How hot or warm should the T7222 chip get. The aluminum chassis is pretty warm, but when I touched the chip plate it was hot enough to burn my finger. It's not discolored like the 3756. Takes about 15-20 minutes to get this hot. I just picked it up today. 1983 Taiwan, hasn't been used in 8 years, meter stuck, unlocked clarifier( D75,R44 and orange wire to reg) I changed back to original. TR 26 was removed and I'm putting that back in next. I replaced the cut R112. All other solders appear untouched. Does 9w pep 3.5 avg. bias is set real low on both driver and final. I see both the 27 & 54 traps were backed all the way out. I unfortunately don't have a way to measure the 54 MHz level. I'm hoping the heat is normal or maybe its just a 10v cap, there's a lot in this one.
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148 audio chip HOT
- Thread starter Sarcasm
- Start date
How does the receive audio sound? Can you turn the volume up real loud and still sound clear? Your audio chip is running way too hot if the metal tab burns your finger. If the chip makes distorted audio, one of its internal transistors is shorted. They can still make audio in this condition but if you look at it on the scope, there is only one half of the sinewave being reproduced.
The color of the metal tab being black doesn't mean anything since most were silver plated and what you see it just tarnish. If the chip makes clean audio but just runs hot, check the plastic screw holding it to the heatsink. Many are cracked or loose and do not dissipate enough of the heat into the aluminum.
As far as tuning the 54 Mhz. trap, do you have a scanner? If so tune it to exactly twice the frequency the CB is transmitting on. You will hear your AM signal sounding somewhat distorted through the FM receiver. If the scanner has a signal meter, tune the trap for minimum signal.
If the scanner does not have a signal meter the goal will be to make this harmonic signal in the scanner weak enough to hear white noise. Maybe disconnect the antenna on the scanner or run the CB into a dummy load. Once you hear white noise with the harmonic signal, tune the trap so the signal gets weaker and the white noise gets louder.
The color of the metal tab being black doesn't mean anything since most were silver plated and what you see it just tarnish. If the chip makes clean audio but just runs hot, check the plastic screw holding it to the heatsink. Many are cracked or loose and do not dissipate enough of the heat into the aluminum.
As far as tuning the 54 Mhz. trap, do you have a scanner? If so tune it to exactly twice the frequency the CB is transmitting on. You will hear your AM signal sounding somewhat distorted through the FM receiver. If the scanner has a signal meter, tune the trap for minimum signal.
If the scanner does not have a signal meter the goal will be to make this harmonic signal in the scanner weak enough to hear white noise. Maybe disconnect the antenna on the scanner or run the CB into a dummy load. Once you hear white noise with the harmonic signal, tune the trap so the signal gets weaker and the white noise gets louder.
No scanner. Tx and RX audio is very good. Actually excellent receive. It was a little crackle till I hit the pots with detox it. Tapping the pot knobs didn't change or effect the crackle I heard the first time I turned it on, like a dirty pot usually does. The screw is ok. It does not have the diode bulletin updates. I will verify pin voltages. When I run my finger over the solder side the RX wil fade out and slowly come back around the vfo and r107. On a good radio it goes out and on quick.
Good idea to check the pin voltages on the audio chip. If they look good the only other thing I can think of is to check and make sure they didn't forget to use white heatsink compound on both sides of the mica insulator.
The slower response you see in this radio when you run your finger over the board, is the VCO coming in and out of lock when you load the circuit down. Proper alignment of the VCO coil should make it lock on frequency faster.
The VCO alignment is outlined in the service manual available on cbtricks.com. You'll need a plastic alignment screwdriver and a volt meter to do this.
The slower response you see in this radio when you run your finger over the board, is the VCO coming in and out of lock when you load the circuit down. Proper alignment of the VCO coil should make it lock on frequency faster.
The VCO alignment is outlined in the service manual available on cbtricks.com. You'll need a plastic alignment screwdriver and a volt meter to do this.
I did think of one other reason this chip could get hot. If C175 is leaky or shorted it will allow DC current to flow through the speaker. That will cause a good deal of heat. The easiest way to check for this is with the radio on and the volume turned down. You'll want to use your DC volt meter to see if there is any DC voltage present at the speaker terminals.
This should be done without the speaker connected so it does not load down any DC voltage that may be leaking past C175. I'd plug an 1/8 inch jack right into the external speaker jack with just the meter connected and no speaker. If you see anything over 0.5 volts DC, the cap is bad.
This should be done without the speaker connected so it does not load down any DC voltage that may be leaking past C175. I'd plug an 1/8 inch jack right into the external speaker jack with just the meter connected and no speaker. If you see anything over 0.5 volts DC, the cap is bad.
