National Panasonic 5410 humming

[BoomboxLover48]

My new addition 5410 was humming when I was turning on. I switched to batteries and it worked well.
Now I think the issue is with the AC/DC converter area of it. I was unable to find any large smoothing capacitors and found a small cap there near the diode area.
The circuit diagram is not showing the value of it.

I can't locate that smoothing cap on the main PC board.

The green cap looks like a ceramic cap. It is marked 1, 2, 3 near the rectifier diodes.
Any help with solving the humming issue with the power supply is greatly appreciated.

 

[stynger007]

My new addition 5410 was humming when I was turning on. I switched to batteries and it worked well.
Now I think the issue is with the AC/DC converter area of it. I was unable to find any large smoothing capacitors and found a small cap there near the diode area.
The circuit diagram is not showing the value of it.

I can't locate that smoothing cap on the main PC board.

The green cap looks like a ceramic cap. It is marked 3 near the diodes.
Any help with solving the humming issue with the power supply is greatly appreciated.
View attachment 52069




View attachment 52067

Not familiar with this box. Check for a missed ground wire connection- if you have 2 separate panels as in cassette and pcb , try holding using your body as a ground, one finger and thumb. If that doesn`t help, have seen some funky things happen when record slider is cleaned. All functions pass through the record slider regardless of the circuit being in use. If no caps show an expulsion, chances may be it`s the record slider.

 

[BoomboxLover48]

Not familiar with this box. Check for a missed ground wire connection- if you have 2 separate panels as in cassette and pcb , try holding using your body as a ground, one finger and thumb. If that doesn`t help, have seen some funky things happen when record slider is cleaned. All functions pass through the record slider regardless of the circuit being in use. If no caps show an expulsion, chances may be it`s the record slider.

Thanks! I cleaned the record switch so well. I will clean it again and see if that helps. But if I use external DC power it has no hum. It only does when the 12V DC power supply from the transformer is connected to the rails.

 

[stynger007]

Thanks! I cleaned the record switch so well. I will clean it again and see if that helps. But if I use external DC power it has no hum. It only does when the DC power supply from the transformer is connected to the rails.

Thanks! I cleaned the record switch so well. I will clean it again and see if that helps. But if I use external DC power it has no hum. It only does when the 12V DC power supply from the transformer is connected to the rails.

Does this unit have selectable power ? 110/120/230/240 volt selector? Try cleaning that- as well, does instance occur in both cassette play and tuner selection?

 

[BoomboxLover48]

Does this unit have selectable power ? 110/120/230/240 volt selector? Try cleaning that- as well, does instance occur in both cassette play and tuner selection?

Yes it does. This humming is there in all modes like tape, radio or line in.

 

[BoomboxLover48]

Once I unplug the positive and negative leads from the 12V built in power supply and connect to an external one, humming disappear and all work fine.

 

[Valde]

I don't know this particularly box, but for general trouble shooting, you could start with the points marked in the schematic. The green disc cap on the power supply board is most likely ok. Fuse wrapped in tin foil?? The main filter cap (1) is tied to the center tap of the audio output transformer. The filter cap (2) after the voltage regulator. The filter cap (3) for the pre-amp voltage supply. It's difficult to see the text and capacitor value in the schematic, but I would start by checking these caps. Also check for cracked solder joints and loose wiring.

 

[BoomboxLover48]

I don't know this particularly box, but for general trouble shooting, you could start with the points marked in the schematic. The green disc cap on the power supply board is most likely ok. Fuse wrapped in tin foil?? The main filter cap (1) is tied to the center tap of the audio output transformer. The filter cap (2) after the voltage regulator. The filter cap (3) for the pre-amp voltage supply. It's difficult to see the text and capacitor value in the schematic, but I would start by checking these caps. Also check for cracked solder joints and loose wiring.
View attachment 52100

Thanks so much for your suggestions.
I remember the humming going away once I removed that main PC board with the amp section and all and put it back. It was okay for a while and then started again to make that loud humming noise. I will find time to check for any breakage and those 3 caps you pointed out. It will be a good idea to replace all caps in that section. I will order those and replace one by one marking the polarity.

Found the values of those caps.

C131- 0.015mfd/16V
C160- 100mfd/10V
C 145- 470mfd/10V

 

[Valde]

The one cap you found (C131), I think should be C151. 2200uF/16V. This is the main filter cap. When you say that it was working and suddenly blow the fuse, I suspect that you don't have a capacitor problem. The hum can be an result of heavy current draw. Most likely one of your output transistors or the audio output transformer is defective. Both will result in the output transformer getting hot. By the way, the DC input jack is bypassing the fuse... If you check the schematic and find the Colector and Emitter of the output transistors you can messure across C to E. The voltage should be about battery voltage -1 Volt over each of the transistors. The point where the Emitters meets, the voltage should read about 1Volt with reference to battery negative ÷ All messurments should be done with the volume at minimum. Don't run it for long time if things are getting hot! If the voltage over one of the transistors is much lower than the other, or the voltage in the Emitter to Emitter point is much higher than 1Volt, one of the transistors is probably shorted. An safer way is to test the transistors out of circuit, but it requires more work. To messure the transistors you need to solder them out. Make sure to not swap the two, or put them back in the wrong way! You can test the transistors with an multimeter. You can google how to test them.

 

[BoomboxLover48]

I was wondering why it all works fine on battery power with no issues!
Humming happens when I plug in to the AC power source.
If the output transistor is defective it has to show problems running on batteries also right? No issues at all with battery power and works so good.
It all happens with the AC is plugged in as a power source.

 

[Valde]

High current draw trough the output transformer will not result in any hum from battery power, only redused output power. On AC an high current draw will pull down the supply voltage resulting in the smoothing capasitor having no effect. Result is 120Hz sine shaped DC. Would have started by messuring the the current draw, both om battery and from AC. Idel current should definitely not eceed 1Amp. Difficult to remote diagnosis an boombox, you can try to do voltage messurments and compare with the voltage in the schematic (usually reference to battery ÷). Dont run it if the current draw is excessive. Anyway, it's night in Norway now.

 

[BoomboxLover48]

Thanks.

 

[BoomboxLover48]

Just measured the voltage and and current from the power supply. It shows voltage 12.76 and current 3.76 Amps. So I think the internal power supply is messed up. These are measurements at the pins where we connect to the main power amplifier board. Now running on an external power supply and everything is working like new. Power transformer is staying at room temp 69F. What can cause this high current flow from the built in power supply. It looks like a small rectifier circuit with 2 diodes and a decent size transformer with several AC selection taps.