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What's the best approach to troubleshooting this Model I Level II cassette problem?

K

kengr

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Sep 11, 2017
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I acquired a very clean, working TRS-80 Model I Level II unit, and the only repair it needed was resoldering one of the keyboard switches. I have the cassette cable (which has been tested) hooked up to a CTR-117 cassette recorder.

For many months, I was able to CSAVE programs to cassette, and successfully CLOAD them. Listening the recorded program, the audio was very clear. It only took me one try to get the volume setting right, and then saving and loading worked 100% of the time.

Suddenly, and now consistently, programs recorded to the cassette have a very loud constant hum, and I can hear the program signal very faintly in the background. No adjustments to the cassette controls were made between when it worked and when it started behaving this way. The relay to control the motor works perfectly. And I can save a program, but can't load it because of the loud hum, faint program that was recorded on the tape. No adjustments to the volume help. (I inadvertently erased my tape containing earlier good program recordings, and I don't have any commercial software tapes, so I don't know if I can still load well-recorded programs. But I suspect that when I solve the bad recording issue, I'll be able to read tapes again. At least, that's my hope.)

I remember from my original system that there was a ground loop problem, which I fixed with a little switch box that would effectively break the ground on either the input data or output data cable, depending on whether your were writing to tape or reading from tape. (You'd throw a switch on the box to choose which operation you were about to perform.) I built one of these, but it's long gone. While the loud hum is reminiscent of that issue, I don't think the ground loop is the issue here, because it worked just fine and then suddenly stopped working, without any changes to cabling or cassette settings. And I haven't changed where everything is plugged in.

I suspect some part of the TRS-80's cassette recording circuit suddenly failed, but am not sure where to start looking. I do have the technical reference manual available, if we need to refer to specific components.

Any advice on steps to troubleshoot this issue? My test equipment is limited, and I'm mainly a software person, so be gentle. (That said, I am willing to use this issue as an excuse to purchase some additional test equipment.)

Thanks.
 
Is the hum present when you connect it to a computer mic or set of speakers? if not, then look at the deck. That way you at least can narrow this down to which is causing it before you tear into the Model I.

Edit to add: Check the heads on the deck? Cotton swap with some alcohol?

Cable "could" have a weak wire and causing the hum.

I'd isolate all the simple things before getting into the Model I.
 
Thanks for your reply and your suggestions.

The loud hum is present when I connect a small amplified speaker to the cassette output while doing a CSAVE operation, without the cassette deck in the equation. When the problem started, I hooked up the speaker to listen to what was actually coming out of the cassette cable from the TRS-80.

I cleaned the heads of the deck just before using it with this TRS-80 for the first time. Even though I had only run recorded on the deck for a total of 45 minutes before this problem showed up, I also cleaned the heads again when this problem showed up. No effect.

I have tried two different cassette cables: an original vintage cable and one I built from scratch. Both have been tested to ensure that they are wired correctly. Both demonstrate the exact same behavior.

I agree that I would like the cause to be something external to the TRS-80. I realize now I should have included these details in my original post.
 
kengr said:
I acquired a very clean, working TRS-80 Model I Level II unit, and the only repair it needed was resoldering one of the keyboard switches. I have the cassette cable (which has been tested) hooked up to a CTR-117 cassette recorder.

For many months, I was able to CSAVE programs to cassette, and successfully CLOAD them. Listening the recorded program, the audio was very clear. It only took me one try to get the volume setting right, and then saving and loading worked 100% of the time.

Suddenly, and now consistently, programs recorded to the cassette have a very loud constant hum, and I can hear the program signal very faintly in the background. No adjustments to the cassette controls were made between when it worked and when it started behaving this way. The relay to control the motor works perfectly. And I can save a program, but can't load it because of the loud hum, faint program that was recorded on the tape. No adjustments to the volume help. (I inadvertently erased my tape containing earlier good program recordings, and I don't have any commercial software tapes, so I don't know if I can still load well-recorded programs. But I suspect that when I solve the bad recording issue, I'll be able to read tapes again. At least, that's my hope.)

I remember from my original system that there was a ground loop problem, which I fixed with a little switch box that would effectively break the ground on either the input data or output data cable, depending on whether your were writing to tape or reading from tape. (You'd throw a switch on the box to choose which operation you were about to perform.) I built one of these, but it's long gone. While the loud hum is reminiscent of that issue, I don't think the ground loop is the issue here, because it worked just fine and then suddenly stopped working, without any changes to cabling or cassette settings. And I haven't changed where everything is plugged in.

