TI99 with no sound

[mike_z]

I've been retired for quite some time and recently came across some of my kids stuff in the attic. Thought maybe the grandkids would be interested in some of this stuff. I came across a TI99 4A that I had forgotten about. Pulled it out, cleaned connected it up, nothing. The power supply was working. I found a few broken wires in the video cord. After repairing those the video came on and I could type in some BASIC commands and it worked. I tried a cartridge and that worked also, but there is no sound. You can turn up the volume on the monitor and it sounds like an amplifier without an input. I opened up the TI99. Cleaned out the insides,re seated the IC in sockets. Some of the TI chips have black leads, tried to clean them some. Cleaned the heat goo from the Video chip (?) and put some new stuff on it. Looked for poor connections or CB cracks, nothing. Reassembled the TI and although the video improved there is still no sound. The TMS9919 is one of the TI chips with black leads. I wonder if that may still be the problem. Could not find a replacement, but it looks like a SN75489 would work? Any ideas, thanks, Mike

[RickyDean]

I would suggest checking the audio wire on the 5 pin din that plugs into the back of the ti and the rca jack that comes from there, testing for continuity between them. Eliminate the cable from being a possible issue before changing out components. Check for possible solder issues on both the din plug and jack too.

[mike_z]

Rick, thanks for the reply, I have checked the video cable/cord. There were two wires broken that I had to resolder. I'm pretty sure that the cable is ok. I had thought of attempting to inject an audio signal on the audio pin, to see if that makes it though the modulator. Mike

[HOME AUTOMATION]

If you are getting a louder hum when touching the otherwise unconnected input wire to the modulator, it's fair to assume it's working...

I wouldn't inject anything into the din though, as I can attest to the ease with which that IC's output can be permanently defeated!

The computer will continue to run w/o the IC inserted.

[AwkwardPotato]

2 hours ago, mike_z said:

The TMS9919 is one of the TI chips with black leads. I wonder if that may still be the problem. Could not find a replacement, but it looks like a SN75489 would work?

If yours is actually labelled TMS9919 (which is actually pretty rare in 4As) it needs to be replaced with either an SN94624(A) or SN76494(A). Other chips like the SN76489 would need a higher clock rate than is found on a board that originally came with a 9919.

[mike_z]

Well, well, well...... You guys were correct. I have a SN7694. My 75 year old eye sight can see the number on the diagram better than on the chip. So, I pulled the chip and injected an audio on pin 9, nothing. I opened up the modulator and applied the tone on the yellow wire and the monitor played the sound. Next I re looked at the cable connections and they are OK, but there was no continuity between the ends. After wiggling the cable for a while, and not getting any connection or determine which end the open is on. I just cut off both ends and re soldered the wired to the board and the plug. The sound came back. Thanks a million. Now I can dazzle the grand kids, as I did my kids so many years ago. Well.... maybe the grand kids may seem to think it's a little hokey? Mike

[mike_z]

Surprisingly, after some additional attic mining, I found more parts of the old TI99. I found the tape recorder, cable, a bunch of old tapes and a few books. So, I thought I'd try the recorder and see it any of the old tape survived. No soap. All the tapes were blank. So, I thought of trying to make some new tapes. After a lot of cleaning and connecting (and remembering how to make this stuff work). I wrote a short basic program and then attempted to save it using SAVE CS1. I heard a tone, which I think is a leader and then hash which is the data. It didn't last long (short program) and there was no trailer tone. I played the tape back and it was on the tape, with good volume. Next I tried to read the tape using OLD CS1, this didn't work, kept getting errors. So I figured that maybe the cable was bad as was the video cable. Sure enough there were breaks in the wires. Some wiggling revealed high resistance on some of the connections. I did get to read the tape correctly once, by holding the cable in a certain way. Seemed that most of the problems were at the 9 pin plug end. Then I realized that this plug is just a DB9 plug. I cut the old plug off and soldered on a new DB9 plug. Being ever hopeful I tried the record a test program, but it didn't work. I checked the cable and it was OK, but no leader tone and no data hash. Tried many times, nothing. It was as if the entire circuit as off. Dragged out the Scope and connected it to pin 5 of the cassette cable and there is no VDC or VAC signal on it at all. Dug into the computer and tried pin 28 on the 9901, nothing there either. I assume that there should be some signal here. Unless I'm forgetting something. What kind of signal should I be seeing here. The interface between the 9901 and the DB9 plug is just some passive components, which all seem OK. Any suggestions? Thanks Mike

[mike_z]

Well...... I have to back track a little. I found that I have a bad scope probe. Let me find a better one and then report back, Mike

[mike_z]

Seems my old cable and probes are a problem. Anyway, now I see some signals of significance. At pin 28 of the 9901 I see lots of signal, about 5vpp. But then it attenuates rapidly. The RC network of C403, C407, R400 and R401, reduce the signal to about 2 milli volts pp across R401. The resistors have the proper values.  The resistor divider gives about a drop to 3.5% and it would produce about 170 milli volts. The two capacitors look odd to me, they look like resistors with color codes on them. They are not shorted, but that is all I can tell, yet I suspect that maybe they are the trouble. What do you think? Mike

[mike_z]

I replaced C403 and C407. Now when I SAVE CS1, I get sound on the recording tape. BUT, I still can not read the tape. There must be some more problems. Progress!, Mike

[+acadiel]

Loading is finicky. Make sure you play with the volume and tone controls and try different settings.

[mike_z]

Here is the next problem in the ongoing saga of the attic TI99. I have restored the ability to record and the recording looks OK. There are nice square waves of good pp voltages and I can hear the leader and data on the tape. Next I connected my scope to Pin 30 of the 9901. Then when reading the tape, I get nice pulses on this pin, but the TI99 never acknowledges the read. My guess is that there is something wrong with the 9901 chip. I don't know how it works, so for now I stuck. Any ideas? Thanks for the help, Mike