Servicing a Fender SRM 6302 PA Amplifier.

Just thought that I would start a thread about some servicing work I'm doing on a mate's Fender SRM 6302 PA Amplifier, the complaint was that it was producing a loud hum when switched on, so I opened up the back panel and had a look inside, I couldn't see anything obvious, like burnt-out components so I decided to have a more careful look around the main PCB that has the power supply and power amp circuitry on it, making sure that the amp was completely disconnected from the mains, and that was when I noticed that one of the main power supply filter caps felt like it was a bit loose compared to the other, and I immediately knew where the cause of the fault was most likely to be, so I completely removed the back panel of the amp from the cabinet after carefully noting where wires were connected, and then removed some screws that held the PCB in place on the heatsink, then I looked at the underside of the PCB and found that I was correct, there were two solder joints on the loose filter cap that had cracked and broken so I proceeded to remove the old solder using some 3mm Goot Solder-Wick, and then re-soldered the joints, I did the same to the solder joints on the other filter cap as a matter of course, I think that should fix the loud hum fault, but I'm going to go through and test all the power output transistors with my in-circuit transistor tester first before I put the back panel in the cabinet just to be sure that all the power output transistors are still all good.


As a side note, just before I was about to put the amp back together I noticed an electrical safety-hazard that I needed to correct before proceeding any further, whoever had previously owned the amp had replaced the original 240 V AC Cable Clamp (presumably because said Cable-Clamp broke, or was missing) with a Zip-Lock Cable-Tie and some Electrical Tape which was wound around the 240 V AC cable and Cable-Tie, I have now removed the Cable-Tie and Electrical-Tape because at some point the 240V AC cable will chafe against the metal back panel and create a short-circuit, I fitted a Black Plastic 240V AC Cable Gland as a replacement, which should prevent any chafing of the 240 V AC Cable in future, making it safer, it also holds the 240V AC cable more securely too.

Update:

One more thing on the PA amp needs attention before I can give the amp back to my mate, the front-mounted pot for controlling the effects level is broken and needs fixing, or replacing, so that's what I'm going to do next, but first it's time for a break.

Some happy snaps of your wonderful work would be nice there Doc...

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Originally Posted by Marcel

Some happy snaps of your wonderful work would be nice there Doc...

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I'll take a few pics a bit later on and post them here then.

Update:


I've just received an email from my mate who owns the PA amplifier that I worked on in this thread, he reckons that it is working properly and rockin after he's tried it out, I actually had a feeling that it was going to work properly after having fixed it.

There are some pics which I'll be posting for you in the morning, stay tuned.


Update 2:

Just got an email from my mate saying that the PA amp isn't powering up, I'm thinking that it could be a blown fuse.

Update 3:

As it turns out, the cause of the PA not powering up is something mundane......a cracked solder joint on the PCB where the 240 V AC mains connects to the PCB via two spade-connectors, it was relatively easy to fix, just resolder the connection on the back of the PCB.

Update 4:


I've just given my mate's PA amp a proper test-out and it is indeed powering-up and working perfectly, I've got some pics that I will be uploading shortly....stay tuned.


Here we go, some pics of the Fender PA amp that I worked on for my mate:


The Fender PA Amplifier:

Attachment 20579


A Closeup Of The New Cable Gland I Installed:

Attachment 20580

This was done because someone had replaced a broken,or missing, 240 V AC cable-clamp with a zip-tie and a few turns of blue electrical tape, I fitted the new cable gland after removing the zip-tie and sticky electrical tape to prevent chafing of the 240 V AC cable against the metalwork of the panel, at best a short circuit here would blow a fuse, at worst, a short-circuit here could kill someone, it should be all safe now though.

New 240 V AC Mains Plug:

Attachment 20581

In order to fit the new cable gland that was needed to protect the 240 V AC cable from chafing on the metalwork, I had to cut off the original 3-pin plug because the other end had spade connectors crimped onto each wire and I didn't have any spare spade terminals, so I fitted a new 3-pin plug instead, making sure that I quadruple-checked to ensure that I had gotten it correct, fortunately it was all correct.

Solder Joints On The Main Supply Filter Caps:

Attachment 20582

The two on the right are the ones that were cracked, I re-soldered all four of them as a matter of course.

Solder Joints On The 240V AC spade Connector Terminals:

Attachment 20583

I used some thick stranded wire that I twisted together, tinned, and bent into shape to reinforce each of the solder joints for the spade-connector terminals, this was to stop the solder-joints from cracking again under mechanical stress, one of the spade-connector terminals for the 240V AC input had cracked solder joints, this prevented the PA amp from powering up, it's all good now.

Update 5:


My mate came and picked up his PA amp, he was really happy that I was able to fix it for him, he and I went out to lunch and had some really nice seafood out at a place called the Warf, I'll tell you something, I had the best piece of cooked fish ever at the Warf, it was cooked perfect and tasted awesome, not sure what kind of fish it was but it was deliciously awesome.

Glad to hear the patient made a full recovery Doc!

