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).

 

[Hawkeye351]

Wow, I just got home from Sunday Diner with my locals. We do this every weekend. Anyway, I had a text from another local who is also a ham stating he had some for me on my porch. I went to check and sure enough there was a box sitting there. I brought it in, opened it and was shocked to see a brand new Rework station (over 100 bucks on Amazon). I asked him what he needed me to do with it (I figured he wanted me to fix something on it) and he told me it's for me, it's mine and to enjoy it.

Yes, I was very appreciative to him for this. I think it's a hint, lol, what do you guys think, lol... I've been needing to learn how to do surface mount work, guess now is a good a time as any.

He said it's because I'm always available to help others by not charging much for my work. Good deeds do pay off.

Still gotta put this other 7222 in this radio, just had some things going on.

 

[loosecannon]

Seems to me that the 10.75 volts on pin 1 of the audio chip is your issue.

that comes directly off of the main V+ into the radio.

something is pulling that voltage down as it should be almost 14 volts.

First, you can try lifting pin 1 off of the PC board and checking that trace, to see if it is now back up to normal. (whatever your power supply is set at).
if that works, you problem is with the audio chip itself and associated circuitry.

If it doesn't change the voltage, start at the power supply, and look for components coming off that line that may be pulling the voltage down.

C243, C173, and Q37 seem like good places to start, unless its the power supply itself.
LC

 

[nomadradio]

Seems to me that the 10.75 volts on pin 1 of the audio chip is your issue.

May not be "the" issue, but it's an issued. Pin 1 connects directly to the main B+ power coming in to the radio's circuit board. Means the radio is running from under 11 Volts. Needs more than that.

73

 

[brandon7861]

May not be "the" issue, but it's an issued. Pin 1 connects directly to the main B+ power coming in to the radio's circuit board. Means the radio is running from under 11 Volts. Needs more than that.

73

I was guessing they got measured/numbered backwards because it would be hard to explain pin 10 being higher than the supply voltage otherwise.

 

[nomadradio]

got measured/numbered backwards

Yeah, my idiot light's shining pretty brightly right now. Voltages look okay in the right order.

A 'scope would instantly reveal if the audio level is the same on both sides of C179, the fat 330uf cap connected to pin 9. Not a common fault, but on the list of suspects. If it fails open circuit it can do this.

73

 

[brandon7861]

it would be interesting to pull a good one out and put pins 1-10 on a curve tracer to get an idea whats inside and if that several volt difference is telling. Mine only has what appears to be one diode drop between them when powered.

 

[Hawkeye351]

I've already got another 7222 yanked out and ready to put in this radio, but before I remove the one that I think may be bad I figured I would check the things you guys mentioned above:

Lifted pin 1 (the pin showing 10v). Checked the pad and had 0v, checked the lifted pin 1 lead itself and had 10v. Pin 10 (closest to 8v regulator) was still showing 14.10v. All the other leads still showed as above.

Both leads of C175 now show 0v while pin 1 of the 7222 is lifted.

I lifted all the leads of the 7222 from the board and checked just the pads. All pads are 0v except for pin 10 which still shows 14.10v with no IC leads connected at all.

I'm counting the leads as, the pin closest to the Passthrough regulator (754) is pin 1, the pin closest to the 8v regulator is pin 10. I could have them backwards.

What do you guys think? Everything look right without any of the IC leads soldered?