Output transistors sockets SWR SM-400

[MarkusBass]

I'm looking for the sockets used in SWR SM-400 bass amp (for output transistors).

I'm also looking for the schematic of the version of the amp with a fan: SM-400S - upgrade to the earlier version. I think that fan is necessary in this amp.

Mark

[slawkowski]

hi
take look on the pdf catalog
http://www.keyelco.com/pdfs/prod22-trans-sockets.pdf
and online store
TO-3 Transistor Socket - Thermalloy 8113PF603
good luck
slaw

[MarkusBass]

Thank you very much but the one that I need are printed circuit mounted - slightly different. Maybe I should ask SWR directly.

Mark

[Enzo]

They wouldn't have been custom made. Keep searching for TO-3 transistor sockets. Fender of course now makes SWR products, they may have them, and they could probably provide the schematic you need. Is ther a brand name molded into the connector body?

[MarkusBass]

They wouldn't have been custom made. Keep searching for TO-3 transistor sockets. Fender of course now makes SWR products, they may have them, and they could probably provide the schematic you need.

I cannot find on Fender's web site any e-mail that I could ask technical questions.

Is there a brand name molded into the connector body?

There are "MD" letters in a small circle.

Mark

[Enzo]

Oh... I bet your serial number is lower than 5105.

Now that I look into it a little, I see you must have one of the early style power amps. This WAAAAAAY predates any Fender involvement. I tend now to think they won't have them.


If it were mine, I might try to find something similar and adapt it to fit.

[MarkusBass]

Thanks for the answer. I cleaned the old sockets and asembled the amp. I still have a problem with quiescent current - I get only 2.1V between bases of the output transistors. And this is slightly too low. I'm looking for the schematic of the amp because there are may versions with different components (e.g. sometimes 180 Ohms resistor is replaced with 270 Ohms). I replaced 2SD424 output transistors with MJ15003. This should be OK, I think?

Mark

[Enzo]

Too low? This is a quasi-complementary output, so the plus side bases are at a half a volt or so above the output, whule the negative side bases are that same amount positive from the negative rail. So I'd expect about 50v between them. Now the bases of the drivers I would expect to be about 2v apart. How did you determine you wanted more than 2v, and exactly where is it measured?

I have only seen the one version of the TO3 power amp. REvision C. Basic drawing 6/6/84, revision 2/5/91.

SWR didn't put out schematics for complete models. They had specific drawings for preamps, many of which were shared by models. They they had their basic power amp. The later power amp when they got rid of the TO3s was drawing #1011.

The 1991 drawing says it is used on all models prior to July 1992. ANy minor changes such as a resistor value are not probably done as model specific, it is more likely done to make that standard design more reliable.

[MarkusBass]

Too low? This is a quasi-complementary output, so the plus side bases are at a half a volt or so above the output, whule the negative side bases are that same amount positive from the negative rail. So I'd expect about 50v between them. Now the bases of the drivers I would expect to be about 2v apart. How did you determine you wanted more than 2v, and exactly where is it measured?

Yes, I made a mistake. I ment bases of the drivers. And this is measured between emitter and collector of the temperature sensing transistor (or directly between bases of the drivers). I can change the voltage between 1.5V and 2.1V with a trim pot. But the change does not cause increase of the quiescent current of the output transistors. Let me do some more measurements.
The problem is that I replaced many resistors, capacitors and I'm not sure wheather i did this correctly (in some cases it was not possible to measure the compoments - there was a fire in the amp).

Mark

[Enzo]

The only downside to running cool would be the POSSIBLE increase in crossover distortion. If you cannot hear the crossover distortion, then the cooler the amp can run, the better. The lower that bias voltage is the cooler the amp runs.

[MarkusBass]

The only downside to running cool would be the POSSIBLE increase in crossover distortion. If you cannot hear the crossover distortion, then the cooler the amp can run, the better. The lower that bias voltage is the cooler the amp runs.

I wonder if this amp is in class B by design. I can increase the voltage between bases of the drivers to 2.5V but this is still not enough to cause a quiescent current through the output transistors that I can measure. I get only 0.5 mV on one of 0.1 Ohm ballast resistors. On other resistors I get 0 mV. I think it's time to use a dummy load and a generator. Does anyone know how to set up the quiescent current in this amp? Should the drivers be only considered?
The serial is below 3200. The amp must be quite old. It does not have a fan, which in my opinion is required with this output power.

Mark

[Enzo]

I never use idle current through the outputs as a biasing approach.

I want the outputs set up to be just on the verge of conducting, so any signal will push them into it. But at idle, at most I want a tiny current just so they are ready to conduct. Send a sine wave through, and adjust for minimal crossover notch.

[slawkowski]

hi
this is bias procedure for sm-400/sm-900/st-800
file upload manager does not work for me

set amp to stereo mode
insert generator to mono return jack
connect 2 ohm load to one output side
set balance controll to mid-position
adjust bias trim to full counter-clock
turn amp on
raise signal generator (from 0v) level so that 2volts rms appears at speaker output
monitor signal on scope:
load 2ohms
sweep time 50us volts/div 0.2v
signal generator freq 1khz
adjust bias trimpot just past the point the crossover notch disappears
repeat for other side

slaw

[MarkusBass]

Thanks Slaw. Unfotunatelly I cannot use this pocedure because the amp still has some problems. I checked it yesterday with a dummy load and a generator and there is high crossover distortion. I can decrease it with the bias trim-pot but it is still visible. I'll keep you informed.
BTW, are you Polish (you have definitely Polish name)?

Mark

[slawkowski]

hi
yes, I am Polish amateur noob,and my experience
it is only dozen of vintage hifi tube and transistors amps which I had to
restore or repair,
unfortunately I do have sm-400, years back I had to rebuilt power supply in this amp
which is bad design from factory, this voltage dropping resistors are overheating
surrounding diodes and caps, and indeed this amp should be fan cooled,
now my sm-400 (which is the PITA to work on it) has dc offset to high on one side and
needs service again


BTW I would like to say thank you to moderator and every person in this forum for
sharing their knowledge

slaw