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Old 19 November 2013, 14:56   #1
Druon
 
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A500 Revision 3 Repair

I'm looking for some help restoring an A500 back to life. It's an early model (revision 3 without the X1 crystal).

Update:
The resistor at EMI406 appears to be bad, I will replace it and see what happens...

Symptoms:
Power LED comes on, then after several seconds it might go out, or the disk drive LED might come on (without the drive actually spinning). This is random.
Caps lock light comes on normally and can be toggled.
There's no video output at all.
No disk activity.

Components verified to be in working condition by testing in another A500:
PSU
CPU
Kickstart ROM
Denise
Floppy drive
Keyboard

During testing I noticed that Denise was not warming up at all, so I took out my multimeter and I'm only measuring between 0V and 1V on Denise and the ICs surrounding it. The voltage does not fluctuate constantly, but it might be 1V and then drop to 0.1V after plugging in the composite cable. Other components like the CPU and floppy drive are being supplied with 5V. I understood that A500s are not known for cap problems, but are the larger caps C401/C402 a likely cause for this? Is there anything else I should be checking?

Any help is greatly appreciated!

Last edited by Druon; 24 November 2013 at 10:48.
 
Old 21 November 2013, 10:26   #2
Loedown
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Many of the Amiga computers I have in for repair are showing signs of cracked joints around the larger chips, Kickstart / 68000 etc. I don't know if the early rev of A500 boards have most or all of the chips socketted but those too can cause issues. Look for bent pins on any socketted chips ( got an A4000 at the moment that had one socketted chip with one pin bent but making contact )

You may also have a grounding issue with your wall socket where you are seeing variations in voltage or a broken wire in the PSU loom.
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Old 21 November 2013, 11:02   #3
zipper
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After all very sturdy stuff - I yesterday launched my A500T with Blizzard 2060 etc after two years of no use. Worked straight away - after I connected a tube monitor because all RTG modes were tube trimmed and far away from what a LCD can display. So, a 23 years old mobo kicking a$$...
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Old 01 November 2018, 14:04   #4
RiP
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I have just received an A500 Rev 3 that has the exact problem too. All of the removable chips were fine, PSU and A520 too
There's no video signal at all and Denise is quite cold. Caps Lock will stop responding after pressing about 12 times.
I can reset the system though. Hope someone can help me
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Old 01 November 2018, 15:08   #5
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I don't know if it works on a rev 3 but the standard answer these days is to replace kickstart with diagrom: http://www.diagrom.com/ and then check the output via serial connection for clues.
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Old 01 November 2018, 17:06   #6
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If there's no video signal at all and the caps lock light behaves like that, I would expect that the CPU isn't running. If that's the case, DiagROM won't run either, since it needs a CPU. You would need to check the reset and other control lines - some of them will cause a complete stop if the pull-up resistors are damaged for example. There are also potential issues with the non-removable parts, like the reset circuit and the various control logic chips around the board.
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Old 01 November 2018, 19:52   #7
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Thanks for reply
I have noticed that the 1-Ohm resistor in E406 (EMI) was about 200-Ohm, so that's why Denise is cold and receives only 1-Volt rather than 5-Volt.



But you're right, there should be another problem with CPU that the caps lock doesn't work properly.
I'll replace the resistor Saturday, hope it shows a screen color
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Old 03 November 2018, 03:27   #8
Hewitson
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I'm not sure why people keep referring to E406 as a resistor when it clearly isn't one.

It's prefix is E and not R, it has three terminals, and it's measured in microfarads, not ohms! It looks to be some sort of EMI (electromagnetic interference) filter.
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Old 03 November 2018, 13:24   #9
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I know but the manufacture was used a resistor for it. Well, I replaced it and got picture but the RGB output isn't good
It gets brighter every 30 seconds for a second and after few minutes, I get no picture. B&W composite is all fine though

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Old 03 November 2018, 13:30   #10
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It is indeed an EMI filter. Internally it consists of a varistor or similar in series with the signal and a capacitor between the signal and ground, so you can substitute discrete parts (as is done on the later Amigas), or just bypass it entirely for the purpose of testing.

Typically the resistance across these is very low, less than 1 ohm. You need to be careful with this though - if you measure between the ground (middle pin) and either signal terminal while in-circuit, you'll likely get a 100-200 ohm resistance, which is typical of the resistance between the power rails. So be really sure that it's faulty before you go replacing it.

Edit: Ah, possibly the video hybrid then, or one of the components between that and the RGB port. See if the hybrid, your replaced resistor, or anything else after it is getting hot - it's possible something was faulty and that's why your EMI filter failed. Also, measure the voltage across the resistor when it's running. It should be close to 0V.
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Old 06 November 2018, 15:06   #11
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I fixed a Rev 3 with these symptoms today. First I noted that the board hacks on the underside of the PCB around the video power and RGB CSYNC seem to be factory standard. The video power mods serve to supply +VID from VCC via E406. E406 was populated with what appears to be 1-ohm rated resistor but was measuring 200-ohms+.
I bypassed E406 and all works (after re-seating all chips and putting a CIA in the right way round) so... perhaps E406 on Rev 3 boards is under stress and can fail?

I figure it's a 1-ohm 0.25W part. If Denise and the video circuit consume more than 250mA then that's a problem. I haven't measured it, but the whole board consumed just shy of 2A and Denise is one of the warmer chips, so I suspect E406 is loaded beyond its spec.

Now I think perhaps I should have taken a proper EMI filter from a donor board rather than bypass completely, but I suppose it doesn't matter really very much.

EDIT: I just swapped in an EMI filter from a donor board and removed the bypass wire. All happy now. I think E406 is loaded with the incorrect part on Rev 3 boards.

Last edited by kaffer; 06 November 2018 at 15:38.
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Old 06 November 2018, 20:49   #12
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Thank you all, I re-soldered many joints that had poor/cold contacts and finally got the right screen




Quote:
Originally Posted by kaffer View Post
I figure it's a 1-ohm 0.25W part. If Denise and the video circuit consume more than 250mA then that's a problem. I haven't measured it, but the whole board consumed just shy of 2A and Denise is one of the warmer chips, so I suspect E406 is loaded beyond its spec.
Possible to use 1-ohm 0.5W resistor instead?
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Old 06 November 2018, 22:54   #13
kaffer
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Quote:
Possible to use 1-ohm 0.5W resistor instead?
No, a 1-ohm resistor is a bizarre and inappropriate choice in the first place. Unless you can fit a proper 3-pin EMI filter (as used in dozens of other places on the Rev 3 board) you are better off bypassing E406 entirely.
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