SWR workingmans 15 Frying resistors

[bajarage]

A good friend of mine decide that his working man 15 should power his entire collection of speakers while nearly maxed out, it worked for about 5 min. afterwards it blew, on schem 12/15/2004,

R28,R69,R41,R38,R27
D10
C9,C15
Q3,Q9

after replacing all of this, when i turn it on with no load(its been removed from its combo) it burns thru R41 until its burnt then thru R28.
I am at a loss, and if I fix the amp I get to keep it, any insite or steering into the right direction would be greatly appreciated

[Jazz P Bass]

Swr WM15

Did you check Q4 & Q10.(output transistors)
Sounds like they are shorted.
Check Q5 while you are atit.(bias transistor)

[Enzo]

Yes, first look up "light bulb limiter" and make one for your service bench.

And continue to work without a load until we are certain there is no DC on the output.

Your replaced the drivers Q3,9, but how about the outputs, Q4,10?

And check R29,42, those 0.1 ohm 5w power resistors.

This is important. When you replaced R69,27,38, were they actually bad? Or were you just replacing them hoping to find the problem? If they are burnt out, then surely Q5 can;t be healthy.

I really cannot imagine that C9, 15 were bad.

If R41, 278 were bad, I have to worry that R40 is too.

[bajarage]

I only replaced what was visually burnt.
Have not replaced the outputs since I couldn't see anything wrong with them.
I checked r40 it ohmed @100.
Thanks for the quick response, will restart testing after work tomorrow
Also I forgot to mention, the last time it fryed. R41 started to burn then only after it had burned and popped did R28 cook

[Enzo]

Ever hear anyone come from a funeral and claim that the dead person in the casket "looked so life like."

You can see burnt parts, sure, but many many many failed components don;t look any different. You have to get out your meter and other test equipment to find bad parts.

When resistors burn up, there is ALWAYS something else causing it. Usually a bad semiconductor.

[bajarage]

I removed q10/5/4 and this is what I got off of my dmm
Q10 w/ positive on B on both C&E i got 1.5 ohms
Q10 w/ negative on B on both C&E i got 2.2 ohms
Q10 from C to E nothing on B i got .5 ohms

Q5 w/ positive on B on C i got .2 ohms; E i got .1 ohms
Q5 w/ negative on B on both C&E i got .6 ohms
Q5 from C to E nothing on B i got .5 ohms

Q4 w/ positive on B on C i got 324.5 ohms; E i got 15.95 Mega ohms
Q4 w/ negative on B on C i got 324.5 ohms; E i got 15.95 Mega ohms
Q4 from C to E nothing on B i got 15.87 Mega ohms

I am fairly new to electronics diag& repair but if what ive read in the past is correct all of the transistors are bad Q5 and Q10 being shorted with Q4 being shorted with high resistance

Please correct me if I am wrong

Thanks

[Jazz P Bass]

SWR WM15

Checking transistors using resistance measurements is o/k, but it would be better
if you would use the diode check on your DVM.
Yeah, those transistors are toast.

[Enzo]

Agreed, does your meter have a diode test function?


And may I suggest not using the term "nothing" for a reading. SOme people say "no reading," and I don;t like that either. If the reading is the same as holding your probes in the air, then you have an indication of "open," When I hear "nothing, I don;t know if you mean no resistance or no change in the reading. But open is still a legitimate measurement.

[bajarage]

Yes it has a diode test function, im not sure how to use it as I have never attempted circuit board repair before.
.

Understood about the nothing, was just using it to say I didnt have the leads on the base.

Got new transistors and new r29&r42 since the ohmed out a lil high

[Enzo]

The 0.1 ohm resistors? New ones won;t hurt, but when you are measuring a tenth of an ohm, the resistance of your meter probes and wires are a significant factor. Those are wire wound resistors, and prety much they are either open or they are OK. They don;t shift in value much.

[bajarage]

Well after replacing those it turns on without frying anything, now i just need to know what the oe combo speaker impedance is and where its suppose to wire in at.
any input would be greatly appreciated, you mentioned making sure its not leaking voltage to the output, how would you test for that?

[Jazz P Bass]

DC offset

Look at the power board schematic posted above.
Where Q4 & Q10 meet is the amplifier output.
That center connection ultimately goes through F1 (8 Amp) to connection marked W14. This is where you would hook up the positive lead to a speaker.
The connection marked W10 is the negative connection for the speaker.
You want to measure DC offset at these two points. W14 & W10.
If the amp is functioning properly, you should not measure (ideally)more than 100 millivolts (0.100Vdc).
What you do NOT want to see is a high voltage.
That means that you still have a problem.

[bajarage]

did what u suggested and got a steady , 162 mV. This has been quite an experience, now all i need to do is build an enclosure and find a speaker or 2 to hook this up to and ill be set.