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Help needed with MITS ACR

K

kyrbee

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Aug 1, 2026
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I'm bringing my Altair 8800b back to life after 40+ years. I have the unit running with 24K of memory, a 2SIO card and an ACR/SIO-B card. I am able to install and run Basic (started with 4K3.2) by loading it through the 2SIO card and using the appropriate data and loader files from altairclone.com. It's cumbersome in that I have to set the serial port to 9600 8-N-1 to load the software, but then change it to 9600 7-S-1 to run Basic. I could activate the other port on the 2SIO and split it and call it done, but I'd really like to get the ACR working again. (It functioned just fine 40 years ago.) I never knew what the waveforms should be coming out of the demod circuit. Attached is a scope capture of the card playing out 125's recorded from the Altair. The bottom trace is from test point TP (after the filters), and the top trace is the output of the daughter card (signal SRSI). Feeding the front panel the ACR locks up in a second or two every time and the square wave appears solid. My question is "is this what the signal at TP supposed to look like? There is a little bit of clipping at the top but adjusting for that doesn't seem to after the output. When I adjust the two filter pots I can make the 2400 freq equal the 1850 freq, but is that the right approach? Regardless, so far the loads via the ACR all fail.
 

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Yes, I've been using the alignment procedures with a scope. Attached is another screen shot just now of test point TP (filter out) and SRSI (data out of the ACR). Is this what the signals should look like? R29 is very touchy and the PLL lock range is small. I'm thinking of replacing it with something better.
 

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  • ACR TP and SRSI-260812-1035.png
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Further, the SRSI signal is not a uniform square wave. I'm testing by playing back a 125 test signal recorded from the ACR. Adjusting for equal highs and lows while feeding it 125's allows the R29 PLL adjustment to have short range, but making the highs and lows as equal as possible puts the adjustment right at the end of the range and it drops out. The pulse widths are also not completely stable. Preceding that in the circuit, I've been assuming that the adjustment of the two filter pots has a goal of equal amplitudes of the two frequencies in the yellow trace shown in the image. I can do that with minimal clipping on the top of the waveform. I can get it set such that the 125's as seen on the front panel are stable for a while but eventually an error will occur. To try to load Basic I've been using the Basic wave files and loaders from altairclone.com. I have two old cassettes (MITS Ext V4.1) that I used decades ago but the player I used is long gone and the only alternative I have is an old handheld dictation thing and it's quite subpar. Consequently, I reverted to a better wave file. I then tried using a wave file from deramp.com (using loader https://deramp.com/downloads/altair/software/papertape_cassette/Cassette Loaders/LDREX40.PRN and wave file https://deramp.com/downloads/altair/software/papertape_cassette/Tapes as .wav/Basic Tapes.zip and within the file Extended BASIC Ver 4-1.wav). In this last attempt the loader ran and then appeared to continue waiting, tried it twice. Sense switches A12, A9, and A8 on. Did I use the correct loader here? Very much appreciate the help - thanks. FWIW, I can load Extended Basic V4.1 via the 2SIO port and that works well.
 
Maybe you could use a PC with a sound card to record the output of your handheld dictation device and compare that to the original wav file?
 
Thanks. I'm using the app Audible to record and play back the Basic wave files on my win10 'terminal'. (I use Audible for all my music and old time radio files.) I recorded both sides of the two 50-year-old cassettes that came with the Altair using Audible. I also recorded the 125 pattern wave signal from the Altair and then played it back into the Altair. The first simple test was playing back the 125 file and adjusting the ACR to get to the square wave at the ACR output (SRSI signal). I send the live signal to the front panel (single step 333octal and then 007octal) and monitor the output live and watching a scope at test point TP (filter output) at the same time. At first things were looking good, or so I thought. The 125 locked up but watching it for a while errors occur. Too unstable. The files recorded from the cassette are poor; the amplitude varies a bit and there are a couple dropouts. (Audible allows you to visually examine the entire file.) One next step is to ask around for a better cassette player before I decide the cassettes are NG. But putting the cassettes aside, the wave files found online should work much better. That's what I've been struggling with.
 
@kybree So, just to be clear, you continue to have the same problem when playing back a presumably good 'wav file' (audio recording) for the ACR board to interpret, regardless of the audio source (win 10 pc, hndhld dictation)?

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http://www.virtualaltair.com/virtualaltair.com/vac_88-ACR.asp

Could your original cassettes be using the original frequency pair of 2225/2205 Hz? Has your board been modified for 2400/1850Hz operation?
 
I have the same problem regardless of which good file (from deramp) I use. It's possible I have the same problem when playing back the file I copied to Win10 from my old cassettes. But the further complication with the cassette is that the recorded files when viewed in Audible are not as clean: level drifts and there are a couple dropouts. (But the problem seems to manifest itself before the playback gets to the first dropout.) I hadn't thought about the frequency pair being different - I look at that. But, the 125s coming out of the Altair when playing the test file recorded from the Altair I see the 2400/1850 pair. I can tell from the scope that those are the frequencies. I don't recall ever modifying the ACR and it worked for me back then so I think the frequencies are correct wrt the ACR setup and my cassettes. Which frequency pair are the the wave files recorded from deramp.com?
 
The digital output from the demodulator should be a square wave, 3.33ms, 3.33ms low. The output from the from the op-amp filter looks about right, so it’s the PLL demodulator circuit that has issues. Looks like it’s slow on the switch from 1850hz to 2400hz.

Mike D
 
Thanks for the confirmation on the filter output. The demod output is a square wave and about 3.33ms but it is a bit unstable - width randomly changes width about +/- 0.2ms. Just ran the 125 test wave file and the 125's appear to lock up 'almost' every time and I'm guessing the checksum isn't good enough. I'll focus on the PLL circuit.
 
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