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woody
February 21st, 2018, 02:04 AM
Hi all

Bought a M4 Non Gate Array , Model 26-1069 Serial # 333 off Ebay as DOA no Video.

Found No High Voltage, suspect solder joints on HV TRansformer.

Re soldered and now getting Video message " Diskette"

Power supply voltages look good, +5.12 , +12.3 , -12 ,

Issue now wont boot. getting "diskette" message with door open.

With known good copy of LSdos on multiple drives Red light comes on after reset but not seeking

or loading the O/S.

The controller is separate board with 3 pots. What are they for ?

I checked the connecting cable and all pins look ok. Swaped with serial board (?) and no change.

Bad floppy controller of FDC chip (1791 i think ) ?. Can pots be adjusted ?

Ive gone about as far as i can.

Any help would be greatly appreciated .

Woody (Doug )

ldkraemer
February 21st, 2018, 05:57 AM
Woody,
DO NOT MESS WITH THOSE POTS!.

Depress the BREAK KEY, and keep it depressed. POWER ON the Model 4. When the display shows text, release the BREAK KEY, and
tap the ENTER KEY twice. You should see the cursor.

Type in the following and depress ENTER
?mem

You should get 48082

Is this what you are getting?

Does your Model 4 have as Astec Power Supply or does it have the Tandy Power Supply? If it is a Tandy, you
need to remove it and check ALL Solder joints for the Pins on the Connector side of the Power Supply. You will
likely see several of the Pins on the Circuit Board that are not soldered good, and will have a small ring around them,
if you look real close. Flow new solder on all pins if that is what you are seeing.


Larry

woody
February 21st, 2018, 09:49 AM
Hi Ldkraemer,

Yes the power supply is an Astec and i found some solder joints i didn't like

so i resoldered them. All supply voltages are good now.

Didn't touch the pots on the floppy controller.

Boots to basic fine and i get 48082 bytes.

There is a PAL chip PAL16R6 U3 near the connector to the the controller on the motherboard thats getting very hot.

Within 1-2 minutes you cant keep your finger on it.

Do you know anyone who might have the code for that chip.

I don't know if my programmer supports PAL chips. it does support Gal's though.

Beyond that the main problem seems in the booting process after the drive activity light comes on

it isn't moving the head to load the O/S.

Doug

ldkraemer
February 21st, 2018, 04:36 PM
Doug,
I'm not sure how hot that PAL Chip gets. I've never checked mine to see how hot it is. You can always remove the PAL IC and
replace it with a DIP switch. Then set the DIP switch to the configuration so it's a standard Model 4 with 64K RAM versus 128K.
I've got that information somewhere. I'll have to dig a bit.

As for the Floppy Drives, I'd suggest removing both, and then clean and lubricate the Drive Head Guide Rails. Clean with Alcohol,
then I use Dri-Slide to lubricate the Drive Rails. It's a Motorcycle cable lubricant, that doesn't attract dirt. It will keep them lubed
for years. If the slides are dirty and gummy, the stepper motor won't be able to move the head assembly. You can try to move
the head assembly very carefully, and it should glide along. If it moves hard, it needs to be cleaned and lubricated. I'd bet that
is the only problem with the drives.

I'll look for the DIP setting for a standard 64K Model 4.

1..............20
2...............19
3..............18
4..............17
5..............16
6-----------15
7-----------14
8-----------13
9-----------12
10............11

Larry

woody
February 22nd, 2018, 01:19 AM
Hi Larry ,
That’s very helpful, I didn’ t Know it was that simple to to set a bit code

And leave it fixed for 64 k only. I hope you can find the info i’m Eager to try it.

I have some dip switches and if it works I’ll replace it with a dip shunt.

Thanks Larry

Doug

woody
February 23rd, 2018, 10:10 AM
Larry.
I cleaned and lubed the drive rails. I had 4 to do.

