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Columbia Data Systems 1600-VP CRT issues

Coder

Coder

Experienced Member
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Apr 17, 2023
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I recently picked up a Columbia VP, one of the early IBM clones. The CRT in it displays but it's not giving a usable picture (see attached image). I've discharged the high volatage in the anode and removed the CRT board. Inspecting the caps and other components, I don't see anything obviously leaking/burnt/damaged. I get 12 volts at the connector for the board when it's disconnected. I believe I was getting 8v when it was connected to the board, I'll have to double check. The ground and 12v don't appear to be shorted at least.

Does anyone have a service manual for these or can give me a direction to go in? I'd like to get this this up and going.

Although I'm not well versed in electronic components, I'm technically minded and can follow along if someone's willing to guide me.

I only have a multimeter to work with but might get a scope and try to learn how to use it. I've been watching a lot of Adrian's Digital Basement and can see that scopes can really expand the diagnostic capabilities with these early computers.
 

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You have horizontral deflection with foldover and while I can see retrace lines in your your green box either you also have poor vertical deflection or weak high voltage.

Absolutely start with a complete analog board recap. Just because they don't look bad doesn't mean they're still 40 years old.
 
Very nice, I've also got a Columbia Data Product VP. Mine also has an issue with the CRT board - the linearity coil on the CRT board got smushed (the CRT came loose in shipping and damaged it, but fortunately somehow didn't crack the CRT itself).

I'd be interested in seeing a photo of your CRT board to see if it differs from mine.

I doubt I can help much, but at least it looks like you have high voltage and your CRT is working. That strongly suggests it can be fixed.

Does the rest of the machine do anything? If the motherboard is going, it should beep and after a few moments access the floppy drive.

8V? Might check the voltages at other places such as the floppy drive power connector.

Also, check and possibly clean the power connector to the motherboard. My 1600 MPCs that use the same plug have issues making good contact.

Also, just a BTW, if this has a different BIOS rom than the ones that are archived out there, a dump would be appreciated. A later update supposedly fixes compatibility with VGA cards. (VP uses the same BIOS as the desktop 1600 MPC)
 
Thank you NeXT. I've started cataloging the values of the electrolytic caps. I've wondered this for a while; should I be replacing ALL of the capacitors or just the electrolytics? Do the other types have the same issues? I know tantalums can short when they fail, but I don't see anything that I'd identify as tantalum. That said... I don't know all of the types on this board. There seems to be quite a variety.

SomeGuy:

It seems people don't like to take care when shipping these. Mine came loose in an oversized box with only a small roll of scrap bubble wrap to cushion it. Luckily, it arrived in the condition it was described to be in.

20230425_232127.jpg
I've tested the voltages as they come out of the power board. I would like to ask a question about that. There are several blue wires coming out of the power board marked as 24VDC on the PCB. But I only see ~12VDC on my meter. The manual's specifications for the power supply are +5VD 10A, +12VDC 3AMP, and -12VDC 3A. I wonder if I'm testing the power supply wrong. I used the chassis for the negative. I wonder if the "24VDC" that is marked is -12VC to +12VDC. My lack of knowledge fails me. Do you get 24VDC on that wire? The purple wire marked +VDC is the wire where I read 8V with the CRT board connected and 12V without it. I may plug the board back in and see if it drops back to 8V. That would indicate a component is damaged and "pulling down" the voltage if it does, doesn't it?

A few seconds after turning it on, the drive seeks. And if I put a disk in it, it spins. I put a DOS disk in but it didn't seem to do anything past that. There was no beep. The speaker pin header has more pins than my speaker, so I wondered if it was plugged in wrong. The lack of information on these machines is surprising. Can you post a photo of your speaker connection so I can make sure it's connected correctly and is the correct speaker?

I have some Deoxit on the way. I've misplaced the can I was using, but I'll clean the connection when it gets here. Thanks for the heads up.

As far as the BIOS goes, I wouldn't hold my breath. My unit appears to be a very early build. It has a very low serial number (317) and the power cord is non-removable. I am curious if there are other differences in the hardware.

Here's the CRT board:



20230428_203954.jpg20230428_204112.jpg
 
Looks like the BIOS is a disappointment:
(see attachment)
 

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Ok, not too surprised about the BIOS. I think most units out there have this version, but thought I would ask.

