Fender Deluxe 112 plus bad buzz

[solderjoint13]

Im working on a Fender Deluxe 112 plus that a friend gave to me to repair. The amp kept blowing fuses and I suspected the output transistors. I de-soldered the output transistors and the two driver transistors. I checked with a meter and found that the output transistors were shorted. I ordered replacements and I have installed them. Now the amp doesn't blow fuses, but when connected to a speaker load I get a loud buzz on the speaker with no sound.

I've injected a 1Khz tone from my generator on the input of the amplifier and traced it using my oscilloscope and I notice that my signal gets weird distortions once I get past the preamp stage starting on the input of the MC1436p1 IC. The 1K signal gets blurry and I see what looks like little flecks that pop up all over. I assume that my preamp stage is good as I've got a clean signal on the last output stage. I've checked all of my voltages on the supply rail and I'm running at the correct voltage and I am balanced when comparing the two supply voltages.

My question is....besides the bad output transistors on the power amp section what else could be causing this? I suspect that the MC1436p1 amp IC might be bad, but I'm not sure and I want to check as much as I can before replacing that. IF anybody has any ideas let me know!!!

I've included the schematic.

[Enzo]

Nitpick: I hope you mean no signal on the output, since a loud hum is sound...after all.

OK, and welcome to the forum.

First off, disconnect the speaker and see if there is DC on the output. Considering your output stage was blown, it is a very likely possibility. That will damage a speaker.

No system works without proper power supply, so if your main rails are even and clean of ripple, we can move past that.

I assume nothing, so you are aware there are two kinds of outputs there, and installed appropriate new ones in the appropriate spots? NPN and PNP. And whenever output transistors die, ALWAYS check the associated resistors. Any of those 5w 0.47 ohm resistors open? And look at your bias string, are the two 4.7k and 2.2k resistors intact, and the four diodes?

A solid state amp is one massive feedback loop, so if the output is distorted in some way, then the op amp is going to try to "correct" it. So don't be too quick to blame the IC. It is also hard to measure signal right at an op amp input pin. It is more interesting to me whether there is DC offset at the output of the IC, pin 6. Probably OK, but the 2N4401/4403 are limiters, and they and their 1k base resistors COULD have been damaged when the outputs blew.

[solderjoint13]

Nitpick: I hope you mean no signal on the output, since a loud hum is sound...after all.

OK, and welcome to the forum.

First off, disconnect the speaker and see if there is DC on the output. Considering your output stage was blown, it is a very likely possibility. That will damage a speaker.

No system works without proper power supply, so if your main rails are even and clean of ripple, we can move past that.

I assume nothing, so you are aware there are two kinds of outputs there, and installed appropriate new ones in the appropriate spots? NPN and PNP. And whenever output transistors die, ALWAYS check the associated resistors. Any of those 5w 0.47 ohm resistors open? And look at your bias string, are the two 4.7k and 2.2k resistors intact, and the four diodes?

A solid state amp is one massive feedback loop, so if the output is distorted in some way, then the op amp is going to try to "correct" it. So don't be too quick to blame the IC. It is also hard to measure signal right at an op amp input pin. It is more interesting to me whether there is DC offset at the output of the IC, pin 6. Probably OK, but the 2N4401/4403 are limiters, and they and their 1k base resistors COULD have been damaged when the outputs blew.

1) Okay I checked the output going to the speaker and I'm not getting DC voltage to the speaker. I'm reading 0.17Vdc.

2) The negative and positive rails have a ripple of approximately 2.2V or so and has a strange pattern. Measurement taken @ TP3 and TP4 on schematic.



Could this be from the large electrolytics going bad? I did remove both of these caps and checked them with my multi-meter and one read around 4200uF and the other closer to 4700uF. Or could it also be that I may have a bad rectifying diode? Even so, the buzz I have is not controllable by any volume pot and it is buzzing with a hint of frying as well. Would this extra ac ripple cause this?

3) Would measuring the resistors on the board and getting a close measurement be enough to determine if the components are good? For the .47ohm 5W resistors, the two 2.2K and 4.7K resistors, and the diodes? Should I go ahead and de-solder them and check each component individually?\

I also wanted to point out that I have not powered this amp up without a light bulb in series with the amplifier.


