swr workingman 15 trouble

[zairec14]

Hi Im in the process of trying to repair this amp that initially worked great but lost it sound somehow. It would turn on but i wouldnt get sound. im still using the speakers so i know that wasnt it I checked them first . Also i didnt have sound through the earphone jack with or without horn or in full range. Plugged in passive or passive/active jack, nothing. Not a hum or pop.
After being in the back burner until i could collect enough info and learn the basics of amplifiers i went in and found that one of the power transistors 2sc3264 @ q10 was not reading from the emitter with the dmm while power was on. And visually i saw burns on r31 and r43 and so i replaced them. Along with Q10 i changed Q4 they came as a pair. I figured it was good measure even though it was giving me readings on all 3 point, base, collector, and emitter. I cant say that thats what caused the sound loss and i feel it might of not been directly related but thats my question to the more experienced.
And now how should i safley switch it on to test so that if and only when im sure theres nothing im missing i could feel the acomplishment. I could really use some advice about what to do next.?
SChematics attached below. btw im in los angeles california

[52 Bill]

Have you tested the speaker fuse?

You can build a light bulb limiter to help save the new parts on power up, if there is still something wrong with the circuit. Have you tested the other transistors in the power amp?

The two resistors that you mentioned are part of the speaker output circuit. R31 will only overheat if the coil that parallels it comes unsoldered. R43 is part of the Zobel network and may overheat if the amp oscillates at a high frequency.

[zairec14]

i have tested the other transistors to see if they were operating but not for exact values. the speakers work fine i use them now from my peavey century 200h. if you mean the board mounted fuse it looks in tact and there is continuity. i did notice the solder pads were iffy at best so i resoldered when i first dove in.
Now before i rread this i got ancy and powered it up and this is what i assesed. Nothing blew up in smoke so thats good. The built in limiter light was on consistantly though. Not the way it should work i know that. I also monitered the board for temperature and r1 and r2 ceramic resistors became extremly hot. It was within a matter of minutes so i wasnt able to test for values on that account. i immediatly powered down. this was night before last.
Im going to look up what the zorbel network is and what componants they are .
Thank you for your respons Bill does any of the information i just presented help in your assesment? I will wait for more advice, patiently this time.

[zairec14]

i am now informed on the zobel network. I powered up with an 8 ohm 12" speaker and muy bass plugged in to the input. I think i should of mentioned that. im feeling the transformer might be suspect here . im going to test for dc just at the rectifier and see what values i attain.

[DrGonz78]

I'm feeling the transformer might be suspect here.

Why do you suspect the transformer?

Measure for DC voltage after the rectifier. Also, measure to check if any DC voltage is present on the speaker output. Use a the light bulb limiter while testing these things first and no speaker plugged in, no load yet. You can also test diodes or Bridge rectifiers with your meter too.

[zairec14]

will do thanks gonz . thats what i meant after rectifier before pcb. will a 75w 250v bulb work , it should right? ill rig one up in abit .
well i have a feeling that its not the original transformer. ive looked up images and its hard to tell that way but the transformer there has only to mains in and on the out to rectifier it has 3 . two ac in red and center yellow. and onthe schematic it looks like it starts center tapped at the input. I mean it worked when i recieved for idk a year with it. i didnt know much then. but just an observation for now. ill be back with some more clues later . thanks again

[DrGonz78]

will do thanks gonz . thats what i meant after rectifier before pcb. will a 75w 250v bulb work , it should right? ill rig one up in abit .
well i have a feeling that its not the original transformer. ive looked up images and its hard to tell that way but the transformer there has only to mains in and on the out to rectifier it has 3 . two ac in red and center yellow. and onthe schematic it looks like it starts center tapped at the input. I mean it worked when i recieved for idk a year with it. i didnt know much then. but just an observation for now. ill be back with some more clues later . thanks again

Yes 75w bulb will be sufficient. The primary input to the transformer only has two wires and the secondary has three wires, 2 reds and 1 red/yellow center tap. Not sure what you mean by center tapped at the input?

Edit: If the amp worked okay when you first owned it then I would not get too concerned that the Power transformer is not correct.

[zairec14]

update: 58 vdc from rectifier and from output 4.3vdc. r1 and r2 gradually became real hot ???

