Mimeo 1 – video problems

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A few days ago I posted that I expected problems similar to Sherlock's when I started to bring up my Mimeo 1. Well, I have got problems, but they are different from Sherlock's.

I tested the power rails before populating the board and they all seemed OK. I have now just populated the video section. Unfortunately, when I power up, I don't get any characters, just a flickery display in which you can sometimes get vertical stripes:

I can't find any obvious shorts or bad connections. All the IC pins look to be in place. Nothing is running hot (except the heatsink, which seems to be as hot as others describe it).

However, the 74166 at D1 is running pretty warm – much warmer than the other ICs.

It sort of looks (to my uninformed mind) like it might be a timing problem with the video output (so some fault in the area of the 74166 may make sense).

My understanding of electronics is pretty limited; so any pointers as to how to go about diagnosing this would be greatly appreciated.

Many thanks for looking.

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speedyG's picture
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Joined: Nov 16 2011
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Re: Mimeo 1 – video problems

Hello lazarusr,

there are 2 possible directions that you must examine:
there seems to be no access to the character ROM
or you have bad RAM Dataline ( D0 to D7 ) access.

So first to solve would be to replace that 74166 at D1 and then examine the datalines D0 to D7.

If one of that lines stays permanent at 0 Volt with no "live signals"
you should trace for shortcut.
Checkout the IC´s that are related to the Character Generator ROM
within the circuitplan of the manual with focus to the 2519 at postion C3 and the 74161 at position D8 and the 74157 at position C7 and also the 74161 at position D11.
This includes the CE ( Chip Enable) signal and the adressing lines too ( A0 to A8 )...

Due to the fact that at the moment the CPU is not populated
you may at least be able to pick up some data signals to be tested from that socket...

speedyG

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Re: Mimeo 1 – video problems

Thanks for the advice.

The first thing I did was to swap out the 74166 from one I had in a working Apple II. That made no difference.

I tried swapping some of the other suspect parts around the board. But that didn't really get me anywhere.

I am not sure the problem is necessarily to do with 2513 character ROM. I still think it has something to do with video timing and sync. At one stage, I was able to get a screen full of characters that looked like @'s (could have been O's) and _'s. However, they were rolling vertically very fast and were difficult to make out.

I also followed some advice from IEEE-802 in Sherlock's thread and removed all the 25xx devices except the 2513. When I power on I can momentarily see a screen full of @'s before the vertical stripes take over. I don't know whether the stripes are actually columns of blocks (as would be the case without the 2513) or columns of @'s which are just 'rolling' very fast.

When all the sockets are populated, the vertical stripes are more wave-like than straight columns. It is difficult to explain and hard to capture in a still photo.

I don't have an oscilloscope or even a logic probe (so my testing abilities are very limited). I did visit my local electronics store today to get a logic probe, but they had sold out. So it will be a few days before I can do anything more to test the board in more detail.

speedyG's picture
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Posts: 2414
Re: Mimeo 1 – video problems

Hello lazarusr,

beeing located in England
getting an oscilloscope would be a bit expensive and time eating....
if you want a cheap but rather satisfying replacement instead take a look at this:
http://www.ebay.de/itm/SainSmart-DSO138-2-4-TFT-Digital-Oscilloscope-DIY-Parts-Kits-1Msps-Probe-DE-/231587113116?hash=item35ebaae89c:g:hWYAAOSw3ydVzEp3
- there are similar cheaper offers, but that other cheaper offers are located in china
and additional transportfees and much more important far longer delivery time -
make the more expensive offer attractiv:

the guy seems to act as distributor for the chinese factory and he is located in Berlin in Germany
so transport time is far faster ( within 1 week instead of the 4 to 5 weeks demanded for mailing from China )
and there are no additional fees in Europe....

I have ordered such a device few weeks ago and the description for mounting and later for adjustment
is realy simple, so a amateur with medium skills of soldering can manage that task.
TIP: pick up the documents that are availiable at the offering page by links
and view them by yourself prior to ordering
and you may be able to decide if your skills are good enough to solve the soldering tasks and understanding the instructions...

The device is fairly usable up to 1 Mhz which is the highest frequency used in the Mimeo....

The fact that the result without the Char ROM is nearly same like with the Char ROM just
confirms my suspect..... there is trouble with the access to the Char ROM !

It is either not selected and activated correct or the output from that ROM is bad....
maybe the content has not been programmed correct in the ROM or the chip is damaged ....
that are also possible reasons for the bad display....

The first point to solve is to get correct Chars to the screen then you might attempt to the next step
in diagnosis of the "wavy" display and disturbed sync....

Did you test the "clear screen" task ? What kind of result ?

sincerely speedyG

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Re: Mimeo 1 – video problems

If you got the kit from Unicorn, you might try swapping out the three 7400 Quad two input NAND gates with replacements from another source. He has been shipping a lot of bad 7400's. I had him replace mine, and still had similar problems which were positively diagnosed with a scope. I replaced them with 7400's from another source and multiple issues (including one similar to this) were solved.

