Peavey KB100 - overheating power transistor

[52 Bill]

Power supply voltages look OK. I'm reading 33.5 VAC per side at P104/106 (red wires), and 18.8 VAC per side at P107/109 (orange wires).

AC readings are good to know, but what is more important for us to know are the DC voltages.

Injecting the signal straight into the power amp in does the trick- no hum, normal output.

Then the problem is in the pre-amp section. First check the low voltage supplies. Plus and minus 15-16 volts.

I'd suspect the last IC in the pre-amp, just before the power amp in, as that is where you had mis-plugged one of the wires from the power amp board. I don't have a schematic for your amp so I can't tell you what IC it would be, but follow the circuit path from the output connector that you had plugged in wrong and you'll find it.

[dissonance]

I've got a stable +/-24.3 VDC, and a stable +/-15.6 VDC. I also checked pins 4 and 8 on all the op amps, and they've also got a stable +/-15.6 VDC.

I think the IC you're referring to is U5. The +/- supply to it is fine, but I'm not sure what else to look for.

Thanks for the feedback!

[Enzo]

Isolate the problem. DO the EQ controls affect the sound of the hum? If not, then the hum is from after them. U5 is a great place to start. Look at the output pins, pins 1 and 7. Is there DC voltage on either? Shouldn;t be.

ANd there may not be anything to measure, the IC could have an internal problem. ICs are cheap, replace it.

Reverb control has no effect, right?

[dissonance]

Neither the EQ or reverb has any effect on the hum. As far as U5 is concerned, I've got zero VDC at pin 1, but I do see about 2.9 VDC at pin 7. I'm going to track down a 4558 or equivalent and swap it out.

Thanks!

[dissonance]

So, I went through and swapped out U5 as well as every other 4558 op amp on the board with a replacement part, and the problem persists.

I did retrace my steps to try and determine exactly what I did when I had the connectors on the output board mistakenly swapped. Since there was only one other way that I could have connected them, it appears that the end result was that I dead-shorted the speaker output coming off the main board.

[52 Bill]

Since there was only one other way that I could have connected them, it appears that the end result was that I dead-shorted the speaker output coming off the main board.

And that is why the power transistor overheated and failed. Changing out all of the opamps is called the shotgun approach. Sometimes it works and sometimes it's a waste of money and time. And sometimes it can cause more damage than it cures.

As Enzo suggested earlier, try and isolate where the hum is coming from to find the problem. If the power amp is putting out a clean strong signal, then the problem comes before the power amp input jack.

If none of the controls cause a change in the hum, then it could be after the first stages of the pre-amp or it could be power supply related. Look at the schematic and try to follow the signal path through the amp. If none of the controls are involved what sections are left to check? Is there a limiter or compressor stage? An FX loop? Have you tried to send the signal out from the pre-amp and send it to another amp?

Have you tried to see if there is any ac ripple on the dc power supplies to the pre-amp? You may have a bad filter cap in the pre-amp power supply.

[dissonance]

Thanks. I can measure some AC at all of the filter caps, but I'm not clear on how much is too much. I've searched around trying to find a definitive procedure for measuring ripple in power supplies, but haven't turned up anything useful. I'm seeing about 250 mVAC p-p on my scope when measuring at the the filter caps for the pre-amp. Is is it better to just use a meter for this test, rather than a scope?

[52 Bill]

I've searched around trying to find a definitive procedure for measuring ripple in power supplies, but haven't turned up anything useful. I'm seeing about 250 mVAC p-p on my scope when measuring at the the filter caps for the pre-amp. Is is it better to just use a meter for this test, rather than a scope?

I don't think that it matters meter or scope, just what you have and prefer to use.

A quarter volt of ripple in the pre-amp seems high to me, does this amp have Zener or IC regulators in the power supplies?

[Enzo]

I am not too concerned over a quaret volt of ripple.

Scope is fine, or set your meter to AC volts and measure the DC supply with it. On AC it ignores the DC and just reads the ripple.

[dissonance]

@52 Bill: It uses Zener regulators.

I just measured again at filter caps C149 and 153 using a meter. I'm seeing 123 mVAC at the positive lead of C149 and negative lead of C153, and about zero on the other side of each.

[Enzo]

Well the other end of each filter cap is ground, so I'd expect very low ripple there.

[dissonance]

Here's a question: Should the ripple increase when there is a signal driving an input? In other words, all of my measurements up until now have been with the amp at "idle", and nothing connected to any inputs. Today I decided to connect a signal source to one of the inputs and take some measurements (I just used a synthesizer generating a constant tone). I found that I could measure almost 1 VAC of ripple at either of the large filter caps. Ripple remained unchanged at the pre-amp filter caps, however. In addition, I also noticed that for any given settings on my scope (when AC-coupled), the waveform appears the same when probing at the main filter caps as it does when reading at the + speaker output on the board. Should it?

[Enzo]

Look at the schematic. This amp uses "flying rails." The emitters of the outputs are grounded, and the speaker lead is the junction of the filterr caps. Yes, your signal will appear on those caps.

i reserve the term "ripple" for the 120Hz bumpiness from the rectified AC only.