SWR 350 bass head not working?

[DrGonz78]

Test all transistors, diodes, caps, resistors etc etc... I would be looking at the transistors first then test the diodes. Do you know how to test diodes with your meter? Look at that bias pot VR1 test from leg 1 to 3 to see check the resistance. I would not make any turns on the pot, but need to make sure it is not open. Basically take a lot of care to inspect each component in the power amp circuit before we power it up again with the light bulb limiter. Good news is that the bulb did go dim, so I feel your getting close. I remember a time I replaced a bunch of output transistors in a power amp and powered it up with a light bulb limiter only to see it shining bright. In that case I went back and started to test a bunch of parts with my meter. I found an open diode and that solved all my problems. In this case I am pretty sure (hoping) you looked all over the power amp circuit already for any open components? My gut says that you did and now we might be looking for a smaller harder to find problem.

BTW once you do the thorough tests of all the components then I personally would feel better about testing voltages with the light bulb limiter connected, but thats just me.

[Jam]

I have been testing everything and I have not found anything bad should I pull every part to get a true reading? How do you test the little bipolar transistors ? Do I need to even test the little square film caps too and little resistors? Should I focus on the diodes and transistors? Thanks

[DrGonz78]

I would focus mostly on the transistors and then diodes too. The diodes are easy to test and most of the time don't even need to be removed from the board. However, with diode and resistors just pop up one leg of the component from the board. There is no need to remove the entire component, but just one leg pushed up from the board to break the circuit. Many resistors will test fine in circuit, but then when you get a funny reading it usually because of the circuit. Mainly concerned to test the transistors and here is an easy to follow instructable for bipolar transistor type tests.

Test a transistor with a multimeter | Vetco Electronics

[J M Fahey]

Testing all parts at random with no specific clues is a very frustrating and error prone method of repair.
I much prefer functional testing, and only when something "functions bad" then I imagine why and test what might cause that.
In this case the only bad symptom is +2V DC at the output, so let's concentrate on that first.
1) I'm not that certain you actually have +2V there, please recheck, state what multimeter scale you are using and write the exact display indication, as well as any label it shows (such as "mV", "V" , "DC", whatever.
2) with the scale set first to 20V DC measure voltage to ground at Q1 and Q2 , then repeat on the 2V scale , then on the 200mV scale.
A blank screen or an "overrange" or "1" followed by blanks *is* an indication, so please post it.
"Nothing" is not an indication.
We rely on your measurements and we must all speak the same language or we'll go nowhere.

[DrGonz78]

My rationale about testing parts was to ensure that you really did look over the circuit before we start functional testing. Of course, Juan is right on the money! Sorry if I pushed in that direction of frustration and randomness. Just wanted to make sure there was nothing else open or obviously bad before taking further voltage tests. I guess I move slowly on the power amp stages, but moreso in cases where I am not there in person. There is always that element of not knowing what you looked over on the circuit before testing begins.

Edit: Definitely should double check measurements of DC on the output again to make sure.

[Jam]

Thanks guys heres what I have

The output
1.54.5 on MV, 1.70 on 2Vdc , 0.16 on 20Vdc, 00.1 on 200 setting

Q1 Leg 1 1 on MV, .887 on 2Vdc, 0.89 on 20V, 00.8 on 200 setting
leg 2 -1 on mV, -1 on2Vdc, -1 on 20Vdc, -50.7 on 200setting
leg3 1 on mV, ..293 on 2Vdc, 0.29 on 20Vdc, 00.2 on 200 setting

Q2 leg 1 1 on mV, .896 on 2vdc, 0.89 on 20Vdc, 00.8 on 200 setting
leg 2 -1 on mV, -1 on 2Vdc, -1 on 20Vdc, 51.7 on 200 setting
leg3 1 on mV,.326 on 2vdc, 0.32 on 20Vdc, 00.3 on 200 setting

[J M Fahey]

Thanks guys heres what I have

The output
1.54.5 on MV, wrong, you can't have 2 decimal points at the same time
1.70 on 2Vdc , wrong, you can't have a decimal point there, you should have 0.170 on the 2V scale, meaning 170 mV , which is consistent with the next two measurements:
0.16 on 20Vdc, right
00.1 on 200 setting right. Incomplete because we have run out of digits.

