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Mod my SM900?

David Jayne

David Jayne

Jan 16, 2006
16,337
8,096
Brookfield, CT
#1
Would the techies among us chime in here please: I've got a pre Fender SWR SM900 which has always been a little weak power-wise, according to a tech with a meter for measuring it. I don't have the numbers, but anyway he fed it sine waves and measured the output and told me it was sub-standard. That was several years ago. And it seems to have gotten worse, based on my seat-of -the pants, non-scientific measurements(ears). I goes into clipping quite easily, and the pwr amp clipping indicator indicated as such. Same speakers, etc as always. Any chance I could swap out the caps for better ones? Or, do they wear out? They are 80v, 6800uF jobbies, 4 of them. I can supply a schematic if it helps. Or maybe I'm dreaming, whaddya think? Thanks. EDIT: I no longer have the schematic.
 
#2
Not exactly what you are asking, but I worked away at my SM-900 for years, then went to Markbass.Don't want to start a MB praise thing, but the big differences were a) the MB was much easier to EQ for a good sound b) the MB was much less sterile sounding and c) the MB had much more useable volume. It was like night and day, and it was pretty much a straight swap with what I got for the SWR. Just my .02.
 
#3
dmusic148 said:
Would the techies among us chime in here please: I've got a pre Fender SWR SM900 which has always been a little weak power-wise, according to a tech with a meter for measuring it. I don't have the numbers, but anyway he fed it sine waves and measured the output and told me it was sub-standard. That was several years ago. And it seems to have gotten worse, based on my seat-of -the pants, non-scientific measurements(ears). I goes into clipping quite easily, and the pwr amp clipping indicator indicated as such. Same speakers, etc as always. Any chance I could swap out the caps for better ones? Or, do they wear out? They are 80v, 6800uF jobbies, 4 of them. I can supply a schematic if it helps. Or maybe I'm dreaming, whaddya think? Thanks. EDIT: I no longer have the schematic.
Click to expand...

Does it sound fine at lower volumes? Any hum (60 Hz or 120Hz), any distortion? SOMETHING is limiting the power. It COULD be the filter caps, they can go bad with age (that's why many people recommend recapping old tube amps, but there heat is more of an issue and it exacerbates the problem). This is really a job for a tech who really knows what he's doing. If I were doing it I would check the power supply rail voltages with no load (no signal to the amp) and under full power with a dummy load just until the amp clips at maximum power. If there is a significant droop in power supply voltage it could be the filter caps. He could also look at the ripple on the supply rails with a scope to see if it is very large under load. Not knowing anything specifically about SWR amps, the power transformer could be substandard in which it couldn't supply enough voltage at the maximum output power. That isn't very likely though. If there are paralleled output transistors, one could be bad (hard to say what would happen without seeing the schematic though. That could cause a large imbalance and offset voltage at the speaker terminals). It could also be a problem somewhere in the driver board where any component could fail or partially fail that would limit the drive to the output transistors.

There are a number of scenarios but as I said, it requires a pretty savvy tech who could trace through the power amp to find a problem like this. It could be as simple as replacing the filter caps, but it could take quite a bit of time ($$$) trying to trace the problem, even if it's something cheap like a bad resistor or cap.
 
#5
Thanks for the replies. No issues at low volumes, sounds terrific. Sounds great right up to the point where it clips, which really seems to happen too easily-hard to believe I'm really using all of the rated power, that's all. Stereo or bridged, same. More power bridged of course.
No, he didn't bias it(I didn't know there was biasing to be done with a SS amp).
And it has gotten very, very hot many times. Stinking hot. Couldn't I just replace the caps for the hell of it? Or would adjustments then have to be made?
 
#6
dmusic148 said:
Thanks for the replies. No issues at low volumes, sounds terrific. Sounds great right up to the point where it clips, which really seems to happen too easily-hard to believe I'm really using all of the rated power, that's all. Stereo or bridged, same. More power bridged of course.
No, he didn't bias it(I didn't know there was biasing to be done with a SS amp).
And it has gotten very, very hot many times. Stinking hot. Couldn't I just replace the caps for the hell of it? Or would adjustments then have to be made?
Click to expand...

Does it get hot if the amp is just idling? If not, it's not the caps because they could overheat if they are really leaky and that would happen even if the amp is just idling. Leave it on for awhile without playing anything through it. Can you tell exactly what is getting hot? Is it the heatsinks for the output transistors, the power transformer, the caps themselves? Be VERY VERY careful if you start poking around. ALWAYS with the unit unplugged. DISCLAIMER: I'm not recommending that you open the amp. That's at your own risk and only if you know what you're doing.
 
#7
Hi.

Yes several SS designs needs periodical biasing, Steves included. I do remember seeing SM900 procedure in TB, but couldn't find it. Only ST220/SM400.

