SWR 750

[e_c_mccollum]

Anyone have a schematic for a SWR 750 bass head?? Mainly need power amp... Thanks

[Enzo]

I have three at least, but it is the same circuit they used in most amps.

DO you have the "blue board" version or a later one? Is there a year or a revision number on the board?

[mozwell]

This may help

[J M Fahey]

Wow !!! I find it amazing that they didn´t use ballast resistors on Q12/13/14 emitters, but put them on the collectors, where they have no current stabilizing effect.
Oh well.

[e_c_mccollum]

I have the blue board.

[Enzo]

Here, see if this matches your board.

[J M Fahey]

Well, they did correct the mistake on these blue boards.

[Enzo]

I think the blue boards are the older ones.

[J M Fahey]

Oh well, in that case the Blue ones were better designed, at least on that aspect.
Mind you, you *still* need one current sensing resistor in one of the lower side NPN transistors, for protection/current sensing purposes, which was properly done on the Blue one.
There you have 4 ballast resistors: 3 on emitters (correct) and 1 on 1 collector, for reasons I mentioned (right).
On "the other" board you have 3 ballast resistors in the collectors (2 of them are useless) and none on the emitters.
There is no guarantee those 3 transistors will share current evenly; much less so after they heat up at stage levels.

[zero cool]

Well, they did correct the mistake on these blue boards.

LOL! well they DID make a massive mistake on these Revision A blue boards! I have 2 of them sitting in front of me and Thanks to Enzo (once again!!!) for the schematics, I now understand WTF is going on and WHY ALL of these WILL fail! DOH!

It seems in Fenders infinite wisdom that they saw no reason to connect the collector of Q16 to ANYTHING! that's right, its just flappin in the breeze!~

On both of my boards the foil trace for the collector of Q16 was originally connected to the collector of Q11 which effectively would be a short across R28, so the factory cut the trace right above Q11 But never bothered to place a jumper wire from the collector of Q16 to the collector of Q12, but the trace does loop all around the board but never goes anywhere OOOPS!

So the negative side of the amp only has 2 output transistors in circuit and 3 on the positive side! push these amps hard and im sure you will reach the SOA of the 2 output devices and BLAMMO you have a cheech n Chong moment (up in smoke).

LOL....sheesh!


Zc

[MarkusBass]

That's very interesting - can you make a photo?

Mark

[zero cool]

If you look closely at the photo's, you can see the factory cut just above Q11's collector. also not the path that makes double loops but never goes anywhere from Q16's collector to what was originally Q11's collector before the cut. the factory made the cut but never put in a jumper to connect Q16 to Q12 as per the schematic!!!

So I added a jumper as shown. Q12's emitter has a great big pad on it and the back side is the collector of Q12 where it connects to the Inductor. So i made a cut with a dremel tool and drilled through it for a connection point. So now Q16 is connected the way it is shown in the schematic.

The amp is running now but still has some issues...man fender really messed this thing up! I hate these blue board versions! Now, when running without a load and you reach clipping, the positive side of the waveform gets this nasty little downward spike on the back edge of the flat topped wave form. and something around the area just above C19 and just below Q16's emitter starts smoking. I need to dig it a bit further...I think it is that resistor on top and to the right of R22 but it almost looks as if it is actually the short junction between Q16's emitter and the emitter resistor! maybe that point is getting hot and burning off some flux or something??? or if it is that top 2 watt resistor which i think is R17 then im not sure what the heck is going on. I need to look into the current limiter section and maybe something is funky there i don't know...Enzo??

[zero cool]

and i just noticed that R16-180ohm and R17 270ohm are reversed on this version as compared to every other SWR.

[MarkusBass]

Thanks for the photos. It's clearly a mistake in the design of the board and most probably a mistake during assembly of the module. I'm not sure why blame Fender for this. SWR was sold to Fender in 2003 and this board was designed before year 2000. It looks like the designer of the amp used Sziklai/compound pair: Compound vs Darlington . I'm not sure what is the role of the D8 diode - it looks like it's shorting R22 resistor when the signal is greater than 0.7V (?). If I had time, I'd simulate the amp in SPICE. Amps that use Sziklai pair are prone to oscillations when poorly designed so I wouldn't test it without a load. I would rather check the voltages without signal, especially currents in the output stage. If the resistor (is it R23?) starts burning, it is caused by to high current. Have you measured the currents (R23, R31, R32, R44 and positive rail too)? Maybe the bias is set up way to high, or it does not work at all?

Mark

[zero cool]

R17 is what smokes, Bias works fine, amp runs fine. all voltages are fine, BUT these amps have a speaker on/off switch on the front. if for some reason someone forgets or for any reason the amp should clip with the load off, R17 will smoke and the amp WILL fail again! I have not yet tried to run it to clip with a load. I don't want to take the chance of burning the amp up again!

The interesting this is that R16 at clip has 30vac RMS across it, that's 5 watts across a 2 watt resistor! I replaced C19, made no difference, with one end of R16 lifted the amp clips very heavily on the negative side first but R17 does not even get hot!

SO... at this point I am assuming I have found another design failure (besides the good with Q16 collector) that causes these amps to fail!

I now need to decide how far I need to redesign this thing to get it to run reliably?