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Fender Amp Power Supply Problem
By Gary | June 18, 2008
This video found on YouTube is a perfect example of a filter cap problem. On solid state units like this one, through vibration and assembly techniques, components can work themselves lose from the circuit board. The owner greg1485 from this video literally has his finger on the problem.
In a perfect situation with a trained tech this would be a quick repair but for someone with no experience or just starting out it would take awhile or it’s time to take it to a shop. What you’re hearing is when he applies pressure to the long barrel cylinder looking things, is non-rectified (AC) getting into the audio circuits via the power supply feeds.
This is the main job of the power supply capacitors, to smooth the AC being rectified by power diodes that are looking at the secondary of the power transformer.
If I remember correctly on this unit the entire circuit board would need to come out of the chasis. This is done by all knobs and nuts and lock washers on the front. Screws holding the board from the top and screws on the bottom holding the power transistors heat sink in place. Pull the board up and resolder the caps to the board. (may have some wires to disconnect also, so you would make a note where everything is connected)
Replace the mica thermal film on the power transistors. Clean and replace the heat sink compound, carefully reinstall the board. Triple check your work was done properly. (a quick check with your volt/ohm meter from the output transistor to ground should tell you everything is ok. For instance you should have a resistance reading from the metal of the transistor touching the mica film, to the chasis. How much can vary, the important thing is not to see zero ohms)
Bring up unit on a variac slowly to check your work one more time. Inject signal and watch for perfect sine wave on the scope, then test out the amp with a guitar. You can always do a power check to make sure the amp is ready to go.
And wa-la you have a happy customer!
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Topics: amp repair (Fender) |
June 24th, 2008 at 2:54 pm
Hi Gary and what a cool site I want to Thank you for the effort and time and info that you are sharing with me and tube heads like me I am looking forward to your newsletters
I also wanted to ask you about adding an extra tube to the pre amp circuit do you have any info on this subject or know of any sites or books or any where to find out about this their is no info that I can find and have emailed several people about it with no response and I will add that I do not mind paying for any kind of knowledge about this subject and anything involving tube amps and the expansion of my knowledge of them I have been tinkering with tube amps for the last five years and I know alot about them but still have a long way to go any info on this would be very helpful in my quest for my ultimate tone
PLEASE RESPOND and Thanks
Dan still tubing in Indy!!!
June 24th, 2008 at 3:57 pm
Hello,
Thanks for commenting on my site. Feel free to use the blog to leave a post or I can always paste this email there if you don’t mind, I won’t use your real name..it helps build traffic…
I assume your extra tube is for more gain? Let me know what you want it to do…are you adding it to an existing amp or building it from scratch? Probably the reason you’re not getting many answers at this point it the question is too vague. I have friends that would use the older fenders with the 2 channels and gut the bass channel. Keep in mind when you add a tube you’re gong to tax the power supply so if you get an amp with it already in place like an old fender you’re doing well. If you add a tube to an existing amp, you have to make sure the supply has enough gas and regulation to take it on, especially in the high gain area…if you build from scratch you can figure all this in.
I’ve been reading the book that is in my resource area…The guitar amp handbook..a guy over at groove tubes turned me on to it after I did a review of their mod output tube plugins…it won’t tell you how to do a mod but when you get through with it, you will know what the ciurcuits are doing in an amp (except for the power supply, he doesn’t go into that,,people can hurt themselves if they don’t know what they’re doing)
Another thing is to study schematics of amps that you like after you read the book above and compare circuits and tinker…
And then you will get into the black art of components which this book does also…and then you can sit online and argue with people LOL
Hope this helps,
Gary
June 29th, 2008 at 9:10 pm
Hi Gary and thanks for the quick response. The tube amp hand book is already in my library I am actually rereading it right now I am wanting to add the extra tube in my marshall 800.
I know the transformer can take it I just need to know where to attach the connections to I already know to tie the heaters together to the closest pre amp tube the other connections are where I am lacking the knowledge I have a general idea but am still not positively sure and do not want to cause any damage any tips would help Thanks Dan still tubing in Indy!!! Ps.
I also know that I will be using only one side of the new pre amp tube the plate pin1 the grid pin2 the cathode pin3 and the heaters pins 4-5-9.
Thanks,
Dan
July 12th, 2008 at 2:41 am
Hello Dan,
Don’t take this wrong because I’m only speaking from experience…I wouldn’t experiment or learn your chops on a Marshall 800 that is working…one little mistake and it won’t be working…..now since it’s in the preamp section you probably won’t do much harm, like take out a transformer but you possibly could….
What you’re talking about is the input grid, plate and cathode…look at tube theory more in your book. You’re getting there but I would find another old tube amp or something on eBay and build a prototype of what you want to do…you will learn tons of stuff by doing this.
You know how to twist the wire and hook up your heater. Now you have to decide phase on the tube, this will determine how you hook up the input to the tube, and how you hook up the plate and cathode…the components around the tube will determine what it does and how..(also a tip) I’m not sure if the tube is already in place that you’re going to use but anytime you modify you can induce noise so beware. …there will be test equipment also that you probably don’t have…like a variac to slowly bring up your voltage so you don’t blow something….
since you’re theory may have a ways to go, you might be the type of tech that experiments by replacing one component at a time..take careful notes…see what each one does…for example with the book you’re reading…build a circuit that he talks about and change values of resistors and see how it effects the sound….
Again I admire you enthusiasm, but I wouldn’t hack on your marshall till you know what you’re doing…
my 2 cents worth…
Gary
September 24th, 2008 at 3:52 am
Hello,
I love the site, you helped me through fixing one of the two problems with my fender performer 1000… sticky pots. It still has it’s major problem however…
When plugged in, the amp powers up just fine. Unfortunately, when you plug a guitar in to the input jack, you get almost no sound what-so-ever. Only when everything is cranked, and i mean on 11, do you hear a faint sound of the guitar playing. I used to get around this by plugging in through the return jack in the rear, but this no longer works.
Any suggestions?
Thanks,
Andy
September 24th, 2008 at 1:58 pm
Hi Andy,
This one “could” get involved. You have 2 problems at this point I would say being an internet tech instead of having it on my bench.
If it’s a simple repair then both of your jacks are bad or need to be resoldered where they connect to the circuit board. (It’s been awhile since I’ve been in a fender performer 1000 so I could be wrong if it’s not pc mount or transistors for that matter).
If you can get in there and eyeball where the jacks meet the circuit board you might see the problem. OR
your jacks might need replacing, they could be bent up inside. OR you have a power supply problem for one and a bad jack for the other OR in the front end there is an IC or amp component that is bad. IF it’s tube make sure they are lighting up if they are you might have lost your plate voltage, (open resistor or it’s pulled away from the board).
Without proper troubleshooting equipment, about the only thing you can do is eyeball the board and you may see the problem or replace the jacks if you can see that they are gimped up inside. This usually starts by the guitar cord jack not snapping in properly or you have to wiggle it to work.
Hope this helps…
Gary