SWR Strudio 220 5kHz Oacillation Problems

[nevetslab]

One of the staff members brought me a friend of his' SWR Studio 220 Bass Head for repair. I was told it didn't power up, but it came up just fine. What also came just fine is a 5khz oscillation, when the Treble Control is Pulled out into it's Transparency' position, and set anywhere from 12 o'clock or higher, Master Vol anywhere from 12 o'clock and higher, Gain anywhere above 3 o'clock, and the Aural Enhancement anywhere above CCW.

These are built as poorly as the SM400. Nothing I really care to have to work on, but, I agreed to take a look at it. I first replaced the 12AX7 preamp tube, as the one installed also had a nasty white noise problem, alternate tube got rid of that. I tried a 12AY7...a bit less gain, which dropped the oscillation level, but didn't remove it. A 12AT7 lowered it more, but also didn't remove it. Tried swapping out the TL072 BiFET op amps, with no success. Tried some slower dual op amps.....around the Treble Control stage, where all the EQ's pots are common, as well as input stage, output stage, etc. Couldn't kill it with swapping IC's out...didn't expect to, but easy to get at. I did NOT see any evidence of power supply bypass around any of the op amps, unless tack-soldered on the rear side of the PCB.

So, reluctantly, I pulled all the pot knobs & hardware off and front panel off in order to gain access to the bottom side of the PCB. Really built like kaka as far as servicing goes. Added 0.1uF/100V mono caps across all the op amp supply connections to ground. Had serious doubts that would cure anything, but.....they should be there anyhow. As expected, that made no difference at all (had put the board and hardware back together, so there was solid connections and grounding of the panel, jacks, etc.

The biggest offender appears to be the Transparency control....but, even leaving that control dis-engaged, I can dial up oscillation by turning the Master up along with the Treble up. Turning the Gain down, Master down a touch, Treble down a touch, Aural Enhancement down all the way will kill the oscillation.

I didn't find any solder joint issues on the bottom of the PCB, all connections appear ok It behaves as though 5kHz oscillation is designed in.

Anyone run into this issue with this model, or the SM400, which is very similar? Presently sorry I ever said yes to service this horribly built amp.

[Enzo]

The amp doesn't need to be modified, it needs to be repaired. Poor design or not, it used to work, they all work.

Isolate the problem. Is your power amp loaded? If so, does the symptom remain when unloaded? I realize the preamp seems to be the center of it, but if the power amp is modulating the power supply, it could feed back through the rails. Does poking a plug into the FX return to disconnect the PA have an effect? Scope the preamp power rails, is your oscillation riding either rail? If you said it, I missed it, but is this osc signal present all through the preamp? Or only some stages? Where does it seem to arise?

[nevetslab]

In trying to isolate the problem....if I dead-patch the power amp with a 600 ohm terminator, which also breaks the Normal from the line amp, no oscillation. The power amp on its' own isn't oscillating, nor is the preamp circuits. Looking with my scope, when I remove the dead patch, I find the oscillation on the line amp circuit, the EQ summing stage before it (common differential circuit to all the boost/cut pots from Bass, Treble and Graphic EQ pots), the buffer stage following the Aural Enhancer tube stage. I don't see it on the preamp input tube (12AX7, nor the gain stage following it, or the Limiter circuit, nor on the plate of the Aural Enhancer tube stage. Not sure if the biFET op amp stage following the Aural Enhancer has it or not, not having a true road map of the preamp circuit to guide me. Dead-patching the Effects Feturn also kills the oscillation.

With the power amp input dead-patched, I do see some lower level oscillation signal on the line amp stage following the master volume, but not the rest of the circuits, until I remove the dead patch to the power amp input.

I don't have the actual preamp schematic, so I'm working off of an SM400 Preamp Schematic....which has no reference designators anywhere on it.

When the overall circuit is oscillating, I do see it at low level on the bipolar supplies. It will oscillate whether the solid state power amp is loaded or not (using a 4 ohm speaker when loaded).

It's NOT sensitive to wire placement of the input cables, power supply cables, line amp cables to the rear panel power amp input jacks. Loading the preamp output doesn't affect the instability either.

I looked at an SWR SM400 in our rental department....very similar to this Studio 220 Bass Amp, with minor differences. It was stable, no hint of the problem this amp has.

