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TRC-459 no receive audio

TM86

TM86

Supporting Member
Jul 6, 2014
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Payson, AZ
Title pretty much sums it up. I've traced the audio signal as far down as entering R97, but not hitting the base of Q25. I've got audio signal at the junction of C92 and R97. Then it just disappears.

I've checked R97, it tests good (82 K +/- 10 %). The Q25 transistor tests good. At -50 dBm input the S-meter is showing S9 like it should. Transmit audio looks good on the scope, PA audio works fine. Electrolytics are new (Thanks, Klondike Mike!).

BTW, the schematic in the service manual (https://bit.ly/3hNP86p, don't even bother with SAMS CB-278) has the order reversed on the parts in the signal path, C92 is definitely in the path before R97, and R97 is connected directly to the base of Q25. At least that's how my version of the radio was built.

I've got about .8 V DC on the base of Q25, .2 V DC on the emitter, and 5.2 V DC on the collector. The audio signal at the junction of C92 and R97 is about 70 mV AC (assuming I'm reading the scope right). Anyone happen to know if those values are anywhere close to correct? I didn't bother checking them before i had the problem.

I'll be more than happy to provide any more detail if requested.
 

Thanks for the link...

upload_2021-7-9_9-42-31.png

In answer to your question, er - no.


When you have 8 volts on the Base, the biasing network for the part needs to be checked - you have WAY too much voltage on Q25, look for and measure R100 - 10K - should put it at 4V max

What "pulls it down" is the Signal and the transistor conduction. Once "1/2 way" starts, the transistor conducts and then stabilizes just below "on - set point" the resistor network provides.

The caps then, provide more of a path for Signal gain - the Resistors keep it's DC stable.


upload_2021-7-9_10-3-6.png
This is a classic Class A amp
 
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Thanks for the response. I have 0.8 V DC at the base. Didn't put the leading zero in the original post. I'll check R100 anyways. Probably check R101 as well, because it's there and part of the voltage divider.
 
How is Q23 doing?

Q24 is a "buffer amp" to use VR 12 to adjust S/RF meter RX side, so does the radio show S-meter - if not you may need to work further back.

Or...the Things got SSB too so...

Q25 being a buffer amp for the detected audio SSB detector - also uses D34 as an RF switch.

So guess that means ... does the thing switch over to SSB RX? It may just be looking for an SSB signal to detect, but there is nothing there - else there should be enough to make the AM side work just fine.

I don't know, because from what I read you're not seeing signal - figuring that means no to everything...
 

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To help

When the transistor is accepting a signal - trying to measure DC at the same time an AC signal is present, the transistor doesn't always cooperate. So you may be seeing "full conduction" on the BASE - with signal present - telling the Transistor to turn on, and at the same time - it's DC value will DROP because it's on.

Which brings up another question - has the radio been recapped?

Why? Well, look carefully at the Schematic and your silkscreen on the main PCB - note the ANL cap polarity and the 1N60 next to it. And then look at C99 (corrected) an Electrolytic - maybe the thing is working but it's too far "on" so it doesn't send a signal - it's too distorted - C90 can fail shorted, causing this circuit to work like a switch.
 
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Sorry, was out of town for a few days and now sick as dog. (No idea why, I don't have a dog right now).

Radio was recapped, and I did double check that the new caps are in the right way to the best of my ability. Once I'm feeling better I'll check Q23 input and output. I do know signal is getting though, just not sure if it's enough.

I do have the same symptom in all modes, so at least it's consistent.
 
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Getting back into this. At some point I managed to get it to blow a fuse. My fault, I did something stupid and the fuse saved me a trip to the garbage can. Today I finally went out and bought a new fuse, so here we go.

Getting S9 on the LED bar meter with a 50 uV signal, so VR12 probably needs tweaking, but there's enough signal to drive Q24 and make that work. Adjusting the RF gain does affect the meter reading, so I'm calling that good as well.

Checked C90 out of circuit, 42pf. Checked resistance in circuit, not shorted, at least, which is nice.

DC voltages on Q23, if I've got the pinout right, 2.00 VDC base, 5.97 VDC collector, 1.30 VDC emitter. 7.8 MHz if signal, 20 mV P-P base, a little over 1 V P-P collector.

I've got a 1kHz audio signal at about 300 mV P-P at the junction of D32 and R93.

But then I have no signal where R93, R95, and C88 all come together. I've checked all three parts out of circuit. R93 is about 850 k and R95 is about 220 k, so both are within 20% of the value on the schematic. C88 measures in at 100nF with an ESR of 37 Ohms. It's a tantalum, so I'm not sure if that ESR is good or not, since I've never gotten into reading tantalum data sheets.

So you can see why I'm a little confused on this one. Which is good as usually I'm a lot confused.

For the record, I'm taking these measurements in AM mode. I believe I mentioned it before, but it's been a while, the symptom is the same in all modes, no audio on receive.
 
Wow! Just found an error in the published schematic of that radio. Pin 5 of IC3 has no connection showing. That pin is the ground connection for that chip.

Oops.

But not too helpful to you. If you don't have a 'scope handy, that's a real handicap. But if you'll find IC12, the TA72222P audio power-amp chip, lay your finger along the shaft of a small screwdriver and touch the tip to pin 4 of IC12. This is the input pin and will get you a loud hum, and maybe a squeal if the audio-power chip is functioning and there is no break in the connection to the speaker.

And if you hear nothing, either the chip is bad or there's a broken connection to the speaker.

Or maybe both. Especially if someone has already attempted to get it working without success.

73
 
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Tantalums of 37 ohms ESR? - got a spare?

As said earlier, the condition shows the COLLECTOR voltage appears low - like the part is in full conduction - which if the Tantalum is bad, places the stage in FULL conduction - like a switch - no signal gets out - is pushing DC instead.

Remove the cap, see if the stages' Collector voltage rises and you may even hear a signal - then you'll know.
 
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Pulled that tantalum (C88). Signal flowed a little farther down the chain. Which means I'll have to order another one since I don't happen to have a spare. Also found that C92 and R97 are out of order on the schematic compared to the circuit. Signal flows from R95 through C92, then R97 and onto the base of Q25. Except, of course, this radio. It gets as far as R97 and doesn't seem to bother coming out the other side.

I'm looking at the voltage chart for Q23, what I posted earlier for DC measurements seems pretty close.

What I did find is I only have about 0.9 Volts on the base of Q25. Should be closer to 2.1 Volts. The transistor is recognized as BJT device with an hFE of about 125 by the tester I have. Which points to the transistor being OK, at least out of circuit. So that's my current mystery to be solved.

Per Nomad's suggestion I did do a touch test on that IC, gave me a nice buzz. From the speaker, of course.
 
TM86 said:
gave me a nice buzz.
Click to expand...

So we know everything between that pin number 4 and the speaker is working. And the trouble is upstream from there.

Consider that all your receiver audio goes through IC11, a 4066 quad analog switch. If it goes bad, no audio. Only three of its four sections are used in this radio. The factory manual posted at cbtricks.com has the pinout for that chip. The control inputs are pins 6,12 and 13. If they are all low, no audio will pass. I'm pretty sure the one that has to be a high level to hear receiver audio is pin 6. If it's not close to the supply voltage at pin 14, that will kill receiver audio.

I think.

73
 

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