Don't know if it's correct, but found the SM900 procedure on this site:
Invalid Link Removed

"This is the SWR bias method as follows:
1KHz on signal generator
2 ohm load - one channel at a time
Signal in to Mono Return
Master pot full CW
Effects pot full CW
Bias trimmers full CCW
Variac set to 115VAC RMS
Signal generator output full CCW
Amp on Stereo mode

Raise signal to get 2 volts RMS at output of amp

Set Sweep to 50 µ seconds
Set Volts/Div to .2 volts

Position waveform to find tiny notch in waveform,
turn trimmer CW until notch removed. Same
procedure
for each channel. "

Your tech will probably know, at least he should ;).

The OP devices produce considerably more heat if the bias point is off, so that could be the reason.

Regards
Sam
 
#8
You guys rule, thanks again. I know electronics in general but not amps in particular-they are strange. I know there's high voltage in there, at least for the preamp tube supply-I'll be careful.
I'm letting it idle for a while. In the past when it got really hot it was during use.

When you say 'leaky cap' you mean electrically leaky, right?
 
#9
dmusic148 said:
You guys rule, thanks again. I know electronics in general but not amps in particular-they are strange. I know there's high voltage in there, at least for the preamp tube supply-I'll be careful.
I'm letting it idle for a while. In the past when it got really hot it was during use.

When you say 'leaky cap' you mean electrically leaky, right?
Click to expand...

Yes. But it might not hurt to put a bucket under the amp... Just in case :D
 
#11
OK, after about two hours or so of idling, I took the lid off and measured the temperature with an infrared doohickey. Pretty much everything, including the transformer, output section/heatsinks, and caps had reached about 120˚.
And now that I was paying attention I noticed that yes, it does hum pretty good through the speakers, but not until it warms up for a few minutes. It was a lot worse before I changed the 12ax7 a couple weeks ago.

Had a look at that procedure, looks like I have yet another excuse to get myself a scope.

Probably not useful but:
http://i219.photobucket.com/albums/cc108/dmusic148/DSC01058.jpg
http://i219.photobucket.com/albums/cc108/dmusic148/DSC01059.jpg
 
#12
Hmm... Was it idling while you had the cover off? It's hard to say if everything just heated equally of their own accord or if the heat was just spread because of one thing heating up (e.g. transformer heat conducting through the chassis, etc.). On the other hand, if the heat sinks were that hot of their own accord, it could be that the bias IS set too high (forgot to mention that, one of the other posters did). Seems kind of unusual in a SS amp. My experience has been set it and forget it. Transistors don't drift nearly as much as tubes and so periodic re-biasing isn't really necessary. However, if someone mucked with it at some point, or it was repaired at some point (output devices replaced with out re-biasing?) it could be set wrong... OR... maybe the design is such that it DOES need periodic rebiasing. It might be a good idea to have a tech check the biasing according the earlier posted info.
 
#13
Good point Jerry-hard to say what heated what.
So just now I removed the lid and turned it on for 5 min. It is very obvious that the output section(transistors and heatsink) for the right channel only are getting quite warm. Hmm. And if I lay a finger on the heatsink I can feel it vibrating. The cool one (left channel) does not vibrate. And two of the four large caps are getting warm.EDIT: I'm pretty sure the caps are warming up because they're very close to some big, hot resistors.
 
#15
dmusic148 said:
Good point Jerry-hard to say what heated what.
So just now I removed the lid and turned it on for 5 min. It is very obvious that the output section(transistors and heatsink) for the right channel only are getting quite warm. Hmm. And if I lay a finger on the heatsink I can feel it vibrating. The cool one (left channel) does not vibrate. And two of the four large caps are getting warm.EDIT: I'm pretty sure the caps are warming up because they're very close to some big, hot resistors.
Click to expand...
Well if its that hot you had better read the temperature spec on all the
Lytic caps and replace them all for United chemicon or Nichicon 105 degree types.
I hope we are not talking celsius here on your infra red doohickey at 120. (It had better be old money).
Then re bias it and enjoy for about 25 more years.
Or flog it and get a big saucy old tube amp to re-cap instead, much more fun on turret board than pcb.:ninja:
 
#18
Me thinks new Caps and correct Bias. Caps first then set Bias.

If you havent worked in amps much I suggest you let a pro do it. Those caps can kill ya ifen they aint discharged aforin you take em out.


BTW those Amps sound great!
 
#19
Pre CBS Fenders, Whoops:eek:, I mean pre Fender SWRs owner's manuals used to state that the bias could be knocked out of adjustment by overheating. Conversely correct biasing and correct cabinet selection obeying ohms and proper ventilation prevent mishaps.
The chassis is supposed to get quite warm. It is a heat sink. Not a flaw but a intended design function to pull heat away from internal components.
 
#20
Thanks Chadds, yes, I know the chassis is a heat sink-it's aluminum which helps in this regard. But the design IS flawed, in that if you put it in a 2 space rack box it WILL overheat. This is because the fan blows hot air out the side, then draws it right back in through the top, stupid. It just keep recycling hot air from inside the rack box.
Anyway it has gotten very hot a bunch of times but never enough to shut down. I will replace caps and rebias, I just need a source for the caps-can anyone help? 80v, 6800uF. Cylindrical, end leads(typical style).
 

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