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Well a big thanks for the help. 2 of the e-caps in front of the audio chip were bad. Replacing C175 stopped the chip from getting hot. It now is barely Luke warm. But pin 2 is 6.5 volts instead of 2.45 like the schematic says, and that only has a 2.2 50v cap going directly to ground. All the other voltages are are good except pins 5&6 are a couple tenths low. 1.19 instead of 1.84. Could be the supply was only 13.01 coming in.?.??? I now get what I think is too much noise from the power source, an old 3a power supply. Making headway. What kind of output should I be seeing and still be clean?
Found some more bad caps, but they were 50v. As soon as I put the limiter back in, she's doing 3.5-4.5w but sounding very good. Receive is crazy clear and now the noise is way down and with the NBC on its gone. My voltage on the 1419 reg is a little low, .8v low. I'm getting 5.28 on the collector of TR42 which is giving the 4.8v instead of 5.6v on the 1419 reg. I had 5.6 before I put the TR26 limiter back in.
Just curious about the diodes soldered to on back near the audio chip, this one doesn't have them, but they are on all the other ones I have here. The audio chip finally went and the new one has perfect pin voltages, where the old one was a little low on 5&6. I'm still getting a crackle in the RX and occasionally it fades out and comes back.
When keyed it will take a a few seconds as the power climbs up to two watts. Meters, both show the needle climbing slow, like a cap charging. Only did it a couple times
When keyed it will take a a few seconds as the power climbs up to two watts. Meters, both show the needle climbing slow, like a cap charging. Only did it a couple times
If it is an old radio -like older than 20yo- and you want to keep and use it; then spend the $25/ebay on a complete set of caps and go through it. Don't expect that to cure all of your problems, but I wouldn't be surprised if it didn't sort a few problems out either.
It's a1983 radio, its all recapped now. But every time I fix a cap that was bad I find another problem. I have found several transistors also with gain much lower than a new part near these bad caps. The hard part is they test & perform good until a load is placed on them. Just like that audio chip. I am wondering if the volume knob and band switches ever go bad. I can't get detox it in the band switch pots. But when I touch the dial or slowly switch it I get the crackle and or fading RX.
Is there an updated 3756 chip? The NTE one is almost $20, wondering if there is another option.
Are the 2 diodes on the back near the audio chip a service update?
Is there an updated 3756 chip? The NTE one is almost $20, wondering if there is another option.
Are the 2 diodes on the back near the audio chip a service update?
Thanks Robb. I seen that and that's what confused me, because this rig has an original 18v D44 and a 4003 in D78 on the board. Not the add on to solder side. Thing is I have a board here that is just like this 83' board but also has the diodes on the solder side too. At the audio chip not the reg.
Oh well.....I appreciate the help. I replaced the AM reg with NTE 152 and the radio worked excellent for about 20 minutes. Then the receive was crazy sensitive and loud, little bit of crackle and RX faded away. Haha fun.. Pin voltages are all good, but when I check IC1 pins 1-9 give me feed back, squeal and buzzing when the multimeter lead touches the pin. Pin 7 takes the RX out completely.
When it did work tx was very clear and pretty darn loud. DK 1.8w and 5.5-6w pep. The meter is stuck, anyway that could do anything if it was shorted out inside?
23ma&39ma on the bias
I am wondering if when they pulled TR26 they also changed something else.
Again thanks
Oh well.....I appreciate the help. I replaced the AM reg with NTE 152 and the radio worked excellent for about 20 minutes. Then the receive was crazy sensitive and loud, little bit of crackle and RX faded away. Haha fun.. Pin voltages are all good, but when I check IC1 pins 1-9 give me feed back, squeal and buzzing when the multimeter lead touches the pin. Pin 7 takes the RX out completely.
When it did work tx was very clear and pretty darn loud. DK 1.8w and 5.5-6w pep. The meter is stuck, anyway that could do anything if it was shorted out inside?
23ma&39ma on the bias
I am wondering if when they pulled TR26 they also changed something else.
Again thanks
Doubtful the TR26 was the problem. Doubtful that the meter is a problem.
Think you should check all of the solder joints and see if there isn't any bad wires too. Wiggle parts and look for some change.
After a cool down it works fine again. Maybe a little too good on the RX. I used freeze spray after it did it again and that worked but I couldn't narrow down the component. It's just slowly came back but not like it is at first. It wasn't instant like I have found in past troubleshooting spray tests. When I tx it speeds up the fading out process. I've wiggled everything individually with a probe, also tapped nothing. It just crackles no matter what you touch , chassis wires, components faceplate. Ill try a reg tommarrow