I lifted out the power amp module, cleaned all the flux off the PCB, as it was never cleaned from manufacturing, but as I expected, that made no difference...the oscillation frequency being quite low (5kHz)

While moving wiring around...notably the four wiper leads from the graphic EQ slide pots, each with a resistor in series to a PCB-mounted cap.....touching any of them with finger capacitance to ground killed the oscillation. Touching the Cut rail with finger capacitance also killed it. For shits and giggles, I placed a 3300 pF cap from Ground to that Cut rail, and with no speaker attached, I didn't get oscillation...but as soon as I DID connect the speaker, the oscillation was present and Lower in frequency, so removed that.

One thought was to add a build-out resistor from the output of the EQ stage to the top of the Master Volume...that being the next circuit following it....if this amp is like the schematic of the SM400 I have. I tried removing series RC circuit associated with the 'Transparency Pull Switch on the Treble Boost/Cut pot.....there is a series RC network that looks like it lowers the effective capacitor value of the Treble Circuit....there's one in the SWR Redhead preamp schematic that does that....or remove that circuit. Unfortunately, I can still dial in the oscillation..., and tune it, using the Frequency controls of the Graphic EQ circuit.

So, for now, the oscillation is a permanent feature of this amp. A good mate to the Mesa Dual Rectifier Triple Solo head I still have here in the shop that's unresolved.

I have not tried replacing the main power supply filter caps for the power amp circuit.

[Enzo]

When I get to my shop next, I'll check my paper files for the Studio 220, I believe it is in there. I don't have it on my hard drive here.

Sounds like the amp is borderline stable, and the power amp is part of the feedback loop.

[Jazz P Bass]

Here is the schematic.

Studio 220.zip

[MarkusBass]

I would check 100pF capacitor in the anode of the tube.
There were many complaints regarding the Aural Enhancer circuit in this (as well as SM-400) amp. Several guys (including myself) were modifying the circuit to get better frequency response of the preamp. Here are some details: SWR Aural Enhancer Bypass | TalkBass.com
Another issue with this amp is that the ground reference for the input of the power amp is provided with the power amp input wire (and it's separated from the power ground from the power supply). So it is not allowed to disconnect the input wire and expect the power amp to work correctly.
Yet another issue is that the ground is connected to the enclosure in several points (preamp input and power amp output at least).
EDIT: there are also several capacitors missing (that reduce the frequency response of the amp at high frequencies). Look for example at the opamp after the MASTER pot and just before the power amp input. This is really very bad design.

Mark

[nevetslab]

Ah, many thanks on the Studio 220 Schematic!

Steven

[nevetslab]

Many thanks on all this input as well. I had a feeling modifications were going to be needed to shut this amp up. I'll check into the grounding scheme first, and report back. Many thanks

Steven

[nevetslab]

Power Amp input Grounding issues

After downloading the schematics Jazz P Bass provided, and pulled the schematic for the power amp I had in my SWR Redhead folder (for the Studio 220, and other models), and had the power amp lifted out along with removing the front panel, preamp assembly attached so all could be viewed....I didn't see a direct path from the Power Amp's input ground to Chassis Gnd, where the output ground/power supply ground common up. Chopped the wiring off the Preamp output/Power Amp input jacks, both being insulated Hi-D jacks., The input jacks' bushing fractured and broke on me. I hadn't intended on replacing it with a metal bushing Hi-D jack, having the input Normal switching contact on it, but...that's what I had on hand and used. Re-terminated all the wiring as they had it wired up. Difference now being the Signal Ground is connected to the Power Amp Output Ground at the rear panel Ground plane. There were also four bottom panel screws missing. Power Supply and Safety Ground are common to the Power Amp's Output Ground by mechanical connections of those two panels only...no wiring between the two panels.

So, with the added screws installed, and all those tightened securely, the power amp's input circuit now grounded (and the preamp circuit ground also grounded from this end as well......I put it back together to hear how bad the hum was going to be. It might have come up a little bit, but.....no way to tell, as all I ever had in the operating mode was 5kHz oscillation. Now, the oscillation is totally gone. What little bit of hum there is, totally acceptable...way in the background relative to signal level.

It WAS a grounding issue. Good call, Mark!! I didn't change anything else....just installed a grounded bushing switching input jack, freshly terminated wiring, added the missing chassis bottom panel screws, and it sounds just fine now. Back to billable hours again.