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Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: Megantonneke on November 21, 2017, 10:30:13 PM

Title: Can anyone tell which A500 modification this is? (Noob)
Post by: Megantonneke on November 21, 2017, 10:30:13 PM

Hi there Amiga enthusiasts,

I just started collecting and working on some Amigas 500, so I probably have a noob question:

I have one dead A500 rev 5 board (most likely ram chip error) but I also noticed a modification to the board. It looks like an added transistor just below Denise, but I don't know what it represents.

I also noticed it some Amiga 500 repair video's  on Youtube, but it is never pointed out and explained.

Here is an image:
(http://www.meganton.com/wp-content/uploads/2017/11/20171119_002531-1024x576.jpg)

Any info on this I would appriciate!

Greetings,

Megantonneke

Title: Re: Can anyone tell which A500 modification this is? (Noob)
Post by: curtis on November 21, 2017, 11:06:50 PM

You forgot to add the image!

Title: Re: Can anyone tell which A500 modification this is? (Noob)
Post by: Oldsmobile_Mike on November 21, 2017, 11:29:56 PM

Try using the forum's built-in image attachment function.  ;)

Title: Re: Can anyone tell which A500 modification this is? (Noob)
Post by: BLTCON0 on November 22, 2017, 12:04:40 AM

Quote from: Megantonneke;833438

... but I also noticed a modification to the board. It looks like an added transistor just below Denise, but I don't know what it represents.

I also noticed it some Amiga 500 repair video's  on Youtube, but it is never pointed out and explained.

It's the DTACK fix for the earlier Toshiba Gary version. It was factory installed on early rev5 boards because some of them might still ship with the earlier Gary chip. If you have the corrected MOS/CSG Gary version you don't need this transistor, but it won't hurt anything either.

Later rev5's (when stock of the Toshiba Gary had dried out completely) obviously dropped this factory mod but introduced a different (not 100% essential) one (a 68 Ohm resistor from 68000's E clock line to the first corresponding CIA pin - a trace must be cut to implement the fix).

Title: Re: Can anyone tell which A500 modification this is? (Noob)
Post by: scuzzb494 on November 28, 2017, 12:53:21 AM

Quote from: Megantonneke;833438

Hi there Amiga enthusiasts,

I just started collecting and working on some Amigas 500, so I probably have a noob question:

I have one dead A500 rev 5 board (most likely ram chip error) but I also noticed a modification to the board. It looks like an added transistor just below Denise, but I don't know what it represents.

I also noticed it some Amiga 500 repair video's  on Youtube, but it is never pointed out and explained.

Here is an image:
(http://www.meganton.com/wp-content/uploads/2017/11/20171119_002531-1024x576.jpg)

Any info on this I would appriciate!

Greetings,

Megantonneke

Hadn't realised what it was for but have noticed it on some of my motherboards

http://www.scuzzscink.com/amiga/scuzzblog_november17_2/car_sbd_091117_38.jpg

http://www.scuzzscink.com/amiga/scuzzblog_november17/scuzzblogdnovember17_0903.htm

Title: Re: Can anyone tell which A500 modification this is? (Noob)
Post by: BLTCON0 on November 28, 2017, 01:08:13 PM

Quote from: scuzzb494;833603

Hadn't realised what it was for but have noticed it on some of my motherboards

And indeed your particular rev5 does have the problematic Gary, must be one of the earlier ones.

(BTW all rev5 boards have that solder blob on the oscillator, nothing nasty happened to yours)