Ok, so the voltages I see on the power supply with everything hooked up and running:
Blue: 11.87v
Red: -12.31v
Purple: 11.90v
Black: ground
White:: 5.00v

It looks like the "24v" is the difference between the +12 and -12. Also looks like the 12v on the purple wire to the CRT board may be a separate source.

Mine has a 4 pin header for the speaker, but the plug has only 3 pins
(back of unit)
[] Not plugged in to anything
[] White wire to speaker
[] No wire
[] Black wire to speaker
(front of unit)

At a glance, that CRT board looks about the same as mine.
One small difference, next to the green wire on the board that runs to the CRT socket, yours seems to have a pluggable pin for a ground wire while mine has a black ground wire soldered on to it (that the other end is screwed on to one of the grounded mounting screws on the power supply)

I'm thinking that is redundant ground, but if there was a wire to it, make sure it gets connected.

As far as electrolytic capacitors, the catch is ideally you only want to replace things if you really suspect there is a problem. BUT when electrolytic capacitors start to go bad, they can create seemingly unrelated odd issues that are hard to track down. You risk damaging things by replacing caps willy-nilly.

With tantalum they either short or go boom, so not as much of a headache.

Also, just FYI that one huge low capacitance-value capacitor by the linearity coil has no direct modern electrolytic replacement. It has been suggested to use (ceramic?) capacitors designed for use in microwave ovens. But from the looks of things, I doubt there is a problem with that one.
 
Looks like my original speaker is dead. I took one out of a 386 I have have laying around. That one was a 4 pin connector, but after moving the wire to the 3rd pin like the stock speaker, I now get a beep on boot. I wish I had some way to get a video signal out of this thing without the internal CRT. The only ISA video card I have has a db9 plug and I don't have any monitors that use that connector. I have a VGA card on the way, but I stumbled across your post from your MPC and it sounds like VGA cards won't work. So I'm back to trying to get the CRT working. I do wonder if there's something I could do to get video out of the 3 wires that come from the motherboard to the CRT connector. I have to assume it's video, H sync, and V sync. Is there any way I could utilize these signals to get some form of display working? Something like composite, component, or similar?

Speaking of the CRT, it turns out the Kaypro machines used a very similar CRT by the same manufacturer. I've been reading though this thread. I wonder if they are interchangeable? They look like they use the same connector and the model number is very similar. And you are correct, that's a pluggable pin for the ground. The other end connects to one of the power board's mounting screws. I wonder if that indicates that they had problems with the pluggable design.
 
I started searching for other machines that use this CRT and that might have better documentation. I found the Intel iPDS 100 also uses it and a gentleman named Mark Ogden has a site where he has archived service documents. Luckily, Intel supplies the pinout of the CRT edge card in the Field Service Manual, under the CRT specifications section. Using that as reference, it looks like the VP uses the same pinout. The VP doesn't make use of pins 1 and 5 (Ground and Chassis Ground, respectively). Interestingly, the ground we've been discussing that is pluggable on mine and soldered on yours, looks like it could have been on pin 5, since the Intel pinout shows that as "ARC GND (chassis)". I wonder why they chose not to use the connector pin for it? Perhaps there's other components in the way. Here's the pinout:

Colorized with VP wire colors by yours truely:
CRT Pinout.jpg

The Intel iPDS Documentation:
1682828113805.png
At the very least, between the two of us, we're slowly giving some details about this model of computer. Hopefully it's of some use for someone in the future.
 
First of all, I would take the floppy drives out and verify their functionality on another computer. They may need a good cleaning.

The desktop Columbia Data Product 1600 MPC has a feature in the BIOS that enables text BIOS I/O to output to its serial port. Flipping the first DIP switch on the motherboard enables that. I've booted generic DOS 6.22 on a 1600 MPC before and was able to use most command line tools with it via a terminal program running on another computer.

The BIOS on the VP is the same, but I have never tried that on the VP. The case has to be disassembled to get at the motherboard DIP switches.

The video signal to the CRT board should have the same sync timing as composite video. I'm a little surprised they did not include a composite output port for an external monitor on the machine. The Compaq Portable did.

I had noticed some early Kaypro computers used a similar CRT board. I had also wondered if they would work.
 
