Amiga 500 won't boot if keyboard is connected

[ComputerCompulsive]

Hi there,
I'm having an issue where I connect a hitek keyboard to (any) Amiga 500 and the machine simply will not boot up. Unhook the keyboard and everything is a okay.

I'm thinking/hoping it's a short on the keyboard PCB but wondering if anyone else has had this issue? Any suggestions on the best way to diagnose the issue?

Thanks for any input

 

[RetroTheory]

Could keyboard is connected wrong way round , or maybe the reset is being held ?

Hi there,
I'm having an issue where I connect a hitek keyboard to (any) Amiga 500 and the machine simply will not boot up. Unhook the keyboard and everything is a okay.

I'm thinking/hoping it's a short on the keyboard PCB but wondering if anyone else has had this issue? Any suggestions on the best way to diagnose the issue?

Thanks for any input

 

[ComputerCompulsive]

Could keyboard is connected wrong way round , or maybe the reset is being held ?

I don't believe it is the wrong way around as it is the same behavior in either orientation. The only difference I see is if the cable is in the wrong orientation the light stays on for the disk drive. Other two keyboards work on the same machine but are either missing keys or someone badly retrobrighted it and bleached the keys. Ideally I'd like to get this one working.

 

[RetroTheory]

Could you swap the pcb and cable for one of the working ones , or are they different style/manufacturer keyboards?

 

[ComputerCompulsive]

Other other two are the more common mitsumi keyboard unfortunately. Which is why I think I have to determine the fault with this as I don't have any replacement parts

 

[RetroTheory]

Yes and it's a more desirable keyboard . I suppose you need to findout if keyboard is holding reset low or shorting 5v down maybe isolate the reset pin( or wire) and see if amiga comes up
And measure for shorts across caps , or across 5v snd ground pins

1 DATA
2 CLOCK
3 RESET
4 +5V
5 key, removed pin
6 GROUND
7 STATUS (led +5V power)
8 INUSE -"-

 

[ComputerCompulsive]

Yes and it's a more desirable keyboard . I suppose you need to findout if keyboard is holding reset low or shorting 5v down maybe isolate the reset pin( or wire) and see if amiga comes up
And measure for shorts across caps , or across 5v snd ground pins

1 DATA
2 CLOCK
3 RESET
4 +5V
5 key, removed pin
6 GROUND
7 STATUS (led +5V power)
8 INUSE -"-

Okay, is there a good guide out there you can point me at to get me started please. I'm still at the "know just enough to be dangerous" phase of learning

 

[larsovef]

I have the exact same problem. Hi Tek keyboard connected = black screen. Any other working keyboard = fully functional Amiga 500. Have you gotten any further in your troubleshooting?

 

[ComputerCompulsive]

I have the exact same problem. Hi Tek keyboard connected = black screen. Any other working keyboard = fully functional Amiga 500. Have you gotten any further in your troubleshooting?

Nope afraid I haven't yet. I'm new to this so it's a bit frustrating atm trying to figure out how to find the short. Keep hoping to find some examples of how to trouble shoot it but no luck so far.

 

[larsovef]

I just fixed mine. Turned out it was a bad solder joint that I had missed when I was checking the PCB.
Have you checked the capacitors on yours? the hints I've gotten from others is to look at the _KBRESET signal to see if there might be a dried up/bad capacitor holding that signal down.

 

[ComputerCompulsive]

I just fixed mine. Turned out it was a bad solder joint that I had missed when I was checking the PCB.
Have you checked the capacitors on yours? the hints I've gotten from others is to look at the _KBRESET signal to see if there might be a dried up/bad capacitor holding that signal down.

Interesting! I thought I'd tried all the solder joints already but I'll give it another pass and see what happens. I don't have anything to test the capacitors so I guess I'd have to order replacements and just replace them all. Before I order them I might throw a link in here just for a quick "sanity" check to make sure I'm ordering the right ones if you don't mind.

Always prefer a second set of eyes on those as I'm still new to this.

 

[ComputerCompulsive]

I just fixed mine. Turned out it was a bad solder joint that I had missed when I was checking the PCB.
Have you checked the capacitors on yours? the hints I've gotten from others is to look at the _KBRESET signal to see if there might be a dried up/bad capacitor holding that signal down.

Well I'm not 100% certain but I think I may have a different issue. I went ahead and re-flowed the solder joints on the board where the capacitors are and tried plugging in the keyboard to the logic board. No difference.

I decided to pull out the multi-meter again and check all the capacitors and chips to see if any of them were going to ground that shouldn't be. When I got to U2 (see pic) I noticed it had three grounded pins. I thought it a bit suspect since the other chips only seemed to have one ground pin so I decided to pull him off the board with my desoldering gun.

When I plugged the keyboard into the Amiga with the chip removed it booted up just fine! If I put the chip back in (I put in a socket) the machine fails to boot!

So now I guess I need to source a new HD74LS27P to see if this really is the culprit. Anyone have a spare please?

 

[ComputerCompulsive]

Well shoot...
Put in the new chip and the machine boots up! BUT now the caps lock key light is always on and the board is completely unresponsive.
Wondering if it's a cap issue or something else, any ideas?

Note: a few googles suggested replacing Q2 but that made no difference.

Edit: I should also note that placing a different keyboard in the same amiga the keyboard works fine so it's not a computer issue for certain.

 

[ComputerCompulsive]

Well I've tried taking the keyboard to a local repair guy and he beat his head on it for a few days and has come to the conclusion "It's cursed" lol


Here is where he was able to get the keyboard:

Original problem - Keyboard plugged in computer would not boot up.
Solutions - This was addressed by changing out HD74LS27P

Next problem - The caps lock light stayed lite up and never turned off.
Solution - The repair guy took out his oscilloscope and found some missing grounds (apparently on the top of the board) He also found that one transistor I had tried to swap out was actually the wrong spec. Had the legs reversed and I didn't catch it so he swapped it to the right one.

Next problem - The caps lock light no longer stays lite up and instead now flashes the code for bad memory. Keyboard responds to a reset command but nothing else and keeps flashing. Also noted that now the led's are reversed on the keyboard where the drive light is now indicating power and the power light indicating the drive activity.

At this point the repair guy said his best guess is a bad micro-controller. I suggested I bring another controller from another Amiga but he was pretty much done looking at this keyboard at that point so I said I'd try it myself and give him a break.

I've since taken the keyboard home and desoldered another micro-controller from another Amiga 500 before testing it in the other board though I decided to try the existing micro-controller from the "cursed" 500 keyboard in the known working Mitsumi. The cursed controller actually seems to work just fine!

I moved the known working micro-controller over to the hitek "cursed" keyboard and...right back to flashing.

So I'm hoping someone has another suggestion as to what to look at next because honestly I have no idea at this point. Known good micro-controller made no difference and suspected bad micro-controller seems to work fine in another keyboard. I'm stumped and my repair guy is stumped at this point.

 

[ComputerCompulsive]

At long last the Hitek keyboard is working!
Thanks again to Baptiste Hobbé from Analog Thinker for working on this one. He found multiple faults including bad traces, problem capacitors, and a bad oscillator. With all that repaired we're in business