Need Help with a 1996 SWR Workingman's 12 Combo Amp

[KP11520]

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

 

[Enzo]

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.

 

[KP11520]

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

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.

 

[KP11520]

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

 

[Enzo]

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.

 

[chris661]

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

 

[KP11520]

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

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

 

[KP11520]

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

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.

 

[chris661]

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

 

[Enzo]

Oh geez... I was at the half-century mark in 1996.

 

[KP11520]

Hi Chris and Enzo,

I figured I would use a continuity tester to see if one of the three red leads is a ground but I need to get under the main board. As when I check the 15 Volt rails. BUT (and I'm talikin about a BIG Butt), the three Power Amp transistors are attached to the heatsink on the back. Can I unscrew them and separate them off the heatsink and reassemble them later with no problems?

I can post pictures if that would help!

And sorry Chris, you actually are still in your prime. I am closer to Enzo's end of the spectrum. (although I sure would like to be where you are again! Sorry Enzo)

Kudos Chris, you are helping an old guy with electronics that's almost 3 times your age and you are light years ahead!

And Enzo is just loaded with info and experience and a gift to guys like me.

Regards//Keith

 

[phase_accurate]

Well 80 Watts is not super beefy. Don't know how loud is loud for you. I recently heard someone with such a combo playing live and for that gig it was loud enough but that wasn't a rock band.

I don't know how loud they uaually pop when turned on but somem pop is definitely normal because this amp diesn't have any type of delayed speaker relay. As long as the driver can take it this might be an even better solution form the reliability point of view.

There is a simple test that would show whether a problem is within the first part of the preamp: Feed some signal into the effect return maybe from some MP3 player or the like and check whether you can go louder.

Regards

Charles

 

[chris661]

Hi Chris and Enzo,

I figured I would use a continuity tester to see if one of the three red leads is a ground but I need to get under the main board. As when I check the 15 Volt rails. BUT (and I'm talikin about a BIG Butt), the three Power Amp transistors are attached to the heatsink on the back. Can I unscrew them and separate them off the heatsink and reassemble them later with no problems?

I can post pictures if that would help!

And sorry Chris, you actually are still in your prime. I am closer to Enzo's end of the spectrum. (although I sure would like to be where you are again! Sorry Enzo)

Kudos Chris, you are helping an old guy with electronics that's almost 3 times your age and you are light years ahead!

And Enzo is just loaded with info and experience and a gift to guys like me.

Regards//Keith

You should be fine taking those transistors off. Are you sure they're all transistors? Seems odd that there's 3 of them. Maybe one is a regulator.

Tell you what, switch the amplifier on (speaker connected) and leave it for a while. If the 3 chips on the heatsink are reasonably cool, then you're good to remove them. If they're really hot, then you can still take the heatsink off, but don't switch it on for any length of time: it might be difficult to replace them if they go pop.

When you come to attach them again, some heatsink paste wouldn't go amiss.

Chris

PS - which are the big power supply caps like?

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

If they're like the latter, you can directly measure across them without removing the board.

 

[KP11520]

Hi Charles,

The distortion is with the gain and volume at less than 1/3 with both the passive setting and active setting.

Great idea! Having to go over the front end rather than the whole amp would be easier and less to do! (and less intimidating because of the "guesswork" issue).

I hope the problem lies in the preamp side. (Smaller PCB abd more accessible with less dismantling!

Thanks for another great idea my lack of experience wouldn't have thought of.

Regards//Keith

Well 80 Watts is not super beefy. Don't know how loud is loud for you. I recently heard someone with such a combo playing live and for that gig it was loud enough but that wasn't a rock band.

I don't know how loud they uaually pop when turned on but somem pop is definitely normal because this amp diesn't have any type of delayed speaker relay. As long as the driver can take it this might be an even better solution form the reliability point of view.

There is a simple test that would show whether a problem is within the first part of the preamp: Feed some signal into the effect return maybe from some MP3 player or the like and check whether you can go louder.

Regards

Charles

 

[KP11520]

Hi Chris,

The caps are radial unfortunately. Time to get out the Chop Saw. LOL

Would Arctic Silver thermal paste for computers suffice?

Thanks again!

Regards//Keith

You should be fine taking those transistors off. Are you sure they're all transistors? Seems odd that there's 3 of them. Maybe one is a regulator.

Tell you what, switch the amplifier on (speaker connected) and leave it for a while. If the 3 chips on the heatsink are reasonably cool, then you're good to remove them. If they're really hot, then you can still take the heatsink off, but don't switch it on for any length of time: it might be difficult to replace them if they go pop.

When you come to attach them again, some heatsink paste wouldn't go amiss.

Chris

PS - which are the big power supply caps like?

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

If they're like the latter, you can directly measure across them without removing the board.

 

[chris661]

The thermal paste you've mentioned will be fine.

+1 on Charles's idea on using the fx loop.

You could also try listening to the fx Send via some headphones to confirm your findings.

 

[KP11520]

Well, Alrighty then!

Let the hacking begin.....

Where's that chainsaw?

The thermal paste you've mentioned will be fine.

+1 on Charles's idea on using the fx loop.

You could also try listening to the fx Send via some headphones to confirm your findings.

 

[KP11520]

Hi Gang,

I bought most of the parts to build the AF tracer probe, except I am not having luck finding a "pen" shaped housing and a probe tip that can thread into whatever housing I find. I guess I could use a plastic cigar tube, but I want something a little thinner for more comfort and better maneuvering in tight spots. The metal pobe tip is evading me.

I am also the Executor to an Estate and that is consuming much of my time. Even when trying to decompress, I feel scattered because I know I should be doing something else. I will be able to focus a bit more come the middle of September.

Thanks for your support and I hope we can pick up where we left off!

If anyone has a line on some sort of pen shaped probe housing that will hold a 630 volt 0.1 uf axial cap and 1/2 watt 470K resister, the evasive tip and out to 2 wires, please point me in that direction. The same with the metal probe tip as well! Thanks again!

See you soon!

Regards//Keith

 

[Enzo]

The tracer probe is a special input cable for an amplifier. You may have a amplifier dedicated to the purpose, but really any amp will work. You need a cap to block any DC in the circuit under test from your tracer amp. It is convenient to put it in the probe body, and I have built various probed in cigar tubes and find them comfortable to use. But the cap can just as easily be in the other end of the probe cable. Lots of room inside a big fat #70 Switchcraft plug for example. Or for that matter if your tracer amp is dedicated, put it inside the amp. All that matters is that you have a cap between the tip of the probe and the input of the amp. Doesn't matter where along the wire that cap lives. Same with the resistor. As long as it is on the right end of the cap, it can be anywhere.


A pen shaped housing? How about an empty pen?