Replica 1 Ten #36 no go

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Replica 1 Ten #36 no go

Postby p20vintage » May Wed 28, 2014 12:14 am

Hi

Built #36 last week and got the reset does nothing problem. Checked all my solder joints (a few times) and remade a few. Still no joy.

I then checked the continuity from the IC pins (topside of the PCB) based on the schematic just in case I missed a cold joint. Everything checked out EXCEPT there's always 10K resistance between ground and PIN 40 of the 6502 even though the function of the RESET button is to connect GROUND to pin 40. I assume this 10K is the function of my DMM charging C11 which then connects the 10K resistor between VCC and ground. I changed the RESET pushbutton as these are cheap (and I've had my fair share of cheap Chinese-made component issues elsewhere) in case it was a defective switch. I did observe the C11 pads are VERY close together but inspecting this joint under a maggy lamp showed no short. Without the IC's installed I measured about 4.7V on the various VCC pins on the IC sockets. With the IC's installed VCC drops to 3.95V - I changed out the wall wart (rated at 5V 2A) and no change to the voltages.

Ordered a spare 6502 and 6821 locally. Got them today. Tried them both (one at a time). No change when 6502 changed. Pin 39 on the new 6502 has a voltage of 1.9V.

However when 6821 got changed and I powered on, pressing CLEAR brings up a single '@' as you would expect, then after a few seconds the '@' starts to repeat (as if a keyboard was connected and you held the key down). Pressing RESET briefly interrupts this (for around half a second) then the '@'s start up again. Put the original 6821 back in, same behaviour. Looks like a signal isn't getting through but I'm perplexed.

I do get a magenta/yellow 'tinge' on the characters but am putting this down to the fact I'm using an old analogue LCD TV as a monitor and its NTSC/PAL converter isn't the best.

Any ideas would be welcome.

Thanks

Paul
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Re: Replica 1 Ten #36 no go

Postby vbriel » May Wed 28, 2014 2:44 pm

1.9V on pin 39 shows that the Phase 2 clock is working so the Propeller chip is operating. All else aside, the voltage with chips installed is a major factor. Start with the USB connector and work your way through the 10uF cap and the switch. Check voltage on the USB connector pin 6 is 5V out from the USB connector. With nothing on, what does it read? With no chips, what does it read? If 5V check the switch side and see each side with power what it reads. Actually, the switch out goes to the 10uF cap, so you can just check that. It could be just the switch, but it could be the usb board.

The orange tinge is probably the NTSC/PAL converter but since I don't own one, I can't verify. Still, not a problem section of the circuit.

The button probably was fine, to test, place a lead on ground and to pin 40. 5V should show, pressing RESET should pull that pin to 0V.

If voltages checks out on the switch and USB board, trying one chip at a time to isolate a bad chip works too.

Just some things to get you started. The board is pretty simple so it shouldn't be too difficult to get going :)

Vince



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Re: Replica 1 Ten #36 no go

Postby p20vintage » May Thu 29, 2014 2:55 am

OK here's an update…

Checked everything as Vince suggested, observed that from pin 6 on the USB board, I was getting 4.9V. Remade the joint at Pin 6 and the power switch, then getting 4.9V at the common of the switch. However when the circuit was switched on this dropped to 4.5V with chips out! After some experimentation (and hair tearing) I decided to swap the USB A- USB Mini cable I was using to power the beast (salvaged from an old satnav I think) and that was it. New cable, I now have 5V throughout the circuit. (Incidentally the cable used right now is one pulled from one of those first gen MK802 Android smart TV sticks which from memory also were a bit demanding in their power needs…)

OK chips back in. Original 6502, voltage on pin 39 down to 0.2V - no good! Local replacement 6502 - 2.5V as expected. So I have a functioning clock. Still no go. Reset does nothing.

Looked at the replacement chip again - 6502A - this is a 2MHz version of 6502 and obviously isn't going to work in the circuit designed for 1MHz 6502. A quick check around looks like that most dealers that supply to Australia are unaware of this (they are basically the modern day equivalent of the salvage stores) and sell 'em all as 6502. So it is a lottery.

So now to find a working 6502 to test it all again.
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Re: Replica 1 Ten #36 no go

Postby vbriel » May Fri 30, 2014 4:06 pm

6502A will function at 1MHz, it doesn't work the other way around. A 6502 can't do 2MHz. I have a 14MHz 65C02 in the replica 1 before, and I use the WDC 65C02 14MHz CPU in the new OSI clone the Superboard III. I'm still going with 6502, 6821 or the EPROM. If you can't find anything local, I can send a set of replacement chips out.

Good catch on the cable, I will have to note that!
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Re: Replica 1 Ten #36 no go

Postby p20vintage » Jun Wed 04, 2014 1:19 am

Hi

Next instalment - I found this post http://www.brielcomputers.com/phpBB3/viewtopic.php?f=3&t=1008

which was useful. No 'scope just yet but armed with a DMM that measures frequency I was able at least to do the basic testing mentioned (with the EPROM and RAM removed from circuit).

Pin A15 on the 6502 starts at around 15Hz as mentioned, A14 doubles this to 31Hz, but A13 doesn't. It just moves up 2Hz to 33Hz. Moving onto pin A12, the measurement is 66Hz - with the frequency approximately doubling each pin A11 through A1 as per the attachment in the above post. Results the same no matter whether I measured at the 6502 itself, the slot or the RAM socket. Double-checked soldering of A13 everywhere, looks OK (no shorts either). There doesn't seem to be a corresponding A13 on the 6821, so looks like 6502 to me.

I already replaced the 74LS00 and 74LS138 as it was cheap (less than $2) and easy to do so, with no change to the readings.

PS 6502 clock is good, right on 1Mhz at both pin 37 and 39.

Thanks
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Re: Replica 1 Ten #36 no go

Postby p20vintage » Jun Wed 25, 2014 12:13 am

OK another 6502 received and fitted - no go. Same symptom.

Now I'm at the limit of my expertise and it's annoying (especially as I recently built a Z80 CP/M 80's vintage machine with no issue!!!!) and I fear I'm killing chips somehow. I'm see how much it will cost to send back to the US as I really don't know what to do next (and I don't want to spend more $$$$/time on parts --- or worse, bin the thing). Looks like I'll be getting the Superboard /// as a fully assembled unit when they become available.
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