Chuck(G) said:
A MIHV-to-composite video is pretty easy to construct. I used this circuit over 40 years ago to adapt our system's video output to a 24" monochrome studio monitor for classroom use:
See Option 2
Click to expand...
I took a look at option 2. I'm no electrical engineer (I'm a code monkey!). I can read part of the schematic, but not all of it. My first question is what's the IC labeled 7486? I googled "7486 IC" and "7486 chip" and didn't find anything that sounded correct. I assume it's an IC anyway! Also, I have some questions about other components. Some I think I understand, some I definitely don't:

1682967992036.png

1) What is this component and I see that it connects to the HSYNC, but does the other side of it connect to something else, or is this something that sits in-line?
2) Is this an adjustable 2k2 pot? Is the 2k2 value it's upper, or "highest" resistance value?
3) I don't think I have an intensity output available, although the connector for the video signal where it plugs into the motherboard has an extra, unused pin. Perhaps this is intensity? I don't know how I would verify that.
4) I believe this is a switch to switch between MDA and CGA video input, is that correct? It looks like there would be two of these switches, one at the input (between pins 6 and 7) and one at the IC connecting pings 13 and 14 for CGA and connecting pin 14 to Ground for MDA?
5) That's just a connection to +5VDC, correct? Won't the switch to MDA that goes to ground cause a short? Or will this just "pull the pin down"?
6) I know that this is a diode, but does "2x 1N4148" mean "two 1N4148 diodes" and if so, are these in series or is the second diode the one shown below it going the other direction?

Sorry for all the questions. Just trying to wrap my head around it all.

I bought a GBS8200 hoping it would be able to translate the signal to VGA since it purports to do CGA and EGA. This was unsuccessful. Is that because my signal is actually MDA? I've also read that these GBS8200's need an analog signal and that computers usually output digital. Is that part of my problem?

SomeGuy said:
First of all, I would take the floppy drives out and verify their functionality on another computer. They may need a good cleaning.

The desktop Columbia Data Product 1600 MPC has a feature in the BIOS that enables text BIOS I/O to output to its serial port. Flipping the first DIP switch on the motherboard enables that. I've booted generic DOS 6.22 on a 1600 MPC before and was able to use most command line tools with it via a terminal program running on another computer.

The BIOS on the VP is the same, but I have never tried that on the VP. The case has to be disassembled to get at the motherboard DIP switches.

The video signal to the CRT board should have the same sync timing as composite video. I'm a little surprised they did not include a composite output port for an external monitor on the machine. The Compaq Portable did.

I had noticed some early Kaypro computers used a similar CRT board. I had also wondered if they would work.
Click to expand...
I have plans to tear them down and clean/lube everything. That part I've done with great success on a few machines that had inoperable drives. But I'm focusing on getting some usable video signal of some sort first.

This might be an option, although I'd really like to get the onboard video displaying either to the CRT or to an external monitor of some sort. But it's good to know that it has that capability. At least that way I can see if it's booting and functional.

I had hoped I could just hook the video/sync signals up to the GBS8200 board or directly to a VGA monitor, but neither worked. The GBS8200 flashed a green solid screen a couple times, then went black and stayed there. Hooking the wires directly up to VGA's Green, HSYNC, VSYNC, and GND pins just got a "Signal out of range" message on both LCD screens I have that have VGA in. My understanding is that to get composite out, I would need to combine the HSYNC and VSYNC together using either logic gates or an IC designed for the task. It also sounds like the signal from the board may not be in the right frequency range for my modern hardware to pick up. Is this correct?

I'm kind of surprised someone hasn't come up with a pre-made board for these types of things. Then again, I guess in most cases, you'd just stuff a video card in it and call it a day. I do have a pair of ISA video cards on the way from eBay. One is an 8 bit VGA card that I bought before reading your thread where you mention the newer BIOS being required for VGA, and an 8 bit CGA card with both db-9 and composite output. I'm hoping between the two of them, I can get some sort of output!
 
That schematic is a little oddly drawn. For the IC, search for the more common Texas Instruments part number: SN7486 that is a Quadruple 2-input exclusive-or gate.

I think the part they are trying to show is a polarized capacitor, but 10M is not the right designation for a schematic, it should be "10 µF". However, most small 10 µF tantalum capacitors will be marked "10M". The negative side would connect to ground.

I don't know why they have a switch for MDA. That is a completely different timing and will not work on standard composite. I think there were only a few early oddball monitors and machines that use a composite signal with MDA timings. (Texas Instruments Professional computer, and possibly the Eagle 1600), Perhaps that RGB converter can sync that.

Pin 14 will always be +5, that is the power pin, input on 13 will be either +5 or ground depending on the switch.

Yes, that means two diodes.

