• Please review our updated Terms and Rules here

Tandy 1000 SX strange audio issue

killr0y

Member
Joined
Oct 11, 2021
Messages
13
Hi All,

I have a fully working Tandy 1000 SX where all of a sudden the PC speaker audio for the Tandy sound stopped working. The PC beep works, so its not the speaker. This SEEMS to have coincided with when I was trying to attach a composite monitor to the PC and perhaps brushed the RCA cable from the monitor along some pins (maybe the lightpen port?) on the back. I can't be for sure that this caused anything, but that's all I can think of. I checked to see if maybe there was a short between the signal and ground of the audio out jack just in case the sound chip was detecting an external speaker even when one wasn't attached, but continuity says OL.

The external sound works and I can actually hear sound coming out of it at extremely low volume and quite distorted, but it is there. This leads me to believe that its not the sound chip. I did a swap with other sound chips but same results. I'm wondering if there's some other component I should be looking at as well?
 
Try this BASIC program (in Tandy 1000 BASIC, not generic GW-BASIC). It plays various octaves of sounds through the noise channel and all three voices at maximum volume. Also try putting a SOUND ON statement as line 0 in the program.

 
Same issue: no sound from PC speaker, but if I plug amplified speaker into the external audio port, I can hear the sounds faintly.
 
The PC beep works, so its not the speaker

To be clear on this, the initial boot-time beep works, but beyond that you can only hear sound extremely faintly? So going back to BASIC, if you run the "BEEP" command does that come through the speaker or it also barely audible after the inital boot? (I believe the "BEEP" command should use the PC-compatible sound hardware, not the Tandy sound chip.)

If you've already swapped sound chips my next best guess would be the 14529 (aka NTE4529B, or quite a few other part numbers) multiplexer chip, U25.
 
You have most of it right except that the Tandy sound isn't coming through the PC speaker whatsoever. It can only be heard faintly by connecting an amplified speaker to the external audio jack. I read in the repair manual something about an audio mixer circuit but am confused as to whether that's in the sound chip or another one. The basic beep is perfectly audible out of the built-in PC speaker, unless an external speaker is plugged in and then it gets routed out that way.
 
I have a 1000SX but use a mono audio card. If you want to use the external audio try loading this on boot:
"tdyspkr.com"
You will need to call it through you Autoexec.bat on boot. It may be easier just to find an 8-bit audio card.
 
You have most of it right except that the Tandy sound isn't coming through the PC speaker whatsoever. It can only be heard faintly by connecting an amplified speaker to the external audio jack. I read in the repair manual something about an audio mixer circuit but am confused as to whether that's in the sound chip or another one. The basic beep is perfectly audible out of the built-in PC speaker, unless an external speaker is plugged in and then it gets routed out that way.

The relevant page of the 1000SX's schematic is sheet 7. (You will notice that it doesn't quite match up with the functional diagram in Figure 8; this is confusing.) The NTE4529B I mentioned is a dual 4:1 multiplexer that can also act as a 1:8. IE, it has two sets of four inputs that can independently be switched to select which source goes out an output, or it can also select a single input from all 8 that will go to *both* outputs. Even though there are 8 inputs there are four options:

1: PC sound only
2: PC sound mixed with the output from the SN76496 (this is accomplished before the multiplexer; look at the schematic, a branch of the PPI/TIM line feeds into "AUDIOIN" on the sound chip.)
3: Sound from the proprietary "AUDIOIN" line on the expansion bus. (I don't know if anything ever actually used this.)
4: Grounded, IE, nothing.

And because of the sort of weird way it's wired not every choice can be independently selected for each of the two outputs, which as you can see in the schematic one of which goes to a preamp in front of the audio out jack and the other of which goes to the speaker.

