# Fender EVH 5150 III head. Issues with heater fuse. FYI



## Church-Audio

I have on my bench a EVH 5150 III Head made by fender. This amp brought in after being at another repair shop a few times. The real issues were with this amp 2 open 5 watt resistors and a low heater supply voltage. The customer complained that the amp did not sound right after its last service, I looked at what I always look at first VOLTAGES! Yep start with the basics and look at voltages.. First thing I noticed was a low heater voltage of around 2.5 volts per leg. Thats not where it should be with no power tubes installed. And even with power tubes installed it would be an issue. So first place to check is obviously the power transformer and the two 100 ohm virtual center tap resistors. They were fine. I measured the voltage coming from the transformer it was a healthy 6.3 volts RMS AC. Great where is this voltage drop coming from? It was the fuse for the heater the 12 amp fuse. I thought ok this is strange what can cause a drop in voltage? Resistance So I measured the voltage drop across this fuse guess what it was happening at the fuse. Took the fuse out of the holder and there is was some contamination from the fuse or fuse holder caused heavy corrosion on the fuse holder lowering the voltage. I am not a big fan of these fuse holders that are pcb mounted its been my experience ( Jmp import marshalls for example from the 1970s ) had this issue as well. So I will drill out the chassis and install a chassis mounted fuse holder voltage on the heater is back up to 6.3 volts and the amp sounds much better after a new set of power tubes. If you own one and it seems to not be sounding quite right having a qualified tech look this over might save you from having an amp go down at a show. If you find you want to keep the fuse holder on the pcb board you have to pull the power supply board and remove all the old solder and use fresh solder burnish the contacts of the fuse holder bend them so they are tight and apply silicone grease to the contacts. This will prevent oxidation of the fuse terminal from happening again. This was also an issue for the hand wired low power twin that fender made part of the Eric Clapton series. They used the same fuse holder there for the 5v supply for the dual rectifiers same exact issue.

I hope this will help someone if they have one of these and the problems keep going on and on.


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## nonreverb

That looks more like an arcing/overheating problem due to bad board solder..


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## Church-Audio

It's from a bad solder joint that then oxidizes the fuse connection The shop simply added more solder also this circuit board needed to be removed so that it could be soldered properly and without addressing the oxidation that increased the current draw and resistance to the point where the voltage dropped. The problem is the pcb mounted fuse holders don't have enough surface area for the current and to be soldered well enough to the board. If people want amps like this to be reliable replace the fuse holder with a chassis mounted one and at least 16ga wire leading to that fuse holder.


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## nonreverb

How does one miss something so obvious?


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## Church-Audio

It was really well hidden.. In all fairness you could not see it unless you removed the fuse. I think what happened here was the fuse was removed and solder was added to the top of the fuse holder instead of removing the circuit board and doing it right. Then eventually it failed again. With just the preamp tubes installed now power tubes the fuse was very hot to the touch. I am going to get a flir viewer for circuit boards that's the ultimate way to look at a circuit. And it's so fast you can find hot spots quickly. The cheapest one so far is $500 I am trying to find something cheaper. I already use a non contact temperature gun that works well but it's slow compared to flir that's in real time. Electricians use it to find hot spots in large current power panels.


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## Guest

Amprepair said:


> I am going to get a flir viewer for circuit boards that's the ultimate way to look at a circuit. And it's so fast you can find hot spots quickly. The cheapest one so far is $500 I am trying to find something cheaper.


Cool idea. You can get a FLIR ONE for your iPhone for $330 now: http://flir.com/flirone/ -- have used one at work. It's not amazingly accurate because of the heat from the phone, but for tracing heat issues on an amp it'd be plenty fine.


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## nonreverb

I'd think the blackening around that fuse holder plus the dullness of the metal is pretty telling.
A similar issue seen in the older 5150's and JCM 2000's where one of the heater connectors on the pcb starts to get really hot or arcs due to bad connectivity basically laying waste to the connector assembly and the plastic insulator.


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## LydianGuitars

nonreverb said:


> A similar issue seen in the older 5150's and JCM 2000's where one of the heater connectors on the pcb starts to get really hot or arcs due to bad connectivity basically laying waste to the connector assembly and the plastic insulator.


On the JCM2000, are you referring to the early combos or something else?


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## Church-Audio

Funny you mention that, I just saw that last night looks cool. I have a iPhone 5c I wonder if it will work with my phone?

- - - Updated - - -

I too have seen it on all kinds of marshals. From the power transformer to the pcb board for the heaters. I usually jettison the crip connectors and solder right to the terminals. The bugara amps have this exact same issue as well.


