Help fix my Robotron! RAM error and no video..

Lescell

Active member
Joined
Feb 1, 2015
Messages
250
Reaction score
25
Location
Connecticut
Hey guys, figured I'd start a thread since I've spent lots of hours on this as a noob and not much luck at all.

I have a Robotron that I want to play so bad but it's got a few issues.. One, no video. Two, RAM error.

Lets start with the video..

First thing was to play with all the color/screen/hold pots on the neck and flyback. I can get colors and brightness to work but no game image. I've swapped out the chassis for a working chassis because of a whoops I had and get the same issue, colors and brightness, but no game image. The only thing I didn't try was swapping out the complete monitor, should I?

I'm not sure what would cause no video so this is where you guys come in. Let me know what else I can check, etc. I've rung out the purple video wires from the board to the monitor and all check out good for continuity, as far as the cable goes.

Next issue is a RAM error. I was getting RAM error 1-3-1, and noticed that chip 31 was literally missing. I pulled a 4116 from my Defender, put it in the empty spot and then got a different RAM error, I think it was 1-2-8 or something, but it seems like every time I power down the game and unplug something or swap a chip, I get a different RAM error. So, I had to order some things from Arcade Shop so I decided to order the 4164 chips and the adapter. They sold 4116 in bulk but were more than the 4164 so i decided i might as well upgrade. So I did and now get a 1-1-2 RAM error. I've cleaned a few other socketed chips with no luck.

I'm also getting only 4.79V which is on the low end but should still be ok? This is tested at the chips, but all the way back to the power supply, I get 4.92V or so. I've replaced all of the caps on the power supply expect the large one (not in stock at GPE) and the essential connectors.

So, with all of that said, what are some things I can look at to fix the no video and RAM error?

Thanks!
 
Hi,
Before you can troubleshoot anything, you need a good clean +5.00 to +5.05vdc to the boards. Personally I go for +5.03vdc.
That is where you need to start.
Search around for the Williams P/S pot hack, where you rebuild the power supply and add a pot in lieu of a resistor and that makes the P/S adjustable but be sure that all your power connectors are clean and free of oxidation or you will just have more problems.
Jeff
 
What else could cause a low +5v? All of my power connectors have been replaced. The only component I didn't replace yet was the 18,000uf capacitor on the power supply board..
 
I would replace that cap as it is in the +5v circuit.

Also, as per the manual, you can remove R10 and that will raise the +5v rail.

See attached
 

Attachments

  • Robo PS.JPG
    Robo PS.JPG
    103.2 KB · Views: 38
I've seen the ceramic resistor right next to the DC output plug go open/out of tolerance/physically break... that's for +5.

I replace all power supply headers, but verify that the solder isn't cracked at your bottom heatsink header. those transistors are for the regulated +5 (6057) and +12 (3055) voltages. the 3055 is kind of a common failure part and should be replaced should you redo the power supply.

PM me if you have additional problems.
 
So here's what I've accomplished so far... Nothing! Well, I did get the voltage up very close to 5V (4.93V) by removing R10. Ok, I think that's suitable for now, but still have the RAM error and no video on the screen. I even swapped out monitors for the heck of it and still no video.

What to check next? I'd even consider sending this out to someone who knows their stuff instead of letting this great game just sit here unplayable :/
 
Maybe it is just me but I do not think that +4.93vdc is sufficient.
BTW, where are you reading that voltage?
 
I'm taking that reading right at the RAM chips.. Can low voltage cause a no picture issue?
 
I am not an expert with the Williams boards but as a rule of thumb, I always set the voltage to +5.05vdc, (unless there is an exception). Low voltage can cause all sorts of issues, including the game not running.......
 
Typically, even when there is bad memory, the game will display something on the screen. Make sure the decoder PROMs are in correctly and the right place. Given that you are seeing an error code from the ROM board, the game program is running. And since you are not getting a solid 1-3-1, the CPU path to the memory is also functioning at least partly.

Since you have no output at all, I would suggest checking the video address generator circuitry to be sure you are getting sync and load pulses. Get out your logic probe and check the 9316 counters to be sure they are advancing.

