Swr sm-900 dc offset

[bigdrums925]

What are the causes of DC offset? This being a 2 channel amp, one side settles back to 0.0 v and the other side goes to about 65v DC offset.

SWR_SM-900 Schematic Set for Amps Prior to 1997.pdf

[The Dude]

One side of the amp is blown. Could be a shorted transistor. Could be an open transistor. Could be a missing rail on that side. Could be a number of things. You'll have to open it up and see.

[Enzo]

That isn't "offset", that is just DC on the output, which Dude mentions as the amp being "blown". Blown being a generic term for major failure.

Offset would be a couple volts off center.

What could cause it? Shorted output transistor on 65v side. Open transistor on -65v side. Open resistors, broken traces. See the small fuses in teh main voltage lines? One of those open?

Lower left, Q3, what is on its collector? Whatever is there should be on the output..If your 65v is there, then the problem is there or to the left.

And is that 65v on the output or -65?

[bigdrums925]

I think the DC on the output is taken care of.
As I run up the variac though this amp is drawing quite a bit of current.
Thus far I have stopped short of turning it all the way up but at about 70% it's drawing
about an amp through the variac. I would estimate that by the time I reached 100% variac,
it would be drawing 1.5 amps or more.
On the back of the amp near the power plug it says 960 watts.
Dividing 960 watts by 120 volts I get 8 amps. That seems very high to me.
I realize that would be full on with strong signal being amplified and a load, so what would be safe for start up?
Comments?

By the way I occasionally see the meter swing past an amp but only with big tube amps.

[Enzo]

Take away any speaker load, just run the amp by itself. Does it still draw large current during power up?

[bigdrums925]

Yes, the only thing I've had on the output jacks was a 1/4" phono plug and the meter attached to that.

[Enzo]

960 watts for a 900 watt amp? Doesn't sound high to me.

SO your amp comes up on the variac with increasing current, but the output stays at about zero DC?

If you are adventurous, see if the increase continues or if it hits a peak and drops to low levels.

r even this: get it up to your 70% and trigger the power up reset - the circuit upper left.

[nevetslab]

I was looking in my notes on the two SWR SM-900's I just serviced...one being the last generation model having six or seven pairs of MJW21193/21194 pwr Xstrs on each heat sink, and the earlier one having just 2 pairs of Sanken 2SC3264 NPN's in Quasi-comp mode. As I recall, nominal idle current when all is correct is around 350mA. So, you have a fault condition in one of the two amps.

This generation amp is all soldered together, so not real user-friendly regarding service. you'll have to find the failed semi(s) on that one heat sink assy. A shorted driver on top, or an open driver on the bottom would allow the output xstrs to pull north, giving you the +65VDC on the output. Open emitter resistors in the neg half power xstrs would also let the output go north. You're pulling enough current to tell you there's a failed part or two.

When you get the heat sink assy cured, pull up the power supply PCB assy, as that's a constant source of trouble....I'd hazard a guess you have solder fractures on the snap-in caps on that board. Poorly built. Look at all the diodes and zeners on that board, as well as the DC dropping resistors that set up the low-voltage bipolar supply that runs the preamp board up front. And, the plastic standoffs need to be tightened again at both ends (thru the chassis, and thru the PCB). Pwr Xfmr mtg screws are usually loose, often the Grounding screw is loose...which ties the circuit grounds to chassis.

[bigdrums925]

The DC on the outputs seems ok. DC on the output goes up to 19v, but only briefly and settles down quickly to 12mV at 70% variac.
I cleaned the board looking for broken traces and cleaned up some questionable solder joints.

I removed the P/S board and found a cracked solder joint on one of the filter caps and fixed that. And made sure all the screws and ground points are tight.
I haven't checked all the zeners and diodes on the P/S board because someone covered it in hot glue, and the voltages seem correct and balanced.

I have not found any bad transistors, capacitors, resistors or diodes, in a static state anyway.
But it seems a clue that after ramping up the variac to 70%, and trimming down the bias pot, the amp meter on the variac climbs very, very slowly from about .5 amps to 1.1 amps before I shut it down. Maybe a minute's plus time to climb that much, and leaving the variac steady. The outputs transistors are getting hot at that point but the heatsink is removed too.

