Re: Re: 1526 printer - repair(able) or not?

Ville,

You're saying : "Finally I checked the /RES signal on the 6504 (at last I had some use for my eBay-bought logic tester!). It seems that the /RES signal is being constantly held low. Would this indicate bad ROM or a fault in the reset circuit?" ==> Definitely, if you can confirm a permanent low level at reset's CPU pin, there must be a trouble around there and nothing can start.

From service manual's schematics, this line is driven by U1C (74LS14). From this line (74LS14 pin 4), you have together (6504, pin 1), 6522 (pin 34 TBC), both 6532 (pin 34), and U8B (LM555 pin 4). Reset line is also driven by extrenal reset pin of serial port on U1C pin 1. To go deeper on this option, I would sugest you to remove U1C or to isolate (cut trace) for either (in preference order) pin 4, 3, 6, 5. From there you should be able to manage manually the reset line.

Another approach could be to remove (agin !) all socketed ICs (CPU/6522/both 6532) to confirm that reset line remains low everytime [if not, then one of our 4 ICs is guilty/dead - if yes, we're still in troubles ==> remains U1C or U8B or transistors / Zeners in between]

The LED is driven by 6532 pin 13 (U5D).

Regards - Hervé


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Message du : 06/04/2014 17:41
De : "Ville Laustela " <ville.laustela@gmail.com>
A : cbm-hackers@musoftware.de
Copie à : 
Sujet : Re: 1526 printer - repair(able) or not?


 
Hello, and thanks, Martin, Hervé and Olaf. I was positively suprised to have received this many ideas so quickly.

As I have very little space to work, I had just a quick look on the board today. I had the board on my desk without the printer mechanics so all indicator I had was the paper advance button with the LED.

Today's experiments:

- Into the primary side of the transformer goes three wires: brown, white and black. White is neutral (directly from the mains connector) and black is the live wire (comes thue the fuse and the power switch). Without modifying the wiring I was able to measure that between the black and brown wires there was 100V AC voltage. If some of you who have a 220V version could have a look on the PSU to see how it is wired? The case of the transformer is apparently fixed in place and I can't see where the wires go.

- I pulled out, checked for bent pins and reseated all chips - no change
- I found another copy of the service manual (as PDF) in here: http://www.commodoreman.com/Commodore/Library/Man/1526_MPS802_4023_ServiceManual.pdf
- On page 24 the manual explains that the board accepts 24 or 28 pin ROMs, jumpers J1-J4 (next to the ROM socket) are used to set which size is used. In my board they are set correctly. 

I have no equipment to check the ROM. I have been thinking of getting an EPROM burner (mainly to read some older ROMs for backup and sharing), but can't decide on the model. A quick search on eBay gives just about only two choises: TOP853 and G540, both USB and cheap chinese units. Reviews on both units are not especially good (bad manuals, unstable software, only 32-bit drivers...). Still some say that they are just fine. Perhaps either of these would be just fine for me? Being able to burn Kickstarts for Amigas would be a plus, but not a necessity.

Finally I checked the /RES signal on the 6504 (at last I had some use for my eBay-bought logic tester!). It seems that the /RES signal is being constantly held low. Would this indicate bad ROM or a fault in the reset circuit? I pulled the 6504 out and bent the /RES- pin out of the socket. This had no visible effect (I manually pulled it low after applying power to the unit), at least judging by the front-panel LED.

Please ask if I explained something badly, I am still very new to this level of fault-finding. Again, thanks for your support, guys.

--
Ville


> You can measure the voltage between the brown wire and the other wires. If
> the transformer has a 230V connection you must measure to one other wire
> 115V and to the other one 230V. So you can use the wires where you measure
> 230V for input with 230V. Then if you look at the wire with 115V, you must
> find a junction for two wires out of the transformer. The 115V / 230V
> transformer has two 115V coils. One i used for 115V input, both are used for
> 230V input.

> Have you checked the /RES signal for the CPU? At the picture i see an EPROM.
> The content of an EPROM can be non-permanent over more than ten years. You
> can compare it!
> 
> The CPU needs RAM. This RAM is into the 6532. There are two 6532. Swap these
> chips! The 6532 have no internally programming!

> I would first double check that all socketed ICs are 1) at good location 2) facing the good direction 3) no blent pin. I would then try to force a reset (pulling low the relevant pin). I would also confirm that reset is not low every time ... Also, I would confirm clock operation and or data/address bus activity. As a last try, I would try to confirm status of the eprom (good/bad ?).
> 
> The board looks quite nice and the ribbon should not be an issue even if dried.
> 
> I wish you great pleasure trying to repar this ! The service manual is very nice, I must a pair or 4023 around there if you wish that I have a look at something.


> Interesting that you report about that just at this moment.
> I have an MPS-802 (which is pretty much the same thing, going by the
> pictures) which also doesn't power up. It is a 220 V model though.
> 
> I haven't done any measurements inside since the connections on the
> transformer expose 220 V if the enclosure is taken off. I'll want to do
> something about that first.
> 
> My ROM next to the 6504 is shorter than the socket, just like in the
> reference above (and unlike your dropbox picture).


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