Rci 2970 low voltage on pin 9 of ta7222ap

[That_guy_in_wv]

Hello it's me again, again lol. A friend just brought me an rci 2970 that has very low if any modulation on am (screaming in mic) and almost nothing out, barely moving mod meter and wiggling watt meter. Radios rf power turns wattage up and down and has modulation on fm, lsb, and usb. Adjust from 12 watts to around 75 dead key.

I upgraded all audio amps and regulator 2sb827 and a1869. Before and after testing voltages on the center legs I get 2.9 volts, both 7808's read the voltage they're supposed to also on pin 9 of the audio ic ta7222ap I get 2.9 volts instead of the 6 or 7 vdc that's supposed to be there. I replaced the ta7222ap and same problem 2.9v on pin 9. The collectors of the driver and final is also 2.9 volts. There has to be a problem somewhere in the circuit providing voltage, does anyone have diagrams of these circuits and voltage checks for them? And where does this voltage start from the regulator or audio amps? I'm kinda stuck on which part of the circuit I should go to from here.

Also want to add I replaced both mic amps near the front of the radio by the mic plug on radio PC board. 2x2sb1240's I've had this problem before where low to no modulation was caused by them. Thanks ahead for any and all the help, much appreciated

 

[Handy Andy]

Locate Pin 3
Its a quiescent current offset pin, usually it's grounded, but if left floating to some value it can shut down the chip -- quiet it.

It's a noise abatements and silencing speaker thing...

 

[That_guy_in_wv]

Thanks for the reply Andy, I have 0.0 volts on pin 3. I rechecked pin 9 and voltage is 2.37 volt no clue why I wrote 2.9v.

 

[That_guy_in_wv]

These voltage were all checked in rx mode not in tx. I wanted to make sure no parts were fried before trying to test in tx mode.

 

[That_guy_in_wv]

If I put a 1khz tone into the mic I get around 40% modulation with the mic gain all the way up and vr turned wide open with rf power turned all the way down, it's almost acting like the limiter is taking the modulation down by too much. But voltages are off so I don't think the audio amps are driving the modulation for some reason.

 

[NZ8N]

Which model RCI2970 is it? First gen?
The Microphone amp circuit does not go through the TA7222 chip, except only in PA mode.
You say you have transmit mic audio in FM (on another receiver), and the SSB modes?
There is a transistor that clamps the mic audio path to the AM carrier voltage setting in the Power regulator circuit, this is active in all modes except AM, so there is no Amplitude being transmitted.
If it is a 1st gen (I have one in my storage), look by the transformer for TR56 and TR57 (or Q56 Q57 on Rick Jackson's schematic).
Q56 clamps the Mic audio feeding the AM/FM/CW Carrier settings in all modes except AM. Q57 clamps the voltage feeding Q56 when in AM mode, only. With Q56 disabled in AM by Q57, the Mic audio is superimposed on the carrier voltage settings for the regulator. Desolder the center pin of Q56, see if you get your AM modulation back at this point. Then resolder it, and check Q57, see if the Base has around 0.6v on it.



If it is a DX, check Q71.

 

[That_guy_in_wv]

Thank you NX8N, I'll check those and get back with you. This is the 2970 model with 2x2290 transistors. Does not say dx anywhere on the faceplate and its not an n2.

EPT295013Z main board. Again thank you.

KD8QPV

 

[That_guy_in_wv]

I have no transmit carrier on fm and no sound on tx, radio works fine on lsb and usb wattage looks good and modulation meter swings like it should. I just have very low modulation around 40% with everything wide open with any power setting and no carrier on fm. FM not working at all, I have a galaxy 77 I hooked up to transmit into close by to make sure. 2970 on dummy 77 on imax2000. I'll check what you said and get back. Thanks hopefully this info will help

 

[That_guy_in_wv]

Am modulation 40% not fm.

 

[That_guy_in_wv]

I desoldered the center leg of Q56 and modulation stayed the same, Q57 is reading 0.68 volts on the base.

 

[Handy Andy]

Do you have a TR57, it controls the SSB mode U and Lower sides, swamps out that TR56...it would kill audio in AM mode but not FM nor would in in SSB or CW...

It's a lot like the PC122 or Cobra 146 or even the TRC465 - that method of modulation...

 

[Handy Andy]

Hmmm. Have caps been changed?

 

[Handy Andy]

check C240 - I believe that's the Chips output cap, that being shorted dried out, will make the chip run hot...

As far as the TR or Q - Q56 is that wonderful mixer of DC bias and the Audio - that uses a cap of 100uF

 

[That_guy_in_wv]

Not sure about cap changes, everything looks factory. The radio sit over a year and my buddy can't remember if it worked when he put it in his safe or not or if that's the reason he started using his 2995dx Because the 2970 went out. But I remember him using it when he got it and it worked fine before.

 

[Handy Andy]

Well, in light of "factory" age of the caps usually is what does them in.

Be careful, for if you can replace the Audio out cap of 330uF and the audio and voltage returns?

You may have your answer...