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Re: Mimeo 1 – video problems

whitexkr wrote:

If you got the kit from Unicorn, you might try swapping out the three 7400 Quad two input NAND gates with replacements from another source. He has been shipping a lot of bad 7400's. I had him replace mine, and still had similar problems which were positively diagnosed with a scope. I replaced them with 7400's from another source and multiple issues (including one similar to this) were solved.

So I replaced the 7400 at C15 with a known working chip (actually a 74LS00) from an Apple II.

I now have a screen full of blinking @ symbols. Not exactly what I am meant to have. But it's something more concrete to actually work with.

Based on what IEEE-802 says here: http://www.applefritter.com/content/mimeo-1-build-part-2-video#comment-77497, I suspect I am now dealing with defective 2504s (which is what Sherlock was faced with also, I believe, from Unicorn).

Thanks to everyone for all their help so far.

I am probably going to have to put this aside for a couple of weeks now because of work commitments. In the meantime, I shall order replacement 7400s and 2504s.

P.S. clear screen blanks the screen whilst the contact is made. But as soon as it is released, all the @s come back.

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Re: Mimeo 1 – video problems

A little update. First the monumentally embarrassing bit. The reason the circuit was behaving as if none of the 2504s were plugged in was because they weren't plugged in!! I had stuffed and pulled so many chips, I lost track of where I was up to!!!

Inserting the 2504s gives rise to characters on screen that make no sense. However, that may be unsurprising.

There are three 7400s in the kit from Unicorn. I cannot get the display to work with any of these three chips in the socket at C15. The only chip that will work in that location is the 74LS00 that I pulled from the Apple II. That can only mean one of two things. Either all of the supplied 7400s are defective (which I very much hope and suspect is the case) or there is some more fundamental problem with the circuit such that it can only provide the lower level of power required by the 74LS00 (this seems rather unlikely).

Assuming I am right, this means that the 7400 at D10 is defective and, until this is replaced, it is pointless trying to identify which (if any) of the 2504s may be defective.

So I am going to order some 7400s and pick it up again when they arrive.

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Re: Mimeo 1 – video problems

lazarusr,

Congratulations on getting your Apple I clone to display video with a swapped 74LS00.

Do not rule the possibility of more than one bad 7400 device.
I believe Sherlock had at least 3 x bad 2504 from Unicorn.

Since you have a working Apple II (I assume), you can use it to test your 7400 chips.

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Re: Mimeo 1 – video problems

IEEE-802 wrote:

lazarusr,

Congratulations on getting your Apple I clone to display video with a swapped 74LS00.

Do not rule the possibility of more than one bad 7400 device.
I believe Sherlock had at least 3 x bad 2504 from Unicorn.

Since you have a working Apple II (I assume), you can use it to test your 7400 chips.

Thanks

When I say I have a working Apple II, I neglected to include the word 'barely'. I have only just acquired it and it needs a lot of work including the removal of what I shall simply describe (to avoid breaking any forum etiquette) as a 'substance'.

The kit from Unicorn includes three 7400s. If any one of these chips is placed at C15, the video is unstable and rolls at high speed. The only way I can get stable video, is to use the 74LS00 at C15. I have therefore reasonably assumed that all three 7400s are defective and have ordered three replacements from another source.

There is another 7400 in the video circuit at C10 and I have replaced that with another 74LS00 that I know to be working.

With that I do get a stable display, but all the displayed characters are wrong (much like Sherlock described in an earlier post). If I pull all the 2504v chips like you describe I (appropriately) get a screen of flashing @s. I have therefore assumed that at least some of the 2504v chips are defective. What gets displayed when they are present is much more weird than that described by Sherlock and therefore, for the sake of simplicity, I have simply ordered a full set of seven 2504n chips, again from another source.

I shall update when I receive the replacements.

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Re: Mimeo 1 – video problems

lazurusr,

The good news is the majority of your video section is working (i.e. clock oscillator , the timing generator chain which spans many chips, video shift register, etc)

I agree with your idea to swap the 2504s.
Another common denominator that affects all 2504s is the DS0025 clock driver
A marginal or badly bypassed DS0025 chip could impact all 2504.

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Re: Mimeo 1 – video problems

IEEE-802 wrote:

lazurusr,

The good news is the majority of your video section is working (i.e. clock oscillator , the timing generator chain which spans many chips, video shift register, etc)

I agree with your idea to swap the 2504s.
Another common denominator that affects all 2504s is the DS0025 clock driver
A marginal or badly bypassed DS0025 chip could impact all 2504.

I really hope that the DS0025 is OK. Sources in the UK are expensive and I really don't want the hassle (or the inevitable delay) in persuading Unicorn to ship a replacement.

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Re: Mimeo 1 – video problems

lazarusr wrote:
IEEE-802 wrote:

lazarusr,

Congratulations on getting your Apple I clone to display video with a swapped 74LS00.

