Lambda Mikel Posted July 17, 2022 Share Posted July 17, 2022 Dear all, I have noise on my NTSC new-style motherboard (beige) TI 99/4a. I am just hearing the CPU signals etc. My other beige TI and the silver ones don't have that issue. I also soldered a stereo socket directly to the DIN pins for audio (3 and 2), but same sound as over the DIN socket (so it's not the DIN -> RCA video cable either). How would I go about diagnosing the signal path / faulty component? Capacitors? I can't seem to find any schematics. Thanks Quote Link to comment Share on other sites More sharing options...
Lambda Mikel Posted July 17, 2022 Author Share Posted July 17, 2022 (edited) After a couple of hours and swapping capacitors that I had determined were in the audio patch, with no result. Finally decided to put it back together as is - and in that process I noticed that adjusting trimmer / pot RV1 on the power supply, which is touched by accident in the reassembly process, fixed the issue. So, that was an easy fix! All noise and hissing gone! Edited July 17, 2022 by Lambda Mikel 6 Quote Link to comment Share on other sites More sharing options...
Ed in SoDak Posted July 17, 2022 Share Posted July 17, 2022 Good to know! 2 Quote Link to comment Share on other sites More sharing options...
Lambda Mikel Posted July 17, 2022 Author Share Posted July 17, 2022 (edited) Unfortunately, after 10 minutes when the TI QI heats up, the noise is coming back... now what? New power supply board? And, for whatever reason, I seem to be unable to find the sound chip on board... well it has one, just where? Edited July 17, 2022 by Lambda Mikel Quote Link to comment Share on other sites More sharing options...
HOME AUTOMATION Posted July 17, 2022 Share Posted July 17, 2022 U7... TI-99/4QI SCHEMATIC DIAGRAM TI-99/4QI COMPONENT PLACEMENT DIAGRAM 2 Quote Link to comment Share on other sites More sharing options...
AwkwardPotato Posted July 17, 2022 Share Posted July 17, 2022 (edited) Verify that all the voltages coming off the power supply are still within spec. I mention this not because I think it's the root cause of your sound issue, but because RV1 may be an adjustment for one of the power supply outputs (I don't have one ATM to verify this). The sound IC is right next to the large hole in the motherboard that one of the case standoffs protrudes through. Chances are that it's the fault, rather than the power supply. Edited July 17, 2022 by AwkwardPotato 1 Quote Link to comment Share on other sites More sharing options...
Lambda Mikel Posted July 17, 2022 Author Share Posted July 17, 2022 Thanks! Yes, the power is OK. I just did a full recap... didn't change anything unfortunately. Quote Link to comment Share on other sites More sharing options...
matthew180 Posted July 17, 2022 Share Posted July 17, 2022 Aside from any power supply problems you are having, be aware that the 99/4A has very noisy audio overall. 2 Quote Link to comment Share on other sites More sharing options...
Lambda Mikel Posted July 17, 2022 Author Share Posted July 17, 2022 (edited) Ordered a replacement sound chip now, let see how this goes. Thanks so far. Yeah, I know audio is not the greatest, but none of my other TIs are that bad. So it must be a chip defect I fear. And, the noise is getting louder the longer it is on. Edited July 17, 2022 by Lambda Mikel Type Quote Link to comment Share on other sites More sharing options...
AwkwardPotato Posted July 18, 2022 Share Posted July 18, 2022 2 hours ago, Lambda Mikel said: Thanks! Yes, the power is OK. I just did a full recap... didn't change anything unfortunately. Just for the record, those green parts are film capacitors, and it is *incredibly* uncommon for those to fail in these types of applications. If any of the capacitors are going to fail, it'll be the red/black aluminum-can electrolytics (and even those generally seem to be fine in the QI power supply). Also, if the replacement capacitors are ceramics, be aware that some types of ceramics' capacitance will change significantly depending on the DC voltage applied to them. This absolutely can influence circuit operations, the film caps on the other hand are immune to this effect. 1 Quote Link to comment Share on other sites More sharing options...
Willsy Posted July 18, 2022 Share Posted July 18, 2022 Yes - change those electrolytics out if you're going to change anything. 1 Quote Link to comment Share on other sites More sharing options...
Lambda Mikel Posted July 18, 2022 Author Share Posted July 18, 2022 (edited) It's believe this is not caused by the caps at all, and the effect that changing RV1 had on the sound was a red herring. I desoldered the sound chip and put a socket in. Waiting for a replacement. Swapping eletrolytic caps never made a difference for me with any machine (with the exception of the SMD electrolytic caps in the Amiga 600); they are generally fine. Guess I was just lucky. I exchanged two electrolytics near the DIN socket, in the audio path, but no difference. So I am not going to swap them. Regarding the caps - well they looked a bit like RIFAs to me, and 2 looked a bit "burned", but that might just be the coloring... Yes, the replacement caps are multilayer ceramic (the small ones) and polyester film capacitors (the orange bigger ones). I don't expect any problems. Edited July 18, 2022 by Lambda Mikel 1 Quote Link to comment Share on other sites More sharing options...
Willsy Posted July 18, 2022 Share Posted July 18, 2022 Electrolytics don't normally pose a problem... Until they do The problem is as they age, they turn into resistors. They can can still add capacitance (though they will be off spec) but they develop a (very low) resistance, the equivalent of placing a 0.5 or 1 ohm resistor across them. It wouldn't be so bad if the resistance was very high, but because the resistance is low, it causes the power supply to work much harder (and therefore get much hotter) than one would like. Eventually, something in the power supply will give up. You'll think it was a power supply problem, but in fact, the power supply failure would be a symptom, not a cause. I see this in old amplifiers all the time. 3 Quote Link to comment Share on other sites More sharing options...
Lambda Mikel Posted July 22, 2022 Author Share Posted July 22, 2022 (edited) Got a new soundchip - problem fixed! Edited July 22, 2022 by Lambda Mikel 6 Quote Link to comment Share on other sites More sharing options...
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