Posted By
andysquiz
on 2021-12-16
08:55:59
|  Cartridge port Not working correctly
I have a Plus\4 which appears to be working correctly (tested with diag 264), however if I insert a cartridge game into the machine all I get is a Black screen.
I have cleaned the edge connector with Alcohol and still no difference. The Cartridges I have tried work in another machine so I am not sure where to start looking.
I have replaced the TED and CPU chip but this hasn't helped. Any help would be greatly appreciated
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Posted By
weidi
on 2021-12-16
12:36:21
| Re: Cartridge port Not working correctly
First thing comes to my mind (after TED and CPU replacing) is power supply.
External power supply could be a problem (5V is not stable enough for extra cart load) or internal capacitors on main board.
Regards,
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Posted By
MIK
on 2021-12-16
16:41:43
 | Re: Cartridge port Not working correctly
If it turns out to be a PSU issue, pressing play on the tape deck "may" also show signs something is up, like crash or reset the machine.
You could try pulling the cartridge out a couple of millimetres so the pins touch a different spot on the contacts to see if there is any life.
If it feels like taking the cartridge in and out is looser than your working machine then the port may have some wear, the pins are wider apart. If so try holding the cartridge slightly up while it's plugged in just to add a little more pressure to the contacts and if it works you may need to look at the pins to some how make them tighter to the cart.
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Posted By
MCes
on 2021-12-17
05:49:15
 | Re: Cartridge port Not working correctly
@andysquiz
I suppose that you used the kernal version of DIAG264 for testing your +4.
The scene is a working machine that doesn't work with any cartridge (please confirm this scenario).
Into this case I'm imagining this possible classes of issue:
1) power supply (using the tape drive the system should collapse: the tape motor receives power from the same power supply)
2) the cartridge connector is damaged: one contact spring could be bent and short-circuit with another when the cartridge is inserted, or a small conductive object could be in the connector, into the contacts seats.
3) On the PCB a trace that starts from the pad of the connector and is connected to the internal circuit, is open.
4) the chips generating signals for the cartridge port are not working perfectly, these chips appear to be:
A) U20 (74LS139)
B) U22 (74LS27)
C) U21 (74LS175)
My advice:
-Thorough inspection of the cartridge connector (also with the help of a multimeter).
-desolder/place socket/change ICs into U20, U22, U21
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Posted By
andysquiz
on 2021-12-17
10:37:57
| Re: Cartridge port Not working correctly
Hello all,
Thanks for the quick responses. I have just moded my Plus/4 so that it can use a C64 power supply. Bought a brand new C64 PSU. So the voltages and Current are stable.
The cassette deck works fully.
I have the Diag264 kernal which you have to open up the case and replace another IC for.
The cartridge connector appears to be clean and in very good condition.
I have tried to apply gentle pressure in several different directions to the cartridge and this doesn't change anything.
MCes thanks for pointing me in the direction of which IC's to look at, I will give this a look into when I get back from work.
Thanks
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Posted By
weidi
on 2021-12-17
11:07:26
| Re: Cartridge port Not working correctly
Hi again,
Looking at the Plus/4 schematics I'm little skeptical about Datasette "test".
Power for it is taken not directly from +5V but from separate +9V unregulated dropped down to ~6V thru TIP29 (D313) transistor.
The main +5V "digital" is little affected by Datasette.
Another circuit in main +5V bus is power switch. it happened to me that contact was not perfect and the switch itself become "resistor" dropping voltage below +4.7 and less depend on the load (attached cart increase the load and voltage can drop below +4.5V).
Would be good to measure with multi-meter main voltage with and without cartridge.
Also: U20 (74LS139) could be the problem.
Please keep us updated as it is interesting case...
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Posted By
weidi
on 2021-12-17
11:14:35
| Re: Cartridge port Not working correctly
Ups i forgot:
to test U20, U21, U22 - ROM banking circuit, you can try to use 3-in-1 software build in Plus/4.