My operative theory of what might be happening is that multiplexer is stuck so it's in a mode where "PC sound only" is being provided to both audio outputs regardless of what the control lines are telling it to do. I think the only reason you can hear the Tandy sound faintly on the audio output with an amplified speaker is essentially a small amount of signal is "leaking" through, not enough to be audible on the speaker but you can pick it up when you amplify it enough. If I turn up the volume enough on my Tandy 1000HX I can hear all kinds of subtle interference sounds so it wouldn't surprise me a bit if noise from the Tandy sound chip can leak through even when it's unselected.

The reason I asked if you could still hear the PC beep after the initial bootup or in BASIC when you should be set up to play the Tandy sound is because the way that multiplexer is set up *if* it were correctly switching to the output from the sound chip but the audio from the sound chip were somehow badly attenuated then it should also be muffling the beeps. (Unless the chip itself is broken in such a way that its generated output is trash but the sound it pipes through from AUDIOIN is unaffected, but since you said you swapped sound generators and it didn't make a difference I'm inclined to think that's not the problem.)
 
According to the "A Practical Guide to the Tandy 1000SX", Chapter 2, "Setting Up Your Computer"; page A-8, the only reference to external audio is the connection from the 1000SX audio out RCA jack to the Tandy RF modulator (CAT NO. 15-1273).
I have all of the original 1000SX reference manuals that came with the 1000SX and would be more than glad to look anything up for you. Also, I pulled my SX down off the shelf and it's now up and running if you would like me to run some sort of tests.

On the other hand, if you suspect a short it's anybody's guess. Good luck,
 
According to the "A Practical Guide to the Tandy 1000SX", Chapter 2, "Setting Up Your Computer"; page A-8, the only reference to external audio is the connection from the 1000SX audio out RCA jack to the Tandy RF modulator (CAT NO. 15-1273).
The external sound works

Just to be crystal-clear, when referring to "external sound" we're talking about the RCA jack on the back next to the composite video out jack, not the headphone jack on the front? Just tossing that out there because the schematics alone are a little ambiguous about the routing of the outputs of the two op-amps after the audio multiplexer, but it at least *looks like* the headphone jack is parallel with the speaker while the audio jack on the back is the "other" output.

(It might be an interesting experiment to connect an amplified speaker to the headphone out and jack it up high to see if you can actually also hear the tandy audio faintly through that route, although it wouldn't necessarily disprove my theory about the multiplexer if you couldn't.)

It's sort of fascinating to walk through the different configurations Tandy applied to that audio multiplexer over the years. That diagram that neglects the separate Audio Out connected to the multiplexer is the same one that appears in the 1000EX and HX tech manuals, and for them it's accurate because they don't have the separate jack or the associated op-amp. Meanwhile the original 1000/1000A have the internal speaker/headphone jack skipping the multiplexer, it's hardwired to the mixed PC BEEP/sound chip output and the multiplexer only selects what comes out the back panel jack. Kind of curious why Tandy decided to implement all this in the first place.
 
Just to be crystal-clear, when referring to "external sound" we're talking about the RCA jack on the back next to the composite video out jack, not the headphone jack on the front? Just tossing that out there because the schematics alone are a little ambiguous about the routing of the outputs of the two op-amps after the audio multiplexer, but it at least *looks like* the headphone jack is parallel with the speaker while the audio jack on the back is the "other" output.

(It might be an interesting experiment to connect an amplified speaker to the headphone out and jack it up high to see if you can actually also hear the tandy audio faintly through that route, although it wouldn't necessarily disprove my theory about the multiplexer if you couldn't.)

It's sort of fascinating to walk through the different configurations Tandy applied to that audio multiplexer over the years. That diagram that neglects the separate Audio Out connected to the multiplexer is the same one that appears in the 1000EX and HX tech manuals, and for them it's accurate because they don't have the separate jack or the associated op-amp. Meanwhile the original 1000/1000A have the internal speaker/headphone jack skipping the multiplexer, it's hardwired to the mixed PC BEEP/sound chip output and the multiplexer only selects what comes out the back panel jack. Kind of curious why Tandy decided to implement all this in the first place.
Well, that's what I was referring to, was the black RCA audio jack next to the red composite video jack on the rear.
 
