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Old 13th Jun 2022, 8:14 pm   #1
ScottishColin
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Default Commodore PET Speaker

I cannot for the life of me get any sound out of the User Port on my PET. I have followed the instructions from here and nothing.

https://www.the8bitguy.com/wp-conten...Port-Sound.png

I have checked continuity from Ground to pin N on the User Port and from Pin 19 of the 6522 to Pin M.

I have swapped for another 6522 and still nothing.

In the Nick Hampshire book "The PET Revealed", there's a circuit diagram which uses a PNP - p105 here:

http://www.primrosebank.net/computer...T_revealed.pdf

On Facebook when I asked I got the attached diagram which confused me further as it mentions a headphone jack and a piezo speaker.

I'm completely stumped.

Has anyone added a simple speaker - it's not supposed to be difficult and I did it when I was 18, but it seems to be beyond me now.

Any help gratefully received.

Colin.
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Old 13th Jun 2022, 8:24 pm   #2
SiriusHardware
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Default Re: Commodore PET Speaker

Do you have a link to the speaker you have bought or are using, or a picture if it is something you already have? The 'speaker' you are trying to use for this should specifically be a passive high impedance Piezo electric sounder, not a traditional magnetic / moving coil speaker. Edit: However, you CAN use a low impedance speaker if you use the circuit of P105 in that book you linked to. It's rather badly drawn, if the transistor really is a PNP type it will have to have its emitter connection going to +5V. What you can't do is run one of those low impendance speakers directly from the 'M' connection, the chip pin which drives that connection does not have enough drive strength.

With a little bit of scope-fu you can find out whether the speaker should be working, run something which should be making a noise (ideally something which plays a continuous tune) and scope the 'M' connection on the port with the speaker add-on disconnected.

If you see nice digital activity, connect your speaker and repeat. Is the activity still there?

Last edited by SiriusHardware; 13th Jun 2022 at 8:35 pm.
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Old 13th Jun 2022, 8:28 pm   #3
ScottishColin
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Default Re: Commodore PET Speaker

I know the speaker works as it gives me a nice buzz when I put 5 v through it (the small one in the photo).

I tried a different speaker (again see photo) and I also tried wiring in a jack to plug into the AUX of my radio/cassette player but nothing there either.....

If it's just as easy to point me at a speaker that will work, please let me know and I'll go buy that.

Colin.
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Old 13th Jun 2022, 9:03 pm   #4
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Default Re: Commodore PET Speaker

Of those two you have, the one in the clear enclosure looks the better bet because although it is a low impedance speaker it also includes what looks like an amplifier. The red and black wires are for a DC power supply to the amplifier, and it will not work unless you give it some power. I can't see enough detail to be able to tell you what voltage it should run on.

If you power up the amplifier and connect the blue plug (=audio input) to your phone or other personal audio player you should get sound out of the speaker. If that works, try applying the sound signal output from 'M' to the tip of the blue plug and the 'N' connection to the connection next to the body of the blue plug - leave the middle 'ring' disconnected.

If you just want something you can connect to the M and N outputs via a resistor then you need a Passive high impedance Piezo sounder. Avoid anything which includes the words 'Active', '16 ohm', 'Electromagnetic', all of those are unsuitable.

The difference between active and passive is that an active buzzer makes a continuous sound when you apply steady power to it. That is NOT what you want. Your small buzzer with the red and black wires is an active one, and not suitable.
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Old 13th Jun 2022, 9:07 pm   #5
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Default Re: Commodore PET Speaker

Quote:
I also tried wiring in a jack to plug into the AUX of my radio/cassette player
That should work, so I repeat, use your scope to verify that there is actually output coming out of the 'M' connection, if there is not then even the right sort of sounder won't work.
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Old 13th Jun 2022, 9:08 pm   #6
ScottishColin
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Default Re: Commodore PET Speaker

It's a Kano speaker that came with a Pi. Here:

https://9to5toys.com/2015/06/14/kano...rry-pi-review/

Colin.
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Old 13th Jun 2022, 9:15 pm   #7
SiriusHardware
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Default Re: Commodore PET Speaker

OK, so that is shown plugged into the 5V / Gnd supply pins on the GPIO connector on a Pi. Where might you find a +5V supply and 0V in the PET, I wonder? If you can power it up with +5V from the PET then all you should need to do is connect the tip of the blue plug to 'M' either directly - it might be a bit loud - or via a resistor.

However, you need to check that there is actually activity on the 'M' output when the computer is meant to be making a sound, if there is not then no speaker will work.
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Old 13th Jun 2022, 9:38 pm   #8
ScottishColin
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Default Re: Commodore PET Speaker

Ok. I put 3v through it with 2 1.5v batteries and there's sound. Thank you.

Very quiet though. Any good ideas how to make it louder?

Colin.
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Old 13th Jun 2022, 9:52 pm   #9
SiriusHardware
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Default Re: Commodore PET Speaker

Running it on the correct voltage (5V) may help. Do you have it connected directly to 'M' or via a resistor?
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Old 13th Jun 2022, 9:54 pm   #10
ScottishColin
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Default Re: Commodore PET Speaker

No resistors in play. I'll see if I can find a 5v and ground on one of the expansion ports.

Colin.
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Old 13th Jun 2022, 10:05 pm   #11
SiriusHardware
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Default Re: Commodore PET Speaker

Can you (with all power off and blue plug not connected to the computer) measure the resistance between:

-Tip on blue plug and connection next to plug body *
-Tip on blue plug and middle ring of blue plug
-Middle ring on plug and connection next to body of blue plug. *

* For these connections put the black lead on the connection next to the body of the plug. For the middle measurement polarity is not important.