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## Church-Audio

Here is the solution for this issue. Drill out the chassis and put in a real fuse holder. Run some number 14 ga wire to the new connection, then run one of the transformer leads to the fuse holder and your done. Simple and much better solution then that crappy circuit board mounted fuse holder. You can see the new fuse holder top left with the green heat shrink over the connections. I use some lock tight to make sure the screw will not slip. Incase it does I always put the load side on the side of the fuse holder and supply side on the end of the fuse holder. This way if the nut ever does slip off it will short out the fuse to ground. And stop the flow. If you have it the other way around the fuse will not blow and it will short the transformer output to ground. Most likely just blowing up the 100 ohm virtual center tap resistors. Its a much more serious issue if we are talking line voltage here. I have seen this type of failure on older fender amps where the nut literally shorted out the line to ground. Caused some serious arching inside the amp. Before the 15 amp circuit panel fuse failed. The only tricky part was drilling the hole on the circuit board to allow a #14 wire in there. Scrapped off the traces to allow good solder adhesion.


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## nonreverb

Both....The early ones were more problematic. The early JCM 2000's had connectors that were susceptible to loosening and be literally black they were so bad. 



LydianGuitars said:


> On the JCM2000, are you referring to the early combos or something else?


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## LydianGuitars

nonreverb said:


> Both....The early ones were more problematic. The early JCM 2000's had connectors that were susceptible to loosening and be literally black they were so bad.


Both the heads and combos? 

I've never heard of that on the heads (I have the 1st DSL50 that came into to Steve's Ottawa when they 1st came out in 1997 - even the schemes for the board rev/issue # are slightly different) and also had the 100w head from 1998 and they've been solid performers. The DSL has seen 100's of shows. 

I know that the combos had more issues because of the heat and the Bridge rectifier being under rated.


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## nonreverb

Yessir...I remember one TSL100 that had one connector on it that was so bad that the plastic shield around the crimp end shattered into little blackened pieces when I attempted to remove it. Wound up hard wiring the leads to the board....problem solved.



LydianGuitars said:


> Both the heads and combos?
> 
> I've never heard of that on the heads (I have the 1st DSL50 that came into to Steve's Ottawa when they 1st came out in 1997 - even the schemes for the board rev/issue # are slightly different) and also had the 100w head from 1998 and they've been solid performers. The DSL has seen 100's of shows.
> 
> I know that the combos had more issues because of the heat and the Bridge rectifier being under rated.


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## LydianGuitars

nonreverb said:


> Yessir...I remember one TSL100 that had one connector on it that so bad that the plastic shield around the crimp end shattered into little blackened pieces when I attempted to remove it. Wound up hard wiring the leads to the board....problem solved.


My god... the TSLs were horibble. I bought one when they 1st came out (got it @ Steve's too) and sold it before I had problems with it. I fixed a couple and the problems were a real mix of different issues. Which conn was giving you trouble? The conns that seemed to go bad were related to the accessory cct and switching. The footswitch design was problematic and when a fuse went, you had to change out the PCB soldered fuses.  Now those are a great candidate for chassis mounted fuse holders...

but I digress... and we're derailing the thread.


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## Church-Audio

The other main issue with this amps was the main power tube pcb was prone to carbonization around pin 5 you have to dremel it out so it does not conduct and cause extreme bias drift. This was the major issue with this amp as well as the stupid 16 ohm speaker connector ground issue. On the older amps there is a switch built into the ground on one of the outputs that disconnects the ground to the other unused outputs so you can use them. Only thing is if you use other sockets the 4 ohm socket by it self the ground switch cant handle the current. Remember lower the resistance on the output transformer higher the current will be. For example 20 volts into 4 ohms = 5 amps of current. Thats a ton of current for a little switch built into a 1/2 jack that was never designed to switch current that high. So these jacks failed to close the circuit causing the amp to be disconnected from its load ultimately destroying the output transformer. I have replaced more than a few of them on the 2000 series.


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## nonreverb

I think it would be best to start a whole new thread regarding this discussion. There's far more to the bias drift and other Marshall issues than should be discussed here.....just sayin'



Amprepair said:


> The other main issue with this amps was the main power tube pcb was prone to carbonization around pin 5 you have to dremel it out so it does not conduct and cause extreme bias drift. This was the major issue with this amp as well as the stupid 16 ohm speaker connector ground issue. On the older amps there is a switch built into the ground on one of the outputs that disconnects the ground to the other unused outputs so you can use them. Only thing is if you use other sockets the 4 ohm socket by it self the ground switch cant handle the current. Remember lower the resistance on the output transformer higher the current will be. For example 20 volts into 4 ohms = 5 amps of current. Thats a ton of current for a little switch built into a 1/2 jack that was never designed to switch current that high. So these jacks failed to close the circuit causing the amp to be disconnected from its load ultimately destroying the output transformer. I have replaced more than a few of them on the 2000 series.


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## kinder5150

Anyone still have pics of this mod as I am running into this problem on my 5150? I am not an electrical genius, but can do this. Just don't want to blow up the amp. Basically, I get new fuse holder and run the new wires where the old clips came out of to the new fuse holder? Thanks!


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## jb welder

Yes. Remove the clips from the board. Run wires from the holes where the clips were to your new fuse holder.
You may need to enlarge the holes on the board with a drill. This will depend on your wire size, he recommended minimum 14 gauge in post #10.


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