Marshall guitar amp keeps blowing internal fuse

Thread Starter

Kinoton

Joined Jan 20, 2014
25
Hi ,

I have been given a Marshall transistorised guitar amp , model G80RCD.

It keeps blowing its internal T1A /230 v fuse every time it’s switched on . Visually nothing seems amiss but I was wondering if anyone had any ideas before I take it to a repair shop. Could it be the switch or transformer at fault ?

Any help or advice will be gratefully received.

Best wishes

Kinoton
 

MrChips

Joined Oct 2, 2009
20,243
You can do some trouble shooting on your own before taking it to a shop.

Step 1 - Get the circuit schematics. How good are you at reading schematics?

Step 2 - With an ohmmeter, measure the resistance across the AC plug LINE and NEUTRAL, (disconnected from the mains, of course), with power switch OFF and then ON.

Step 3 - Disconnect one side of the transformer primary winding and measure Step 2 again.
 

Thread Starter

Kinoton

Joined Jan 20, 2014
25
You can do some trouble shooting on your own before taking it to a shop.

Step 1 - Get the circuit schematics. How good are you at reading schematics?

Step 2 - With an ohmmeter, measure the resistance across the AC plug LINE and NEUTRAL, (disconnected from the mains, of course), with power switch OFF and then ON.

Step 3 - Disconnect one side of the transformer primary winding and measure Step 2 again.
Thanks very much for your replies guys. Re the above step 1,2 - what values should I be looking for? I guess anything but a short ? Yes, the schematic is the one I have thanks.
 

Dodgydave

Joined Jun 22, 2012
8,824
If the circuit is the same as post4,, then measure the Transformer at TxL and TxN with mains removed!! and switched off you should get approx 100 to 150ohms, and measure the Secondary side across the bridge rectifier input and output, should get around 10 to 30 ohms ..
 

Tonyr1084

Joined Sep 24, 2015
4,169
1) Stop throwing fuses at it. All you're doing is wasting fuses and potentially damaging the amp.
2) NEVER go up in amperage with a fuse. You'll SURELY damage the amp.

Had an amp blow a 7.5 amp fuse. When I disconnected the BR (Bridge Rectifier) the fuse did not blow. Checked the BR using a DMM set on Diode Check and looked for around 0.450V one way and "OL" the other way. Did that for each segment of the BR. Found one shorted. Replaced the BR and the amp has been working happily since then.

Of course MY issue was the BR. Before you start throwing parts at the problem FULLY DIAGNOSE the problem so you only replace what needs replacing. I doubt the issue is with the transformer. Not saying it isn't, just saying it's uncommon for a transformer to short.
 

DarthVolta

Joined Jan 27, 2015
247
That schematic G50RCD is hard on my eyes. For a fuse to keeping blowing, it must be a major short circuit. So on the bottom page it could be the 2 caps on the primary side, or the primary winding. Or the secondary windings or the FWR.

If they are good, have a good look at D2 section of the schematic, any of those caps, or zeners could go short.

Seems there's a lot of stuff off the FWR that could go short around the op-amps too.

I wish I had something to troubleshoot and fix. Most of the broken stuff I have is like mp3 players, or motherboards, and that's all too hard.
 

dl324

Joined Mar 30, 2015
10,040
what values should I be looking for? I guess anything but a short ? Yes, the schematic is the one I have thanks.
Anything that seems too low to be right.

Relevant portion of the schematic with some white space removed and improved contrast:
1573665518906.png
Check the caps to the left too.
 

GetDeviceInfo

Joined Jun 7, 2009
1,681
Personally, I’d desolder your m+/- coming off the power supply bird, and plug it in. Yes, it may cost a fuse, but you’ll know which way to go. My last Marshall was the final driver stage.
 

Ylli

Joined Nov 13, 2015
893
Very likely the output transistors. Remover TR9 & TR10 and test with an ohmmeter in diode mode.
https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07
Then for testing, there is a trick you can use. Add a 100 watt incandescent light bulb in series with the power line input. If the unit is working the bulb will stay relatively cool and the unit will get most of the line voltage. If there is an overcurrent, the light bulb will light up bright, reducing the voltage to the unit under test and protecting it (and the fuse). http://www.geek-tips.com/2015/11/22/dim-bulb-tester/ . A Dim Bulb Tester is connected directly to the power line, and as such use care in constructing it and follow the listed directions so as to produce a safe device.
 

