What does this circuit do?

[simonfr]

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

 

[benta]

It's just an emitter-follower with some low pass filtering.

Benta.

 

[IRstuff]

The last two elements in the path are for AC coupling and input impedance for the next stage, I think.

TTFN
faq731-376

 

[Skogsgurra]

In other words: the circuits cuts out the HF components in the Audio Output and lowers the output impedance while keeping Vp-p same as original output.

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Comcokid]

Don't know anything about old Commodores, but like benta indicated it's a simple amplifier. All of the small pF value capacitors are probably for HF noise or RF emissions. Given you symptoms, I would replace C85 which is to block any DC from going out to the speaker. If the speaker gets a little DC, it will offset the voice coil one direction and typically the sound will be dimished and sound a tinny.

 

[simonfr]

Thanks for your replies guys.
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

 

[MagicSmoker]

C85 is likely bad. You can get a replacement 10uF/16V electrolytic capacitor from Radio Shack, but any EE or tech will have something suitable in his junkbox.

 

[simonfr]

What would be the likely occurence if I removed C85 from the circuit?

 

[simonfr]

Doh. Please ignore the stupid post above.
I meant to write, if I use a polarised capacitor, which way do I install it polarity wise?

 

[geekEE]

On the schematic that you posted, C85 is already polarized. The '+' side is labeled.

Glenn

 

[MagicSmoker]

C85 basically blocks DC from flowing through the speaker. It also sets the low frequency cutoff point in conjunction with the speaker impedance; typically around 100-400Hz for small low-fidelity applications like this.

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.

 

[simonfr]

Thanks guys. Sorry I didn't notice the '+' on the circuit.
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

 

[simonfr]

Gents,
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

 

[ScottyUK]

The emitter-follower is an amplifier, so if it is toast then that would explain your symptoms. The schematic seems to have disappeared, but I seem to remember it was a 2SC**** part. That's a Japanese part code for an NPN transistor of some form; for the duty it is performing there are almost certainly dozens of suitable transistors available in whatever part of the world you're in.

 

[simonfr]

Yeah, the schematic seems to have disappeared.

The transistor is a 25C1815 as marked on the drawing. Need to find one or an equivalent!!

Cheers,

Simon

 

[ScottyUK]

BC182 is common as dirt over here, near enough equivalent for this purpose.

 

[VEBill]

Any small signal NPN transistor would probably work. E.g. 2N2222A.

Worst case is often that you'd need to twist the legs around to match the ordering of the E-B-C leads.

 

[simonfr]

Gents!
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

 

[simonfr]

Guys, Please ignore the above post. I used the Toshiba datasheet to identify the pinouts. Either its wrong or i dont have the toshiba ones, but ive turned it around and replaced back into the circuit (another new one) and im back to where i started. Sound at a very low level.
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

 

[simonfr]

It was the 6581 SID chip at fault. The volume part of the IC wasnt operational.

Cheers for your help gents.

Simon