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Old 13th July 2012, 07:04 PM   #1
KP11520 is offline KP11520  United States
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Default Need Help with a 1996 SWR Workingman's 12 Combo Amp

Hi Gang,

So, like a dummy, I went looking for a cheap used Bass amp that had decent reviews to learn and practice on........

$80.

Now I know why........

It has a loud pop when powering up and down.

When it is turned up to less than half way with a somewhat aggressive attack on the strings, I get a distortion clipping sound (way too early with a passive Bass) that just shouldn't be there.

I thought it might be the piezo as it has a high ring, but it is still there when switched off.

Then, I thought it might be the 12 inch driver because that is considered the weak spot of the combo, but when I bypass it and go to an external Bass Cabinet, the same.

It is SS with a bunch of op amps for all the pre and effects stages and Sanken Transistors for the power amp.

All I have is a decent Agilent DMM (Thank You Chris for recommending it) and I am clueless on how to diagnose it.

Any help would be greatly appreciated!

Thanks Fellas!

Regards//Keith
Attached Files
File Type: pdf Workingman's12-15-2004PowerandPreAmpRev1.pdf (194.7 KB, 70 views)
File Type: pdf Workingman's12ControlPanelPCB.pdf (67.5 KB, 36 views)
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Old 13th July 2012, 11:47 PM   #2
Enzo is offline Enzo  United States
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Start at the start. Verify all your power supplies. The HV is not specified, but make sure both raqils are present, up at about the same voltage, and free of ripple. Then the +/-15v rails for the op amps, same thing - are they both up to voltage and free of ripple.

I don;t worry about popping when power is switched.

This is a small combo amp, you really could be clipping it, it is a lot easier to do than most people think. Bass guitar is really peaky as signals go.

COnnect a CD player or some other nice music source to the amp. Does that come through OK, or does it distort in the same way?

You only have a meter? DO you have any other guitar amps? You can use another amp as a signal tracer. You'd need to make up an input probe for it, but that consists of a cap and a resistor. Google "signal tracer" for a large number of hits on the topic. You can thus make any amp into a tracer by the addition of this utterly simple probe as described in many of these articles. Think of a signal tracer as a scope for your ears. With it you can listen to the signal quality stage by stage through the amp.
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Old 14th July 2012, 09:15 PM   #3
KP11520 is offline KP11520  United States
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Hi Enzo,

Thank you for responding (actually, I was hoping you would).

Thank you on the steps to get me started. I am a kinetic learner and throwing me into the fire in a real life situation is where I learn best and fastest! Finally, now I'm going to learn one of the most useful diagnosis practices I have always wanted to.

I will do the CD test over the next few days and also learn about the signal tracer and amps..... I have guitar amps and some Home and a PA Power Amps. I have little Roland Cube 30 that will be perfect for this (and some guitar cables that need either plug replaced, so all is well) LOL

I will do what you said and go as far as I can. (I might have some questions but I will try to answer them myself so I learn better) Please keep an eye out in case I get desperate. LOL

I'm sure you've heard this before......

I'll Be Bock!

Thanks again!

Regards//Keith

Quote:
Originally Posted by Enzo View Post
Start at the start. Verify all your power supplies. The HV is not specified, but make sure both raqils are present, up at about the same voltage, and free of ripple. Then the +/-15v rails for the op amps, same thing - are they both up to voltage and free of ripple.

I don;t worry about popping when power is switched.

This is a small combo amp, you really could be clipping it, it is a lot easier to do than most people think. Bass guitar is really peaky as signals go.

COnnect a CD player or some other nice music source to the amp. Does that come through OK, or does it distort in the same way?

You only have a meter? DO you have any other guitar amps? You can use another amp as a signal tracer. You'd need to make up an input probe for it, but that consists of a cap and a resistor. Google "signal tracer" for a large number of hits on the topic. You can thus make any amp into a tracer by the addition of this utterly simple probe as described in many of these articles. Think of a signal tracer as a scope for your ears. With it you can listen to the signal quality stage by stage through the amp.
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Old 15th July 2012, 07:55 PM   #4
KP11520 is offline KP11520  United States
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Hi Enzo,

As far as I can see for an AF Audio only cheap DIY signal tracer, all I need is Black and Red probes and Banana Jacks, a single Ceramic Disc Cap 0.1 uF 1KV (Vishay) out to a 1/4" TS Jack in a small plastic box with the cap going from Red Banana jack to the Tip conector on the 1/4 inch jack. Black Banana straight through to the sleeve on the 1/4 inch jack. Then run a guitar cable into my amp on the clean setting.

Nothing about a resistor, pot, aditional cap or crystal diode, if only for an AF Signal Tracer.

Pllease advise if I am wrong!

Thanks!

