docs said:
FM works fine but no AM (MW/LW/SW), no static sound or noise whatsoever.
Voltages at Q103 should be C= 6v, B= 1.0, E= 0.4v.
I'm reading Q103 as C= 0v, B= -5.04v, E= -5.76v.
What would be the next step to diagnose why there are minus voltages to the transistor ?
When I check Q101 I have similar results with negative voltages yet FM works fine.
My DMM is configured correctly and I have plus and negative voltages on the main board from the power supply.
2 reasons why you are getting (-) readings on your DMM.
(A) You are not using your meter correctly and have the leads reversed.
(B) You have the negative probe set at a location that is NOT at ground potential.
You must understand that a meter measures NOT VOLTAGE, but rather, voltage potential differences. The voltage reading is only true to the extent that the negative/ground is set to '0' potential. Set the neg at any other potential and you'll have to account for the changes based on the potential differences. The way I see it, the readings seem to be fine IF and I submit that you probably don't even have the ground probe set at ground, but rather at a 6-v rail.
Here is what I'm presuming you are doing:
(1) Meter leads reversed
(2) Measured against a +6v rail (instead of ground)
Collector v is supposed to be at 6v. If you measured 6v to 6v, then you get ZERO.
Base is supposed to be 1.0v. If you measured from a 6v rail, 6v - 1.0v = 5.0v. (you measured -5.04)
Emitter is supposed to be 0.4v. If you measured from 6v rail, 6v - 0.4v = 5.6v. (you measured -5.76)
Because your boombox does not run on AC after the PS, there is no negative (below ground/'0v' rail), then it is impossible to read negative voltages unless either or both of A/B above conditions apply). Think about the above for a moment. Perhaps it will make sense to you.
