Guitar amp fuse blowing

[Joe Shmoe]

I think I might have to join an electricians forum, as I'm having no end of electrical issues lol.

I've came across this old 1978 amp. If had a blow fuse (2a, as instructed on rear panel), so I replaced the fuse and it blew again.

On closer inspection I realised there was a voltage selector on the rear, so I turned it from 120 to 240v, replaced the fuse, and tried again.

The front panel light illuminated for a microsecond before the fuse blew again.

Can anyone point me in the right direction so I perform any basic checks as to why this is happening?

 

[cutting42]

If it was plugged in set to 120v there is a very high chance you have burnt out the transformer.

The first thing the mains will see are the primary coils of the transformer which are the input coils and there are two of them each rated for 120v. The selector switch you mention will switch them in series for 240v or in parallel for 120v.

Disconnect the output wires from the transformer from the rest of the circuit and try to plug in again, if the fuse blows, it is the transformer

If it has burnt out recently or in the past few months it will smell pretty bad and be junk and need replacing. Transformers are hard to test as they have very low resistance anyway and any equipment you have will be unlikely to be sensitive enough to tell you. Best thing to do is replace it, the voltage may be written on it somewhere. What you need to know is the secondary winding voltage in volts and power rating usually marked as VA which is voltage times the amperage. So it might say 200VA and 0-12V for the secondary. It may also have multiple secondaries, so make sure you purchase a replacement that is the same voltage (very important) the same or higher VA rating and has the same number of secondaries. The primary needs to be 230 vac (This is the UK standard now) and you do not need to bother with connecting the switch for dual voltages unless you think it will be used in the USA any time soon. If you want to preserve the original condition you need to purchase a dual primary as well so you can reconnect the voltage switch.

If by some miracle it has not blown the transformer then you can look at the next components. There will be a rectifier circuit of some sort to turn AC into DC. It is hard to see on the photo but it is likely to be one or two diodes or possibly a bridge rectifier which are four diodes in one package. These can fail and can be tested with a multimeter set to either diode test or just resistance test. Diodes only conduct in one direction so if any diode conducts in both directions or does not conduct in any direction it is junk and must be replaced. You will need to take them out of circuit to be certain you are testing just the diode. Usually you can find out the type as it is written on it but they are not critical and any power diode will do the job.

After that there will be smoothing capacitors, the big blue cylinders in the picture. Again they can fail but they are a little harder to test other than checking if there is any signs of burning or stuff leaking out of them. You can use a multi meter set to resistivity to test a capacitor. It should not be a short or open circuit but some resistance should be there and it shoudl go up as the capacitor charges from the action of measuring it but this is not conclusive.

Try this and if it does not help we can move on. However it is most likely to be the power supply causing the problem.

 

[HOJ]

If had a blow fuse (2a, as instructed on rear panel),

I presume you meant to say SLOW blow?

 

[bugbear]

Be like a proper rocker - cut a piece of 6 inch nail to roughly the right length and use it to replace the fuse, which is clearly useless

BugBear

 

[RobinBHM]

Be like a proper rocker - cut a piece of 6 inch nail to roughly the right length and use it to replace the fuse, which is clearly useless

BugBear

There seems to be a big difference in complexity of advice offered -with this being by far the most sophisticated option

 

[Tom K]

Turn it up to 11 and try again it does go up to 11 doesn't it?

 

[Eric The Viking]

Sadly if you plugged it into 240V mains whilst set to 110, there is a chance you've also fried some of the electronics (as well as the transformer), as, for a brief moment, the transformer output would have been slightly more than twice what it should have been. As indicated the bridge rectifier is the most vulnerable, BUT, if your fuse was the right value, AND it's on the mains input side rather than the output side of the transformer, it *might* have saved it (that's what fuses are for, after all).

TBH, most transistor instrument amps of that vintage are pretty horrid things and a bit nasty. It doesn't look too awful though - fibreglass circuit board and decent electrolytics mean it's not the cheapest construction, but still.