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Posted By
MCes
on 2021-12-18
05:43:46
| Re: Cartridge port Not working correctly
It's true: on +4 the tape unit is feeder from a secondary supply branch (I had into the mind the C16 into my previous post...): for testing the 5V line you have to use a multimeter (voltmeter) for reading the +5V line (after the board "ON/OFF" switch).
@andysquiz
testing the U22, U21,U20 chips:
the first step is try to use 3-in-1 software build in Plus/4, if it fail there are a problem into one of this chips, but if doesn't fail may be that the part of this chips that enable the internal ROMs are ok with problem on the part of this chips that enable external ROMS (cartridge):
3-in1 test fail: a chip is damaged (find and replace it)
3-in1 test does not fail: a more in-depth test is needed to exclude a malfunction of U20,22,21
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Posted By
andysquiz
on 2021-12-18
15:16:22
| Re: Cartridge port Not working correctly
The 3+1 software all work fine
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Posted By
andysquiz
on 2021-12-20
09:14:57
| Re: Cartridge port Not working correctly
Morning all,
I have got voltage readings from U20,21 and 22 as requested. I have done the checks with the cartridge fitted and with it removed
U20 74LS139
PIN Volts PIN Volts
1 4.55 16 4.82
2 0.16 15 1.9 No change in voltage cartridge was fitted or removed.
3 0.16 14 0.7
4 5.55 13 0.7
5 5.6 12 1.24
6 5.6 11 5.6
7 5.6 10 5.6
8 EARTH 9 5.6
U21 74LS125
PIN Volts Cart fitted PIN Volts Cart fitted
1 4.8 4.76 16 4.8 4.8
2 0.25 0.15 15 4.24 1.5
3 4.2 4.24 14 0.16 4.2
4 1.8 1.167 13 3 1.6
5 1.2 1.55 12 3 1.3
6 4.28 4.25 11 0.155 4.25
7 0.163 0.162 10 4.2 1.62
8 EARTH EARTH 9 0 1.05
U22 74LS27
PIN Volts Cart fitted PIN Volts Cart fitted
1 4.83 4.76 14 4.83 4.81
2 5.8 2.56 13 EARTH EARTH
3 1.6 4.13 12 0.66 0.104
4 0 1.73 11 0.09 1.74
5 EARTH EARTH 10 EARTH EARTH
6 4.42 0.7 9 0.16 4.24
7 EARTH EARTH 8 4.45 0.72
U20 doesn't change with a cartrdige fitted
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Posted By
weidi
on 2021-12-20
10:01:53
| Re: Cartridge port Not working correctly
Quick look at your measurements on U20 and:
1) I don't like pin 4 to 7 (first 2-to-4 line decoder outputs) voltage are above pin 16 - Vcc (4.82V)
- this is not normal and potential source of problems
Similar story with pin 9,10,11 (second 2-to-4 line decoder)
I have to look at the schematics for possible answers why that voltage is above Vcc
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Posted By
MCes
on 2021-12-20
10:26:28
| Re: Cartridge port Not working correctly
That some lines are higher than Vcc is suspicious, but 4,85V as "5Vcc" appear to be too low, if you have another PSU test the Vcc with this second PSU.
Try also to read the voltage drop of the 5V switch: place the voltmeter probes one before and one after the closed switch and read how many millivolts it drop.
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Posted By
weidi
on 2021-12-20
10:28:26
| Re: Cartridge port Not working correctly
It is really strange, but:
I don't see reason why on "good" system
voltage could be above +5V Vcc.
Plus/4 should be able to work on +5V only (no Datasette use).
So it is possible to test voltages when ~9V AC disconnected, only +5V coming from power supply.
When it works OK on +5V only that means there is "voltage leak" from ~9V AC or +9V unreg.
+9V unregulated can go to the system thru broken TIP 29, U6 (7406) or some shortage on the board.
