Have you tried cleaning the volume? The TDA7284 which is the record/playback circuit is suspect. The electrolyte caps at the top on the other hand work as a filter, so replacing them with audio grade caps might help.

TDA7284 datasheet
https://www.digchip.com/datasheets/parts/data … 456/TDA7284.php

I'll try some contact cleaner into the jacks and the volume pot tonight.

I kind of doubt that the TDA chip is the culprit as low volume is in both channels, and there is some very bad background noise, it almost feels like a dead filtering cap to me?

My card is attached. I'm kind of suspicious about the large blue filtering cap on the SPK out..

the LM386 on it is able to drive headphones to ear hurting levels so it is quite likely the little capacitors around it have to get replaced.

As far as all the interference goes, there isn't much you can do about it. This card doesn't have a differential input on the CD header so the internal ground loop of the machine between mobo and CD dcive etc. will prevail.

Tiido wrote on 2023-08-14, 13:23:

the LM386 on it is able to drive headphones to ear hurting levels so it is quite likely the little capacitors around it have to get replaced.

As far as all the interference goes, there isn't much you can do about it. This card doesn't have a differential input on the CD header so the internal ground loop of the machine between mobo and CD dcive etc. will prevail.

Thanks @Tiido I'll replace EC4, EC7 and EC11 and see if that helps, I should have caps that will do the job. I checked the caps on board and they are not audio grade or anything, just your run of the mill 85C caps with ~400mA Ripple, so using some noname fresh electrolytics probably won't hurt..

Low volume seems unusual. Cleaning the pots and jacks may help, if not could be a component issue (caps, op-amp, etc.)

As for noise, I have a few early Aztech cards and they tend to generate noise based on system activity. I find this is par for the course with a lot of sound cards from the early 90s.

Thanks, I'll do a deox/contact cleaner run tonight and if that doesn't help, I'll replace first EC4,7,11 and if that fails the remainin electrolytics just in case. If that doesn't help I'll look at replacing the LM386 (I should have spares from back when I built an Adlib) and TDA7284 (not so readily available so I may need to find a pin compatible part..)

TDA7284 is only used for microphone input from what I remember, it is very unlikely it has any effect on this problem. Really quiet output without any bass freqs is a symptom of dried out capacitors and I did have to replace them on my card (and I did a few other mods to it too to increase subjective sound quality).

Tiido wrote on 2023-08-14, 15:00:

TDA7284 is only used for microphone input from what I remember, it is very unlikely it has any effect on this problem. Really quiet output without any bass freqs is a symptom of dried out capacitors and I did have to replace them on my card (and I did a few other mods to it too to increase subjective sound quality).

This is exactly what I get, I can only hear the trebles and the max volume is very quiet..

Did you do a full recap? As far as I can see this card's electrolytics are pretty.. mediocre, so I'm guessing they may all have dried up.

Then it is definitely bad capacitors. I replaced everything, in general when one goes all follow sooner or later and there aren't many to begin with so I got them all changed.

Tiido wrote on 2023-08-14, 15:41:

Then it is definitely bad capacitors. I replaced everything, in general when one goes all follow sooner or later and there aren't many to begin with so I got them all changed.

I changed C4 to no effect, only to find out that the blue cap is not original and someone had already replaced that..

I then went ahead and replaced C7 and C11 and the volume skyrocketed to earbleeding levels!

So did the noise.. I think I will go ahead and do a full recap of the board as well, which I think should help with the noise flor and the amount of noise it picks up from the ISA Bus, no?

Doing the others will most likely help, they're not gonna be in much better shape than the ones you just replaced. But if it will help with the noise from system side of things is uncertain. You need to see where the +/-12V lines go to and put some good capacitors on them, like how 5V has tantalums in bottom of PCB.

appiah4 wrote on 2023-08-14, 13:03:

My card is attached. I'm kind of suspicious about the large blue filtering cap on the SPK out..

Maybe I'm wrong, but I see a dent on top of the blue cap.

I have finished the full recap today and evwn though the pretty poor noise floor barely changed the interference from the ISA bus is bow a lot more tolerable, due to both being less loud as well as running the card at 20% volume instead of 100%.

I should probably install the card's drivers. I wonder if it has a mixer that can mute l/i, mic and CD audio..

There is no user controllable mixer on the card, it is just all analog hardwired mixing on it that cannot be controlled. One thing I did to reduce the noisefloor was to get rid of the TDA7284 output, it is a low end cassette stuff amp with a huge noisefloor all on its own and it is doing the duty of high gain amplifier for a microphone, making noise even bigger.

I made a schematic of the analog part of a slightly different model so there may be some differences but overall it should be matter of disconnecting the capacitor or resistor on TDA output to get things much less noisy. I don't have better resolution available to me at the moment, I need to dig up and resurrect a dead machine to get the original data to accessible state...

That sounds a bit like a difficult to reverse mod, I wonder if it would be possible to add something like a jumper at any point in the circuit to enable/disable the TDA Output?

Here is my recapped card, by the way. The new capacitors are a bit taller, and a bit more colorful, but I think it looks good 😀

That jumper goes in place of the resistor/capacitor you got to find and intercept so it is just as "difficult" 🤣