(It might be an interesting experiment to connect an amplified speaker to the headphone out and jack it up high to see if you can actually also hear the tandy audio faintly through that route, although it wouldn't necessarily disprove my theory about the multiplexer if you couldn't.)

It's sort of fascinating to walk through the different configurations Tandy applied to that audio multiplexer over the years.
There's no headphone jack on a 1000SX.

To recap: the 1000SX has two audio outputs - the internal PC speaker and the line level RCA audio output on the rear, next to the composite video output.

The CPU generated system beeps come out loud and clear through the rear RCA jack. Tandy audio can be heard at a very low level through the rear audio as well.

The CPU generated beeps are routed through the Tandy audio chip via some kind of pre-amp circuit on the chip. I'm wondering what drives the post-amp circuitry because it sounds like (no pun intended) that the Tandy audio output level isn't getting enough amplification.

Like I said, it's a strange issue 😭
 
Last edited:
There's no headphone jack on a 1000SX

Doh! Looks like you're right. Apparently I confused the layout of its front panel with the TX. (I saw a "volume control" on the schematic and assumed that was the front panel one you see on a TX, I guess that's an internal trimpot on the SX.) Which I guess also emphasizes how that diagram from the manual is wrong because it also shows a headphone jack hanging on the same multiplexer output as the speaker. (Again, right for an EX and HX.)
 
Doh! Looks like you're right. Apparently I confused the layout of its front panel with the TX. (I saw a "volume control" on the schematic and assumed that was the front panel one you see on a TX, I guess that's an internal trimpot on the SX.) Which I guess also emphasizes how that diagram from the manual is wrong because it also shows a headphone jack hanging on the same multiplexer output as the speaker. (Again, right for an EX and HX.)
Yes, there is in fact an internal trim pot for the PC speaker audio. It has no effect on the Tandy audio though
 
The CPU generated system beeps come out loud and clear through the rear RCA jack. Tandy audio can be heard at a very low level through the rear audio as well.

And as I’ve said several times, this could be the result of the audio switching circuit being stuck in the PC-sound-only setting with just a faint bit of the SN76496 output leaking through. The output amps for both the rear jack and the speaker are on the other side of the multiplexer, and you keep saying you’re hearing the PC beep perfectly clearly through both so it seems like they’re fine.

Something you could try, I guess, is run a clip from the output pin of the sound chip to an amp input (ground the shield line appropriately) and see if you get the mixed PC and Tandy sound at normal relative levels. This would pretty much prove the multiplexer is the issue.
 
Yes, there is in fact an internal trim pot for the PC speaker audio. It has no effect on the Tandy audio though

Here's an annotated schematic of the sound routing through the multiplexer chip, showing the two output chains. I've marked that volume pot on the schematic (I watched a video showing adjustment of it so I'm confident this is the correct component), so you can see that if the multiplexer was set to output the mixed Tandy/"PC Compatible" sound to the internal speaker turning that knob would affect both of them. There doesn't seem to be any other separate trim knob for either the PC sound or the Tandy sound chip before you get to the multiplexer.

TandySXsound.jpg
 
... If you have a logic probe it might be worthwhile to check the state of pins 15, 6, and 7 on U25 when you think the system should be outputting Tandy sound through the internal speaker. If I'm reading the truth table in the 4529B datasheet correctly the only circumstance under which you should get *only* PC sound from the internal speaker would be if all three of those pins were logic low. (Which would also only feed PC sound to the rear jack.) If the pins are in any other state then the multiplexer is almost certainly broken.

If those pins *are* all zero when you think the machine should be outputting Tandy sound then that may indicate the source of the SNDCNTL signals is broken. That would be bad because they come from the proprietary Tandy keyboard controller IC.
 