If the amp still seems quiet running on 5V leave it running on that voltage and plug the blue plug into the headphone socket of some other audio source (phone, walkman, etc) into it. Is it also quiet when driven by one of those sources?
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Old 13th Jun 2022, 10:17 pm   #12
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Default Re: Commodore PET Speaker

Another question: Do you have an old set of powered PC speakers?
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Old 14th Jun 2022, 1:04 pm   #13
ScottishColin
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Default Re: Commodore PET Speaker

Meter readings respectively as follows:

0.L
150 k ohms
0.L

I tried it plugged into the headphone socket of the cd player and it's so quiet to be not audible really so I think I'll give up on that one.

I do have an old set of speakers in answer to your other question.

Colin.



Quote:
Originally Posted by SiriusHardware View Post
Running it on the correct voltage (5V) may help. Do you have it connected directly to 'M' or via a resistor?
Quote:
Originally Posted by SiriusHardware View Post
Can you (with all power off and blue plug not connected to the computer) measure the resistance between:

-Tip on blue plug and connection next to plug body *
-Tip on blue plug and middle ring of blue plug
-Middle ring on plug and connection next to body of blue plug. *

* For these connections put the black lead on the connection next to the body of the plug. For the middle measurement polarity is not important.

If the amp still seems quiet running on 5V leave it running on that voltage and plug the blue plug into the headphone socket of some other audio source (phone, walkman, etc) into it. Is it also quiet when driven by one of those sources?
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Old 14th Jun 2022, 1:20 pm   #14
SiriusHardware
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Default Re: Commodore PET Speaker

Quote:
it's so quiet to be not audible really
In a way I can understand that because the 5V supply on some Pis doesn't have a lot of headroom, ie, you were never going to get 25W RMS out of it. The power output may be intentionally limited for that reason although to be honest it sounds just plain faulty if it doesn't even work well with a known good audio source. It doesn't have a volume control preset somewhere on that little PCB inside it does it?

Quote:
I do have an old set of speakers in answer to your other question
And they are powered ones, not passive? They have either a mains lead going into one of the speaker units or a Wall-Wart type power supply? If so try connecting the 'M' terminal to the tip of the 3.5mm jack plug and the 'N' terminal to the connection closest to the plug body. If that works you will get audio out of one speaker only initially.

If it does work try connecting 'M' to the tip and the centre ring connection of the 3.5mm jack plug, and connect 'N' to the connection closest to the plug body as before. You should then get sound from both speakers.

If you can get PC speakers to work, one immediate advantage you'll have is that they invariably have a volume control built in. Good for late-night space invaders.
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Old 14th Jun 2022, 3:15 pm   #15
Mark1960
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Default Re: Commodore PET Speaker

The speaker for the Pi is probably intended to bem driven from a Pi gpio, which is probably 5v peak to peak. The headphone socket from the cd player might be a lot less than that.
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Old 14th Jun 2022, 3:40 pm   #16
duncanlowe
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Default Re: Commodore PET Speaker

That little one isn't a speaker. It's a buzzer. Which is why it buzzes when fed DC. A speaker would just click once.
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Old 14th Jun 2022, 3:44 pm   #17
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Default Re: Commodore PET Speaker

No, if you backtrack through Colin's posts you can clearly see the blue 3.5mm audio-in plug plugged into the headphone socket of the Pi it is mounted on so it is meant to be driven by headphone-level audio and it should work just as well with the headphone output from a phone or a CD player plugged into it.

The headphone output level from the Pi will be set at something similar to the level from a typical personal audio player or phone so that the Pi can be connected to actual headphones or aux-in on a bluetooth speaker or anything else to which a headphone-level output can be connected.
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Old 14th Jun 2022, 3:45 pm   #18
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Default Re: Commodore PET Speaker

Quote:
Originally Posted by duncanlowe View Post
That little one isn't a speaker. It's a buzzer. Which is why it buzzes when fed DC. A speaker would just click once.
Indeed.

Quote:
...Your small buzzer with the red and black wires is an active one, and not suitable...
Just to recap, the simplest schemes for sound from a PET have one of the digital outputs driving a high impedance Piezo sounder (no buzzer) through a resistor. Colin does not currently possess that type of sounder. The digital output in question does not have enough grunt to be able to drive a low impedance / magnetic sounder directly but I think it should be able to drive the inputs of a set of active / powered PC speakers provided their input termination resistance is not too low.
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Old 21st Jun 2022, 3:31 pm   #19
ScottishColin
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Default Re: Commodore PET Speaker

Hi. Quick update. I bought a Copper Piezo Disk Passive Drum beeper and it works, but it's very very quiet.

Should I just aim at a bigger speaker? Or are there measurements I can take to see what's going on?

Colin.
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Old 21st Jun 2022, 11:39 pm   #20
SiriusHardware
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Default Re: Commodore PET Speaker

Disconnect the sounder and try scoping the 'M' output as advised earlier to see if that signal line is actually producing the expected logic 0 - logic 1 level output swing. If it is, there isn't anything you can do to make that simple arrangement of 'M' output + Piezo sounder sound louder.

What you could do is build a little logic circuit which takes in the 0V-5V level signal and applies a 'normal' version of it to one side of the sounder and an inverted version of it to the other side of the sounder so that the sounder alternately has +5V then -5V placed across it for a total voltage swing of 10V, rather than the 5V it is being driven with at the moment. You could also use something like a MAX232 (TTL to RS232 level converter) IC and feed that with the 0V-5V level output from the 'M' output, it would generate an output voltage swing of at least +/- 5V, possibly more, to drive the sounder.

However my feeling is that if you are prepared to build something it might as well be a proper audio amplifier based on something like the LM386 IC as you can make an amplifier with one of those and only 3-4 other components. To simplify things try finding a kit for a mono audio amplifier which already includes a PCB and a volume control and a speaker in the kit.
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