A500+ burning resistor r401 when mouse plugged in

Evilged · 12 July 2020, 22:29

Hi,
Just repairing an A500+ I recently bought, the Gary socket was badly corroded so I replaced that and repaired a few tracks, and everything check outs there.
Two resistors on the board were also fried, E502 47 ohm on the+12v rail going to the serial port above the keyboard connector and R401 4.7 Ohm going to joystick port 1, pin 5.
I replaced both and after some cleaning of the Agnus socket its alive again.
But when I plug a tank mouse in joystick port 1 it burns out R401 again, the surrounding components check out OK (RP401) and there are no obvious shorts.
The tank mouse seems good too as it works without issue on the two other A500s I have(cable checks out,and internals look ok).
The mouse port works OK with a DIY PS2 adaptor (prefer not to use it as it is not always detected and is somewhat cumbersome) so I presume U15 74LS157 is OK?
The EMI filter seem OK too not shorting.
I haven't got another old mouse to test with but as the other Amiga work fine without any smoking resistors I presume its OK unless the older non plus models are more tolerant.
U36 (SN74LS05N) chips markings look quite brown like it may have got hot could that have any impact?
Any ideas gratefully received.
Thanks

lesta_smsc · 12 July 2020, 22:53

What value resistor are you replacing it with? I think it may vary depending on revision of board. I suspect it is a 4.7 Ohm 0.5 watt though. I think it relates to 5V pin since joystick should work OK as doesn't require 5V.

Evilged · 12 July 2020, 23:04

Hi
Yes its 4.7 ohm 1/2 watt as in the manual although the original ones on the board seem chunkier.
Works fine with joystick in or the ps2 adaptor but the tank mouse just toasts it, fire up the older A500 boards and the resistor doesn't get warm on those.
Measuring and comparing voltages across the boards all looks equivalent give or take 0.1v so not quite sure whats doing it.
Wanted to get this board just right to run my recently assemble TF536 accelerator in to take advantage of the extra chip memory.
Thanks
Ged

lesta_smsc · 12 July 2020, 23:07

Maybe there is a short which is causing resistor to burn. Continuity test to neighbouring components. Did you do any work near the resistor?

Evilged · 12 July 2020, 23:28

No other work around there done, its fine without the mouse plugged in, or any other device( joystick, gamepad & PS2 mouse) if there was a short I would have expected it burn out on its own, but it doesn't get warm.
The board is very clean except the battery area and Gary socket, even that it was more the socket than the tracks, it was just RST and HLT not making good contact so easier to redo with fine wire than re-heat under the socket as the via had gone.
The seller obviously powered it up with all the corrosion which won't have done it any good but insure if the resistors were burn out before that.
Thanks

Evilged · 12 July 2020, 23:33

All I can think of is the EMI filter is damaged and shorting under load?

hooverphonique · 13 July 2020, 10:13

solder just one end of the resistor in, and put an ammeter in series to check current - if current is zero when mouse not plugged in, your board should be fine.

could there be a mechanical issue that causes a short when you plug in the DB9 connector?

Evilged · 13 July 2020, 10:27

The port looks fine, no bent pins or stress on the board and all check out with no short on the meter, same with the mouse.
I will follow your suggestion and see if the is any load with nothing plugged in.
Thanks

DigitalKeeper · 14 July 2020, 11:55

Check -> E406 <- for Short's, coz - sometimes they are defective

That is the typical failure, if someone have the Power Connector of the Floppy plugged on false Position.

They are more "EMI"'s, that can be defective - all must be checked; have a look:

-> www.amigapcb.org <-

Evilged · 14 July 2020, 13:33

Thanks I will check.
I imagined some damage has been done as when I opened it keyboard connector was plugged in one pin down and a rom switcher wire had also been attached to keyboard connector (can't remember which pin) I thought those usually go on pin 41 on Gary etc.
The rom switcher also incorrectly put in from pin 1, amazingly the roms on it still work.
Probably what blew or contributed to the two resistors blowing in the first place, it wouldn't surprise me if other parts have been plugged in wrong.
Thanks

solarmon · 14 July 2020, 16:51

Check E401 - check continuity and resistance to ground (middle pin) on its outer pins. Compare with E408, which is the equivalent one for the other port.

Evilged · 09 April 2022, 13:57

Just to update it was the EMI filter only just managed to get round to getting some and taking it all apart again and work perfectly now with a new filter.
Oddly it reads fine compare to the others on and off the board no corrosion etc.
similar odd issue to GadgetUK in a recent video he did.
Thanks,
G

