SWR SM-900 transistor question

[dragracer]

I am repairing a SWR SM900 - I have traced the problem down to the right channel output board where two of the 4 output transistors are bad. I pulled the two bad ones out of the PCB and tested them. Does anyone know what transistors these are? ( see pics ). I was expecting them to be 2SC3264's. Does anyone know if these are equivalent - the markings on them are ERS58? I believe someone replaced the original ones with these. They also had an issue with emitter leg... putting a wire between it and the PCB - so i am guessing they aren't equivalent and this could be a 2SC3258?

https://music-electronics-forum.com/...&thumb=1&stc=1

[DrGonz78]

ERS is a replacement parts company just like ECG or NTE. Looking at this ebay listing it shows "1 Piece | 2SC2773 Sanken NPN Transistors Equiv: NTE58 ECG58" in the items description. ERS58 should be equal to ECG58 or NTE58 as far as I can tell, so that would make your part a 2SC2773 if correct. I would just get the correct replacement parts from a reputable vendor, digikey might have these or B&D.

https://www.ebay.com/itm/1-Piece-2SC...5.c10#viTabs_0

[dragracer]

Thanks for the info on ERS! I think I am going to put 2SC3264's in there based on the schematic. The only reason for the hesitation is because SM900 schematics always seem to be a little inconsistent.

[TimmyP1955]

According to the schematics I have, they are 2SC3264 in later ones, 2SD424 or 2SC3482 in older ones.

[dragracer]

Thanks! i have the same schematics except for that left side schematic. Thanks again!

[nevetslab]

SWR SM900 Evolution

Thanks for the info on ERS! I think I am going to put 2SC3264's in there based on the schematic. The only reason for the hesitation is because SM900 schematics always seem to be a little inconsistent.

There have been 3 versions of the SM900, as far as I've seen in our rental inventory. From the front, they all pretty much look the same, but radically different inside. Some images of the amp modules follow:



The 1st image used 2SC3264 NPN's in a quasi-comp design, 2 pairs per channel. The second image used the four pairs of 2SA1295/2SC3264's, though I've seen 2SA1494/2SC3858's used in this one, and the third image used the seven pairs of MJW21193/MJW21194's.

SANKEN_2SA1295.pdf
SANKEN_2SC3264.pdf
SANKEN_2SA1494.pdf
SANKEN_2SC3858.pdf

[dragracer]

That is really good information. The one I am working on is the 1st image you posted. 2SC3264's are the right transistors to put in there. The real question is now why did those two transistors die and why was there a wire between the emitter and the PCB? What have you found in your experience with these amps?

[DrGonz78]

Possible that the wire running from the emitter to the pcb was a fix due to a cracked transistor leg.

[nevetslab]

What DrGonz 78 said ^^^^^^, or perhaps a broken trace, to restore the xstr's emitter to the circuit?

As to WHY the pair on the one side failed........were these the non-Sanken Substitute parts? If so, I haven't seen any details on that non-standard part. As a general rule, all of us here stay with standard industry parts, and not the NTE or equiv substitute parts. When we have the original part's specs, and those parts are no longer available, we'll be looking to use equal/better replacement parts, again industry standard parts. There are re-marked 'fake' parts out there on ebay and other places, so beware. I'm just guessing that the substitute parts weren't a proper equivalent to the Sanken parts, and failed when pushed?

[The Dude]

^^^^^^Yes, that. Another possiblility is that side of the amp wasn't biased properly after the repairs were made and substitute transistors were installed. There are lots of "techs" out there that just throw the parts in, see if the amp makes sound, and kick it out the door.

[dragracer]

Yup, non-Sanken parts... There is so much evidence that previous work has been done on this PCB. I can see a few pads lifted too so I am going to make sure that everything is connected properly. I'm a little concerned that a continuity check may be good when it's cold but then when I apply power, it truly isn't connected and there goes the transistors.. I would bet that all of you are correct and the parts were thrown in without biasing.

[nevetslab]

Yup, non-Sanken parts... There is so much evidence that previous work has been done on this PCB. I can see a few pads lifted too so I am going to make sure that everything is connected properly. I'm a little concerned that a continuity check may be good when it's cold but then when I apply power, it truly isn't connected and there goes the transistors.. I would bet that all of you are correct and the parts were thrown in without biasing.

While you're working on that 1st generation SM 900, take a good look at the power supply assembly. It's a PITA to get up off the chassis, often hampered with the standoffs spinnning to fight you while attempting to remove the mounting hardware from the bottom, but, invariably, the snap-in filter caps are no longer making good solder contact, along with the 5W power resistors that are standing off the surface of the PCB having the ability to start lifting the traces. Cleaning up that poorly designed PCB, which at best was a prototype layout that made it into production by being behind in schedule, so all of us servicing them years later are doing the best we can to keep it running.

Power Transformer mtg hardware is Always loose when the arrive at the shop. The dual concentric EQ pots' inside knob/shaft gets bent with the greatest of ease. I've been able to knock it back into alignment by carefully hitting the knob in the opposite direction lightly enough to not break the knob, nor break the shaft, and must have the magic touch, as so far, after years of keeping these amps alive, I've yet to break one of those pot assemblies. NO MORE Parts available for those, since Fender bought up SWR, and discontinued everything.

[dragracer]

First things i checked where the transformer and the power supply assembly. The complaint was that the main fuse blows as soon as power is applied. You could see on the power supply pcb where it had burned previously. After reading your reply, I think I might reflow those capacitor contacts just in case. I isolated the transformer from the rest of the circuit and applied power (variac) and it's fine. I checked the caps, resistors and diodes on the power supply assembly and then tested the transformer and power supply assembly together and the voltages all checked out. I then disconnected the output PCBS and only connected the preamp pcbs and did some probing and it seems ok - this lead me to the output channels.

[dragracer]

Good news! Replaced the transistors, double-checked everything and it works! However, tried to bias it with the SWR method and I can't tell if there is any crossover notch there at all. I turned the two pots ccw and left them there for now.

[The Dude]

Probably too late, but I'd have measured the output emitter resistor voltage drop on the good channel and matched the repaired channel to that. It would still be a good idea to check that there isn't too much current flow at idle on the repaired channel. Often, amps will make sound when there is still a problem.