The Robotron/Stargate troubleshooting guide has some good tips on where to start.
 
Headers have been replaced. Right now it'll flash 1-2-1 twice, and then boot to "0".

When I got the game, it game me a 1-3-1, but realized RAM 31 was literally missing. I replaced it with one from my Defender and got a 1-8-1, swapped some RAMS around and got a 1-2-1. 4164s and adapter were installed and now a 1-2-1 every time I boot it.

"Since you have no output at all, I would suggest checking the video address generator circuitry to be sure you are getting sync and load pulses. Get out your logic probe and check the 9316 counters to be sure they are advancing."

This is a different language for me, I don't own a logic probe and don't know how to check for sync and load pulses.
 
I don't own a logic probe and don't know how to check for sync and load pulses.

Don't own a logic probe?
That's OK, use your oscilloscope. It's a much better device to use anyway.
Won't get too far debuggin' these boards if you don't have the correct tools. :cool:
 
Won't get too far debuggin' these boards if you don't have the correct tools. :cool:

+1
If you don't have an o-scope or logic probe you have probably gotten as far as your going to. Without the tools about the only thing you can do is look for visible problems. Burnt parts/traces, open/cut traces etc., re-seat parts in sockets/connectors and cross your fingers. BTW 4.93V at the parts is PLENTY good, you can rule out low voltage. I assume you are measuring with a DMM, if so one other thing you can do is set it for AC volts and measure the +5V. This will tell you how much ripple voltage you have which tells how well the PS caps are working. You shouldn't get any more that .1-.2VAC if the output cap is good.
 
so, I've encountered several Robotrons (why those, I don't know) where removing R10 didn't really help the voltage much. I'm looking at the Sinistar drawing set now and it says...

if TP1 is still less than +4.95 VDC, then check precision resistors R25 & R26, if they are within the 1% tolerance then check IC2.

R25 is 2.15K, and R26 is 5.11K, 1% tolerance. there's a ceramic resistor next to the +5 pins on the DC output header, it's a 0.12 ohm 5W ... I've seen these go wonky too. on a Sinistar the middle broke out and it was causing intermittent +5. :)

as has been noted however, 4.93V is plenty for normal operation. in fact, the Robotron at work has the same exact problem, and I replaced the caps, voltage regulators, and ceramics all around.

check the 74LS257 chips, all pins should pulse except 8, 15 (low) and 16 (high). check the 7489s as well, all pins pulsing except 2, 8 (low) and 16 (high). beyond those, it gets really F-ing silly. your Q1 (blue), Q2 (green), and Q3 (red) (RGB outputs) also have to be working, they're 2N4033s.
 
take a few pics of the CPU board making sure that the video cable is in the pic and another pic showing how that cable is connected to the monitor. Post those pics here.

Even a completely dead WMS board set will usually draw something on the screen.

also, as mentioned, make sure the decoder ROMs are in the correct sockets.

I've seen WMS boards run with the +5 as low as 4.2 so your low +5 is likely not the problem.
 
Here's what I've done since my last post..

RAM error I get is 1-1-2. Swapped RAM 1-2 with 1-3 and get same error. Took RAM out of socket and checked a few traces, they are good. Put RAM chip back in and checked continuity from chip pins to back side of socket pins, all checked out so socket seems fine. Reflowed socket pins for the hell of it, all good. What next? Should I check ALL traces for that socket?

L87uCBu.jpg


hbHsF3S.jpg


PxfbGEh.jpg


KF3h6kd.jpg


4I3WLfK.jpg
 
The only thing I haven't done that i can do myself is replace the header and connector on the chassis for the purple video cables. I should also replace that 0.12 ohm 5% 5W resistor on the power supply as that one gets pretty warm while the other 2 ceramic resistors stay quite cool. Would this cause any of the issues I'm seeing?

"arcadepartsandrepair.com" carries a 0.10 ohm resistor, would this work as a substitute or is 0.12 required?
 
Last edited:
Back
Top Bottom