Pioneer sa-420 v2 blowing 240v fuse

[sel_199]

Hi, I bought this old amp to try and fix it. I have very limited knowledge but I like to tinker.

I haven't tried it myself but have looked inside and the 240v fuse is blown, I'm taking his word that it keeps doing it.

There are five glass fuses, 2 on the main board which or ok and three on the mains supply board one of which is blown.

The 240v 2.5 amp fuse is fine.
The 220v t630ma fuse is fine.
The 240v t630ma fuse has blown.

What confuses me is that the transformer has  two different voltages supplied to it at the same time if both t630ma fuse holders are populated.
Looking at the shematic it shows two fuse holders but only one populated.

I have a link for the schematic.  https://www.hilberink.nl/luxman/pioneer/pioneer-sa420v2.pdf
There are two power supplies shown, mine is the one in the full schematic not the isolted one in the black border.

Can any one confirm if it needs two fuses or just one,  thanks.
 

[commongrounder]

Only one fuse should be installed in the holder that corresponds to the mains voltage in your area. Having both fuses installed would most certainly result in one of them blowing, as this would constitute a short circuit of transformer windings

[sel_199]

Thanks for the reply, I'm not sure it would be a short in the true sense as it's the same power line just running through two fuses but connecting to different inputs on the transformer.
I also think if there is a fault on the transformer or amp both fuses would have blown.

I think I'll try one fuse in the 240v holder and test with a dim bulb.

Thanks for reading.

Update:-

Just tried it with only the 220v fuse in place and then checked the output from the rectifier and I'm getting the correct voltage -31v and +31v.

All I have to do now is find out why it's output is totally dead.

[David_AVD]

With old audio gear that's been idle for a long time, a no audio fault can be as simple as dirty switches and controls.

As for the multiple mains fuses, I've seen this before where someone has tried replacing the "missing" ones! Sometimes the text on the PCB doesn't make it clear (to the novice) that there should only be one.

[sel_199]

yea I think it's developed a fault at some point and someone has opend it up and stuck another fuse in which keeps blowing.
I think I should change it to the 240volt one as it's I'm getting 32 volts when it should be 30 volts.

However I have found out that it's actually getting 32volts at the function bulbs which are rated for 8volts so they have blown.
I'm trying to work out what has failed to put 32volts on the bulbs and has it destryed the rest of the amp.

If I'm reading the schematic correctly the real voltage should be 30volts reduced to 25.5 after the two diodes and drop to 8 volts after the two resistors which are in series 270ohm and 120ohm. However right now they seem to do nothing, as if something is bypassing them.

[james_s]

It's an unregulated power supply so the fact that you're seeing 32V instead of 30V does not necessarily indicate a fault, it's likely that your mains voltage is just a tad hot, or it could be that the 30V is specified with the amplifier outputting full rated power rather than idle. The voltage rating of the fuse is a safety thing, it has absolutely no effect on the voltage you measure in circuit. The lamps appear to be powered through a simple dropper resistor in series so if they are burned out then the full supply voltage will appear across them. It's very likely the lamps burned out due to normal use and not from any fault in the circuit.

[sel_199]

Thanks for that info,  I don't have any 8v bulbs  to test with so I may try an led with resistor.
 

[james_s]

You could buy some 8V bulbs. The circuit is very simple, if you have 30V (or something close) present, and you install a bulb of the correct current rating, Ohms law will do its thing and limit the voltage to the bulb to the designed value.

[sel_199]

I'm using this amp as bit of a test, I have a few amps where the bulbs are hard to get or daft expensive.

Anyway I did get an led working, with about a 500ohm resistor it's giving 2.2volts at the led which is about right if the original voltage was for 8 volts.

so at least I can get the function lights working.

Now all I need is for it make some noise.

 

[David_AVD]

Any resistor for the LED needs to be calculated to give the correct current draw, not a voltage across the LED (which is more or less fixed anyway).

So, in your case it will be 32V - 2V (LED forward voltage) = 30V. Divide that by 0.02 (20mA LED current) to get 1500 Ohms. Subtract any existing resistor from that to get the value of the additional series resistor.

The excessive hiss is likely bad transistors.

[sel_199]

I think that was what I calculated 1500ohm - 470ohms existing resistors
So I tried 1k but it didn't even light, I may have had it the wrong so I tried a variable resistor and set it to 500ohm and that worked and wasn't very bright.  I will revist this later as I need to do the other lights and need to recalculate for a white led as I can't see the red one.

I'm not getting any hiss, I was getting nothing.

I swapped fuses from 220v to 240v as it should be and volts have dropped to 29.5 which is correct for this unit.

I also found out that one of my speaker wires was broken on the right channel and my good speaker is connected to the left channel which is dead.

So basicaly the unit has a bad left channel, I swapped the output transistors but left channel is still dead.
Caps all seem to read the same on both channels capasitance wise as well as voltages.
I also tested the other transistors in diode mode and they seem to match the good side.

I also tried to test the transistors while in circuit using a tester but that didn't work (not surprising), however on the right channel you can hear the test running from the speaker, I get nothing on the left channel.

I wonder if the fault is between output transitor and the speaker socket.

Cheers Sel.


 
   

[David_AVD]

Trace the speaker wiring to see if you have continuity right back to the amp board outputs. Bad speaker relays and switches are super common.

[sel_199]

Trace the speaker wiring to see if you have continuity right back to the amp board outputs. Bad speaker relays and switches are super common.

The last thing I did was go around all the resistors just to see if any had gone open but they seemed ok.

I then read your post, these speakers have no relays they're just cheap basic speakers to test with, the amp doesn't have any relays either but I thought I'd go and swap them over one more time.

Before doing that I just switched on and surprise surprise both channels are now working for some reason.

All I can come up with is that there's a bad joint around a resistor even though I can't see it.

Should I put the original output transistors back to where they were because of biasing, that looks a little beyond me. 

Thanks for all the help so far.   

[David_AVD]

Checking and setting the bias is not hard to do.

As I said earlier, dirty switches are the major cause of intermittent audio in old audio gear.

After that I would re-seat any internal connectors and look for cracked solder joints.

[sel_199]

It could be switches, I did spray them a few days ago. It may have taken some time to break down the dirt.

I remember doing the same with an oscilloscope, spraying the switches didn't seem to work and I left it for a year when I tried again it worked.

I do need to learn how to setup the bias as I have a few other amps I saved that definatley have blown output transistors and burnt resistors.

I'm busy for the next few days so it will have to wait.

 Thanks for all the help and advice.