[DrGonz78]

update: 58 vdc from rectifier and from output 4.3vdc. r1 and r2 gradually became real hot ???

R1 and R2 drop a considerable amount of voltage (drop drown resistors) and that is normally expected to be hot. Check DC voltages both sides of R1 and R2 with respect to ground. This way you should see -/+58vDC and then the other side of those resistors will be the lower voltage preamp supply. Definitely something is wrong in the circuit if you have 4.3vdc on the output of the amp, so keep lamp limiter connected and No loads yet. With the amp off you will need to check for shorted transistors. Once you confirm that you have -/+58vdc then you know your bridge rectifier is not the problem.

[zairec14]

workingman2004.pdf
tested all transistors as suggested q12 was not giving me a read any which way. Also i tried to attach more schematics that were missing from the fisrt attachments i posted.
not sure if they stuck let me know ill try again. as far as q12 what do say gonz thats where the inputs come into the pcb from the jacks , right.

[DrGonz78]

Confirm negative and positive rail power supply voltages for both power amp and preamp. "-/+58vDC and then the other side of those resistors will be the lower voltage preamp supply." Just make sure that power supply circuit is creating the voltages, which will read lower than schematic with light bulb limiter attached. Also, with this amp keep a look out for anymore cracked solder joints.

As far as Q12 how are you testing it? What reading does it indicate on your meter, OL = open? The meter is displaying something when you test it so report exactly what it shows. Simply saying not giving a reading is not enough in this instance. Check & confirm power supply voltages and report Q12 readings to make it clearer please.

[zairec14]

yes sir . will do asap. transistors i check for emittter to drain activity not resistance. red probe on base to emitter and then base to drain. i did not check how well they were working but just that there was some passage of current and that one of the contacts was not blown open.
I apologize for my inexperience and appreciate your knowledge. I figure i have to start somewhere and at some point just research was not enough Teaching myself to read sschematics was late nights of staring at these symbols and lines literally until they decoded and began to make sense. This is why i decided it was time for hands on , since i do learn best this way.
Back to q12 . do you mean? that once once i have the transformer connected into circiut , then confirm rails ?

[DrGonz78]

Yes only confirm the power rails with the amp put fully back together and with the light bulb limiter connected, no load. If you have it apart then keep testing for bad transistors but try to not remove parts from the board. Sometimes you need to remove a part to test it out of circuit but don't carried away soldering up the board. Also, don't replace parts that are not bad or with the hope that it will fix the problem, shotgun repair. Really we need to test for bad transistors that are in circuit and find them. Then remove them to test out of circuit to confirm, then replace if bad. If you found one then yes replace it but then we need to go back to check the power rails, which should have been done first.

The easiest way to check the transistor is by using the diode setting on a multimeter which I guess is how your testing. You can test it using ohms resistance readings as well.
How to Test a Transistor

Or diode testing method
https://www.youtube.com/watch?v=Fq9Amy2iAIg

[zairec14]

i apologize for not updating sooner . You may know life throws curves but did you know it also throws a mean change-up?
Not funny, i finally got around to testing the power rails. I can confirm that there is a -/+ rail. What im measuring at r1 and r2 is now under 7 volts dc . I have not checked transisitors im hoping ill have time today or tonight . Ill see. Also i checked voltage at output and its under 1 volt dc approx 30 mv at 59hz.
Gonz , is that normal abouts? until i get around to transistors i dont have much to go on as far as what real measurements i should be getting . im going to further investigate this with the schematics by looking for revised versions . what else can i do?

[DrGonz78]

I can confirm that there is a -/+ rail. What im measuring at r1 and r2 is now under 7 volts dc. Ill see. Also i checked voltage at output and its under 1 volt dc approx 30 mv at 59hz.

What readings do you have for the -/+ rails voltages? Is the 7vDC measured across the each resistor? I thought we had 4.3vDC on the output? Did you change that transistor yet and this is after replacing Q12?

We need to know what the voltages read at -/+HV side of the R1 & R2 and also -/+15 sides of too. Need to confirm all voltages before going forward. If we are now only seeing 30mv on the output then that is great as that is what we want to see.