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Bad Tech, no cookie!
- Thread starter TM86
- Start date
Resurrecting my own dead thread. Attempted to repair that trace today. Not my best work, but then I haven't soldered professionally in at least 20 years. Yeah, lame, but what excuse isn't?
Just needs a little epoxy and it'll be all set.
I also tried to fix the crap with the cap leads on the right side of the picture. Best I can say is I got the little copper donuts around the leads and down to the board.
What I thought was a cold solder joint was actually a loose screw holding the main board to the metal frame. Backed each screw out about a half turn and re-tightened, weird noise issue stopped happening.
The meter I had unstuck is now stuck again. Ordered four of them off fleaBay. I'll probably need the other three later and they're cheaper this way. Once the meter is replaced I may attempt an alignment. Right now the receiver is good down to about -100 dBm, maybe -110.
Just needs a little epoxy and it'll be all set.
I also tried to fix the crap with the cap leads on the right side of the picture. Best I can say is I got the little copper donuts around the leads and down to the board.
What I thought was a cold solder joint was actually a loose screw holding the main board to the metal frame. Backed each screw out about a half turn and re-tightened, weird noise issue stopped happening.
The meter I had unstuck is now stuck again. Ordered four of them off fleaBay. I'll probably need the other three later and they're cheaper this way. Once the meter is replaced I may attempt an alignment. Right now the receiver is good down to about -100 dBm, maybe -110.
You are Awesome!
Since we haven't visited this thread in a while thank you for helping to bring it back to the top.
Looks like you beat Tim at his own game...
Since we haven't visited this thread in a while thank you for helping to bring it back to the top.
Looks like you beat Tim at his own game...
Thanks for the compliment, Andy, but you're only feeding my delusions of adequacy.
I have the time to spend on this one radio. Not a lot. An hour here, 20 minutes there, but it's enough.
If I were deriving an income from working on radios this would have probably been tossed aside for more profitable quick turn around work.
Which, I do believe, is why this thing ended up on my bench in the first place.
Well I did want to advise you on some of the many quirks I've seen across my desk that not too many people seem to know about - and that is an issue with instability of the audio amp in these radios.
Although several "theories" have been tossed about thru the years as to the squeal onto the whine and why they tend to happen - I've seem to be able to fix specific ones - like that which I eluded to in the VID, which deals more with ground currents, open loop gains and the substrate to packaging and the sheer gain of the amplifier inside it (open loop) and it's use of op amps nearly exclusively and lack of details for us to figure out the reason for it.
The MB3710 is what I'm talking about - if you have a datasheet for it - hold onto it - it's gold...
You know you're going to be in trouble when the only "CLOSE ENOUGH" service manual uses the uPC1156 Audio Amp chip...
(I enclosed the UHIC-005 chip for the technically curious)
The amp (3710) is the Fujitsu entry to compete against the likes of uPC 1156, Toshibas' 7222ap and the Korean Electronics Company (KEC) (Cobra users take note) 7217AP chips - all of them exhibit some form of instability due to their designs and notably the nearly universal use of a specific type of Tantalum and it's values to offset the inverted input (Audio in).
IT (Adams) also had the a famous Clipper compressor design that Unit 399 goes into more detail about on his website...
You can find that here...
http://unit399.wixsite.com/858ssb
In a Cobra 19 plus the KTA7217 chip used the same Tantalum idea but with a 47 ~100 ohm resistor thru the years. IF I know the user was going to use a power mike I usually sub in the higher resistor value to offset the gain of the internal amp and it's chain. It also improved the tonal bandwidth just enough to offset the lack of gain you got from the swap. And if they were to use it with power mic - I would also have to "kill" the power feed to the mic line from the Radio - as it used a "Grey Key mic" and so that told he rest of us that it had an Electret Condensor element and any attempts to use a Dynamic mic will be met with Captain Oblivious' Cruddy Performance Certificate.
