What does this circuit do?
What does this circuit do?
(OP)
Ladies and Gents.
I am looking for some help regarding the function of this circuit (attached).
Its the output of the SID (sound chip) of a commodore 128 computer.
The sound out of the monitor speakers is crackly and faded. I would like to test the output at points along the circuit if i can.
The external cable and monitor speakers are good. The connector itself is assumed good (at the moment). The collector of the transistor is tied to +9V.
The output line then goes directly to the output connections.
Any hep would be great!!
Cheers,
Simon
I am looking for some help regarding the function of this circuit (attached).
Its the output of the SID (sound chip) of a commodore 128 computer.
The sound out of the monitor speakers is crackly and faded. I would like to test the output at points along the circuit if i can.
The external cable and monitor speakers are good. The connector itself is assumed good (at the moment). The collector of the transistor is tied to +9V.
The output line then goes directly to the output connections.
Any hep would be great!!
Cheers,
Simon





RE: What does this circuit do?
Benta.
RE: What does this circuit do?
TTFN
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RE: What does this circuit do?
Look at the signals with an oscilloscope. Start at the output and trace back. Signal should be more or less identical up to transistor emitter (with DC shifts) and then there may be some HF at the transistor base while the computer output mat have lots of HF (PWM, probably).
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: What does this circuit do?
RE: What does this circuit do?
Im not an Electronic Engineer myself, so did not know the purpose of many of the elements in this circuit, especially taken as a 'group'.
I will see if i can blag an oscilloscope from someone to have greater depth and understanding if the preliminary track/component/solder joint check does not reveal the problem.
Im reluctant to think that its is the SID device at fault as some of the sound is aubible. Just.
Thanks again guys, i will give me something to work with.
Cheers,
Simon
RE: What does this circuit do?
RE: What does this circuit do?
RE: What does this circuit do?
I meant to write, if I use a polarised capacitor, which way do I install it polarity wise?
RE: What does this circuit do?
Glenn
RE: What does this circuit do?
C85 is almost certainly an aluminum electrolytic type. This kind of capacitor commonly fails by literally drying out over time (the electrolyte slowly leaks and/or evaporates from the imperfect seal around the leads; running at high currents/temperature hastens this process). The capacitance value will usually remain stable up until almost all of the electrolyte has left but the ESR (equivalent series resistance) it exhibits will shoot up. The result is crackly, low volume sound from the speaker that is practically devoid of any low frequency components (and "low" in this case might extend well into the midrange, say 1kHz to 3kHz).
PS - After giving the schematic a second look it seems that C85 might be 20uF, not 10uF, but 20uF is not a standard size so use 22uF instead.
RE: What does this circuit do?
I did think it was part of a 2 in the drawing. I have ordered 10uF capacitors off eBay. So was planning to put two of them in parallel to achieve 20uF.
But it souls like C85 from what you guys describe.
Will hopefully get it done at the weekend. Thanks
Simon
RE: What does this circuit do?
Changed out C85, and it was a 10uF capacitor, and weirdly it didn't actually make an instant difference. But a few minutes later I now get 'perfect' sound but it's very faint. Need to ramp up the volume to the max on the monitor. Tested lead and monitor on other computer (commodore 16) and works as it should.
If the transistor were not working as an amplifier, would you get any sound from it?
No oscilloscope available
Any help/ideas appreciated
RE: What does this circuit do?
RE: What does this circuit do?
The transistor is a 25C1815 as marked on the drawing. Need to find one or an equivalent!!
Cheers,
Simon
RE: What does this circuit do?
RE: What does this circuit do?
Worst case is often that you'd need to twist the legs around to match the ordering of the E-B-C leads.
RE: What does this circuit do?
Drawing says 2SC1815, got some 2SC1815s off eBay.
Now fitted matching up the base collector emitter, but now have no sound period.
BUT, a 2SC1815 didn't come out. A C945 came out.
Are these equivalent?
Cheers
Simon
RE: What does this circuit do?
I have removed all the components and tested them and they seem to be ok when measured on test equipment.
I have also isolated the RF module from the sound output incase it was dragging the signal down, but didnt make a difference.
Some voltages measued on a multimeter.... +5V at pin 25, +12V at pin 28. Output level DC volts on pin 27 at +3.6V (no AC signal showing). On the transistor, volts in +3.6V, Volts out (and at C85 input) +3.0V, and +12V available at collector.
No DC on the output on C85.
Im now wondering if its the SID chip itself. As it would seem (to me) that Q2 - the transistor - isnt amplifying the signal?
Cheers,
Simon
RE: What does this circuit do?
Cheers for your help gents.
Simon