If you ever drop into Rocky Doc I'll take you to a fish-n-chip shop that sells a most truly awesome battered Barra.

Fine job on the power lead.... Brown wire to the Left pin (when viewed from the cord side of the plug) as us Aussies Always keep Left... Same goes for the wall socket as you look at it ready to plug your appliance in, Earth is the bottom hole, Active is the Left hole and Neutral is the Right hole.

A clean up tip for your soldering. A cotton bud soaked in Acetone will remove/dissolve most if not all of the burnt/used flux. Use with caution as the highly flammable Acetone will also melt some plastic stems that some cotton buds use, it will remove all oil from your skin and some labels off components, but it will not harm the solder joint or the fibreglass PCB.

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Originally Posted by Marcel

If you ever drop into Rocky Doc I'll take you to a fish-n-chip shop that sells a most truly awesome battered Barra.

Fine job on the power lead.... Brown wire to the Left pin (when viewed from the cord side of the plug) as us Aussies Always keep Left... Same goes for the wall socket as you look at it ready to plug your appliance in, Earth is the bottom hole, Active is the Left hole and Neutral is the Right hole.

A clean up tip for your soldering. A cotton bud soaked in Acetone will remove/dissolve most if not all of the burnt/used flux. Use with caution as the highly flammable Acetone will also melt some plastic stems that some cotton buds use, it will remove all oil from your skin and some labels off components, but it will not harm the solder joint or the fibreglass PCB.

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That battered barra sounds nice so I'll definitely keep that in mind, barra is always good eating, the convention with Australia's 240 V AC wiring goes like this, the brown wire is Active, the blue wire is Neutral, and the Green/Yellow wire is the Earth wire, and it is standard practice to put the 240 V AC mains fuse in line with the Active wire but after the power switch, reason for this is that if for some reason a short-circuit develops in the mains transformer, the fuse will blow and cut the power off preventing the chassis from becoming live, and the Earth wire is always securely connected to the chassis too as an extra safety measure.

The whole input mains wiring thing is a bit of a nightmare for international manufacturers like Marshall & Vox & Fender. What you can do in one country is often not permitted in another. So if you have multiple models going to many different countries and each has a permanent power lead attached then each item would need to be wired to the country of destination, which is a real PITA!!! Probably why the 'figure 8' for double insulated and IEC socket for Earthed devices on the back of amps became so popular so quickly as all those colour and wiring issues are dealt with in the separate power lead that is thrown in with the product packaging, so therefore the product itself can be made in a standard way for world wide distribution right at the factory of manufacture.

While having the power switch before the fuse is safer in that there will be no power on the fuse socket if you turn the device off while you change a blown fuse there are some manufacturers that don't do it. To save a few cents in wire and to not having mains power going back and forth from the front to the back of a chassis there are devices out there that are wired to the fuse holder first and then to the main power switch. While they are not 100% correct they are also not illegal in most countries, however they are less safe. And which side of the fuse holder is connected to the incoming mains contributes massively to the eventual safety of the device user.

I once had a old VOX amp come across my work bench that a guy had bought in the UK and brought with him to Australia when he emigrated here. Apart from the UK plug it had the standard UK colouring of Black as active and White as Neutral and a solid Green earth on a fixed cord to the amp. He wanted it change to an AU plug as the little adapter he was using to power it was falling apart. It was an easy fix but the whole flexible lead had to be replaced so that it complied with the AU colour scheme for mains powered fixed cord portable devices.

White/Black wasn't even the old UK colour code. Prior to 1977 it was Red - live, Black - neutral and Green earth/ground. Since 1977 it's been Brown - live, Blue - neutral and Green & Yellow - earth/ground.

White/Black/Green from what I can make out, is a US power cord. Neutral must be white or grey, Ground is green or green/yellow and live in theory is any other colour, though black is normally used.

Vox may simply have used the same cord for US and UK models.

My bad,
Thanks Simon for clearing that up. It seems the UK and Australian standards are in step as Australia changed from Red/Black/Green to Brown/Blue/Green-Yellow stripe at about the same time as you state the UK did. I suppose that is one of the joys of being in team Commonwealth...

As the amp I worked on was a VOX amp I honestly thought it was wired to the UK standard. Obviously it wasn't. More weird is that he did bring it from the UK and not from the USA. A side thought, It may have been earmarked for UK export and as the USA would be the biggest and most likely export market destination it was configured to suit that market at the factory, and too bad if the amp finds itself in any other country.... All adds weight to the idea of plonking an IEC connector on it and throwing a bunch of different power leads in the packing box.

One of my Electronics lecturers that taught us basic Electronics at uni told us that the reason why Australia adopted Brown for active, Blue for neutral, and Green/Yellow for earth, for the 240 V AC wiring colour-coding standard was to make it easier for people with colour-blindness to distinguish the different coloured wires.

Where I used to work the discussion on this topic was repeatedly covered numerous times, so I think I have the response down almost down pat...

