Newbie seeking transistor Advice

[Appaloosa]

Hi, I have a fender Bassman 200 combo. the amp works for a while then quits. if you shut it off and turn it on again it will work again for a bit.

I opened the amp up and the only physical problem I noticed was some indication of heat on the power amp board under Q11 and Q10.

I pulled these transistors out. they are both MPSW42 NPN transistors. I tested them like a simple diode. And they seemed to pass the test but on Q11 I got different resistence values between the base+collector and Base+emitter. When I tested Q10 i got the same values (about 800ohms) from B+C and B+E.

Do I have a bad/failing Q11? Or is this OK?

Any advice is greatly appreciated

[Jazz P Bass]

The transistor that is reading 800 ohms is bad,
You should be getting a reading in the mega ohms B/E, B/C.
Just so we are on the same page.
Q10 is listed as a MPSW92/ PNP transistor.
Q11 is a MPSW42/ NPN transistor.
They are both rated at 1 watt.
http://www.onsemi.com/pub_link/Collateral/MPSW42-D.PDF
http://www.onsemi.com/pub_link/Collateral/MPSW92-D.PDF

[52 Bill]

I'm not sure that the transistor is bad. I'd say retest or explain what you did better.

I've seen these go into thermal cycling when the outputs get too hot. In one case it was just loose screws on the heatsink. Run the amp and keep an eye on the temperature of the heatsink. If shouldn't get hot unless you're really driving the amp hard. If it gets hot, then check the usual suspects like bias adjustment, bad parts, loose screws, etc.

[km6xz]

Without knowing the input impedance of the meter and the resistance scale terminal voltage, the readings don't mean much. It is doubtful either of those transistors is faulty. There is no need to pull transistors out of a circuit to test their operation, their operation can be determined by measuring their operation in circuit. Since the symptom is time, temperature or some other variable related, passive testing of components will not get you any closer to the proper diagnosis. Take measurements during operation, before and after the fault occurs.
In what way does it stop working? Is it power dead, or still has LED indicators on but no sound, on but no signal although a little hiss? It the preamp still working, as tested from the effects send jack. Can you inject a signal into the return jack and get a signal through the power amp when the amp otherwise seems dead?
My advice, not just to you but anyone working on gear, don't start pulling parts out unless you prove there is a reason to. Pulling or swapping parts before a diagnosis is determined just adds to confusion and introduces more problems. Repair attempt induced problems are harder to track down than normal problems. The amp, all assembled, can offer the best clues just by observation and a few measurements.
What test instruments do you have available?

[jmaf]

If he is using the diode scale the 800 he's getting is probably mV not ohms.

[nashvillebill]

Granted, I don't claim to be an expert, but if the amp operates fine cold, then quits when warm...I'd try running the amp till it shuts down, then hit components one by one with freeze spray (with power still on of course).

[Appaloosa]

Sorry for the delay, due to my work I only get a few precious days to get to my computer and projects. Thank you all for your tips.

I followed your advice and put the old transistors back in. I powered up the amp with the chasis removed from the cabinet. The amp is sounding and running fine for 30 min. so far. I notice no tempurature rise on the main external heat sink.

I do notice however that Q10 and Q11 get very hot. I can only hold my finger on them for about 2 seconds before it gets too hot for touch. I also notice some darker coloration under these transistors on the board.

Would running the amp with chasis removed keep the tranistors just cool enough before failure vs. in the cabnet?

I assume these transistors are running too hot? what would cause this? I tried testing the voltages around the transistors and I seem to only get 57 VDC. Acording to the schematic i think I should see about 48 VDC at Test Point 11 and Test Point 12. Its hard for me to tell where exactly to probe for this voltage cause with the board in the chasis I can only view from above, but I get 57 vdc on both sides of R14 for example, I think I have a problem here? could someone verifiy this?

