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Need help figuring out probable oscillation in SWR Studio 220 Bass Amp.

tusntuk2

tusntuk2

Active Member
I'm restoring an SWR Studio 220 bass amp. The amp had a bad hum, so I replaced all diodes, zener diodes and all electrolytics. I boosted the main caps from 4700 to 6800. Grounding is a possible hum issue due to the design, but more on that later. When I powered it up (DBT) it seemed to whine low to high and the speaker popped foward.

There is some online discussion (old) about making sure the rail voltages are both correct, or the amp is known to oscillate and go quickly to max output. There is a known problem with the bias driver that fails, and another problem with resistor pairs used to knock down main current that easily overheat. I have addressed these known issues by replacing the bias transistor, the pairs of power resistors, and all diodes and electrolytics.

I fired up with a DBT, the bulb dimmed quickly, and measured +/- 14.78v for the rails, (Spec is 15v) and a steady 25.8mv for DC offset at the speaker out. Both under DBT, I know, but the voltages were steady and encouraging. But as soon as I loaded the output with a speaker the telltale whine and speaker cone travel (visible) happened as before. I hesitate to give it full AC.

Any old bass players out there that might have played with this amp before?
 
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Hi Ylli, thanks for joining in. You have the correct unit, the voltage I'm referring to is for the preamp. Googling the problem brings up the discussions about the common failures with the bias and the big resistors I mentioned, so I tried to address those first. Because it seems to happen quickly I have not gotten as far as measuring the power amp rails or the tube voltages. Playing it cautious. Ser.# 344 3339 Take a look at the PS schematic
 

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I think you need to feed the full voltage . I had a similar problem in a SWR "California Blonde" amplifier (solid state): Bright DBT under load, but the problem was I was using a DBT. I think the thread is somewhere here at AK. Try a larger bulb, and a larger dummy load (In my case, loading 10 ohm was OK, connecting 5 ohms wasn't). But everything went fine with full power.

If you are in doubt , fuse the unit and fuse the dummy load or speaker to do this test. But at one point you need to test the unit at full voltage.

Also, I see you can split the unit at the Preamp OUT, Power amp IN, (inserting a plug at the POWER AMP IN jack) so you could test it that whine is produced at the preamp or power amp.

Do you have an oscilloscope or some way to test for an oscillation? Perhaps a large value resistor in series with a test speaker to listen to that "whine" without damaging something.

( BTW, I have my test speakers protected with a polyswitch, any DC or high current event will trip the polyswitch and save the speakers)
 
Thanks elnaldo! My DBT went dim, I'll try larger bulbs. Great idea about the polyswitch - I just stuck some of those on my tweeters, so I have a couple spares to use. (I never buy just one of anything!) And yes, I will try to split the amps. Now, I do have a scope, but it was a gift and I do not know how to use it yet. It's a decent scope, a Textronix 2213, 60mhz dual. I would like to learn about it, but things being as they are...you know, never enough time. Could you detail that speaker/resistor test a little more please?

This amp has a fused speaker output, not a common feature in guitar amps but very welcome.

I do not have a dummy load for the speaker output, which I know I should work on getting. I use cheap computer speakers, so if they pop it's usually free. I've got lots of old cheap computer speakers!

The schematic is minimal, it lacks voltage measurements everywhere, it's in pieces, and it's incomplete. It's hung me up more than once, but I keep trucking on the PCB. I've worked on tubes and analog transistors, so the IC chips are new, too. In short, this is a great learning project.

Sure appreciate the help!
Jim
 
I mean , connecting a resistor in series with the speakers, something like 50 or 100 ohm, it will attenuate the signal or any DC at a level that you can hear the noises, but at a lower level, so it won't damage the speaker.

But if you have an oscilloscope, you can actually "see" what's happening, without the need to guess.

Whay I'd do: a brief test at full voltage, no load, measure DC at the speaker output. If OK, I'd connect a 8 ohm, 5 to 25Watt resistor at the output (you can series-parallel some resistors you have at hand to increase the wattage and approx to 8-10 ohm), and do a brief test at full voltage. If resistor doesn't overheat, no fuses blown, check with some test speakers.

If the unit is not shorted (DBT went dim), I assume it's safe to feed full voltage. Many circuits doesn't work well under a DBT.

Or, invest some time learning the oscilloscope basics, and test the output with the oscilloscope to see if it's oscillating.
 
Very good elnado, I'll get to it and record some output voltage and see if I can apply the scope a bit. I do have the factory manual for the O-scope, just not the time invested in digesting it. Looks like a good opportunity.

Meanwhile, I wish I knew what voltage (or range) is appropriate for the bias on the 220. There is nothing on the schematic, and the O-scope test to limit the "bump" at 1K seems pretty objective. Any ideas? Or maybe it's not sp important? I've been trying to apply my knowledge gained for recapping stereos. and setting bias was important.

Thanks again.
 
20 to 50mA seems to be an average value that shouldn't damage any amplifier. I'd try 20mA and check if everything works OK.
 
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