Mistery PCB

[AshtrayWasp]

I found this pcb on my grandad's closet and from looking at info on the MC338 I thought it might be a radio power amp, I've sketched the pcb in Multisim in different ways but I can't figure out what type of amp it is, as I can only guess some of the transistors orientations as there isn't a lot of info online about them, any help would be appreciated in deciphering this, cheers

 

[Bo Deadly]

Your schem has to be a little off. Consider the net at Q1 R4 C2. DC bias current flows out of the emitted of Q1 and out of the base of Q3 but that current has nowhere to go and therefore those transistors will not bias.

Post a pic.

 

[AshtrayWasp]

Youre right I didn't even notice, I think this one should be right

 

[Walrus]

How about a picture of both sides of the PCB?

 

[AshtrayWasp]

Thats the pcb

 

[AshtrayWasp]

And here

 

[Bo Deadly]

Still wrong.

 

[AshtrayWasp]

Hopefully this is it, also realised the MC338 is an NPN not PNP, not sure about the 4290 though, just guessing its an NPN

 

[AshtrayWasp]

Actually I think the 4290 might be a PNP from what this website says, but I still dont know the pinout, Ill desolder it tomorrow and test it with the multimeter to check

 

[Bo Deadly]

You shouldn't need to do that. The circuit's just not that complicated. It looks old enough that part numbers might be useless.

The first thing I would do is draw everything so that the positive terminals of parts are pointing upward. So you have one capacitor that is upside down. For NPN transistors the collector and base are positive terminals because current flows into them. For PNP transistors, the emitter is the positive terminal. I think the 4290 is actually PNP.

From that you should be able to figure out which wirepad is for the positive supply and which is 0V. Add "V+" and "0V" labels to your drawing.

Then you should be able to redraw your schematic and visualize current trickling down through the positive supply into the positive terminals of transistors and in/out of the bases down to ground and it will suddenly all make sense.

I could probably figure it out from your drawing but I don't want to ruin the experience for you. It will be much more satisfying (or at least as satisfying as such a simple circuit can be) when you figure it out and make a horrible sounding guitar distortion pedal out of it.

 

[analogguru]

..... I thought it might be a radio power amp,.....

No, it is an electronic flasher relais, probably from your granddads car or home-disco

"Astable Multivibrator" is the searchword for a matching schematic.

 

[Bo Deadly]

No, it is an electronic flasher relais, probably from your granddads car or home-disco

"Astable Multivibrator" is the searchword for a matching schematic.

I don't see that. There would be a lot more symmetry.

 

[analogguru]

I don't see that. There would be a lot more symmetry.

Sorry when you are unable to see the obvious, Maybe an oculist has better skills than me ?

 

[AshtrayWasp]

Well I'll try redrawing it accordingly when I've got some time off work and see if I can make sense of it, not too good with transistors quite yet

I did think of making a distortion module with it but mainly for synths and drum machines so the voltages would probably be at line level.

 

[swpaskett]

In deference to the multivib debate and stepping in with trepidation, I don't see a multivib there either, so I redrew it as one normally sees a multivib drawn, with the bases of Q1 and Q2 facing each other, and nope, not a miltivib. What is it? Heck if I know but I would guess it was intended to oscillate and maybe it was intended to be a multivib.
So mysteries remain... did the OP get the schematic right, which might suggest the original builder did not get the build right. A real possibility that might explain finding it unused (in the junk drawer maybe?) all these years later. Or did the OP get it wrong and maybe it is a multivib? There are the right number of Rs, Cs, and Qs (ignoring Q3) but in the wrong places. There are no inputs, which points to oscillator. Q3 buffers the output (also pointing to oscillator) and questionable values for Rs (non-standard as in R1 and R4 way too high value unless Q3 is a Darlington). C1 runs from base to base of Q1-Q2, C2 from emitter to emitter which I think would kill any attempt to oscillate.

To the OP: I suggest you take a look at an astable multivibrator topology on any number of websites and see if you can organize your schematic more like that, if you can. In the process you may find you made some mistakes, or that you got it right and your grandfather did not. Whatever the case, isn't wonderful he left you with this mystery to work out, and that you care enough to try to figure it out?
Bo Deadly gave you some good pointers on how to redraw it. That explains why I did not post my redraw; I did not want to push you too hard in any one direction or take the fun of figuring it out, but if you request I will post it. I believe I have a faithful redraw of your post #8. I think you are finding that back engineering even simple stuff is hard. We've all been there. I can throw you a few more hints if you want.
One last suggestion: Don't expect this thing to work or to become something it is not now.

Good luck, and Live Long and Perspire!

Edit: removed embedded link. Imjur and my browser both barfed on it.