2 worked but 1 is out of alignment and the other 2 needed too much pressure

to move the head along the guide rails and made a lot of noise trying to move the heads.

I found out last night that on power up or reset, the heads are moving to track 0. I previously

thought it was the drive that automatically did that but it is the computer moving the heads to track 0.

Also the computer knows if the drive is connected to the cable and whether there is a disk inserted.

So it looks like my controller may be ok.

After all the drive cleaning the M4 does not seem to be reading the (boot sector ? ) and loading the O/S.

I'm running out of ideas as to why the computer isn't loading the O/S.

Any luck finding the PAL code yet.

Doug

ldkraemer
February 23rd, 2018, 02:17 PM
Doug,
I think you have the wrong information about setting some bit code. All you have to do is get a 4 Position DIP switch, toggle all 4 switches
so they are closed and plug that switch so Pin 6 of the PAL is shorted to Pin 15, and 7 to 14, and 8 to 13, and 9 to 12, as the picture above
shows.

I think Ian is about the only one who still has the PAL IC's available.


43949

Larry

lowen
February 23rd, 2018, 02:33 PM
Doug,
I'm not sure how hot that PAL Chip gets. I've never checked mine to see how hot it is. You can always remove the PAL IC and
replace it with a DIP switch. Then set the DIP switch to the configuration so it's a standard Model 4 with 64K RAM versus 128K.
I've got that information somewhere. I'll have to dig a bit....

I've just followed this a little bit, but, this is an NGA Model 4, and the PAL in question is U3, right? (let's see, quoting OP:

...
There is a PAL chip PAL16R6 U3 near the connector to the the controller on the motherboard thats getting very hot.
...
) so yes, U3.

The RAM select PAL that is a DIP shunt in the 64K version is U72, not U3. U3 is a fundamental part of the clock generator logic and must be working, since you're getting the dotclock to get characters on the CRT. With the clock rates U3 is running, I would expect it to be pretty warm, but I don't recall hot hot it should get. A DIPswitch in socket U3 would be a very bad thing. U3's position in the circuit is on page 6 of 6 in the schematics.

If I've counted right, there are five PALs in the NGA Model 4, one of which is a DIP shunt in the 64K version at U72.

ldkraemer
February 23rd, 2018, 03:11 PM
lowen,
THANKS for that catch. I totally missed the reference to U3 several posts back. That wouldn't be a good thing to do at U3.

Larry

woody
February 23rd, 2018, 03:31 PM
Hi Guys
I found a replacement rom at U70 marked "C" Rom C ?

it's code is different than a spare Rom C i have.

My rom c doesn't work properly in there so i put the old one back.

doesn't make sense to me the controller moves the drive heads to track "0" and not load the o/s ?

I have checked the white flex cable "controller to MB " for continuity , swapped it with the serial cable,used different floppy cables

Checked traces on the Controller that go to the floppy connector , used multiple known good drives with the O/S written to disk on that drive to make sure

the system isn't trying to load the o/s from a disk written to by a drive that is out of alignment.

The answer is probably staring me right in the face and I can't see it.

It's probably something simple iv'e overlooked

Doug

ldkraemer
February 24th, 2018, 04:18 AM
Woody,
The best thing you can do is compare the three (plus extra U70) ROMS in your Model 4 with a dump from a known good set of ROMS.

I've got this information on the ROMS:
The model 4 contains 14K of ROM (in Model III Mode) divided into:
U68 ROMA MCM68A364 8K Manuf Part Number 8048364 0000-1FFF = 8192
U69 ROMB MCM68A332 4K Manuf Part Number 8040332 2000-2FFF = 4096
U70 ROMC MCM68A316 2K Manuf Part Number 8042316 3000-37FF = 2048 less 37E8-37E9 = 0xFF

I wouldn't be plugging in any unknown ROM's into a system without knowing exactly which one was defective (suspect).

If you have a file of what is contained in the ROMS, I can quickly compare with my known good set.

Larry