I can remove either of the +/- 75 volt rails to the affected board and the problem disappears, meaning I can ramp up the variac to 80% with no excessive current draw.
Only drawing .2 amps and steady. Both the rail voltages are good at +/- 57v at 80% variac
I would think if there were a bad component on the amp board, with one rail or the other removed, a shorted part would show itself via the amp meter this way. But the 2 sides (+/- sides of the suspect board) draw the same current and steady. Again about .2 amps.

Looked at the voltage drop across the .1 ohm resistors and they seemed ok too, about .3 - .4 volts. Also removed the 12ax7 to see if that was causing the current to increase slowly. Although it lowered the draw a bit, it did not stop it from climbing.

The current draw stays down low until I reach 70% variac, then rising to .4 amps. If I leave it there the current increases very slowly, to how much I don't know.
So what can be derived from the current draw climbing slowly?

[nevetslab]

Based on your increasing mains current after the output has settled down to nominal DC output level, it may be that you either have a leaky transistor or leaky electrolytic caps. I’d leave the two supply lines connected to this module in question, so it’s running in it’s normal power configuration, and not upset that. You could also run the module at reduced supply levels, of say +/- 50V or so. I don’t recall what the level is where everything becomes stable and will run normally, apart from lacking full headroom.

There are a couple caps that aren’t obvious to me in the circuit. The pair of 47uF/100V electrolytics, both tied to the output line, one (C6) coupling to the base of the upper driver thru a 470 ohm resistor R13, and the other (C7, tying to the neg rail thru R14 270 ohms. It’s not AC bypass around the bias xstr Q4. C6 is tied to the Turn-on circuit via Q16. I’d look at or even replace C6 & C7. I’ve had problems with the LF feedback cap (C4) in various amp circuits that when aged and leaky, will behave much like a DC Amplifier, so that cap is suspect. For that matter, the 47uF cap C3 on the input stage current source might be leaky

You can also lift the two current limiting diodes D2 & D3, just to get that circuit out of the picture. I’ve had miller integrator caps in the driver stage go bad, and if leaky, they’ll drive ya nuts trying to find it (C9 & C12), as the output stage will suddenly latch up north or south at full line voltage (Crest CA-9’s in their upper drive rails).

You’re in for a challenge in tracing it down, but go slowly….find WHERE the current is increasing. Record a set of DC voltages on the 3 stages (input stage Q1, voltage gain stage Q3, driver stage Q7 & Q8, where it’s stable, then increase the supplies to see what’s changing.

[Enzo]

One thing concerning me is this thing has not as far as I read been up to full mains. A lot of amps are unstable at low voltages, and go through a swing between rails as they power up. The loud noise at power up. If you turn the variac only up to that point, the voltages swing away and cannot center themselves. I have not yet been dissuaded from thinking this is part of the creeping current, running on 90v mains. I could be wrong.

[Tubebass]

Current could be creeping up, and transistors getting hot, because the output transistors are not connected to the heatsink.

[bigdrums925]

Working through each of the suggested possible problems.
But how might I check for leaky transistors?

[nevetslab]

Watching to see where the current is increasing from. for example, if the collector current in the second stage (following the collector of the input diff pair output is increasing, but there's no change in the collector voltage of the input stage that drives it, then I'd suspect that secoind stage xstr. If you have one of the two driver xstrs that seems to be the stage that is increasing, driving the following output stage higher and higher, I'd suspect that driver. Short of using a curve tracer, which would require removing the xstrs, it tends to be easier to find WHERE the problem is originating from . Once a suspect is found, then replacing that part would be the next step.

On Caps.....easiest method is replacing them. As 'Tubebass' suggested, make sure the power xstrs and the drivers, bias xstrs are properly mounted to the heat sink (with insulators, of course).

[bigdrums925]

2nd stage drivers:

I monitor the collector voltage on Q6 and it steadily drops from a high of 76mV down to 2.9mV before I detect any increasing current draw on the variac.

Monitoring Q5 collector voltage reaches a high of 22V and drops to 8V with the same sweep on the variac.

Which one seems correct, and shouldn't they be similar?