But to get back to that cap - there was also the problem of the bootstrap or bypass the amplifier had used and the quality as well as the values of the Electrolytics had to be considered and used accordingly. You can't use an Electrolytic - the best results were from the Tantalums and low values exclusively. 4/7uF at 6.3 volts was also a required replacement for many of those chips - else the gain factor made them squeal even when other types and voltage rated ones were used.
For RX squeal even with SQ on? C89 is what I'm referring to - so if you use LARGER values DON"T - if you can accept the performance of a smaller value or if you get pushed to the wall over needed replacement subs - the better way to go is to use smaller (Lesser) values to reduce the open loop gain problems in both RF and TX problems.
Thanks to Nomad for the help he's given this site and for his efforts on other sites...
Although several "theories" have been tossed about thru the years as to the squeal onto the whine and why they tend to happen - I've seem to be able to fix specific ones - like that which I eluded to in the VID, which deals more with ground currents, open loop gains and the substrate to packaging and the sheer gain of the amplifier inside it (open loop) and it's use of op amps nearly exclusively and lack of details for us to figure out the reason for it.
The MB3710 is what I'm talking about - if you have a datasheet for it - hold onto it - it's gold...
You know you're going to be in trouble when the only "CLOSE ENOUGH" service manual uses the uPC1156 Audio Amp chip...
(I enclosed the UHIC-005 chip for the technically curious)
The amp (3710) is the Fujitsu entry to compete against the likes of uPC 1156, Toshibas' 7222ap and the Korean Electronics Company (KEC) (Cobra users take note) 7217AP chips - all of them exhibit some form of instability due to their designs and notably the nearly universal use of a specific type of Tantalum and it's values to offset the inverted input (Audio in).
IT (Adams) also had the a famous Clipper compressor design that Unit 399 goes into more detail about on his website...
You can find that here...
http://unit399.wixsite.com/858ssb
In a Cobra 19 plus the KTA7217 chip used the same Tantalum idea but with a 47 ~100 ohm resistor thru the years. IF I know the user was going to use a power mike I usually sub in the higher resistor value to offset the gain of the internal amp and it's chain. It also improved the tonal bandwidth just enough to offset the lack of gain you got from the swap. And if they were to use it with power mic - I would also have to "kill" the power feed to the mic line from the Radio - as it used a "Grey Key mic" and so that told he rest of us that it had an Electret Condensor element and any attempts to use a Dynamic mic will be met with Captain Oblivious' Cruddy Performance Certificate.
But to get back to that cap - there was also the problem of the bootstrap or bypass the amplifier had used and the quality as well as the values of the Electrolytics had to be considered and used accordingly. You can't use an Electrolytic - the best results were from the Tantalums and low values exclusively. 4/7uF at 6.3 volts was also a required replacement for many of those chips - else the gain factor made them squeal even when other types and voltage rated ones were used.
For RX squeal even with SQ on? C89 is what I'm referring to - so if you use LARGER values DON"T - if you can accept the performance of a smaller value or if you get pushed to the wall over needed replacement subs - the better way to go is to use smaller (Lesser) values to reduce the open loop gain problems in both RF and TX problems.
Thanks to Nomad for the help he's given this site and for his efforts on other sites...
Attachments
Received the new meters tonight. Being an excitable boy I put one in the radio, and then the channel display quit working. Figured out it gets power and ground through the ribbon cable. The one I'd been casually pushing on a bit when reattaching the meter lamp wires. Reseated the cable at the mainboard end, channel display is back on!
New meter works. I'll have to adjust the trimpots that feed it, I'm sure.
Played a bit with receive signal level. Feeding in down to about -120 dBm, I can hear a signal. At -123 I think I can hear a signal, but that could be the power of suggestion. Point being, the radio's not as deaf as I initially thought.
New meter works. I'll have to adjust the trimpots that feed it, I'm sure.
Played a bit with receive signal level. Feeding in down to about -120 dBm, I can hear a signal. At -123 I think I can hear a signal, but that could be the power of suggestion. Point being, the radio's not as deaf as I initially thought.