So, on this CDP, you don't have an intensity line. Instead you have an analog "video" line that, I think, should have two different voltage levels depending on intensity. With the above circuit you would just get black and white, no in between, but that should be enough for testing.

Also, aside from a few special ones, almost no VGA monitors will sync composite or MDA frequencies.

BTW, because the CDP VP has the CGA card built-in, it would be necessary to disable that in order to use VGA. I don't know if that is possible. Now, MDA or Hercules cards can co-exist with CGA, so that MIGHT work if the BIOS can default to using that instead of CGA.
 
Okay, for those who didn't grow up with a soldering iron in their hand:

1. SN7486 (74LS86 or 74S86 will do just fine) Quad XOR gate here
2. That's a 2.2K trimmer resistor--you adjust for correct sync amplitude
3. If you want, you can leave that branch of the circuit, from the INTENSITY input to the point where it joins the VIDEO OUT line. Or leave it in--no matter.
4. The switch is ganged, so a DPDT switch; the idea is that it inverts the green signal. If you wish, you can ignore the CGA circuitry.
5. Yes, it's +5, but the switch common that goes to pin 13 never connects the +5 to ground.
6. The writer is just noting that his circuit has 2 1N4148 diodes in the intensity circuit branch. Of course, if you have no intensity pin, the question is academic.

Hope this helps--if you're still bedaferated, I'll draw up a KiCAD schematic.
 
So would the 3 wire video signal coming from the board be CGA, or is this some other format meant specifically for the CRT? I'm trying to understand what I have and what I don't have. Thanks for all of your help!
 
Interestingly, there is an unused 6-pin header next to the BIOS by the video section. Could be anything, but makes me wonder if it could be RGB CGA output related.

I'd keep fiddling with that converter. The sync rate should be the same as composite/RGB CGA (note that in the video game world, CGA means something a bit different). Unless the signals are inverted or something the sync probably would work. Then you would just get the video in one color. It could be that the "analog" video signal voltage is not quite right for the TTL input, so you are just getting a blank or filled screen.
 
Chuck(G) said:
Okay, for those who didn't grow up with a soldering iron in their hand:

1. SN7486 (74LS86 or 74S86 will do just fine) Quad XOR gate here
2. That's a 2.2K trimmer resistor--you adjust for correct sync amplitude
3. If you want, you can leave that branch of the circuit, from the INTENSITY input to the point where it joins the VIDEO OUT line. Or leave it in--no matter.
4. The switch is ganged, so a DPDT switch; the idea is that it inverts the green signal. If you wish, you can ignore the CGA circuitry.
5. Yes, it's +5, but the switch common that goes to pin 13 never connects the +5 to ground.
6. The writer is just noting that his circuit has 2 1N4148 diodes in the intensity circuit branch. Of course, if you have no intensity pin, the question is academic.

Hope this helps--if you're still bedaferated, I'll draw up a KiCAD schematic.
Click to expand...
Thank you Chuck. I grew up taking apart everything I could and *sometimes* managed to put it back together (sometimes they even worked when I was done!), but never got quite to the circuit level with any real understanding. I've played with Arduinos and have done some circuit repair on laptops, but only basic things like DC power ports and the occasional obviously bad component (burnt). I want to dive deeper into circuit design and repair though, so I appreciate your patience and the explanation. I took the software path professionally, but I've always felt that circuits were very similar. It's all logic. You have expected inputs, they get transformed in some way, and you get an expected output. Obviously there's more nuance than that, but I think the mindset is the same. And I get an incredible high when I finally get things working, so I'm motivated to learn. So thank you both for your help so far!

On the capacitor front, I was going to order them through Mouser but quickly hit the knowledge wall again. It never ceases to surprise me how many options there are and I have no idea what matters and what doesn't. So I have some assumptions I'd like verified, and I have some questions. For all of this, I was looking to replace a 10uf 200v 85°C electrolytic capacitor:

Assumptions: Are these correct?
  1. I believe I'm looking for capacitors of the "Aluminum Electrolytic radial leaded" variety, as far as I can tell
  2. The voltage rating can be MORE but not LESS (220v is ok, 100v is not, in this example)
  3. The uf rating (microfared, if I understand correctly) must match exactly
  4. The temperature rating is a minimum operating temperature, so higher values are ok (105°C instead of 85°C cap would be ok)
  5. Some descriptions include and "hour" rating like "2k hour" or "10k hour". I assume the higher this number the higher the quality/reliability? I think it's the expected service life.
  6. Some are marked "AEC-Q200", I understand this to be an automotive standard that ensures the capacitor can handle rough environments. All things being the same, I don't NEED a capacitor with this rating, but it's nice if it does.
  7. Some mention 3.5LS or 5LS. I think this is "lead spacing" (I assume in mm). Do I need to measure this too? Is it ok to just bend the leads to get them where you need them? Nobody seems to talk about this value when talking about replacing caps, so I assume it's not that important.

