The power supply is putting out to many volts. I have changed the two electrolytic capacitors. C302 C304 I have checked the value of the resistors. When I adjust RT 301 variable resistor if I go to the right the volts start to climb if I go to the left they start to drop when I get to center of RT 301 the volts drops to 15.66 volt but if I keep going to the left the volts doesn't drop any more. I am new to trying to repair these radios. This is one of my radios from back in the 70s. I don't have a lot of equipment but do have a multi meter. I am going to recap this radio if I can get the power supply to work right. It works on A external power supply. Thanks in advance for your help it takes me a little while to catch on. Dale
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Robyn SB 520 D
- Thread starter Stuck in the mud
- Start date
If you have a way to test the zener diode and transistors in the regulator circuit, that's where I would start.
The trimmer pot itself is suspect. You might try a dribble of solvent on it, and turn the thing back and forth a few times. Always try the cheapest fix first.
If that fixes the problem, you're good to go.
If it doesn't then it's time to test semiconductor parts. Troubleshooting a voltage regulator with a voltmeter can be more time consuming than identifying the failed part or parts by testing.
73
The trimmer pot itself is suspect. You might try a dribble of solvent on it, and turn the thing back and forth a few times. Always try the cheapest fix first.
If that fixes the problem, you're good to go.
If it doesn't then it's time to test semiconductor parts. Troubleshooting a voltage regulator with a voltmeter can be more time consuming than identifying the failed part or parts by testing.
73
Thanks a lot for the reply checked the zener diodes and transistors they check ok.. Cleaned the trimmer pot it is working. Where do we go from here ? Thanks again Dale
If it won't work this means you missed something. If all the parts really, really are good, it will work.
If it won't work, one part (or more) is not right. The circuit's too simple to break this rule.
What voltages do you read on collector, base and emitter of TR301?
This is the error amplifier stage, and those 3 readings might suggest what's still broken.
73
This is the first radio I have ever tried to trouble shoot so hang in there with me.
D406 Pin 1 24.37 DCV P 2 12.55 P 3 00.00 P 4 12.56
TR401 C 24.33 E 15.68 B 16.30
TR302 C 24.32 E 5.968 B 16.33
D302 16.38
TR301 C 5.970 E 6.71 B 5.926
D301 5.911
D406 Pin 1 24.37 DCV P 2 12.55 P 3 00.00 P 4 12.56
TR401 C 24.33 E 15.68 B 16.30
TR302 C 24.32 E 5.968 B 16.33
D302 16.38
TR301 C 5.970 E 6.71 B 5.926
D301 5.911
The Collector of TR301 and the base of TR302 are directly connected, so they should both read the same voltage. You show TR301 Collector as 5.97 and the base of TR302 as 16.33. Something wrong there. Open solder connection maybe ??
- 399
- 399
To start, I believe you have misidentified the terminals of TR302, swapping the base and emitter leads in your table of readings. This takes care of 399's observation, making the emitter reading show for TR302 what the base should show, same as collector of TR301.
But it also looks as if you have switched base and emitter of TR301.
Just the same, in light of this "adjustment", the (real) emitter of TR302 is way higher than the base terminal. All wrong. Should always be about 6/10 of a Volt less than the (actual) base voltage. That high voltage can come from only two sources. Either TR401 has leakage resistance from collector to emitter, or TR302 has leakage from C to E.
No other logical source for that reading. Your "transistors are fine" suggests that your test did not include this possible fault.
73
But it also looks as if you have switched base and emitter of TR301.
Just the same, in light of this "adjustment", the (real) emitter of TR302 is way higher than the base terminal. All wrong. Should always be about 6/10 of a Volt less than the (actual) base voltage. That high voltage can come from only two sources. Either TR401 has leakage resistance from collector to emitter, or TR302 has leakage from C to E.
No other logical source for that reading. Your "transistors are fine" suggests that your test did not include this possible fault.
73
Sorry yes I did misidentified the terminals of TR302 and yes I did switched base and emitter of TR301. I looked at the schematic and it shows a picture of TR302 and shows indent then says front view I was confused.On TR 301 is says bottom view confused again.
If i check the voltage at T401 red wire to red wire I get 19.70 ac volts
At #21 I get 16.39 Dc volts.
D 302 16.39 V.
TR 302 C 24.59 E 16.53 B 5.97.
TR 401 C 24.62 E 15.59 B 16.53
So since my voltages at the B is over 2 volts high and at E almost 2 should I be suspect of TR 401 for replacement ? Thanks Dale
If i check the voltage at T401 red wire to red wire I get 19.70 ac volts
At #21 I get 16.39 Dc volts.
D 302 16.39 V.
TR 302 C 24.59 E 16.53 B 5.97.
TR 401 C 24.62 E 15.59 B 16.53
So since my voltages at the B is over 2 volts high and at E almost 2 should I be suspect of TR 401 for replacement ? Thanks Dale
The base voltage of TR302 should be about 6/10 of a Volt higher than the emitter voltage. But it's just over ten Volts lower. Something is "pulling up" the voltage at TR302 emitter. Maybe leakage current in TR302. A tester that can't test for that may show the part as "good". It's also possible that TR401 is responsible for the fault current that is pulling up the voltage at TR302 emitter, this seems less likely.
73
73
Thanks for all the help got the power supply fixed. It was the capacitor C418 .1 on TR401 Voltage Regulator. Thanks Again Dale
Rock on! That wasn't an easy one. Congrats for sticking with it!
73
73
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