Servicing a Fender SRM 6302 PA Amplifier.

[DrNomis_44]

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.

[DrNomis_44]

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.

[Marcel]

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

[DrNomis_44]

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

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

[DrNomis_44]

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.

[DrNomis_44]

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:




A Closeup Of The New Cable Gland I Installed:



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:



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:



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:



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.

[DrNomis_44]

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.

[Dedman]

Glad to hear the patient made a full recovery Doc!

[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.

[DrNomis_44]

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.

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.