Do the voltages on the schematic indicate the powered-on standby values? I'm looking to repair the audio output of a amateur radio handheld transceiver...Volume has to be turn to max to hear it. Partial schematic attached. Q13 at the top of the image is the AF pre-amp. IC3 is the AF power amp, with pin 5 being the output going to the speaker. I've already tested the speaker and it is fine.



Block diagram:

 

I think that standby values would usually be labeled as such if placed on a schematic, but I don’t recall ever seeing such a thing.

 

Voltages would be what is known as "quiescent" readings, i.e. idle DC mode.

 

So, the device powered up normally then?

 

The voltages are for diagnostic purposes. They are what one would expect to measure when the power is turned on and the unit is in working condition.

 

Is the sound undistorted and not missing the low or high frequencies?

 

I was surprised to see Q16 has a base voltage of 2.9V, but on checking the 2N2214 datasheet that seems fine, because there is a built-in 10k base resistor.

 

I would think the voltages are with the set turned on and sitting, unless otherwise stated.
For example, in this circuit of a 2way radio, various voltages are marked, along with the condition for measuring them...


The service manual (if you can find one) may state how to read them.

 

Is the sound undistorted and not missing the low or high frequencies?

As far as I can tell, yes. At max volume it sounds like how it would normally sound at min volume. I can scope it, though.

 

I would think the voltages are with the set turned on and sitting, unless otherwise stated.

And, I assume, with the squelch turned up so that audio isn't being passed through?

The service manual (if you can find one) may state how to read them.

Well, the schematic actually is from the service manual, but no mention of how to read the voltages.

 

There are tables on page 40 that tell you the setup conditions for measuring various readings.

 

I think those are all for RF adjustments.

 

I'm not sure this is even doable...at least not easily and not with what I have. The audio circuitry is on the top side of the bottom board. There is a shield on top of that and then the upper (CPU) board is on top of that. Power comes in to the CPU board and then passes to the bottom board through a small multi-pin connector. I would need to connect the two board together during testing, since the CPU controls everything.

 

So I went ahead with testing the 4 audio-related transistors in-circuit. Three check out, with diode drops of between 0.64V and 0.72V.

I could not, however, test the UN2214 (Q16 in post #1). Base-Emitter and Base-Collector, in both directions, measure open circuit (using multimeter diode function). I found that's a pre-biased transistor. Diagram attached. Shouldn't I still be able to measure the diode drops?

 

Shouldn't I still be able to measure the diode drops?

Try using the ohm meter function on a 20K scale.
Does the "beep" sound level seem normal?
Are you able to get voltage readings?

 

Try using the ohm meter function on a 20K scale.

No 20K scale. My Uni-T UT61E is auto-ranging.

I get:
Base-to-Emitter: 64K ohms
Base-to-Collector: open
Collector-to-Base: 0.4Meg ohms
Emitter-to-Collector: open
Collector-to-Emitter: 2Meg ohms

Does the "beep" sound level seem normal?

What beep? The multimeter beep in diode mode?...There was no beeping.

 

What beep?

The beep in the radio when buttons are pressed.

 

The beep in the radio when buttons are pressed.

Hmm...No beep at all.

 

Hmm...No beep at all.

It might be disabled. The manual says to hold down the Tone MW button when turning the radio ON to enable or disable.

 

It might be disabled. The manual says to hold down the Tone MW button when turning the radio ON to enable or disable.

That had no effect.