Peavey KB2

[andyjazz]

I was gifted this amp. I don't have that much experience working with SS amps and although I have some electrical training and do some fix it work for friends I still consider myself something of a knuckle-dragger electronically, especially with some of the people here whose knowledge I am very admirable of. Anyway the amp just hums loudly when you turn it on. I first checked and resoldered the two main caps and a few other connections that looked suspect on the board. I have put a DVM on the speaker outs and have DC voltage at the speaker. I'm guessing it is the op amps that are fried? I'm guessing these are the two big ones with heatsinks attached? I'm just wondering do I check DC voltage acroos the legs of these things? Is that how you determine if they are faulty? Any help is greatly appreciated.

[Jazz P Bass]

What was the polarity of the Vdc on the speaker out.
That can be a clue as to the failure mode.
Transistors are usually checked for a shorted condition.
Use the diode check on your DVM to check the forward voltage drop of all junctions.
Link:Electronics Tips: Measurements: Testing Diodes and Transistors

[J M Fahey]

And disconnect those speakers *now* until everything is solved.

[andyjazz]

Thanks. I'll check these op amps with the diode checker for short. I forgot about the whole voltage drop thing with transistors. I learned a lot of this stuff in school but I just have never had any practical experience. I'll also call peavey today and get a schematic and parts list.

[andyjazz]

OK i pulled the transistors (Op amps) with the big heat sinks on them. I've been dicking aroung for an hour now on the internet trying to figure out what pins are which. As your looking at it with the printing facing you there is a black dot over the middle pin and the pins are from L to R: B, C, and E. According to an old book I'm reading the forward and back R's between E and C should be nearly equal and one is checking out ok. the other not. So I've found one bad part. Cool. Before I order a replacement I'll check out the other chips. Any suggestions? I'm very grateful for your help.

[Jazz P Bass]

The parts in question are Transistors.
They are Not Opamps.
Check the resistors around the failed device.
There are two Driver transistors before the output transistors.
Check them for shorts.

[Jazz P Bass]

Checking Transistors

According to an old book I'm reading the forward and back R's between E and C should be nearly equal and one is checking out ok. the other not.

What does forward / backward R being equal mean.
They should Not be equal.
Not even close.
I think the one that you acertain to be the good one is in fact the failed one.

Use your diode check function to check the transistors.

NPN transistor
Red Lead on Base. Black On Emitter.
Red Lead on Base. Black Lead on Collector.
Forward voltage drop is displayed.
Reversing the base/ emitter , base collector leads should read O.L.

PNP transistor.
Same thing with the meter leads (Black on Base)reversed.

[andyjazz]

I have this old book on electronic troubleshooting. The book describes a couple of ways one can test transistors. The book says:

"You can use the ohms function on a VTVM or high input resistance VOM to check transistors by measuring the emitter-collector, base-emitter, and base-collector forward and back resistances. A backward-to-forward resistance ratio of 100:1 should be obtained for the collector-to-base and emitter-to-base measurements. The forward and back resistances between the emitter and collector should be about equal." It also says: "It is advisable to make all three of these measurments for all transistors you check because experience shows that transistors can develop shorts between collector and emitter and still have satisfactory forward and reverse resistances for the other two measurments."

Maybe this is a dumb question but I've never used the diode checker on my DVOM. Does the "beep" mean O.L.? What is the V drop supposed to be? .7V? ok, got that on one but not the other. I'll probably replace them both anyway but it'd be nice to know what the fuck these things are sposed to read. As far as "forward" and "reverse" we're just talking electron flow right? neg to pos? God I feel really stupid having to ask these fundamental questions but...you know troubleshooting is completely different from knuckle-dragger electronic installation.

I'll do some checking on the components around these transistors. Gotta be a reason why they fried. thanks

[Jazz P Bass]

On diode check, a short "Beep" is a good sign.
A long beep is bad. That means a shorted condition.
The next step is to look at the meter.
What is displayed is the forward voltage that made that particular junction conduct.
Typically you will see .5-- something on NPN transistors.
A little less on PNP.
It may help to visualize the NPN base / emitter as a diode.
The base is the anode. The emitter is the cathode.
The same is true for the base/ collector.
When you use the diode check function of your DMM, the red lead goes on the anode & the black lead goes on the cathode.
This will result in a short beep & a forward voltage reading.
PNP transistors test the same but with reversed meter leads.
On my DMM I showed OL (open line) when checking collector to emitter or emitter to collector.