Regards//Keith
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Old 17th July 2012, 01:57 AM   #5
Enzo is offline Enzo  United States
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A signal tracer is a system to listen to audio at various points in the amp. All it need is an amplifier itself, plug a cap at the input to block DC, and a resistor to make sure the cap gets charged. A crystal diode sounds to me like you were also reading about "RF probes", which is a thing to let your mete measure signal levels at RF frequencies that it would not otherwise respond to. OOORRR you got a signal tracer article that included a "detector" (which is what an RF probe really is) so you can listen to modulation on an IF or RF signal while working on radios. Stick to the very basic probe deal as you desribed.

Probes today tend to be molded plastic or rubber things - not hollow. In the old days, probes were usually these hollow plastic bodies, and there was room inside them for the cap and resistor. Alternatively, since your blocking cap only needs to be in between the unit under test and the tracer amplifier input, instead of at the probe end, you can often mount the cap inside the plug end of the probe cable.

However, absolutely nothing wrong with making a little box with terminations. However, the best results will be by keeping the unshielded portion of your hot probe as short as possible to reduce picked up noise.


Hmmm, I thought the first few google hits were better than they turn out to be. Your series cap will be fine, I just recommend adding a high value resistor - 1 meg, 470k, whatever - on the tracer side of that cap to ground.
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Old 17th July 2012, 09:57 AM   #6
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I think given the age of the amplifier, I'd consider replacing all electrolytic capacitors (replace the ones around the power supply as a minimum).

They lose value as they age (especially in hot environments), so when you consider that this amplifier is ~16 years old, I'd expect those caps to be well out of spec.

Chris
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Old 18th July 2012, 04:58 AM   #7
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Hi Chris,

Thanks for the reminder. 16+ ain't like we were in our prime LOL

After I get it stable, addressing all the obvious maintenance upgrades sounds like a good idea!

Regards//Keith

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Originally Posted by chris661 View Post
I think given the age of the amplifier, I'd consider replacing all electrolytic capacitors (replace the ones around the power supply as a minimum).

They lose value as they age (especially in hot environments), so when you consider that this amplifier is ~16 years old, I'd expect those caps to be well out of spec.

Chris
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Old 18th July 2012, 05:21 AM   #8
KP11520 is offline KP11520  United States
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Hi Enzo,

Thanks for the info as to where the resistor goes. From the red probe (+) to the black (-) before the cap. After the cap, the wires go straight to the amp.

Yes, most of the schematics I found were for both RF and AF signal tracers and for the AF side, just a Cap. When you said a cap and a resistor, I knew I better check back with you. Thanks again.

Also, thanks for the lead length heads up I would have plugged in two DMM probes (too long) and picked up 20 meters Ham Radio band. LOL

I'll get creative, order the right parts and build something suitable.

In the meantime, I'll loosen the main board and do what it takes to measure the +/- 15 volt rails. Wouldn't you know it, the transformer has three secondary RED wires and I can't find any info about what the secondary puts out. Earth to Mercury (Magnetics)

Thanks for the hand holding. You don't know how much I appreciate this! And I am learning!

Regards//Keith

Quote:
Originally Posted by Enzo View Post
A signal tracer is a system to listen to audio at various points in the amp. All it need is an amplifier itself, plug a cap at the input to block DC, and a resistor to make sure the cap gets charged. A crystal diode sounds to me like you were also reading about "RF probes", which is a thing to let your mete measure signal levels at RF frequencies that it would not otherwise respond to. OOORRR you got a signal tracer article that included a "detector" (which is what an RF probe really is) so you can listen to modulation on an IF or RF signal while working on radios. Stick to the very basic probe deal as you desribed.

Probes today tend to be molded plastic or rubber things - not hollow. In the old days, probes were usually these hollow plastic bodies, and there was room inside them for the cap and resistor. Alternatively, since your blocking cap only needs to be in between the unit under test and the tracer amplifier input, instead of at the probe end, you can often mount the cap inside the plug end of the probe cable.

However, absolutely nothing wrong with making a little box with terminations. However, the best results will be by keeping the unshielded portion of your hot probe as short as possible to reduce picked up noise.


Hmmm, I thought the first few google hits were better than they turn out to be. Your series cap will be fine, I just recommend adding a high value resistor - 1 meg, 470k, whatever - on the tracer side of that cap to ground.
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Old 18th July 2012, 10:19 AM   #9
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Quote:
Originally Posted by KP11520 View Post
Hi Chris,

Thanks for the reminder. 16+ ain't like we were in our prime LOL

After I get it stable, addressing all the obvious maintenance upgrades sounds like a good idea!

Regards//Keith
You'll speak for yourself, sir: I was 2 years old when your amplifier was made.


To figure out what the 3 secondaries do, measure the AC voltage on each one ref. to ground. Chances are one of them is grounded, resulting in a split supply.
Add some masking tape to label which wire does what for future reference.

Chris
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Old 19th July 2012, 12:50 AM   #10
Enzo is offline Enzo  United States
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Oh geez... I was at the half-century mark in 1996.
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