I tried to check the souronding resitors in circuit and they seem to be within tolerance. I did the same with the zeener diodes, but the only way I know how to test these is to turn my DMM to diode mode which is 2kohm scale and test both ends of the diode. on each diode I get from .680 to about .750 in only one direction. is this a good test method? I also noticed while I tested in this manner the speaker would pop once and a while when I probed some of these diodes with the AC line cord unpluged, is this normal? caps discharging or something?

For testing tools I also have a Fluke 23 and a Fluke 77, that I have never used. I belive they have some graphing ability is their something I can use on these meters that would be better for testing then the basic non auto rangeing one im using? what tools should I get? scope? signal generator? can I build something useful?

Does any one know of a good electronics basics or troubleshoot book/manual they would recomend for a self tought novice like myself?

thanks again for any advice!

[Appaloosa]

Granted, I don't claim to be an expert, but if the amp operates fine cold, then quits when warm...I'd try running the amp till it shuts down, then hit components one by one with freeze spray (with power still on of course).

Can you explain this Freeze Spray method further? I have never heard of this. thanks

[gbono]

Electronic Troubleshooting by Olesky

[nashvillebill]

Can you explain this Freeze Spray method further? I have never heard of this. thanks

The theory is that if a component is failing after it reaches operating temperature, but works OK when cool, then the freeze spray can be used to methodically check suspect coomponents. If the circuit starts working when a particular component is cooled back down, logically it must be that component.

Well that's the theory at least! I suspect having a good schematic on hand, showing the expected voltages at certain locations, could at least point one in the general direction and then the freeze spray perhaps could narrow it down.

[Jazz P Bass]

If TP 11/ Q10 (+48Vdc) & TP 12/ Q11 (-48Vdc) are not true then, yes. something is wrong.
Both of these transistors are "feeding" Q9.
What is the voltage reading at TP 13 (-.473 Vdc)
The readings that you took & the fact that Q10 & Q11 are getting hot (1 watt transistors) leads me to suspect one or more (Q9,10,11) are shorted.
As to the DVM, use the Fluke for transistor checking (set it on diode check)

[Appaloosa]

The theory is that if a component is failing after it reaches operating temperature, but works OK when cool, then the freeze spray can be used to methodically check suspect coomponents. If the circuit starts working when a particular component is cooled back down, logically it must be that component.

Well that's the theory at least! I suspect having a good schematic on hand, showing the expected voltages at certain locations, could at least point one in the general direction and then the freeze spray perhaps could narrow it down.

What do you use for the actual spray? Aerosol air?

[Jazz P Bass]

Tech Spray 167210S Envirotech Freezer - 10 oz.

[Appaloosa]

If TP 11/ Q10 (+48Vdc) & TP 12/ Q11 (-48Vdc) are not true then, yes. something is wrong.
Both of these transistors are "feeding" Q9.
What is the voltage reading at TP 13 (-.473 Vdc)
The readings that you took & the fact that Q10 & Q11 are getting hot (1 watt transistors) leads me to suspect one or more (Q9,10,11) are shorted.
As to the DVM, use the Fluke for transistor checking (set it on diode check)

So maybe this is a dumb question but what's the best way to track down the short? Is their a good way to test Q9 in circuit? When I get back to the amp I will see what my test point 13 voltage is. Thank you

[Jazz P Bass]

Check the voltages of Q10 & 11 while you are in there reading the base of Q9.
Look at the schematic.
Q10 is a PNP.
A low voltage on the base will turn it on.
The collector is at +57Vdc.
The base should be at +45Vdc.
That means that Q10 is just barely turned on.
So, the emitter should read just barely on.
The same is true for Q11 (negative voltages/ PNP transistor)
If this is true, then how can Q10 & 11 get hot.(remember they are spec'd at 1 watt dissipation devices)
They are barely on.
Use your Fluke to read the transistor junctions.
Base to Collector & Base to Emitter.
With the Fluke on diode test, you will see .600 (about) on a good junction.
Collector to Emitter should read nothing.(out of circuit)
By the way, this is the "bias" circuit for the output transistors.
When everything is correct the bases of the outputs should be "barely" on)