Thread Starter

Kinoton

Joined Jan 20, 2014
25
Very likely the output transistors. Remover TR9 & TR10 and test with an ohmmeter in diode mode.
https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07
Then for testing, there is a trick you can use. Add a 100 watt incandescent light bulb in series with the power line input. If the unit is working the bulb will stay relatively cool and the unit will get most of the line voltage. If there is an overcurrent, the light bulb will light up bright, reducing the voltage to the unit under test and protecting it (and the fuse). http://www.geek-tips.com/2015/11/22/dim-bulb-tester/ . A Dim Bulb Tester is connected directly to the power line, and as such use care in constructing it and follow the listed directions so as to produce a safe device.
 

Thread Starter

Kinoton

Joined Jan 20, 2014
25
Hi guys,

Thanks so much for all your replies. I’ve finally got some time to have a look at the amp.

Firstly and please excuse this 101 question , but am I ok testing components in circuit or do I really need to remove them to check properly?

I’ve had a look at the bridge rectifier and using a DMM in diode check mode I get a voltage drop of 0.53v across each diode , in both directions? Should I get OL in one direction and a drop in the other ? These readings were obtained with the component still in situ.

Secondly I checked the two large caps in the power supply next to the rectifier, once again in situ . With the meter set to cap check I obtained a reading of 6.25 from both ;from what I can see they are both 2200micro farad caps. Does this sound right ? Or am I just delaying the inevitable trip to the repair shop!

Cheers
 

Ylli

Joined Nov 13, 2015
893
You are getting some phantom readings from other components connected to the part under test. That's why in most cases you need to remove the parts for an accurate test.
 

Tonyr1084

Joined Sep 24, 2015
4,169
Yes, as @Ylli said, you're getting readings from other circuit components. The bridge must be isolated from the transformer and from the caps. And you should get a good reading in one direction but an OL in the other. However, fact that you reported NOT getting a dead short (zero ohms) in any of the tests seems to suggest the BR is not shorted. However, you may still have an open rectifier.

If your rectifier tests good then keeping it out of circuit install a fuse and see if it blows when you turn it on. If so - then you probably have either a bad transformer or a short circuit on the circuit board the transformer directly connects to.

Caps need to be tested out of circuit unless you have a tester that can test in circuit. I know most of us do not have such a tester. ME? I'd remove the cap and dead short it through a resistor of a few KΩ's. Then once it's drained, I'd dead short it. THEN I'd put my probes on it in the ohms reading. It should start out as very low ohms and should climb slowly and seemingly steady. Over time the climb will lessen. Then you can reverse the leads and you should see the ohms drop and drop and drop back to zero, cross zero and then start up again. And there's no such thing of negative ohms. None I'm aware of anyway. If both caps behave in this manor then the next thing I'd look for is shorted output transistors. You can YouTube how to check transistors in circuit to get a decent idea of how to handle the task. If I don't find a shorted output transistor then it's time to refer to qualified service personnel. Or to scrap the unit and replace it. But that's MY skill level. And on a scale from 1 to 10 my level is no higher than a 3.

Good luck.
 

MrChips

Joined Oct 2, 2009
20,243
...And there's no such thing of negative ohms. None I'm aware of anyway.
Off topic - Yes, there is such a thing as negative resistance and they are encountered in real life. A tunnel diode exhibits negative ohms in its behaviour. A dynamic braking circuit presents a negative resistance to the motor in order to stop the motor.
 

Thread Starter

Kinoton

Joined Jan 20, 2014
25
Yes, as @Ylli said, you're getting readings from other circuit components. The bridge must be isolated from the transformer and from the caps. And you should get a good reading in one direction but an OL in the other. However, fact that you reported NOT getting a dead short (zero ohms) in any of the tests seems to suggest the BR is not shorted. However, you may still have an open rectifier.