Questions:
  1. Some capacitors have a percentage with them. It seems 20% is very common. I've read that this could mean that the actual value ranged +-20% of the rated value. It sounds bad. Should I avoid capacitors that have this listed? If it's not listed, is the value "low enough not to matter" or is it just a difference in what they decided to include in the description for each cap?
  2. Similarly, many do not include the "2k hour" or "10k hour" rating. Are these lower quality, so the hour rating is not advertised, or is there a "default value" for this that I don't know about?
  3. I've heard the names Rubycon and Nichicon thrown around when people discuss buying caps. Are these decent brands?
  4. Beyond the uf/v/°C, are there any other values you think I should consider when ordering?
 
A qualified "yes and no" on this.

1. Aluminum electrolytic are the most common and cheapest option in this case. Do bear in mind that eventually all wet electrolytic capacitors will fail. The question is "after how many hours of service". Today's caps are rated at 10K hours or more.
2. Generally, that's the case, so long as you don't go overboard. A 600V capacitor has different operating characteristics than a 16V capacitor. ESR is the biggest thing to pay attention to.
3. Depends on the application. Know thy circuits. Recall that electrolytics can have values that range up to ±30 percent of the marked value and still be considered "good". A circuit that uses a 1000 µF cap will probably work just as well with a 2200 µF one. On the other hand, if this is a timing-dependent circuit, you should select a value that matches precisely.
4. Generally yes--but again, watch that ESR.
5. Generally MTBF, which tends to be a bathtub-curve sort of thing.
6. If you're working in automotive electronics, use the caps with that rating. Automotive stuff tends to get exposed to more extreme conditions.
7. For replacements, you can fudge lead spacing quite a bit. When repairing very old electronics, this can be problematical, as old components tend to be physically much larger than newer ones.

Questions:

1. Aluminum electrolytics are not precision devices. ±20 percent variation is to be expected. The circuit designer takes that in mind during the design process. If you want precision, use a different technology, such as film, ceramic or tantalum.
2. I wouldn't let it bother you--that rating is the MTBF of operating hours. But see my (4) below.
3. Rubycon, Nichicon, NCC, Panasonic are all top-line brands; however, be aware that there are Chinese forgeries. I prefer NCC myself--seems to be a happy medium between cost and reliability.
4. Uh, yes, you left out ESR, a very important parameter. The gist is that a capacitor isn't purely capacitive--it can be modeled as a capacitive element in series with an inductor and a resistor. The inductance probably doesn't matter too much in most circuits, but can be an issue in high-frequency applications (you're probably better off going with a different technology). The ESR or Equivalent Series Resistance is a very important aspect, particularly when there's a strong AC component to the applied voltage. That implied series resistance will cause internal heating, which eventually does the capacitor in. It can also affect the way a capacitor performs in a circuit. Low ESR is generally best.

I'll add that solid-electrolyte capacitors are becoming more common particularly in surface-mount applications. They can't absorb voltage spikes as well as the wet electrolyte type (so-called "self healing"), but they're pretty robust otherwise.
 
Coder said:
So would the 3 wire video signal coming from the board be CGA, or is this some other format meant specifically for the CRT? I'm trying to understand what I have and what I don't have. Thanks for all of your help!
Click to expand...

Are we sure this isn't just straight composite? (Granted that only should require two wires, but the third could just be a "monitor turn-on" signal or something.) I don't have one of these machines but the photos I've seen indicate that the monitor can output CGA in shades of gray, which it would not be able to do if this was a fully digital connection. I assume this three-wire connection runs to some intermediate board upstream of the monitor's 10-wire edge connector? I don't suppose anyone has any good photos or (better) a schematic of that board? It may well be a composite add-on for the monitor chassis.

An in-between possibility might be that the three wires are an analog "intensity" line, composite sync, and a ground, IE, not quite regular composite, but close. If that were the case you might be able to run an alternate monitor with little more than some mixing resistors.
 
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