As far as I am aware, the majority of the Minstrel kits that are built work first time, or are quickly fixed due to the wrong jumper settings, a missed solder joint or solder bridge, or misplaced component etc.
When there are problems, I try to troubleshoot via email, and usually get things working.
Every now and then, despite the best efforts of the builder, one doesn't work.
This is the case here, a Minstrel 3 was built (quite a while ago I think), and unfortunately didn't work.
They have now sent the Minstrel to me, along with something else for repair.
At first glace, all looks well.
I think this was sold as a Minstrel PCB only, the builder sourced their own parts. (Minstrel PCB only options now available on Tindie - https://www.tindie.com/products/tynemouth/minstrel-2-or-3-pcbs/)
It is good to see a nice set of Texas Instruments logic chips. Whenever I see a photo of a board with a random assortment of vintage logic chips, my heart sinks as I have seen too many times where one or more of those is broken or a fake, remarked etc, or is just old and tired and not running to spec any more.
For example this Minstrel 2 that came in for repair a while ago.
On that board, the 74LS08, 74LS10 and 74LS86 were all bad - http://blog.tynemouthsoftware.co.uk/2017/08/minstrel-zx80-clone-troubleshooting.html
I did note that on today's board, the three big chips, the Z80, RAM and ROM, were all vintage, so my suspicions will fall on those first.
A Z80 from 1983, a Sony CXK58257P RAM chip (from 2008?), an apparently pin compatible with the 62256 required, and finally an unknown EPROM, probably a 27C64, probably of similar vintage.
Before I do anything, I will fire it up and see what it does.
What lovely wallpaper.
Seems an appropriate point to mention if you ever see a screen like this:
This can be caused by the wrong ROM settings. This is what happens if you have the ZXpand ROM without the ZXpand connected. The ZXpand ROM has it's extra code in place of the character bitmaps, hence the display corruption.
You need to move the A15 jumper to the right (or the A13 on some older versions).
I did try a different ROM chip with the correct jumpers, but it made no difference, it is probably just a ZX81 ROM image on the EPROM.
I also looked at the Z80. I have seen quite a few vintage Z80s with bad M1 lines, almost entirely on Spectrums - http://blog.tynemouthsoftware.co.uk/2024/09/divmmc-troubleshooting.html
I have never seen one in a Minstrel, but unlike the ZX Spectrum, the ZX80/81 does need the M1 line to function properly.
The ZX80 based designs need the M1 line to know when to enable the NOP generator, so if you have a Z80 with a bad M1 line, you will get a white screen or just nothing. If you are using a vintage Z80, one good place to check is the Z80 M1 pin.
If it is just a flat line wiggling around ground, you need to get another Z80.
It should look like this, with a low pulse whenever the Z80 is reading an instruction.
This Z80's M1 line was fine.
I did try replacing all three big chips with known working versions, but it also made no difference.
I checked the usual culprits as well, power, clock, reset. All doing what they should be. Nice clean 3.25MHz clock, one of the benefits of 74HC series logic is you always get nice strong, clean traces like this.
I looked around address and data lines, and all seemed to be wiggling appropriately, no bus conflicts as far as I can see.
Hmm .... Scratches head.....
The design is such that pretty much everything needs to work to get a display, since that is mostly what it does.
I had an all socketed Minstrel 3 board on the bench, so I thought it would be a quick way to divide and conquer the problem if I took out all the chips and tried them in the known good board. I would then move onto trying a known good set of chips in the bad board and seeing where the problem was.
I removed all the chip and trued them on the known good board with, and it all worked fine, so the problem was on the board.
The builder was very critical of their soldering and suspected that. Before I put in a set of known working chips, I had a good look over the board and it all looked perfectly fine to me, I couldn't see any issues. They have done quite a neat job.
I probed around for a bit looking for shorts or other issues and then something caught my eye.
Hello, what's going on there.
Did you spot it?
Oooh nasty.
I thought I would dig out the USB microscope and get some closer shots for the blog post.
Definitely damage to the solder mask, the copper is showing through.
The tracks damage on the right doesn't seem to have actually broken any traces.
However on the left, it does look like a break in the middle of those five traces. The one highlighted in blue below.
I had to search around for the V3.5.2 PCB layout, that was quite a while ago.
That trace carries A14 from the diodes on the right to the 74HC251 on the left.
Or rather, it should. In this case there is no continuity there. The others test out fine, just that one trace.
Not the neatest bodge wire. I'll tidy that up later, just wanted to test it out.
And there we are, K prompt, ready to go.
I attached the owners keyboard to test it further (they didn't send the overlay PCBs).
They have the parts to build a Minstrel ZXpand, but have not built that yet, so I tested with one of mine to make sure everything was working.
All looks good. Rex says Hallo.
One final thing I had to do was swap out the 74HC04. For some reason (probably share holders and accountants), Texas Instruments make some ICs with a fraction of a gram less plastic, and they just don't look right. I have generally managed to avoid those, but I do have several tubes of 74HC02 and 74HC165s that I won't use in kits because they just look wrong).
Ah, that's better.
I also swapped out two of the 74HC257s for 74LS257s, as per the latest Minstrel 3 design, to improve loading from tape.
And that's all done, ready to go back.
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Minstrel 2 and 3 kits are available from my Tindie store, with worldwide shipping and 5% off for the next month. Order soon to beat the usual postal slow downs in December, and avoid impacts of the GPSR regulations that come into force on the 13th December, and whatever happens in the USA when the new administration takes over.
Versions are available for ZX81 case:
Or standalone with keyboard:
And also Minstrel 3 with ZXpand microSD card interface:
I have listed the Minstrel 2 and Minstrel 3 PCBs with and without keyboards or overlays on Tindie. Just in case you want to source your own parts and don't like the ease and simplicity of having them all supplied in neat little bags and a foam pad with all the ICs in order.
I am in the process of moving things over there from my SellMyRetro store, so if there is anything that you want, let me know and I'll add it.
I have recently added lots of PET repair and upgrade parts, more to follow.
Bluesky
For those interested in such things, I can now be found on Bluesky, and I expect to be posting more there.
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