I suspect some part of the TRS-80's cassette recording circuit suddenly failed, but am not sure where to start looking. I do have the technical reference manual available, if we need to refer to specific components.

Any advice on steps to troubleshoot this issue? My test equipment is limited, and I'm mainly a software person, so be gentle. (That said, I am willing to use this issue as an excuse to purchase some additional test equipment.)

Thanks.
Click to expand...

Before you you tear everything apart, I would check cables as it may be some simple as a ground problem.
 
Thanks.

Yes, I have checked all the cables for correct wiring to each DIN pin, and ensured that (except for the common data-in and data-out ground connected to DIN pin 2) there are no shorts between any of the wires, and none are shorting against DIN connector shield. Both the original vintage cable and the custom-built cable are wired identically. The custom-built cable was wired per instructions at http://www.8bit-micro.com/data-cable.htm before I acquired the vintage cable. Both cables worked perfectly before this problem showed up, and now both behave the same since this problem showed up. I have just re-checked the wiring in both cables. Good to go; no shorts or breaks.
 
Yeah, sounds like it could be inside. One last thing I would try and to just shoot some contact cleaner into the din jack just in case there is some corrosion there.

Tearing into the Model I isn't a hard endeavor, you just need to be very careful with the ribbon cable between the keyboard and CPU board. I'm sure some on here can help you with the troubleshooting, but first google TRS-80 Technical Reference Manual and you'll find the black one for the Model I. It is really good at helping to describe how the whole circuit works. You might be able to figure out by looking at it where to look. I'd hope it was something simple like a bad capacitor or resistor, because they are far easier to change than pulling out chips, not that its hard, it just takes practice to get good at it. If I get some time today I might look at the schematics and see where would be a good place to look.

Does it only happen while its saving or is it constant? Play a game on it and see if it does it when making sound? This would still help determine of the relay is somehow causing the issue.
 
Thanks for the advice.

I'll need to go get some contact cleaner to try that. Will report results.

I have opened up the Model I before, to re-solder a key switch that was working intermittently. (Frankly, the hardest part of opening the case was more emotional than anything...breaking that nice orange nail polish seal.) Yes, the ribbon cable is very brittle, and I was careful not to flex it too much. (I'd like to replace it with something more durable someday.) I do have the Technical Reference manual in PDF form.

As far as I can tell, the hum occurs only when saving. Unfortunately, I don't have any games or other software that produces sound. If there is a small sample BASIC program somewhere I can try to generate sound, I could test that.
 
I don't know much about the TRS-80, but a loud hum on anything audio suggests a bad filter capacitor allowing AC ripple through.
 
kengr said:
As far as I can tell, the hum occurs only when saving. Unfortunately, I don't have any games or other software that produces sound. If there is a small sample BASIC program somewhere I can try to generate sound, I could test that.
Click to expand...

Here's simple BASIC program that will generate a low buzzing noise:
Code: 
10 OUT255,1:OUT255,0:GOTO10

That doesn't turn on the cassette relay. This will make the buzzing noise with the relay activated:
Code: 
10 OUT255,5:OUT255,4:GOTO10

The buzzing is at a low frequency that may make it hard to distinguish from the 60 Hz hum. I can give you a slightly longer BASIC program that will produce clear tones if you like.

There might be some diagnostic value in checking the output voltage levels. "OUT 255,0" should register as 0.46 V. "OUT 255,1" should show 0.85 V. "OUT 255,2" will be 0 V. Best to do this in a loop just to be sure:

Code: 
10 INPUT N
20 OUT 255,N:GOTO 20

That program will prompt for the output value and then continuously send it to the cassette port. I believe the loop is necessary because BASIC will reset the cassette port when it returns to the READY prompt.
 
Thanks for all this.

I tried the first loop you wrote, with an amplified speaker attached to the cassette output. What I got was a rapid click-click-click sound, where the clicks are very distinct from each other. To make sure this is what I was getting from the program, I stopped and started the program several times. When the program isn't running, I get silence. When the program is running, I get the rapid click-click-click sound. This is very different from the loud hum I was getting on CSAVE.

I tried the second loop you wrote, with an amplified speaker on the cassette output and the motor control attached to the cassette deck. I started and stopped the program a few times, to verify that (a) the motor starts and stops as expected, and (b) the sound I get from the speaker is the rapid click-click-click as described above. No loud hum is present.

I need to hunt down my meter to test the voltage, but thought I'd reply at this point, in case these results tell us something.
 
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