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Posted By
andysquiz
on 2021-12-20
10:29:08
| Re: Cartridge port Not working correctly
Weidi,
Thanks for the quick response, got to admit I was more worried about why the chip didnt switch/change when the cartridge was put into the machine. I don't have schematics to hand. But I would really appreciate knowing why the outputs are so high!
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Posted By
andysquiz
on 2021-12-20
10:38:21
| Re: Cartridge port Not working correctly
MCres - I am getting 4.85v at one side of the switch and 4.85v on the other side.
Tried 3 different PSUs and the readings are all the same
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Posted By
weidi
on 2021-12-20
10:44:51
| Re: Cartridge port Not working correctly
Sorry I'm probably getting nervous but another idea to trace fault:
Highest voltage +5.8V is on the U22 pin2 - it is R/W line coming from CPU.
CPU controls Datasette motor thru U6 (7406).
You can trace high voltage on:
1) U2 pin 23 - CPU R/W output
2) U2 pin 27 - CPU P3 output
3) U6 pin 3 - 7406 input
4) U6 pin 4 - 7406 output
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Posted By
weidi
on 2021-12-20
11:31:30
| Re: Cartridge port Not working correctly
I have to correct myself:
U2 - CPU - R/W output is on pin 39 (not 23)
7406 is U7 (not U6 which is 7404 on my Plus/4).
Correct measurement would be:
1) U2 pin 39 - CPU R/W output
2) U2 pin 27 - CPU P3 output
3) U7 pin 3 - 7406 input (should be the same as U2 pin27)
4) U7 pin 4 - 7406 OC output (can be +6.8V when Datasette is on)
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Posted By
andysquiz
on 2021-12-20
14:00:58
| Re: Cartridge port Not working correctly
weidi,
on U2 pin 39 = 5.75v
U2 pin 27 = 2.79v
U7 pin 3 = 2.79v
U7 pin 4 = 0.1v
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Posted By
weidi
on 2021-12-20
15:19:17
| Re: Cartridge port Not working correctly
Hmmm,
U7 looks normal that means no problem there.
That complicates situation.
Could you remove the big glass fuse and test against that higher voltage (>5V)?
I'm curious where on the main board that higher voltage is leaking in to +5v.
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Posted By
Andysquiz
on 2021-12-21
02:22:51
| Re: Cartridge port Not working correctly
Weidi,
Glass fuse removed voltage on U2 pin 39 still 5.75v
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Posted By
weidi
on 2021-12-21
10:33:32
| Re: Cartridge port Not working correctly
I'm out of ideas.
Something must be wrong unless your Plus/4 generates voltage by itself 
Either your power connector or multmeter has something wrong.
On my Plus/4 U20 pin 4,5,6,7 +4.62V; 9,10,11 +4.65V only pin12 has +1.98V (computer idle on welcome screen) .
I have no higher voltage than Vcc (in my case +5.18V), but I'm using small 15W 5V impulse power supply from old wifi router. U2 pin 39 - +4.87V
I connect only Pin5,4 for +5v pin1,2 and 3 for GND on round C64 style power connector. No ~9VAC at all.
This is for mother board Assy No.310163, where power socket conversion is easy (solder out-solder in).
I know there are older or other mother board for example Assy No 250455 where such a conversion is not easy.
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Posted By
andysquiz
on 2021-12-21
10:46:27
| Re: Cartridge port Not working correctly
Hi Weidi,
My motherboard is assy 310163. It has been converted to take a C64 round power connector. Is there a way to upload photos to this forum
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Posted By
Andysquiz
on 2021-12-21
11:36:31
| Re: Cartridge port Not working correctly
This is probably not the best troubleshooting method but..
I removed the cpu and did voltage check on the socket of U2 pin 39 5.75v
Left the cpu out and did voltage check on pin 7 U1 5.75v
So I removed both CPU and Ted u2and U2 voltage now 4.72v on pin 39 U2 and pin 7 U1.
Could the ted be at fault with this?
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