I really appreciate the help!

[Enzo]

I am not worried about the resistor values being wrong, I am concerned they might be open.

[Jazz P Bass]

If the lamp limiter is not glowing brightly with the amp at idle (no signal) then I would ditch it.

Then remeasure the Vac ripple on the supply capacitors.
2.2Vac is pretty high.

[solderjoint13]

I am not worried about the resistor values being wrong, I am concerned they might be open.

I did not find any of the resistors to be open.

If the lamp limiter is not glowing brightly with the amp at idle (no signal) then I would ditch it.

Then remeasure the Vac ripple on the supply capacitors.
2.2Vac is pretty high.

Okay I'll ditch the limiter and check again. If the 2.2 Vac ripple is still present, what components should I expect to be in failure. Would other components beyond the power supply section affect the 40VDC supply rails greatly to cause this or does this mean that I may need to change out my two electrolytics and possibly the rectifiers. I also noticed that when I move the pcb away from the chassis ground (no continuity from the pcb grounding to the chassis) my strange ripple goes away and I get a maybe .2Vac ripple in a sawtooth pattern.

[g1]

I also noticed that when I move the pcb away from the chassis ground (no continuity from the pcb grounding to the chassis) my strange ripple goes away and I get a maybe .2Vac ripple in a sawtooth pattern.

Is the sawtooth showing double the frequency of the 2.2VAC ripple? If so, the 2.2V is 60Hz hum and not related to the filter caps.

[solderjoint13]

I removed the limiter lamp and check the supply rail again. I'm still getting the ripple but now the ripple's swing is not as large at around 1.2Vac. Is this normal for a small solid state amp, if so, I need to move on to figure out what else could be wrong with the amp. Any suggestions?

[solderjoint13]

I am not worried about the resistor values being wrong, I am concerned they might be open.

Okay I desoldered all the components you mentioned and all resistors are good and the avalanche rectifier diodes (cr5,6,7, and 8) read as follows: 0.489Vdc, 0.493Vdc, 0.491Vdc, and 0.492Vdc. Should the avalanche diodes initially read one value and then slowly climb to a stable voltage value? I haven't worked with these types of diodes so I didn't know if this was a correct characteristic of thier function. Example: Initial voltage reading 0.481 then final 0.491

[solderjoint13]

Is the sawtooth showing double the frequency of the 2.2VAC ripple? If so, the 2.2V is 60Hz hum and not related to the filter caps.

Yes it does look as though it is double the frequency. I noticed I would get the same pattern even with the amp turned off but not as high in amplitude. So you are right it is 60Hz hum.

[solderjoint13]

Ok guys I'm hoping that someone would give me some more help here. I literally laid this amp to the side and worked on some other equipment. I can run the amp without the limiter (lightbulb in series) with no speaker or load. All the DC voltages look good as far as I can tell. There is a slight ac ripple as I described above. I'm using a function generator on the input with a 1K sinewave and the gain lowered as far as I could get it roughly 200mV. When I check the input signal it is a clean and focused looking sinewave on my scope, but when I check the signal further in the chain, the sinewave gets fuzzy. I have also connected an audio probe to different sections in the chain and there is a noticeable buzzing past the first opamp stage in the preamp. I suspect that the opamp past the first stage then might be bad. Is this possible or should something else in the circuit be causing this issue.

Also, when I connect a speaker to the output and turn the amp on with the volume all the way down. The amp gives a loud squeal at full volume and I immediately turn it off. Once the power is turned off one can hear a crackling sound from the speaker and it slowly dies down in volume as the caps discharge in the amp.

If anybody has anymore suggestions I am all ears because I'm at a loss as to what to try next other than start replacing just about every semiconductor on the board other than diodes.

Recap:
When I received the amp the fuse was blown, a open coil speaker, and all the ouput transistors were bad. I replaced the fuse, all 4 output power transistors, and the two driver transistors (Q8 and Q9). After that I was able to run the amp without blowing a fuse.