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Elenco AM/FM-108ck Radio kit build. Failed Output Bias Test

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Glassman:
“The volume control resistance measurements seem off. That looks like the volume control is defective (open wiper)? I could find no data on Elenco 622050 potentiometer, it might be 10kΩ or 50kΩ or 100kΩ etc. but 10kΩ is shown in Fig. 4's.”

:  Page 9 of the manual shows,  Volume/S2 (50k Pot / SW). Without de soldering I’m sure this was marked on the pot before soldering in place.

“You only ever measure resistance with an ohmmeter when power is off. This is because the multimeter is injecting current which will clash with anything that already has power and current flowing. Always have power off, and wait a bit for caps to discharge when measuring ohms or diode-test."

:  Thankyou, shows how much of a newbie I am.

"On ohms function with power off (no battery connected), between the wiper (middle pin) and ground side, the resistance should go down to near zero at min. volume and climb up when you turn up the volume. Leakage current from C40 will interfere at the higher ohm readings but removing the IC (it's in a socket right) open circuits C40 at pin 3."

:  With meter set at 200k & no battery connected & IC removed, zero reading, powered on and eventually peaked at 52k @ full volume.

“Look at the IC markings, there are many LM386 copies coming out of china and they might not perform so great.”

:  Markings on top, “LM386N” “14L01S” bottom is K11 in a round circle. I have been reading some reviews about these chips and yes, understand that these can be hit and miss in quality / performance. I’m considering buying some more for potential future projects and spares. National Semiconducter seem to be a good brand / manufacturer?

james_s:
You could just try building the rest of the thing and see if it works properly, it's possible that there's nothing wrong with it. You could also buy a LM386 from Digikey or some other reputable supplier and try that, it's possible it was defective to begin with or it could have been damaged by static or something.

Glassman:
I will likely be buying some extra 386's and am now thinking that I may copy the circuit on a breadboard and test. Will have a look at digikey & mouser etc from what I've read so far I'll try and avoid Aliexpress / Ebay etc

james_s:

--- Quote from: Glassman on October 05, 2022, 02:10:25 am ---I will likely be buying some extra 386's and am now thinking that I may copy the circuit on a breadboard and test. Will have a look at digikey & mouser etc from what I've read so far I'll try and avoid Aliexpress / Ebay etc

--- End quote ---

A LM368 on a piece of perfboard driving a little speaker can be handy as a bench amplifier, so you could make a dual purpose test amp/LM386 tester.

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