SWR Workingmn's 15 bias adjustment

[mokomon]

Looking for info on the proper bias adjustment and limiter adjustment for a SWR workingman's 15.I have repaired this amp , the zener diodes went and the HV took out resisters and all the IC's, and the jfet in the limiter circuit.

[Jazz P Bass]

Post the schematic?

[mokomon]

Can't seem to get the pdf of the schematic to attach . I'll try again latter . . The amp I am working on is dated 1998 , very close to schematic I have , which says Fender on it . Amp works fine , over 100 watts clean until limiter defeat is pulled . But I still have -.56 vdc on speaker output , with no signal applied . Should this not be 0 vdc ?

[Jazz P Bass]

560 mv's is a tad high.

It is a little bit difficult to walk through how to check were the offset is originating.

Your best bet is to carefully take a few measurements.
Start with the power supply.
The + & - HV should be the same with very little ripple.
The + & - 16 should have minimal ripple.

You could also check across the ballast resistors in the output stage. (R29 & R42)
See if they are the same.
(This is were you will monitor the bias when you set it)

If push comes to shove & nothing untoward is found, replace the LTP transistors. (Q6 & Q7)

[mozwell]

workingman 12 7 15 share the same sch
here are various workingman schematics that may help

WM4004 lists the bias adjustment, its the old "set bias by monitoring the crossover distortion notch & adjust the bias until the notch just disappears" trick.
This is best done at low output levels from the power amp, SWR use 2V rms output for WM4004.

[Enzo]

Bias and offset are not the same thing.

[mokomon]

The workingman's 12 is pretty much the same schematic I have . The HV and the 15v rails are all good with minimal ripple .I did replace both Q6 & Q7 already , the old ones tested ok with diode check even when I took them out .I will check the ballast resistors again to see what was there.

[Jazz P Bass]

Well, the - sign on the offset may be a hint.
That would imply that the negative side is turning on harder than the plus side.
Maybe take some measurements on the finals.
The driver bases in particular.
Q5 and/ or Q8 may have taken a hit.

[mokomon]

Sorry for the delay in responding. I adjusted the bias according to these instructions I found and here are the voltage readings I get now.
Q6 B .158 ,C -55.3,E .70
Q7 B.176,C-54.5,E.70
Q8 B-55,C-10.15,E-55.5
Q5 B-.970,C.60,E-1.55
Q3 B.60,C61.3,E.084
Q4 B .084,C66.5,E-.473
Q9 B-1.55,C-60.9,E-.473
Q10 B-60.9,C-.473,E-61.4
So the speaker out still has -.473 v. Is that ok or is something else still wrong ?
I still can't seem to figure out how to insert a file to this post. I go into Manage Attachments , and try to upload a PDF file , but it never seems to finish what it is trying to do . So anyway , what I did as per the pdf instructions was , use a 2 ohm load , insert 1khz signal , adjust master to get 2 v out, and adjust bias control to just remove notch in wave form. So the readings I got were from after this adjustment was made . Any comments ? Am I on the right track here ?

[Jazz P Bass]

"I still can't seem to figure out how to insert a file to this post. I go into Manage Attachments , and try to upload a PDF file , but it never seems to finish what it is trying to do"

What computer OS are you using?
Windows XP & IE 8 do not work right.
Try a different browser, Firefox or Google Chrome.

[mozwell]

I quit using IE to upload files, i now use firefox, & have no problems.

Your voltage measurements look ok.

Personally, if you have -0.473V DC on the output & the amp sounds ok, i would live with it. It is most likely just a gain mismatch on the input differential transistor pair, and not much you can do about it. Certainly this offset wont hurt the speaker.
The voltage across Q5 (the vbe multiplier) look ok with 2.1V collector to emitter
The power amp is a fairly basic design, and for a bass amp it is more than adequate

Let the amp warm up for 20 minutes, with power on, & no input signal.
Measure the voltage across the two 5W emitter resistors R29 & R42, these should be roughly the same and maybe only about 10mV or so.

If these are ok, i would say you are good to go.

[Enzo]

If you are trying to upload this amp schematic, it may already be in the system as one of our dead files, so when you try to post, it won;t let you. We are all bumping into this now and then.

[mokomon]

I was trying to attach the PDF I found of how to set the bias on the Working mans 15 amp . Anyway , aside from that , did I set the bias correct by adjusting it to get rid of the notch in the waveform ? What is the minimum vdc on the output for a SS amp ? And what is the best method for adjusting the limiter circuit for this amp ? I just looked at the output wattage as I adjusted the volume , and set the limiter pot so it was turning on the limit led at about 80 watts . This was just a guess on part as to how to set this . oh , and by the way , after letting the amp warm up for at least 20 minutes the voltage drop across R42 was .0025v & R29 was .0019v.

[olddawg]

If it is a funky input difamp, you could always monitor the offset and "tickle" them one at a time with some freeze. (You might want to use a load bank so you don't invert a speaker) If you have a meter that measures hfe you can match the beta out of a few replacements. If it is a 5 pin dual package you just replace it. In the old days, beta matched transistors had a dab of red or white paint on them. I think PNP were red and NPN white. You also can replace a 5 pin package with 2 matched transistors if the dual package is unavailable.

[mokomon]

Anyone know how to adjust the limiter circuit ?