[andyjazz]

I've been digging around and I finally found the owners manual for my DM. It's a craftsman and the diode checker is set up a little different than what you described above. On this thing the "beep" means resistance less than 40ohms. When I put black on the emitter and red on base I get an "open" on of the tran's and 1.8v on the other. These devices have different #'s, one is 7815A and one is 7915A. The 7815 is the one that is open. So either it is designed that way or it's fried. On my schematic it looks like the 7915 is on the + side and the 7815 is on the - side. If I'm getting this right the transistor on the - side is fried and thats the problem. I've got another issue to deal with right now so I'll get to the surrounding components tomorrow. Does this sound like I'm on the right track?

[Enzo]

7815 and 7915 are not transistors, they are integrated circuits, specifically, 15 volt voltage regulators. They may look like transistors, but they are not and cannot be tested as transistors. However, if one tests shorted, that would be bad.

[Zipslack]

Let's simplify

Let's try to simplify things...

Put your meter on a high Ohms setting (20K would probably be good). Look for at least 2 metal things about the size of a nickel or 2 black plastic packages with a metal tab that are connected to a large piece of aluminum. The black things would look like bigger versions of the two regulators that Enzo identified. You can test and identify transistors with the ohmmeter setting by checking for resistance between legs and then rotating the leads ( red-black, then black-red).

Put the black lead on one leg and then use the red lead to check connection from the other two legs. Then move the black lead to another leg and check the other legs. Then move the black lead to the last leg and measure again. Two of the measurements should read low, the other four should read high. If you do not get the 2 and 4 readings, you may have a problem. Notice I said "may". If you do not remove the transistors, other circuit connections and components will affect your values, but you can probably still get a general idea. Here's a few links to peruse...

Powered by Google Docs

Transistor Test Type and Polarity

Meter check of a transistor : BIPOLAR JUNCTION TRANSISTORS

I teach high school students how to identify NPN and PNP transistors and identify the base leg using this method. After you do a few, it will make more sense. I would recommend you find a good transistor to practice with so you will have an idea of what to look for on the meter.

If the transistors check okay, I would say go back to your power supply section and check the 4 rectifier diodes and the filter capacitors. Diodes are easy, capacitors are much harder to troubleshoot with a DMM.

[andyjazz]

Thanks Enzo. I just downloaded the data sheet for the 7815 and the 7915 V regulators. One is a + regulator the other is -. I shoulda known by what I see on the schematic. Stuff was right in front of me and I didn't see it. They do make transistors that look similar to these things, but these are not op-amps or V followers or transistors. No wonder I was getting all this wierd shit with the diode checker.

I was really good at math in electronic school - not in identifying discreet components. Also you'd think they would have a schematic reading class in Vo-Tech huh? Not at my school, it was all about theory and math and commercial/residential wiring. Everything I've learned about reading schematics I've done by asking my brother-in-law (He's a radiotech for the city) and by reading books on my own and just figuring it out.

You think I should just order some from Digikey or should I order them through Peavey? Also, I know I'm not done because I haven't found the reason that these things fried. So stay tuned. Thanks to all for your help.

[Enzo]

ANdy, you are beiing misled. I have to point out to the well-meaning folks posting above that this amp uses a LM1875 to drive its tweeter, and an LM3886 to drive its woofer. So those won;t be in three-leg TO218 cases, or for that matter, three leg anything. There is a small push pull discrete amp for the headphones, runs off +/-15.

If there is DC on the woofer, your LM3886 is almost certainly shot. The two ICs I mention are indeed on the heatsink, but you won;t be making meaningful tests on them with a meter unless you find the output pin shorted to a supply pin. We still don;t call them op amps, they are full fledged power amp ICs.

The LM3886 is that 11-leg thing. I'd remove it, and check that there are both +20 and -20 volt power supplies getting to it.

I attached the powr amp diagrams for those who need them.

[Jazz P Bass]

Misled indeed!
Hopefully now we know how to check a transistor.
One of my first posts on this I requested what polarity the Vdc was on the speaker out.
Knowing this would lead me to check the 3886 output pin to the + or - power rail.