Amp makes high picth sound but not feedback

[wizard333]

A bad filter cap can cause that. Not many people know about that one but I've seen it several times.

Something gone wrong in the negative feedback loop would also be a suspect. check the feedback resistor and make sure its not gone open or otherwise damaged. Check for bad solder joints etc.

[52 Bill]

A bad filter cap can cause that. Not many people know about that one but I've seen it several times.

Yes, this can be true.

Jam: When you are testing the amp after replacing the outputs, are you bolting down the heatsink bar and the transistors? What brand transistors are you using as replacements?

From my own personal experiences I always remount the transistors and the heatsink bar after replacements, even for testing, as all of the Fender SS amps can quickly kill outputs if they aren't heatsinked.

As Bob mentioned, the outputs are very high gain and should be matched. If you must replace singles, at least try to use matching brands to the originals.

[Jam]

stage 160

I bolting down the heatsink bar with two screws and the transistors. I got the parts from Mousers (511-tip147 +142) When I took at look at it only Q15 is bad. I did notice that I didnt tighten the transistor all the way would that cause it to get to hot and damage it? What do you mean should be matched?


Which ones would be the bad filter caps and feedback resistor ?

[52 Bill]

I did notice that I didnt tighten the transistor all the way would that cause it to get to hot and damage it?

Yes, they can overheat very quickly.

What do you mean should be matched?

Whenever multiple transistors are paralleled the power load should be shared equally by all of the transistors. If one transistor is different from the others, it could either do too much of the work or not enough of the work. This mismatch will either cause the new transistor to work itself to death, or will cause the others to work too hard and die.

[Jam]

Stage 160

I replaced Q 15 that was bad when I was done turned it on and it flew the fuse. I tested the transistors and they test out ok any idea?

[52 Bill]

I replaced Q 15 that was bad when I was done turned it on and it flew the fuse. I tested the transistors and they test out ok any idea?

There is still something shorted in there. It could be a power supply diode, a driver transistor, bias diode, etc.

Carefully double check all of the outputs for shorts, then check the drivers Q10 & Q11. Also check the two protection transistors Q12 & Q13.

Be sure to also check all of the diodes in the power amp section. Have you been checking the big emitter resistors?

If you can't find anything in the output section, check the 4 power supply diodes for the power amp D54 - D57. It would be unusual for a filter cap to be damaged from this but one could be shorted, so if you can't find anything else, check C63 & C64 as well.

[Jam]

stage 160

The big emitter resistors are the six big white like r94 and so on whats the best way to test them? One more thing last night when I blew that last fuse one of the wire from led light on D64 was broke off with one end broke off could that open something up or short something out too?

[52 Bill]

The big emitter resistors are the six big white like r94 and so on whats the best way to test them?

Use your ohmmeter. You may not be able to get a true accurate reading, but what you need to know is have they been damaged by the shorted outputs and are they still close to the rated value 0.47 ohms.

One more thing last night when I blew that last fuse one of the wire from led light on D64 was broke off with one end broke off could that open something up or short something out too?

Depending upon which wire was broken, it could have caused the fuse to blow, but probably would not cause anything other than the LED or D67 to be damaged.

[Jam]

stage 160

When you check the drivers Q10 & Q11. Also the two protection transistors Q12 & Q13 are they test with the diode setting? and all these part do I need to take them out to test them?

Ok after last night I tested the 6 power transistors and now Q15,17,19 are reading bad if one is bad does it burn out the other . I dont know what iam doing wrong. Could something else cause them to burn out ?

[52 Bill]

When you check the drivers Q10 & Q11. Also the two protection transistors Q12 & Q13 are they test with the diode setting? and all these part do I need to take them out to test them?

Test all transistors and diodes using the diode test function. Test them in circuit and only remove them if you can't get a reliable reading in circuit. (See the answer below.)

Ok after last night I tested the 6 power transistors and now Q15,17,19 are reading bad if one is bad does it burn out the other. I dont know what iam doing wrong. Could something else cause them to burn out ?

All of the outputs are connected in parallel. The only thing that separates them are the emitter restistors. When one shorts it will make them all read like they are shorted. When one shorts, it will place an abnormal load on the other transistors so it is possible that another one can be damaged before the fuse blows and kills the power to the amp.

When you test them, test them all. If one is shorted they will all test as shorted. The usual short is between the collector and the emitter, these are the center and right side pins as you look at them from the top. Unsoldering only the center pins of the transistors on the side that shows a short, will allow you to test the transistors without having to remove them all. When you find the shorted one (or ones) remove it and replace it with another of the same type and manufacturer (if possible). Or as we had discussed before remove them all and replace them with a matched set.

Be sure to clean off all of the old heat sink grease from the transistor, the heatsink and the mica insulator. Or if your amp has thin grey Silpad insulators, carefully inspect them for tears or cuts. If your amp has the mica insulators, install the new transistors with new heat sink grease, reusing the old insulators.

Pre-bend the leads of the new transistor to fit the pc board. Coat the back of the new transistor with a thin coat of heat sink grease. Next place the mica washer on the back of the transistor using the grease to "glue" it in place. Then apply a thin coat of grease to the back side of the mica insulator.

If you have Silpads, just clean them carefully with a soft rag and reuse them.

Finally install the transistor to the heatsink and screw it down with moderate pressure. A simple test to do after installation is to use your meter and check for resistance from the center collector lead to the metal heatsink. If you get a zero or a low ohm reading, you will need to check the insulator washers, etc.

If you have one, this is the perfect place to use a light bulb limiter when you power up the amp again.

[Jam]

light bulb limiter

How do you make a light bulb limiter ? and how does it work to test gear?

[52 Bill]

How do you make a light bulb limiter ? and how does it work to test gear?

Search the forum or google it there are lots of instructions for these.

All it basically is, is a standard light bulb wired in series with an ac socket. You plug the test amp into the socket and if the amp draws too much current the light bulb lights up and limits the current going to the amp.

By limiting the current to the test amp, you can save the cost of a blown fuse, and it allows you to test voltages to the circuit to find out where a circuit fault is.

It won't work for all amps, but it can be quite helpful to have one around and it doesn't cost much to build one.

[Jam]

stage 160

So I test the transistors with the prongs out and only had one bad one to replace what do you think I should do before I power up?

How do you know if the light bulb deal will work with a amp? what watt of bulb? Sorry for all the questions THanks

[52 Bill]

So I test the transistors with the prongs out and only had one bad one to replace what do you think I should do before I power up?

Replace the bad one. Check all of the parts that we've been discussing Transistors, Diodes, etc. Reassemble the amp, check for any loose or broken wires. Plug it into a light bulb limiter and turn it on.

How do you know if the light bulb deal will work with a amp? what watt of bulb? Sorry for all the questions THanks

You try it. The only amps that I can think of that don't work well with the light bulb limiter are the SWR amps.

60-100 watt incandescent.

[Jam]

Stage 160 with light bulb limiter

Ok I made a light bulb limiter and then I double checked some parts on my amp. So I hooked it up the to the light bulb limiter and it was weird it keeped going bright then dim then bright so I was checking voltage on the 6 + and -rails seem like I was getting the - side but the + side was reading very low and seem like my reading were up and down