Repair is probably doable, and components or substitutes can be found, but you need a specialist. Expect it to cost at least 100 plus parts, if it's a simple matter (but some parts, like the transformer, will be expensive). There's at least one pot (knob) physically broken I can see, and the output transistors might have been damaged by the overvoltage.



A few more (and better) pics would help.

 

[Joe Shmoe]

Firstly, let me say thanks for some awesome advice and for taking the time to write those replies.

I paid a pittance for the amp in a junk shop as I guessed it was broken, but if I can get it working that would be good, but more than that, this is a great learning experience.

The amp had a blown fuse when I got it, I'm guessing someone made the same mistake as me and just plugged into 240v when the amp was set to120v. I think it may have been imported from America originally as I have another amp head that I purchased at the same time, This one says 120v only and it has american coloured wires inside (green, black, white). The plug cable has been cut and the fuse removed, so obviously suffered the same fate as this marshall. Bit annoying as it's actually quite a nice bass head, an acoustic 220 from the sixties I belive.

Anyway, back to this one....

I cut the output wires from the transformer, and the fuse didn't blow and the on/off switch stayed illuminated, so I guess that's okay.

I then tested what I think we're the diodes.... The four blue things that have D1 through D4 printed on the board. I set the multimeter to resistance and they all read 1 in one direction, and roughly 850 in the other. I'm guessing that's okay? I then tested what I think is a Zener diode (printed with ZD1 on the board). This read 1 in one direction, and 1000 odd in the other. I read that you cannot test the Zener function with a multimeter alone?

I then checked the two capacitors, which appear in okay condition, and the reading on the multimeter continued to rise as they charged from the multimeter.

So I'm not really sure what to do next, as clearly, I havent a clue what I'm doing, but it's great learning. Sorry about the images.. I'm struggling to take low resolution on the ipad, so having to crop it serverely.

 

[Joe Shmoe]

Oops, forgot to check the bridge rectifier as you instructed, and it appears to be giving a high reading in both directions when I connect yellow to yellow, and yellow to black, so I'm guessing this is fried and needs replacing?

 

[Eric The Viking]

AC in across the yellow ones, DC out across the white/black ones (full wave rectified). White seems to be positive, so you should get high resistance with the meter facing one way (yellow-to-white) and short circuit (approx) the other way round across the same terminals. Same forthe other yellow-wired terminal, and the opposite way round for black to-yellow connections.

Also check the primary and secondary of the transformer. With the power switch 'on' (but obviously not plugged into the mains), you should get a low-ish resistance across live and neutral through the transformer and the voltage changer -- I'd go off the back of the fuseholder to eliminate a dead fuse. If it's open circuit, double check what you're doing, but it may mean a dead transformer. Similarly check the secondary (the yellow leads that would go to the bridge rectifier).
. . .
You can work out what the power rails ought to be by looking at the data plate - what RMS wattage is it supposed to be, and into 2, 4 or 8 Ohms?

Rearranging the simple equation of Ohm's law will give you the current:

W = I^2 R, or I^2 = W / R

W = 100 (say)
R = 4 Ohm (say)

So I^2 = 100/4 = 25

and so I (the current flowing) = 5A (sq root of 25)

Simple Ohms Law bit:

V = I R

so V = 5 x 4 = 20V,

which is the voltage across the two power rails. Well, nearly. We've been assuming RMS (root mean square) equivalence between DC and AC power. So you need to increase the voltage, by root2 (1.414, approx.), to get the voltage at the peak of the AC (music) wave: About 28V (say 30V).

The PSU looks like it's balanced so it would be +15 0 -15 approx., coming off the capacitors, or 30V altogether across the rails.

You can plug in the numbers for whatever is on the data plate. So,

for 8 Ohm speakers and 100W, you'd need 3.536A flowing,
V = IR, = 3.536x8 = 28.3V approx.
multiply by 1.414 = 40V approx.

So you can see that for more power, amps usually need bigger voltages inside, or to drive much lower impedance speakers. It's something you can predictably test though.

E.

PS: its also why the "power" outputs for a lot of car stereo systems are gross exaggerations. You're starting with 12V available, so unless you expensively increase that, you're limited as to the power you can deliver to the speakers.