Do not rule the possibility of more than one bad 7400 device.
I believe Sherlock had at least 3 x bad 2504 from Unicorn.

Since you have a working Apple II (I assume), you can use it to test your 7400 chips.

Thanks

When I say I have a working Apple II, I neglected to include the word 'barely'. I have only just acquired it and it needs a lot of work including the removal of what I shall simply describe (to avoid breaking any forum etiquette) as a 'substance'.

The kit from Unicorn includes three 7400s. If any one of these chips is placed at C15, the video is unstable and rolls at high speed. The only way I can get stable video, is to use the 74LS00 at C15. I have therefore reasonably assumed that all three 7400s are defective and have ordered three replacements from another source.

There is another 7400 in the video circuit at C10 and I have replaced that with another 74LS00 that I know to be working.

With that I do get a stable display, but all the displayed characters are wrong (much like Sherlock described in an earlier post). If I pull all the 2504v chips like you describe I (appropriately) get a screen of flashing @s. I have therefore assumed that at least some of the 2504v chips are defective. What gets displayed when they are present is much more weird than that described by Sherlock and therefore, for the sake of simplicity, I have simply ordered a full set of seven 2504n chips, again from another source.

I shall update when I receive the replacements.

lazarusr: Sorry you're having troubles like I was. Sounds like you've gotten lots of good advice from all here on this thread, and hopefully the new parts will show more of what you're expected to see. Phew, I'm glad I didn't have problems with my 7400's in addition to my 2504's.

I was able to trace the bad 2504's on my board back to the bad chips by looking at the relationships of what was actually displayed to what I was expecting to see. When you start up, you're supposed to see alternating columns of stable underscores, and alternating columns of flashing @'s (that is, @'s changing to blanks, and then back to @'s), and then when you short the pins on the keyboard DIP socket to "clear" the screen, the screen should be blank with an @ in the upper left corner.

If you look at the datasheet for the character generator (2513), you can compare the codes for the characters. Once you've tracked down which bits in the code are wrong, you can look in the Apple 1 board schematics in the "Apple 1 Operations Manual", and trace the bad bit down to the 2504v chip position (because each 2504v supplies a specific bit of the display character codes). I found tracking these display problems down in this fashion to be actually quite fun, and gave me at least some understanding of this part of the Apple 1's display functionality.

Hope this helps.

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Re: Mimeo 1 – video problems

Well today I received some replacement ICs. I had found a local supplier who was selling 2504n ICs at a relatively cheap price. I say relatively because (with tax and shipping) they worked out at £3.17 (US$4.82) each.

When they arrived, I was pleased to discover they were actually genuine 2504v ICs. What's more they all had date codes of 7624, which couldn't be more perfect. What's more, they looked (unlike some NOS chips I have seen) as if they were absolutely brand new.

So I swapped the previous ones for the new ones and this is what I was presented with:

Bingo! (The CLS function works correctly too!)

I then set about systematically swapping in the suspect ICs one by one and discovered, somewhat to my surprise, that only one of them was in fact defective. I say surprised because previously I was getting a whole range of random characters on the display, not the 'logical' abnormalities that Sherlock had that allowed him to trace the defective ICs. I realise now that the defective chip was sitting in C11b, which has a somewhat different function from the other six. Had I had the presence of mind to simply swap that position with one of the others, I could have saved myself buying so many. In my defence, in addition to trying to debug my Mimeo 1, I was also preparing for a rather complex trial that started on Monday. Anyway, one lives and learns...

As a result, I now have six additional apparently working 2504v ICs. If anyone (particularly in the UK) is facing similar difficulties, I would be happy to oblige.

The replacement 7400s should arrive tomorrow, so I can swap out the 74LS00s and return them to their original devices.

Populating the computer section may have to wait a while as I have another, more British, 6502 based project that I am going to be working on this weekend.

Thanks to everyone for the advice and encouragement.

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Re: Mimeo 1 – video problems

Glad to hear you're past to the next stage!

One other thing - make sure your keyboard works okay before getting rid of those extra 2504's. I had thoughts my 2504's were all okay, but then when I typed, certain keys were having bits flipped, which made them alternate between two characters. This sort of error was traced back to more bad 2504's, and they still gave me a nice clean start-up and clear screen. The flipped bits were only affecting how the characters were being displayed, and did not affect the commands the Mimeo was receiving.

British 6502 project? Sounds suspiciously like a BBC! If so, I'm a big fan. I dragged one all the way to the USA, converted its power supply to run on 120V, and it's a sweet machine. Really brings back memories.

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Re: Mimeo 1 – video problems

Does that local vendor have more 2504v? PM me their number and address. I can receive post in the UK and am always on the lookout for spares.

Thanks,
Corey

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Re: Mimeo 1 – video problems

Corey986 wrote:

Does that local vendor have more 2504v? PM me their number and address. I can receive post in the UK and am always on the lookout for spares.

Thanks,
Corey

PM sent.