Guitar amp repair help needed

[dubyayoung]

Hi, I am new to the forum, but it looks fantastic!

I just acquired a SWR Strawberry Blonde acoustic guitar amplifier, but it doesn't work (I knew it didn't work, so I got it very cheap!) When you turn the power on, it hums loudly and either pushes the speaker way out, or sucks it way in depending on the position of the speaker phase switch.

I found the R11 resistor badly burned, so I thought that must be the problem and replaced it, along with R12, since they are the same value, the package came with two resistors, and they were next to each other.

Unfortunately that did nothing to fix the problem.

I have attached schematics I received from the factory of the pre amp and power amp.

Does anyone have any thoughts?

 

[Nigel Goodwin]

The resistors were only a symptom, the fault is almost certainly the output IC, U1.

It will have gone S/C and be outputting a high DC voltage, which may have damaged your speaker.

 

[dubyayoung]

OK, I replaced the output IC, U1, and it still hums and makes the speaker extend, but it also pops loudly now, irregularly like popcorn. This is with the volume and gain down all the way. I didn't leave it on but a couple of seconds so as not to burn anything else up.

Any other thoughts, or is it just time to take it to an amp repair shop?

Don't get me wrong, I'll do any troubleshooting you suggest, but I realize it is hard to troubleshoot this without having it in front of you.

Wayne

 

[Nigel Goodwin]

Measure the voltages on the chip - in particular pin 14 should be zero volts, or very close to it. Also check for +45V on 7, 13 and -45V on 8, 15. I would also suggest changing the four small electrolytics round the output IC.

 

[dubyayoung]

Without changing anything, and using pin 4 as a ground, here is what I have:
Pin 14= 38.7 volts
Pins 7 & 13= 42.7V
Pins 8 & 15= -42.5V

 

[dubyayoung]

Don't know why I didn't think of it before, but yes, I can use the chasis as the ground (earth where your are?), but I just looked at the drawing and saw pin 4 went to ground so I used that. Yes, I'm a bit frazzled trying to see if I can make this a Christmas present or not!

 

[dubyayoung]

Oh, and I didn't connect the speakers, in case that makes a difference. I didn't want to burn up the speaker when checking the voltages.

 

[KeepItSimpleStupid]

With 14 so close a power supply rail, it suggests that the IC itself is faulty.

The voltage across pin 1 & 2 of J1 should be nearly zero. This will rule out pre-amp problems.

 

[dubyayoung]

KISS: Are you suggesting the IC I just replaced is faulty? If so what could have caused it to go bad as soon as I put it in? Or is that why you want me to eliminate the pre-amp?

I will check the voltage across pin 1 & 2 of J1 in the morning (PST) as the amp is at my work.

 

[KeepItSimpleStupid]

I missed the fact that you replaced the IC. Sorry. migraine tonight.

It also looks as if you can pull the P1 connector to eliminate the preamp and check the voltage at pin #14 again.

Check the power supplies 1st, including the +-15 V ones.

Capacitors can mess up the DC balance of the amplifier circuit and they are tough to check. At the very least see if the capacitors are shorted with an ohmmeter. Do this before powering up.

 

[Nigel Goodwin]

OK, the output is at one of the supply rails, and both supply rails are OK - which would usually signify a duff IC.

Check the voltages at the two inputs of the IC, pin 3 should be zero - and pin2 would normally be zero, but with the output high, pin 2 'should' be reading positive. If it reads zero, then R3 is probably O/C, and could be your problem - but it would be unusual.

There's really very little else in the circuit that could be duff and cause this, apart from the IC.

Do you have access to a scope?, if you do stick it on the output and see if it's oscillating?.

 

[dubyayoung]

Here's the latest:
I don't have access to a scope.
Voltage across pins 1 & 2 on J1 is .03V
I can't find a connector P1
J3 pin 1=14.96V
J3 pin 2=-14.85
Pin 2 on IC=0.0
Pin 3 on IC=0.0
R3 =21.6K (calls for 22K)

Here's the only problem I've found:
Checked all the caps, C7 appears to be shorted. 0.4 ohms in both directions!

Here's what's new:
While checking voltages, my probe shorted across a couple of pins in the middle of the IC, and a small blue spark emitted. I hope I didnt' fry it. I did buy two yesterday, so I have a spare.
I rechecked the voltages on pins 13, 14, and 15, and pin 14 now reads zero (13 and 15 still the same).
I plugged the speaker back in, and it still hums and extends , except at a somewhat lesser level than yesterday.