... If you have a logic probe it might be worthwhile to check the state of pins 15, 6, and 7 on U25 when you think the system should be outputting Tandy sound through the internal speaker. If I'm reading the truth table in the 4529B datasheet correctly the only circumstance under which you should get *only* PC sound from the internal speaker would be if all three of those pins were logic low. (Which would also only feed PC sound to the rear jack.) If the pins are in any other state then the multiplexer is almost certainly broken.

If those pins *are* all zero when you think the machine should be outputting Tandy sound then that may indicate the source of the SNDCNTL signals is broken. That would be bad because they come from the proprietary Tandy keyboard controller IC.
Thanks this is helpful. I disconnected my external speaker and to my dismay, the PC beep is not being output to the PC speaker either. So loud system beep, quiet 3-voice Tandy sound from rear external output, nothing from the PC speaker. 😭
 
Thanks this is helpful. I disconnected my external speaker and to my dismay, the PC beep is not being output to the PC speaker either. So loud system beep, quiet 3-voice Tandy sound from rear external output, nothing from the PC speaker. 😭

Wait... so was the built-in speaker beeping and now it's not, or were you never getting any sound from the built-in speaker but you thought you were because you were hearing a loud beep from a speaker plugged into the rear port?

Here's a useful blog article about the Tandy 1000's sound multiplexer. It clarifies a thing I was slightly unsure of, which is whether it was even possible to route *just* the PC beep to the internal speaker. (Apparently the datasheet I was reading was a little unclear about one aspect of the settings. That datasheet said both outputs were tied together in 8:1 mode, but maybe the part used in the Tandy is slightly different. Apparently you can't *just* route PC speaker beeps to the internal speaker, it's tandy sound or nothing.) I only have an HX in front of me, not an SX, so I can't replicate everything, but I fired up BASICA and have some stuff for you to try. (Again, do this all from a Tandy version of GWBASIC.)

First, here's the three things to test the sound generation, run these and see what you get, first with just the internal speaker, then with the external speaker plugged in:

Code:
#This will make a PC BEEP:
BEEP
# These will exercise Tandy sound
PLAY "N10","N20","N30"
NOISE 1,15,10

Now let's see what the Tandy thinks the multiplexer is set for:

Code:
# This will print out the current setting for the multiplexer
PRINT INP(&H61)

On my HX this returned "97". This should translate on an SX to "mixed Tandy/PC beep" on both the internal speaker and the external jack. According to the blog entry the default on an SX *would* be "1", but maybe GWBASIC changes this when it starts up?

If you didn't get a "97", do this:

Code:
# This should set the computer up for Tandy + PC speaker on both jacks if it isn't.
# Run this and try the BEEP/PLAY/NOISE commands again.
OUT &H61,97

If this works and you get Tandy sound out the back then I'm going to guess maybe your internal speaker or the amplifier might be dead. (Or it could still be the multiplexer.) If you didn't get Tandy sound but you did hear the beep try this:

Code:
# If you heard the "BEEP" but nothing from PLAY/NOISE externally, do this. It should mute the external jack
# Do it and see if the machine is completely silent now
OUT &H61,65

If you *did* hear the Tandy sound, try this:

Code:
# If by some miracle you *did* hear all the sounds on the external jack when the port was set to 97
# then set it to this and repeat. This should allow only the beep
OUT &H61,1

All this should exercise the multiplexer to see if it's doing "something". (The "mute" test might be the most useful, because if it doesn't mute the external sound then you know the multiplexer isn't listening.)
 
So the BEEP does not make the beep sound for me. The beep on POST works fine though (through the external speaker).

The 3-voice plays low through the external speaker when the mixer is set to 97. It defaulted to 103 and didn't play at all.

Another interesting anomaly is that playing the PLAY function over and over, the sound gets slightly louder each time for 3-4 iterations and then it gets quieter and quieter until no sound at all. The only way to get any sound back is to set the mixer value to 1 and then back to 97. I can hear the circuit audibly click (via external speaker) when changing between the two settings.

None of the mixer settings resulted in any audio from the PC speaker. Sigh...
 
Back
Top