Another 2517 low receive issue

[Hawkeye351]

Check for voltage on the RF gain wiper lead (center) with RF gain turned down to halfway.

Also, check R233 for voltage on the rear lead, should be the same as the voltage on wiper (center lead) of the RF gain control. Also, check Q17 and L5.

 

[BayouRadioAmplifier]

...and here comes the freezing rain, I gotta go salt sidewalks

oh! I was just outside, walking around, in a T-shirt, drinking a brew.

 

[Hawkeye351]

I just got off the phone with the owner of this radio with the low volume (receive signals are great, just low volume) and he informed me that the squelch control is the same way, he has to crank it almost wide open to silence out even the weak stations. I checked the squelch VR adjustment and it's set properly. So I guess once I figure out the volume low issue then the squelch issue should also straighten out, I hope.

Question:
Isn't the squelch and volume tied into the same circuit somewhere that could be affecting both?

Also, does the transmit audio go through the 7222ap audio IC as well as the receive? If so, then could the IC be bad on the receive audio side but just fine on the xmit audio side?

Stupid questions probably, lol...

 

[brandon7861]

No, the transmit audio does not go through the 7222. It looks like it comes off pin 1 of the op amp and goes in by the AM carrier control at Q53.

 

[brandon7861]

You said the audio came back up when touching the ripple reject cap and you already swapped that high ESR one out, so maybe it's the internal bias. At least we know it can amplify, now to figure out why its not.

What I find interesting is that a cap there just smooths ripple and it should amplify without it, so even a damaged trace or dry cap there shouldn't inhibit amplification. However, the audio coming up then fizzling out makes me think that touching it shifted the internal bias momentarily and suggests to me that the internal bias supply may have been damaged.

Measure the voltage on pin 2 and see if it drifts or rails. It may also be worth touching the leads of a cap from pin 8 gnd to pin 2 which would bypass any unseen trace/pad damage, although I doubt that's the issue here..

There is no detailed schematic of the inside of that 7222 chip, but I would assume the ripple reject pin should be about Vcc/2 (since it would be on the DC bias circuitry). A quick measurement on a different radio would tell us what the voltage should be (because I'm guessing on Vcc/2).

I am guessing about how the ripple reject works, I could be wrong. heck, I'd just swap the chip and see.

 

[nomadradio]

There's the manual digital analog signal injector. Lay your finger (digit) along the shaft of a small Philips screwdriver. Touch the tip of the shaft to pin 4 of the TA7222 audio chip. If you get a raucous loud hum, the problem is upstream from that part. And if you don't, your only other hope is one or more electrolytic caps feeding into and out of the audio power chip. A 'scope will quickly reveal if C179 is blocking the tracks. It connects from pin 8 of the 7222 to the speaker circuits. Blocks DC, passes the audio. If it has failed as an open circuit, you would see a lot more audio voltage on the positive side of the cap than you do on the negative side. Should be the same on both sides of that cap.

When it works.

I'm used to 'scoping pin 9 of that chip. If the steady DC voltage on pin 9 isn't nearly half the DC voltage at pin 1, there's a problem with that chip. Or with an attached capacitor.

Or both.

Weak audio is not a common failure for the TA7222, but it could be this time.

73

 

[Hawkeye351]

Sorry for delay. Had some other things to take care of.

TA7222 IC:
1 - 10.75
2 - 7.12
3 - 0
4 - 0
5 - 1.17
6 - 1.17
7 - 0
8 - 0
9 - 6.78
10 - 14.10

That's with RF gain wide open and all the way down, as is with the volume control also.

C175
+ 0
- 7.12

RF gain control, wide open
Outer lead - 3.63
Wiper - 0
Outer lead - 0

RF gain control, all way down
Outer - 8.70
Wiper - 2.24
Outer - 0

R233, with gain all way down
Inside lead - 2.24
Outer edge lead - 8.70

Q44 - nothing on any leads with RF gain wide open or all the way down, as with the volume control.

I don't get a loud ruckus sound when touching the metal shaft of my screwdriver on pin 4, with my finger touching the shaft also. I just get a pulsing bubble sound a little louder than before.

The red wire beside CN13 has 8v on it all the time on both ends of the wire, that is right I think.

Hmmm, I hate receive issues, lol....

 

[brandon7861]

In my washington (which has the same chip and same wiring), I get
1-13.82
2-6.60
3-0v
4-0v
5-1.151v
6-1.156 v
7-gnd
8-gnd
9-6.86v
10-13.01v

Looks like I have one diode drop between pin 1 and pin 10. You have almost 3.5v there. Since the bias voltage at pin 9 looks right, I don't think the output is loading it down, so either there is a bad boodstrap capacitor (which I thought you changed) or the internal bootstrap diode is bad (or, according to chatgpt, the upper drive transistor which is less likely, either way, bad IC).