TOA BG-130 Amplifier power supply fuse blown

[fuxxy]

This amp appears to have a basic unregulated power supply which is protected by a 1A fuse on the input side (F101), and a 3A fuse on the output side (F102).

Power from mains through F101, into the transformer primary
Unknown voltage out of transformer secondary through F102, and into a full wave bridge rectifier, and then through a 63V 4700uF cap. I'm assuming the secondary voltage is lower than 63V, lol

F101 is blown, and If you guys have taught me anything it's not just to replace a fuse and plug it in..

On visual inspection of the board, the area around D109/D110/R179 has gotten hot and discolored the board, and I'm currently trying to trace how those might contribute to my problem.
Do you guys have any ideas where I should troubleshoot?

[petehall347]

Re: TOA BG-130 Amplifier power supply fuse blown

could be shorted output devices or shorted rectifiers

[Andrew F. Ali]

Re: TOA BG-130 Amplifier power supply fuse blown

Since F101 is on the Primary side of the Transformer, it could very well be a 'tired' fuse. Assuming you do have a DMM, measure the DC resistance across the transformer's Primary winding. It should be high. If you get no reading, the winding could be open or there might be a thermal fuse built-in the transformer. If you get low then there is a short in the winding. If there was a problem with the Output Transistors/ICs then the F102 on the Secondary would have blown. Just replace the F101 with the same rating and give it a try....It will work.

[R_J]

Re: TOA BG-130 Amplifier power supply fuse blown

Look at the blown fuse closely, if the fuse element looks like it is still there but broken open, it could just be fuse fatigue. Usually if there was a short the element will be totally missing and the glass will be black.

[Andrew F. Ali]

Re: TOA BG-130 Amplifier power supply fuse blown

look at the blown fuse closely, if the fuse element looks like it is still there but broken open, it could just be fuse fatigue. Usually if there was a short the element will be totally missing and the glass will be black.

+1

[fuxxy]

Re: TOA BG-130 Amplifier power supply fuse blown

the fuse itself actually appears blown, there's a bit of soot inside the glass.

[fuxxy]

Re: TOA BG-130 Amplifier power supply fuse blown

measure the DC resistance across the transformer's Primary winding. It should be high. If you get no reading, the winding could be open or there might be a thermal fuse built-in the transformer. If you get low then there is a short in the winding.

Primaries on the main transformer read 8.8 ohm
secondary on the main transformer read 1.7 ohm

Replaced F101 , and it's blown as soon as power is applied.

[petehall347]

Re: TOA BG-130 Amplifier power supply fuse blown

have you got a link to service manual ?

[fuxxy]

Re: TOA BG-130 Amplifier power supply fuse blown

Sadly, not yet. I'm still searching for one.

[budm]

Re: TOA BG-130 Amplifier power supply fuse blown

Replace the main AC fuse and disconnect the secondary side of the transformer so all you have is the AC feeding the primary side of the power transformer, if the fuse is blown again then you have bad power transformer.
You can also use a 40~60W lamp in place of the main AC fuse, it the lamp comes on and stays on brightly then you have bad transformer.
The DCR reading of the power transformer will show low resistance which is normal for power transformer.

[Andrew F. Ali]

Re: TOA BG-130 Amplifier power supply fuse blown

The transformer Primary windings are shorting. It should be near 100ohms or there about.

[R_J]

Re: TOA BG-130 Amplifier power supply fuse blown

Those resistance readings for the transformer are fine, I dought its the transformer at fault
I have a couple 110 to 20-0-20 the sec. res. is 1 ohm the primary is 3.5 ohms
You could check the output transistors (looks like 4 of them) and see if they are ok

[fuxxy]

Re: TOA BG-130 Amplifier power supply fuse blown

I'm at work at the moment, and don't have access. I'll disconnect the secondaries and see if the fuse shorts when I get home. I did test the rectifier, and I don't remember the forward voltage readings, but I do remember that all 4 individual diodes in the rectifier had roughly the same forward voltage and read open when tested in reverse polarity. All seems fine with the rectifier.

Can you identify the PCB markings for the output transistors? I'm assuming you're talking about the four attached to the big heatsink, but I'd like to be sure we're both looking at the same thing.

[fuxxy]

Re: TOA BG-130 Amplifier power supply fuse blown

Replaced F101 again
Plugged in transformer primary
Unplugged transformer secondary windings
Fuse stays intact, and secondary output is a steady 39.6v

[Andrew F. Ali]

Re: TOA BG-130 Amplifier power supply fuse blown

Look at the Output transistors...Yes the 4 on the big heatsink.....Each channel may have a complementary pair of NPN and PNP transistors.

[fuxxy]

Re: TOA BG-130 Amplifier power supply fuse blown

Look at the Output transistors...Yes the 4 on the big heatsink.....Each channel may have a complementary pair of NPN and PNP transistors.

Disclaimer: I've never been confident testing transistors with a multimeter

I've pulled all 4 transistors from the heatsink and board.
It looks like two complimentary pairs.
Facing the silkscreen on the front of the transistors, they are arranged like this

D2059 D2059 B1367 B1367

Not getting any shorts from any pins on each of the transistor, however, on "ohms" mode, I'm getting different values from the two B1367 transistors. I would have expected the two identical transistors to have identical results.

[Andrew F. Ali]

Re: TOA BG-130 Amplifier power supply fuse blown

If your multimeter is DMM then use the Diode scale to check the transistors. If you have analogue multimeter ( the one with the needle pointer) use the Rx1k scale. You would have to remove them from the circuit for testing, but you said you are not getting any shorts on the pins, so I'll say that is not the cause of blowing fuse. Getting similar readings from the identical transistors is correct to expect. Looking again at Post #1, I see there are two Transformers in this amplifier?

[fuxxy]

Re: TOA BG-130 Amplifier power supply fuse blown

I'll have to repeat the test. I know I tested in "beep" mode, and then again in "pure ohm" mode.

There is two transformers, but the secondary transformer has multiple secondary windings and I'm honestly out of my element as to it's purpose.

[fuxxy]

Re: TOA BG-130 Amplifier power supply fuse blown

Looking at the board layout, it appears that all 4 output wires on the second transformer connect directly to the speaker output terminals of the amp, but at varying voltages.

[R_J]

Re: TOA BG-130 Amplifier power supply fuse blown

That second output transformer is for driving multiple speakers that are a long way from the amp, like in a large store 25 volt or 75 volt line. the speakers would also have a small transformer on them with multiple taps.
or if you want to connect just a speaker direct you would use the com and 4ohm
I would think the outputs are fine.
I wonder if d109 & d110 are maybe zeners and are bad, It looks like the voltage from the main cap. goes through r179 and is droped by the 2 diodes which seem to be connected in series.