If your rectifier tests good then keeping it out of circuit install a fuse and see if it blows when you turn it on. If so - then you probably have either a bad transformer or a short circuit on the circuit board the transformer directly connects to.

Caps need to be tested out of circuit unless you have a tester that can test in circuit. I know most of us do not have such a tester. ME? I'd remove the cap and dead short it through a resistor of a few KΩ's. Then once it's drained, I'd dead short it. THEN I'd put my probes on it in the ohms reading. It should start out as very low ohms and should climb slowly and seemingly steady. Over time the climb will lessen. Then you can reverse the leads and you should see the ohms drop and drop and drop back to zero, cross zero and then start up again. And there's no such thing of negative ohms. None I'm aware of anyway. If both caps behave in this manor then the next thing I'd look for is shorted output transistors. You can YouTube how to check transistors in circuit to get a decent idea of how to handle the task. If I don't find a shorted output transistor then it's time to refer to qualified service personnel. Or to scrap the unit and replace it. But that's MY skill level. And on a scale from 1 to 10 my level is no higher than a 3.

Good luck.
Yes, as @Ylli said, you're getting readings from other circuit components. The bridge must be isolated from the transformer and from the caps. And you should get a good reading in one direction but an OL in the other. However, fact that you reported NOT getting a dead short (zero ohms) in any of the tests seems to suggest the BR is not shorted. However, you may still have an open rectifier.

If your rectifier tests good then keeping it out of circuit install a fuse and see if it blows when you turn it on. If so - then you probably have either a bad transformer or a short circuit on the circuit board the transformer directly connects to.

Caps need to be tested out of circuit unless you have a tester that can test in circuit. I know most of us do not have such a tester. ME? I'd remove the cap and dead short it through a resistor of a few KΩ's. Then once it's drained, I'd dead short it. THEN I'd put my probes on it in the ohms reading. It should start out as very low ohms and should climb slowly and seemingly steady. Over time the climb will lessen. Then you can reverse the leads and you should see the ohms drop and drop and drop back to zero, cross zero and then start up again. And there's no such thing of negative ohms. None I'm aware of anyway. If both caps behave in this manor then the next thing I'd look for is shorted output transistors. You can YouTube how to check transistors in circuit to get a decent idea of how to handle the task. If I don't find a shorted output transistor then it's time to refer to qualified service personnel. Or to scrap the unit and replace it. But that's MY skill level. And on a scale from 1 to 10 my level is no higher than a 3.

Good luck.
Yes, as @Ylli said, you're getting readings from other circuit components. The bridge must be isolated from the transformer and from the caps. And you should get a good reading in one direction but an OL in the other. However, fact that you reported NOT getting a dead short (zero ohms) in any of the tests seems to suggest the BR is not shorted. However, you may still have an open rectifier.

If your rectifier tests good then keeping it out of circuit install a fuse and see if it blows when you turn it on. If so - then you probably have either a bad transformer or a short circuit on the circuit board the transformer directly connects to.

Caps need to be tested out of circuit unless you have a tester that can test in circuit. I know most of us do not have such a tester. ME? I'd remove the cap and dead short it through a resistor of a few KΩ's. Then once it's drained, I'd dead short it. THEN I'd put my probes on it in the ohms reading. It should start out as very low ohms and should climb slowly and seemingly steady. Over time the climb will lessen. Then you can reverse the leads and you should see the ohms drop and drop and drop back to zero, cross zero and then start up again. And there's no such thing of negative ohms. None I'm aware of anyway. If both caps behave in this manor then the next thing I'd look for is shorted output transistors. You can YouTube how to check transistors in circuit to get a decent idea of how to handle the task. If I don't find a shorted output transistor then it's time to refer to qualified service personnel. Or to scrap the unit and replace it. But that's MY skill level. And on a scale from 1 to 10 my level is no higher than a 3.

Good luck.
there might or might not be negative ohms but there is negative skill; if you’re a 3 Tony then I’m definitely minus! Thanks so much for that , I’ll follow your suggestions Tony and if no good it’s off to the repair man . Cheers!
 
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