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Post by williaes on Jul 20, 2021 14:15:16 GMT -5
I have my dad's old Cobra Cam 89. It was stored for 25 plus years and not stored well. I have been working on radios for a little while as a hobby. This one when you power up with AC 110 it lights up but no TX or RX. If you turn the squelch all the way it has a faint high pitch frequency that can barley be heard. I replaced the rectifier diodes due to one being bad. I have not replaced the zener diodes and they didn't test well in circuit so I need to pull one side to test these. I also have a resister that is way out of realm from what the schematic shows. The resistor matches its rating but is not what the schematic is showing on the rectifier board/regulator board.
My main question is what should my voltage be off the secondary of the transformer? One leg to the center tap shows 20 volts currently. Then what should my voltage from the rectifier to the main PCB board be? I would assume 13.8 DCV. I am only getting 5.5DCV on the leg to the main power rail.
I don't have a good schematic that shows voltages and the only one I have is almost not legible. Any help would be grateful and if I can ever assist I will gladly jump in.
I have a significant amount of test equipment besides my eyes and fingers, just need to be more proficient at using them. Thanks!!
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Sandbagger
Administrator/The Boss 
Posts: 6,247 
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Post by Sandbagger on Jul 20, 2021 21:22:23 GMT -5
I have my dad's old Cobra Cam 89. It was stored for 25 plus years and not stored well. I have been working on radios for a little while as a hobby. This one when you power up with AC 110 it lights up but no TX or RX. If you turn the squelch all the way it has a faint high pitch frequency that can barley be heard. I replaced the rectifier diodes due to one being bad. I have not replaced the zener diodes and they didn't test well in circuit so I need to pull one side to test these. I also have a resister that is way out of realm from what the schematic shows. The resistor matches its rating but is not what the schematic is showing on the rectifier board/regulator board. My main question is what should my voltage be off the secondary of the transformer? One leg to the center tap shows 20 volts currently. Then what should my voltage from the rectifier to the main PCB board be? I would assume 13.8 DCV. I am only getting 5.5DCV on the leg to the main power rail. I don't have a good schematic that shows voltages and the only one I have is almost not legible. Any help would be grateful and if I can ever assist I will gladly jump in. I have a significant amount of test equipment besides my eyes and fingers, just need to be more proficient at using them. Thanks!! A regulated power supply will usually have between 18 and 25 volts DC output out of the rectifier and into the collector of the series pass regulator transistor (the large one that gets very warm). On the emitter of that transistor should be 13.8V depending on the voltage adjustment setting. The schematic for the CAM 89 can be found here: CAM 89 |
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Post by 2600 on Jul 22, 2021 0:06:52 GMT -5
Good chance the channel selector switch has a layer of oxide on the contacts. If it's thick enough, this can shut down the radio all by itself. Cleaning the controls and switches is the starting point before digging any deeper when they're this old.
73
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Post by williaes on Jul 22, 2021 16:44:08 GMT -5
Thanks guys!
That schematic is better than any I have been able to find. At least it is legible. Wish it showed some voltages every so often throughout. If I had a starting voltage I can do the math and find it but it doesn't give me enough info to figure it out. The caps have a variety of voltage ratings on them so they are of no help. Both legs coming off the rectifier/regulator board are only showing 5.5 volts ( I am assuming it should be 13.8) I have tested all the transistors on the regulator board and they test okay. I did check the large resistor and it tested fine out of circuit but I will check voltage on it as well. Thanks again, I will keep at it. I will clean the channel selector switch as well.
By the way I keep calling the small PCB board a rectifier/regulator board. It is where the power is rectified from AC to DC and it does get regulated with the transistors and variable resistor before leaving that single PCB. Please correct me if I am wrong on that and making my questions harder than they need be.
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Post by 2600 on Jul 22, 2021 23:26:48 GMT -5
Umm, let's back up a step or two here. Measuring the AC-voltage output of the transformer usually won't reveal a lot. Transformers tend to either work or smoke and blow fuses. You won't see one that only "partly" works as a rule. The transformer's secondary winding gets rectified to DC by D15 and D16.
C114 has the rectified but not-yet-regulated DC voltage on it, around 22 Volts plus or minus a couple. This is the unregulated side of the power supply. The DC voltage here will appear to fall while transmitting, and come back up in receive mode.
This feeds into the collector pin of TR21.
The base terminal of TR21 is controlled by a driver transistor TR22. TR22 is controlled by the error-amplifier transistor TR23, which compares the voltage at the center lug (wiper) of the trimpot VR12 to a fixed voltage on the zener diode D18.
When it all works right, the steady regulated output appears on the emitter of TR21, and should be determined by the setting of the trimpot VR12.
That diagram is still kinda grubby. Has one misleading spot. The wiper of VR12 appears to connect to the zener diode D18.
It doesn't. There would be a fat round dot where the two right-angle lines meet if they were connected. The line from the wiper of VR12 passes over D18 without connecting to it. VR12 wiper lug connects only to the base lead of TR23, and nothing else. At all.
73
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