12 July 2020, 22:29	#1
Evilged Registered User Join Date: Jul 2020 Location: Worcester Posts: 7	A500+ burning resistor r401 when mouse plugged in Hi, Just repairing an A500+ I recently bought, the Gary socket was badly corroded so I replaced that and repaired a few tracks, and everything check outs there. Two resistors on the board were also fried, E502 47 ohm on the+12v rail going to the serial port above the keyboard connector and R401 4.7 Ohm going to joystick port 1, pin 5. I replaced both and after some cleaning of the Agnus socket its alive again. But when I plug a tank mouse in joystick port 1 it burns out R401 again, the surrounding components check out OK (RP401) and there are no obvious shorts. The tank mouse seems good too as it works without issue on the two other A500s I have(cable checks out,and internals look ok). The mouse port works OK with a DIY PS2 adaptor (prefer not to use it as it is not always detected and is somewhat cumbersome) so I presume U15 74LS157 is OK? The EMI filter seem OK too not shorting. I haven't got another old mouse to test with but as the other Amiga work fine without any smoking resistors I presume its OK unless the older non plus models are more tolerant. U36 (SN74LS05N) chips markings look quite brown like it may have got hot could that have any impact? Any ideas gratefully received. Thanks

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12 July 2020, 22:53	#2
lesta_smsc Registered User Join Date: Feb 2012 Location: United Kingdom Posts: 3,196	What value resistor are you replacing it with? I think it may vary depending on revision of board. I suspect it is a 4.7 Ohm 0.5 watt though. I think it relates to 5V pin since joystick should work OK as doesn't require 5V.

12 July 2020, 23:04	#3
Evilged Registered User Join Date: Jul 2020 Location: Worcester Posts: 7	Hi Yes its 4.7 ohm 1/2 watt as in the manual although the original ones on the board seem chunkier. Works fine with joystick in or the ps2 adaptor but the tank mouse just toasts it, fire up the older A500 boards and the resistor doesn't get warm on those. Measuring and comparing voltages across the boards all looks equivalent give or take 0.1v so not quite sure whats doing it. Wanted to get this board just right to run my recently assemble TF536 accelerator in to take advantage of the extra chip memory. Thanks Ged

12 July 2020, 23:07	#4
lesta_smsc Registered User Join Date: Feb 2012 Location: United Kingdom Posts: 3,196	Maybe there is a short which is causing resistor to burn. Continuity test to neighbouring components. Did you do any work near the resistor?

12 July 2020, 23:28	#5
Evilged Registered User Join Date: Jul 2020 Location: Worcester Posts: 7	No other work around there done, its fine without the mouse plugged in, or any other device( joystick, gamepad & PS2 mouse) if there was a short I would have expected it burn out on its own, but it doesn't get warm. The board is very clean except the battery area and Gary socket, even that it was more the socket than the tracks, it was just RST and HLT not making good contact so easier to redo with fine wire than re-heat under the socket as the via had gone. The seller obviously powered it up with all the corrosion which won't have done it any good but insure if the resistors were burn out before that. Thanks

12 July 2020, 23:33	#6
Evilged Registered User Join Date: Jul 2020 Location: Worcester Posts: 7	All I can think of is the EMI filter is damaged and shorting under load?

13 July 2020, 10:13	#7
hooverphonique ex. demoscener "Bigmama" Join Date: Jun 2012 Location: Fyn / Denmark Posts: 1,644	solder just one end of the resistor in, and put an ammeter in series to check current - if current is zero when mouse not plugged in, your board should be fine. could there be a mechanical issue that causes a short when you plug in the DB9 connector?

13 July 2020, 10:27	#8
Evilged Registered User Join Date: Jul 2020 Location: Worcester Posts: 7	The port looks fine, no bent pins or stress on the board and all check out with no short on the meter, same with the mouse. I will follow your suggestion and see if the is any load with nothing plugged in. Thanks

14 July 2020, 11:55	#9
DigitalKeeper _Repair_Nerd_ Join Date: Jun 2020 Location: Germany Posts: 71	Check -> E406 <- for Short's, coz - sometimes they are defective That is the typical failure, if someone have the Power Connector of the Floppy plugged on false Position. They are more "EMI"'s, that can be defective - all must be checked; have a look: -> www.amigapcb.org <-

14 July 2020, 13:33	#10
Evilged Registered User Join Date: Jul 2020 Location: Worcester Posts: 7	Thanks I will check. I imagined some damage has been done as when I opened it keyboard connector was plugged in one pin down and a rom switcher wire had also been attached to keyboard connector (can't remember which pin) I thought those usually go on pin 41 on Gary etc. The rom switcher also incorrectly put in from pin 1, amazingly the roms on it still work. Probably what blew or contributed to the two resistors blowing in the first place, it wouldn't surprise me if other parts have been plugged in wrong. Thanks

14 July 2020, 16:51	#11
solarmon Registered User Join Date: Dec 2018 Location: UK Posts: 1,716	Check E401 - check continuity and resistance to ground (middle pin) on its outer pins. Compare with E408, which is the equivalent one for the other port.

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09 April 2022, 13:57	#12
Evilged Registered User Join Date: Jul 2020 Location: Worcester Posts: 7	Just to update it was the EMI filter only just managed to get round to getting some and taking it all apart again and work perfectly now with a new filter. Oddly it reads fine compare to the others on and off the board no corrosion etc. similar odd issue to GadgetUK in a recent video he did. Thanks, G