Went looking for an alignment procedure and found this thread: https://www.worldwidedx.com/threads/realistic-navaho-trc-458-service-manual.162368/
Thanks, Robb!
Thanks, Robb!
This radio has an odd clarifier mod.
The clarifier has obviously been "unlocked", as R117, R119, and D30 all have one end lifted. R119 and D30 to isolate the clarifier from RX/TX voltages. I'm guessing R117 to get a little more range.
I can adjust the frequency of the AM oscillator in receive with either the clarifier knob or by adjusting CT6, which is as it should be.
I can adjust the frequency of the AM oscillator in transmit with the clarifier knob, but adjusting CT6 has no effect. This doesn't seem right.
Because the clarifier is unlocked, adjusting VR9 has no effect in transmit. Which is expected.
Transmit and receive are not the same frequency, they're about .5 kHz apart.
I did not check the USB or LSB modes, but it would be logical that they're doing the same thing. So fixing the problem for AM should fix it for them as well. If I'm wrong, well, not like it's the first time.
I have figured out that the clarifier control is getting 9.31 volts in transmit, and 9.21 volts in receive. This alone could be the explanation for the TX/RX frequency split. I've also figured out that the bad tech had a spool of white wire, because that's the color of two out three wires going to the clarifier pot. So for that, no cookie!
The clarifier has obviously been "unlocked", as R117, R119, and D30 all have one end lifted. R119 and D30 to isolate the clarifier from RX/TX voltages. I'm guessing R117 to get a little more range.
I can adjust the frequency of the AM oscillator in receive with either the clarifier knob or by adjusting CT6, which is as it should be.
I can adjust the frequency of the AM oscillator in transmit with the clarifier knob, but adjusting CT6 has no effect. This doesn't seem right.
Because the clarifier is unlocked, adjusting VR9 has no effect in transmit. Which is expected.
Transmit and receive are not the same frequency, they're about .5 kHz apart.
I did not check the USB or LSB modes, but it would be logical that they're doing the same thing. So fixing the problem for AM should fix it for them as well. If I'm wrong, well, not like it's the first time.
I have figured out that the clarifier control is getting 9.31 volts in transmit, and 9.21 volts in receive. This alone could be the explanation for the TX/RX frequency split. I've also figured out that the bad tech had a spool of white wire, because that's the color of two out three wires going to the clarifier pot. So for that, no cookie!
More fun with this beast.
Built an outboard 8 volt regulator to feed the clarifier control. OK, bought a 7808 and a couple of caps from Fry's. Installed it and found... no difference.
Poked around a bit, put everything back the way I found it, and just to confirm the symptom hadn't changed, tried adjusting the AM oscillator one more time. Got it dialed in for RX, hit the key, and now it's dead on +/- about 1 HZ in TX.
So now I just need the correct size internal speaker (looks to be about a 3 inch opening) and she'll be good to go... on eBay.
Built an outboard 8 volt regulator to feed the clarifier control. OK, bought a 7808 and a couple of caps from Fry's. Installed it and found... no difference.
Poked around a bit, put everything back the way I found it, and just to confirm the symptom hadn't changed, tried adjusting the AM oscillator one more time. Got it dialed in for RX, hit the key, and now it's dead on +/- about 1 HZ in TX.
So now I just need the correct size internal speaker (looks to be about a 3 inch opening) and she'll be good to go... on eBay.
So what do you think caused that drop/change in the condition it was in?
Mis-wired (routed) clarifier?
Mis-wired (routed) clarifier?
My best guess is that it had a non-obvious cold solder joint.
Congratulations Doctor, you win a cookie...
GOSH DARN IT...
Hmmm...those darn rats...
GOSH DARN IT...
Hmmm...those darn rats...
you know what the 400 pound rat said to the kitty?
(In a deep voice)) HERE KITTY KITTY
