricky especially if it's a strange voltage not used by anything else.

The PS for a model 1 is a PITA

They show up on occasion for too much $$.

I have heard good things about this one and the bare boards are available - seems especially good if you anticipate getting a full blown system with EI.

Decent reference and diy PS sites.

On the hardshell case - if it is commercial (see thread here) it might command a few $$.

Let us know what you come up with.

 

[Divarin]

On the hardshell case - if it is commercial (see thread here) it might command a few $$.

Yeah that photo of the monitor in the case is exactly what I got. Although that photo is a bit dark and I can't make out the foam padding which mine has (see photos below). It also has two cassettes in there. One is backgammon and one looks to be blank with a radio shack label on it.

I think for now I'll build that din plug adapter to enable me to use this monitor on other composite sources but keep an eye out for a good deal on a Model 1.

 

[Eudimorphodon]

Good question, I got the monitor for $40 and it came in a hardshell padded case and seems to have been really well cared for so I suspect I could probably have got the computer for around $100.

When I bought that last Model I (which was only a couple years ago) it wasn't that hard to just lurk on eBay and nab an untested Model I keyboard for $100; I did a little better than average, probably, but that was around the ballpark. Looking at recent auctions now it seems like $200 is more where it's sitting. But at least some of them came with PSUs...

Yeah the DC voltages are easy, if nothing else I can find something that puts out a bit more voltage and use a buck converter to bring it down to what I want. The A/C can be tricky especially if it's a strange voltage not used by anything else.

I mentioned doorbell transformers; the standard output is 16VAC, and you can get a 30VA unit (which should probably be big enough) for around $18. That was the route I was going to look into before I changed plans and nabbed the EI with two PSUs inside.

The voltage isn't that critical on either the AC or DC supplies on the Model I, because all the actual regulation is on the motherboard; it chops the AC input in half with two half-wave rectifiers (diodes) and turns that into the +5/-5v supplies, and the 19VDC (which is also just diode regulated, not smoothed at all, in the PSU) is filtered and regulated down to +12v. You could probably feed it as little as 13.5V on the DC side and maybe just a bit higher for the AC. (The keyboard has a *big* capacitor in it for smoothing the half-wave positive feed for the +5v, that's by far the most heavily loaded of the three feeds. Unless I'm forgetting something I think the +12v and -5v are *only* used for the DRAM chips.)

 

[Divarin]

When I bought that last Model I (which was only a couple years ago) it wasn't that hard to just lurk on eBay and nab an untested Model I keyboard for $100; I did a little better than average, probably, but that was around the ballpark. Looking at recent auctions now it seems like $200 is more where it's sitting. But at least some of them came with PSUs...



I mentioned doorbell transformers; the standard output is 16VAC, and you can get a 30VA unit (which should probably be big enough) for around $18. That was the route I was going to look into before I changed plans and nabbed the EI with two PSUs inside.

The voltage isn't that critical on either the AC or DC supplies on the Model I, because all the actual regulation is on the motherboard; it chops the AC input in half with two half-wave rectifiers (diodes) and turns that into the +5/-5v supplies, and the 19VDC (which is also just diode regulated, not smoothed at all, in the PSU) is filtered and regulated down to +12v. You could probably feed it as little as 13.5V on the DC side and maybe just a bit higher for the AC. (The keyboard has a *big* capacitor in it for smoothing the half-wave positive feed for the +5v, that's by far the most heavily loaded of the three feeds. Unless I'm forgetting something I think the +12v and -5v are *only* used for the DRAM chips.)

That's good to know, if I can get a good deal on a model 1 and it doesn't include the PSU then I can take a look at some doorbell transformers and see if I can find something that will work.

 

[cjs]

Honestly, the regulator circuitry inside the Model I could probably deal with a less exact replacement, but it’s still a pain given it needs both AC and DC at voltages you won’t find in normal wall warts.

The way the previous owner of mine dealt with this was to rip all the PSU circuitry (regulators and a few other things) off the board and just wire it up for a fairly standard +5/+12/-5 switching PSU. It's a bit of a modification, but at least it's an option.

 

[Eudimorphodon]

The way the previous owner of mine dealt with this was to rip all the PSU circuitry (regulators and a few other things) off the board and just wire it up for a fairly standard +5/+12/-5 switching PSU. It's a bit of a modification, but at least it's an option.

It's a shame they dropped -5v on the ATX standard(*), because neutering the power supply circuitry on the board and just hiding one of those Pico ATX PSUs in the box would certainly have its attractions. The system would run a lot cooler, among other things.

(Although you could always just hang an LM7905 off the -12v output and solve that with one additional component.)

 

[Divarin]

The way the previous owner of mine dealt with this was to rip all the PSU circuitry (regulators and a few other things) off the board and just wire it up for a fairly standard +5/+12/-5 switching PSU. It's a bit of a modification, but at least it's an option.

I was thinking that might be a good way to go actually.