Red/Green colour blindness is the most common form of human colour blindness with between 15% to 40% of all people suffering the condition in any given community. The Brown/Blue/Green colour combination is the least common form of colour blindness at less than 0.0001% of all people. And it is the founding reason why for at least the past 40 years the IEC recommendation specifies the Brown/Blue/Green-Yellow stripe for flexible cords on appliances. Most countries in the world have adopted and legislated the IEC recommendations as standard practice.

In isolation the simple addition of the Yellow stripe to the Green earth wire improved safety by an estimated 90% as it made the earth wire visibly different to the Red wire as seen by people who are Red/Green colour blind. Including the Brown/Blue combinations improved safety due to mistaken colour identification to near 100% levels... Which only leaves ignorance, stupidity and the individual (obstinate) country based unique antiquated colour systems as the current main three factors contributing to modern day appliance power wiring errors,...

And undoubtedly why 3-phase wiring colours have been harmonised in the UK with the EU colour choice since 2004 and whereas L1 was red, L2 yellow and L3 was blue, L1 is now brown, L2 is black and L3 is grey. Nothing like as pretty though.

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Originally Posted by Marcel

Where I used to work the discussion on this topic was repeatedly covered numerous times, so I think I have the response down almost down pat...

Red/Green colour blindness is the most common form of human colour blindness with between 15% to 40% of all people suffering the condition in any given community. The Brown/Blue/Green colour combination is the least common form of colour blindness at less than 0.0001% of all people. And it is the founding reason why for at least the past 40 years the IEC recommendation specifies the Brown/Blue/Green-Yellow stripe for flexible cords on appliances. Most countries in the world have adopted and legislated the IEC recommendations as standard practice.

In isolation the simple addition of the Yellow stripe to the Green earth wire improved safety by an estimated 90% as it made the earth wire visibly different to the Red wire as seen by people who are Red/Green colour blind. Including the Brown/Blue combinations improved safety due to mistaken colour identification to near 100% levels... Which only leaves ignorance, stupidity and the individual (obstinate) country based unique antiquated colour systems as the current main three factors contributing to modern day appliance power wiring errors,...

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I can vouch for that, even though I happen to have red-green colour blindness, I can clearly see/distinguish the difference in colour between the brown active wire, the blue neutral wire, and the green/yellow earth wire with no trouble at all.

Update 5:

Along with the fender PA amp, my mate asked me to have a look at his Crate Vintage Club 50 guitar amp, more specifically, the short FX Loop breakout cable I had made for him, the complaint was that it was getting a return signal, but wasn't putting-out a send signal, after doing a couple of tests on it I managed to figure out what was going on, turns out it was simply due to me incorrectly labeling the two 1/4 inch sockets on the little diecast box on one end of the FX Loop breakout cable, I had the send socket mis-labeled as the return, and vice-versa.....how embarrassing for me, eh?, oh well no real harm done and at least that proves that I'm a human-being that makes mistakes after all, anyway it's all good now.


Moral of the story....always make sure that you quadruple-check to make sure you've got it right first.

Are you sure you're just not Send and Return blind as well? ;)

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Originally Posted by Simon Barden

Are you sure you're just not Send and Return blind as well? ;)

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Hahahaha......you never know, I could very well be mate, it looks like I don't know my sends from my returns, which doesn't really surprise me since I've known for a long time that I'm easily confused...lol.


I'm just glad it turned out to be far easier to sort out than I had anticipated, didn't even have to take the chassis out of the cabinet, I just wish all electronic faults were as easy to find and fix as the FX Send socket was.

Update 6:


Decided to do a relatively minor mod to the Send and Return sockets in the Diecast box on the end of the FX Loop breakout cable, this basically involved soldering a 2M2 resistor from each socket's tip connection to circuit-ground, this was done to stop loud popping-noises coming through the amp due to a couple of coupling-caps in the FX Loop circuitry, one end of the caps was tied directly to the tip contact of their respective socket, this provided no path for built-up charges on the caps to leak to ground, adding the two 2M2 resistors now provides the path to ground without a detrimental effect on the signals in the FX Loop, I've tested it out and it is still working perfectly.

The wonderful static discharge abilities of the faithful 2M2 are sadly under appreciated.

I unsuccessfully tried to get a 2M pot today from Jaycar.... Seems RS is the only reasonable alternative...

Quote:

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Originally Posted by Marcel

The wonderful static discharge abilities of the faithful 2M2 are sadly under appreciated.

I unsuccessfully tried to get a 2M pot today from Jaycar.... Seems RS is the only reasonable alternative...

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For sure, I could have used a couple of 1M resistors, but when nothing is plugged into the send and return sockets, the two resistors would effectively be in parallel with each other, giving a load-resistance of around 500k, with two 2M2 resistors the load resistance is around 1.1M, both 500k and 1.1M would constitute a relatively light load to the signal coming from the preamp, but since the preamp signal is buffered by a 2N5087 PNP transistor configured as an emitter-follower stage, the loading would have been inconsequential, I went with the 2M2 resistors anyway.