SWR Workingmans 12 amp

[lurkalot]

I have a old but very nice sounding SWR Workingman's 12 amp, which I bought a while back for my son to practice the bass guitar on.  It's the older one with the controls set back at the top.   All seems to work very well, except for the clipping light, which doesn't seem to do anything regardless of any settings.

I was wondering if maybe his cheap Bass guitar's passive output is a bit weak, or is it the clipping light that's faulty. 

Can anyone give me some advice on how to test this please.

[phatt]

If the Amp works fine and makes no ugly sounds until pushed hard then it's likely working just fine.
Frankly the idea of a clipping led in such a small amps seems rather pointless.
Thousands of amplifiers seem to work just fine without a clipping led,,  :loco go figure?
My Advice is,,,If it ain't broke then don't fix it.
Phil.

[lurkalot]

If the Amp works fine and makes no ugly sounds until pushed hard then it's likely working just fine.
Frankly the idea of a clipping led in such a small amps seems rather pointless.
Thousands of amplifiers seem to work just fine without a clipping led,,  :loco go figure?
My Advice is,,,If it ain't broke then don't fix it.
Phil.

Thanks for the reply Phil..  The amp sounds great even when pushed hard, no horrible sounds except for the guitar playing. lol..  I was just curious about the input clip light, but I'll take your advice and live with it.

[lurkalot]

Frankly the idea of a clipping led in such a small amps seems rather pointless.

For got to say.  I guess it's small, but I wouldn't like to lug it around too much, it weighs about 50lbs. 

SWR Workingman's 12 Bass Combo Specifications:

* POWER OUTPUT
* 120 Watts RMS
* 160 Watts @ 4 Ohms (with 8 ohm extension speaker)

SPEAKER COMPLEMENT
* (1) 12" Custom SWR driver
* (1) Le-Son TLX-1 Piezo tweeter

IMPEDANCE
* 8 Ohms (internal)

DIMENSIONS
* 15-1/2"W x 22-1/4"H x 15-1/4"D

WEIGHT
* 50 lbs.

* Discrete solid state front end
* Gain Control
* Aural Enhancer Control
* Bass Control
* Midrange Control
* Treble Control
* Effects Blend Control
* Automatic compressor/limiter
* Side Chain Effects Loop
* Headphone Jack
* Tuner Output
* Extension Speaker Jack
* Balanced XLR record out
* 3-Way Output Switch

[Roly]

I found the attached circuit at http://www.eserviceinfo.com.

The limiting LED isn't actually shown on the circuit but I assume it is connected to the "LIM" terminal bottom-right on the first sheet, driven by a LM339 comparator (mainly 'cos I can't think what else would be).

There is a limiter adjust pot (preset?) which may have some influence on the situation.

It looks like the LED connects between the point "W1 LIM" and the +15V rail somewhere.  I'd clip a multimeter across the LED and see if you get any voltage difference when the amp is driven hard.  If you do see some voltage across it (more than 2-3Volts) then the LED is boofed (to use the technical term) and simply needs to be replaced (but watch the polarity).  It could also be shorted so, with the power off, measure its resistance.  If you see anything less than about 2k ohms it's shorted.

Failing that, disconnect the LED and clip your multimeter across the leads going to it, and drive the amp into clipping.  If you don't see any voltage then the clipping driver isn't delivering to the LED and this needs a deeper dig back on the circuit board.

The short answer is to simply try replacing the LED.

HTH

[J M Fahey]

1) never ever touch, even dare to *look* at the "Limiter adj" preset, you will mess with the limiter working point and never propoerly set it back where it belongs.
"Best" outcome would be to lose it, and worst would be it working too much and turning your 100W amp into a 5W incredibly heavy bedroom amp, so don't.
2) yes, U2C , 1/4 of an LM339 is a comparator, detects peak rectified voltage sent to the power amp in and above certain value turns the Led on.
3) this SWR "schematic", like what's lately being supplied by Marshall and many others is *HORRIBLE* and an absolute lack if respect for techs, ... because it is NOT "the schematic".
It lacks *all* connections and external parts.
*ALL* of them.
In fact, this is a printout of the ".sch" file generated by the schematic design program (Eagle or any other similar) and ONLY shows *PARTS ON THE PCB ACTUALLY BEING ROUTED*
No pots, jacks, switches, ***Leds*** (as duly noted), XLR jacks, transformers, power fuses, line connections, NOTHING.
Pure, absolute CRAP. :trouble :grr
No, it ain't funny .

[lurkalot]

Thanks, but this stuff is all new to me.  The amp has a Preamp CLIP LED (the dead one I was asking about).

The Preamp Clip LED will light whenever the Preamp, Tone section or output buffer reach clipping (that
is, run out of headroom). In the event the Preamp Clip LED lights, turn down the Gain control. Since the
Preamp Clip also monitors the Tone section, boosting any one of the tone controls can cause the LED to
activate. Again, turn down the Gain control if this happens.
NOTE: Constant clipping of the preamp will not harm the electronics in your Workingman's 12.
However, damage can occur to speakers due to near-DC content present in a clipped waveform.

And a Limiter LED

LIMITER CIRCUIT
The Workingman's 12 Limiter Circuit is a soft knee type limiter that prevents distortion of attack transients
or peaks. The Limiter can be used as an effect and is also helpful in preventing speaker damage.
The circuit is located after (post) the master volume and before (pre) the power amplifier; it is driven by
the Master Volume control. Its threshold (starting point) is preset by the factory so that the user can get
maximum overall apparent volume without unduly overdriving the power amplifier.

LIMITER LED
The Limiter LED lights when your signal has reached the Limiter threshold, letting you know that the
Limiter circuit has been activated.
NOTE: No harm is being done to your amplifier when this LED lights or stays lit.

[Roly]

1) never ever touch, even dare to *look* at the "Limiter adj" preset

Warning noted.  My thinking was that this may have already been twiddled.

{During one of my stints on radio stations I had to deal with a clueless but hyperactive "technician" whos' first response to any problem was to get inside cart machines, and even the station compressor/limiter, and crank every sealed trimpot, and the limiter had dozens - then on the brink of disaster it fell to me (since nobody else on the station could be trusted to operate an electric kettle) to try and bring them back into Broadcast Spec.   :loco }

Yeah @lurkalot, just try replacing the LED.

[J M Fahey]

Oh!! so after I kicked his *ss hard he emigrated to Australia and went to work for you ??  :lmao:

[Roly]

{No, make work for me.  I nearly took him out the back and throttled him.  :grr   Started fitting securi-drive screws to stuff to keep his fingers out.  :tu: }