 

[Nigel Goodwin]

Here's the latest:
I don't have access to a scope.
Voltage across pins 1 & 2 on J1 is .03V
I can't find a connector P1
J3 pin 1=14.96V
J3 pin 2=-14.85
Pin 2 on IC=0.0
Pin 3 on IC=0.0
R3 =21.6K (calls for 22K)

Here's the only problem I've found:
Checked all the caps, C7 appears to be shorted. 0.4 ohms in both directions!

In post #5 you measured the voltage across C7, which was -42.5V, so it couldn't be S/C.

Try removing C22 - which may be S/C?, if there's 38V on pin 14, then there can't be zero volts on pin 2 (unless R3 is O/C or perhaps C22 is S/C - or you've got a short between pins 1 and 2 when you chnaged the chip?).

If you've shorted pins out, it's fairly likely you've killed the chip anyway.

 

[dubyayoung]

I apologize that you are dealing with a rookie here, so I'm sure I am making this harder than it should be.

You are correct about C7, I removed it and checked it off the circuit board and it is fine. When I checked them all before I just put the ohm meter leads across all the caps while they were installed and C7 was the only one that showed no resistance both ways.

I'm afraid that me killing the chip is what is causing the weird new readings, such as that now I have no voltage on pin 14 or pin 2. Do you think all of my readings are suspect now?

I am leary of installing my last spare chip if there is still a problem somewhere else, I don't want to lose this one too.

C22, are referring to the pre-amp section? I don't see a C22 in the power amp section.

 

[Nigel Goodwin]

I apologize that you are dealing with a rookie here, so I'm sure I am making this harder than it should be.

You are correct about C7, I removed it and checked it off the circuit board and it is fine. When I checked them all before I just put the ohm meter leads across all the caps while they were installed and C7 was the only one that showed no resistance both ways.

I'm afraid that me killing the chip is what is causing the weird new readings, such as that now I have no voltage on pin 14 or pin 2. Do you think all of my readings are suspect now?

Looks rather that way.

I am leary of installing my last spare chip if there is still a problem somewhere else, I don't want to lose this one too.

C22, are referring to the pre-amp section? I don't see a C22 in the power amp section.

Sorry, C2 (22uF)

Power amp section, the preamp is totally separate, and in no way responsible for your problems.

 

[dubyayoung]

OK, it's panic time, I'm taking it to a repair place. I really appreciate the help and efforts from Nigel and KISS, but I need this for a Christmas present and am too scared I am not going to make it.

 

[killivolt]

OK, it's panic time, I'm taking it to a repair place. I really appreciate the help and efforts from Nigel and KISS, but I need this for a Christmas present and am too scared I am not going to make it.

I came to the same conclusion with my Marshall. It's a digital mix with a distortion tube. When it went to hell, I thought I would just buy some chips for both channels. But, my gut is telling me to take it to the Pro-Audio guy's before I do; unnecessary damage.

I would think that if I could find someone with a blown Speaker and a good head. I would be far and above the cost of repair. Audio is like a serpent if you don't latch it close to the head it will bite you.

kv

 

[ben7]

I plugged the speaker back in, and it still hums and extends , except at a somewhat lesser level than yesterday.

I actually semi-built a TDA2005 stereo 20watt amplifier, one channel works, and the other one puts about 11volts on the speaker!

I am sure that there are no problems with the circuit, it is still sitting in a box half-finished! :/

-Ben

 

[unclejed613]

C2 is your DC correction cap, if it's leaky i.e. acting more like a resistor than a cap), it will cause offset. since pin 14 is a few volts less than the power rails, it's not likely that the new chip is shorted. if it were shorted it would be very close to the rail voltage. something is driving the output into offset, as i said C2 being bad is the most likely suspect here. the noninverting input goes back to the preamp through a mylar cap, which although i have seen them fail, is much less likely, so unless C1 is shorted, i would rule out a preamp problem. C3 being leaky is a possible problem, but would be more likely to cause problems with signal present, so C2 would still be my first choice.

also recheck your connections to pin 7 and 8. these are the power for the input stage, and loss of one of these rails could drive the output towards one of the rails

 

[dubyayoung]

I wanted to get back to the board with the final disposition of the amp. It took the repair guy a month because he said it was hard to find the right parts because they used off values and sizes. Whatever, I can't verify that, but at least I have it back now. I wish I would have saved the parts he replaced so I could tell you all what he did, but my brain wasn't thinking when I got home and I threw them in the trash.

He replaced the two big caps, I'm pretty sure just because they were "leaking", not completely bad, one yellow cylindrical cap (may have very well been the C2 cap unclejed was talking about), and at least one diode.

The two big caps were the ones that appeared to be hard to find, as once he found the right value they were not the same size, so he had to do an alternate mounting of them to finish the repair.

Thanks for the help. Sorry I couldn't stick with you to finish the repair here. As it turned out, time wise, I probably would have been better off letting you all help me through it!

-Wayne