Ideas to salvage an old amp

[br8086]

hello all,
I've recently been given an 4-5 year old Fender FM65 Amp which does not work. There is some major problem with the electronics, and because i know very little about this kind of thing, i cannot diagnose the problem. i know that the speaker is in good condition, and i'm sure many of the electric components inside could be salvaged.
My question is this, what would i have to do to make the amp work just enough to act as a secondary speaker for my Fender Cyber Deluxe. I don't need tone controll, overdrive, or anything other than volume and a "power in" input.
I know how to solder and can read wiring diagrams, i just don't really know where to start. any help from someone who knows more about this would be greatly appriciated.

[n9voc]

The following is my humble opinions, these with about $5.00 will get you a cup of coffee at most restaurants!

Couple questions you need to address:
1) How much power do you want your new "Auxilliary Amp" to put out?
2) Do you want to use the power supply of the FM-65 to power the "New Amp" or do you want to add your own supply?

The answer to #2 will depend highly upon your electronics skills!

(I believe that it would actually take MORE skill to modify the old supply than to install a totally new one, especially if the Fender has only one board that contains all the electroncs, amp/effects/power suppy)


First, I recommend that if you have any electronically savvy friends, get them to help you with this project!

THEN:
I suggest that you remove the "Guts" from your FM-65, leaving the fuse, power switch, line cord, a volume contol, input jack and line cord intact.

Then I suggest you install a power supply (either modify the old, or build up a new from the old one's parts as well as new parts), and then build up a power amplifier module.

BE CAREFUL!!   The mains voltage can severely damage or even kill!!

There are several good power amplifier kits from Canakit.net that can handily provide you with the power amplifier module.  (also available from several other suppliers).  They and several other suppliers give you a board, components and heat sink for the power supply module section - makes life simpler than building the power amp section on a "vectorboard" solely from a schematic.

Then connect up the power supply assembly.  Then connect the input jack and volume controls to the power amplifier module and the speaker to the output of the PA module.

I do recommend that you get some "aluminum tape" and cover up ALL the extra holes that you will have in the front of your amplifier (to keep out dust and other "junk")

If you take a look at the "Schematics" section and some of the posts there, you will find some ideas for complete amplifiers - including the power supply circuits.  Since you don't need the high impedance input, you can insert a volume pot before the power amplifer module, and this will give you control over the output volume.

Final note - as you build, TEST each section before integrating it into your amplifier, it is a BUNCH easier to correct a miswired connection on a board on a bench than to take the board out of the completed amplifer and try and diagnose where the problem is!  (Yep, I still find miswired connections on my project boards, the final note IS the voice of experience!)

Good luck in your project!

[br8086]

thank you so much. that helps a lot.

to answer your questions, I'm thinking that around 60 Watts is what i'm aiming at.
and, I would like to use the power supply from the FM-65 if possible, and i can get other parts and "Frankenstein" it if needed.

I have a close friend who is an Electrical Engineering Major, so i'll get his help with this.

Thanks again!

[teemuk]

I don't need tone controll, overdrive, or anything other than volume and a "power in" input.

The features you require make up a major part of the amp: You need at least a power amplifier and a power supply. Have you seriously thought this thing through and considered all issues involved. To put if frankly, if you are planning to rebuild an amp from scratch because you do not have enough skills to troubleshoot a circuit then your chances of success are pretty poor.

Sure, there's plenty of stuff to salvage from amps: Speakers, transformer(s), chassis, cabinet, reverb tanks, those big supply capacitors, power transistors and ICs, switches, potentiometers, fuses and fuse holders, wires, knobs, bolts and screws, circuit board stands... pretty much all you can pull apart without destroying things. I just don't know why you should do that, as there's a good chance that the amplifier is totally repairable - even fairly easily.


Anyway, you can start by giving a better description of the fault the amp is having. "Major problem" tells nothing. You can start by telling us:

- Does the preamp work? (Do you get signal output from PRE OUT jack?)
- Does the power amp work? (Do you get sound when you plug into the PWR IN jack? This jack bypasses the preamp.)

Have you tried these tests; sometimes these jacks get corroded and result into intermittency (faint signal or no signal at all). The fault caused by corrosion usually goes away when this simple test to locate the problem is done. To get you started:

- Is there some special fault symptom, i.e. loud hum?
- Is the transformer making loud noise when you power up? Fuses blow repeatedly? These are signs of a short circuit. You can verify this by powering the amp up using a "light bulb current limiter" (you need to build yourself one). A "quick" test is to measure the resistance across common and each supply rail (with amp powered off) do you get a very low reading (few ohms) or something closer to few kilo-ohms?
- When you remove the speaker load does it end or decrease the symptoms of the problem?
- Are there parts that look faulty, i.e. burned resistors, leaking/bulging capacitors?
- Do the output transistors (complementary TIP142 & TIP147 pair) test ok (not short)?
- Are parts of the PCB either "charred" or discoloured? If yes, are the solder joints near those parts intact, how about the components? Can you figure out what part of the circuitry was overheating?
- Are solder joints near heavy parts (big capacitors and resistors) intact? This is a very common problem.
- Are solder joints ok in jacks, potentiometers and switches? Another common problem.
- Preamp regulators: Fender always builds them poorly, which causes plenty of trouble. Have they overheated, melted solder joints etc.
- See: http://members.tripod.com/NatCade/pages/tech_f2.html and the section titled "Overheating Solid State Guitar Amps". Any help with that?

Tests you can do when you have built the light bulb current limiter:

- Supply voltage tests: This is usually about the first test that you should make anyway... Rail voltage of the power amp should likely be about +-30V. Rail voltage of preamp about +-15V. Fender amps usually have test points for these. Likely you can follow the circuit (even without the aid of a schematic) enough to locate these.
- The "finger test": At idle (without input signal), is any semiconductor, IC or transistor, excessively hot. (Touch their case with your finger be careful not to electrocute yorself to something).

I'm not familiar with this amp so I can't offer more specific help. You can search this forum using a keyword "troubleshooting" to find some more stuff. A schematic would be nice but it is obviously rather useless at a point where you haven't given even a simple description of the fault/problem.


Now, I know that next someone will tell that a chip amp can be fitted inside the case very easily and building some chipamp kit doesn't require much special knowledge about power amp topologies and stuff like that. That is partially true. However, building something like that still requires at least basic knowledge of power amplifier electronics and all stuff involved (electronics in general, how amplifiers work, how power supplies work, how to select correct parts, make a good layout etc.) If you have that knowledge then there's a good chance that you have the knowledge for at least TRYING the troubleshooting before cannibalizing that amp. If it just doesn't work out, or if the circuit is completely FUBAR, then you can get a lot of useful stuff from that amp. Hopefully the power transformer is still intact.

[n9voc]

Good Advice from Teemuk above -- first try and see if you and your friend can T/S and repair the amp, THEN, if it can't be fixed, look into building a "new" amp in the old case!
If you decide to build new - I suggest a 3886 chip as the "heart" of the design.
Good Luck!