Q1 Leg 1 1 on MV, .887 on 2Vdc, 0.89 on 20V, 00.8 on 200 setting all 3 match, so I can trust you have around +0.9V on Q 1 base, which is fine.
A good, properly DC tracking amp should have about the same on the output, *because* Q1 base is "pushed up" by said voltage.
leg 2 -1 on mV, -1 on2Vdc, -1 on 20Vdc, -50.7 on 200setting Same here, -1__ means overload, NOT -1V ; 200V scale shows proper value.
leg3 1 on mV, ..293 on 2Vdc, 0.29 on 20Vdc, 00.2 on 200 setting Same here. In fact it's not plain "1" but "1__"meaning 1 and 2 blank digits.
A real "1" would be "1.00"
Interpretation errors such as these can drive us all crazy.
3 digit measurements match: 293mV , 0.29V and 00.2V are all the same,in a digital display.
Sorry for being obsessive but these errors are very common, and doubly so in a Forum, where we don't have the meter before our very eyes.

Q2 leg 1 1 on mV, .896 on 2vdc, 0.89 on 20Vdc, 00.8 on 200 setting
leg 2 -1 on mV, -1 on 2Vdc, -1 on 20Vdc, 51.7 on 200 setting
leg3 1 on mV,.326 on 2vdc, 0.32 on 20Vdc, 00.3 on 200 setting

so in a nutshell you have very reasonable 0.16 V at the output.
If we see that the Q1 Base has a 0.9V offset because of its base current passing through the base to ground resistor, I'm not surprised of having a similar voltage dropped across the Q2 base to output NFB resistor.
That amp is working as expected in the Real World.
So measuring all parts searching for a "problem" which actually does not exist will be, at least frustrating.

Oh !! and what about the "2 Volts" present at the output?
Measurement/interpretation error.

In fact they were very justified 0.17 V or thereabouts.

[Jam]

Sorry about the bad readings and postings the meter I have has auto range and I was getting confussed. So the are you saying the reading s means the amp looks good to go? Thanks

[DrGonz78]

He is saying the amp is looking good. 170mv is in the healthy range for an amp and does not put a significant amount of DC on the output. Would be interesting to test the DC on the output with a load applied. Good job!

[Jazz P Bass]

The readings imply that the output section appears to be stable.

Send a signal through without a load attached & monitor the output.
If it looks good, then it's time to find out whether or not it can provide voltage & current to a speaker.(dummy load at first)

[J M Fahey]

Agree.
*Hopefully* it will drive a load but we won't be certain untilo actual testing.
1) still with bulb limiter, turn amp on without load or speaker, SWR are famous for not "starting" if both loaded and current limited.
Many others share that trait.
2)after turn on, measure output DC.
If still less than 250mV, connect a speaker or load without turning amp off.
Measure again.
You should have your 170mV or less.
3)apply a little audio. Does the speaker sound "normal"?
Up to a couple watts, which is reasonably loud in a bedroom or Lab.
If you drive it **loud** still with current limiter, it may block.
4) if all seems normal, turn o0ff, plug straight in the wall (no limiting) and turn on again.
It should work normal.

[Jam]

Ok I think I still have a issue unless Im doing something wrong. I still hooked up to the bulb limiter and I measured 165mv at the out put. So I plugged a speaker to it and the light bulb went bright and the meter read OL when I tested it?

[J M Fahey]

I bet you turned amp off, connected speaker and turned amp on again.
SWR amps simply don't like that.
Turn it on, read 160mV or close and connect speaker without turning amp off.
What happens?
All this with all controls set to 0.

There's no reason for an amp with just 160mV DC on the output and no signal fed to it to slam hard against a rail just by connecting a speaker.

[Jam]

The first time I did turn the amp off to plug in the speaker. So I did the way you said and this time the bulb stayed dim and I had 151mv on the output

[Jazz P Bass]

Time to get rid of the limiter.