Weird SWR amp problem.

[Axtman]

I take my SWR California Blonde acoustic guitar amplifier to a gig last night. Everything is fine until I tilt the amp back at about a 45 degree angle. Then I start getting crackling noises. I check the instrument cable and power cable and switch them out. When the amp is vertical it works fine, but once it is tilted back it acts up. The overload LED even blinks.

I tear it apart today and check for loose speaker wires. I also do the chopstick tap on components to isolate the faulty item. I kind of suspect a loose wire or cold solder joint. Well I can't seem to get it to act up.

Any ideas?

[The Dude]

Could it have been an intermittent outlet you were plugged into and not even the amp itself. Maybe when you tilted the amp, the power cord moved just enough to break your AC connection. Sounds goofy, but I've had it happen before.

[glebert]

Don't know the California Blonde, but I know that the Workingman's bass combos had issues with cracked powered transistor legs. They are mounted to a heatsink (that is also tied to the cabinet) and thermal expansion essentially pulls them to pieces. Seems like something like that could cause the problem you described.

[DrGonz78]

Don't know the California Blonde, but I know that the Workingman's bass combos had issues with cracked powered transistor legs. They are mounted to a heatsink (that is also tied to the cabinet) and thermal expansion essentially pulls them to pieces. Seems like something like that could cause the problem you described.

It's just the same as those other Workingman combo's with the cracked legs on the output transistors. Probing with a meter on a California Blonde I was touching the soldered portion connected to the board showing voltages. Then I started doing continuity checks since other voltages were missing in places. At that point I tested for voltage high up right on the pin of the transistor and 0v is what I got. Got out the magnifying glass and sure enough hairline cracks just above the joint on the board right through the legs.

Also, there is a crossover board in the back and I imagine something could be loose in there too. There is a drawing diagram of the California Blonde crossover attached to one of the schematics out there, but it was different than the unit I repaired.

[Axtman]

Well I shook and jiggled the amp to get it to reproduce the problem. Well I did.....and in doing so I let the magic smoke out. One of the resistors of the base of the power transistors burned up. I suspect as stated above, the transistor leg cracked.

One of the issues with the first generation of this amp is that it is hard wired to the speakers. SWR used thin meager 24 gauge speaker wire that looks like telephone wire!

[glebert]

Unlike!

[DrGonz78]

Well I did.....and in doing so I let the magic smoke out. One of the resistors of the base of the power transistors burned up.

What was the resistor that smoked?

[Axtman]

What was the resistor that smoked?

Yes.

[DrGonz78]

No I mean which one was it?? R17???

[Axtman]

I think it is R18. There is not identifying printing on the PCB.



Looks like this resistor may have failed previously. Whoever did the half-arse cobbling of the transistor should be ashamed. What a lame repair. Would it kill you to solder the transistor in correctly?! Geez.



The two output transistors are 2SC2837 and not the typical 2SC3264 transistors that SWR always uses and is indicated on the schematic. Any ideas why the switch?

[DrGonz78]

I think it is R18. There is not identifying printing on the PCB.


Looks like this resistor may have failed previously. Whoever did the half-arse cobbling of the transistor should be ashamed. What a lame repair. Would it kill you to solder the transistor in correctly?! Geez.


The two output transistors are 2SC2837 and not the typical 2SC3264 transistors that SWR always uses and is indicated on the schematic. Any ideas why the switch?

Yeah looks like R18 for sure.

Recently when sourcing the transistors I found Digikey to be a great source for Sanken products. I elected to mount the transistors to the chassis heat sink first then carefully slip the board in place mounted and then soldered the pins. Kind of a PIA but I think it will reduce the stress on the pins that way. No idea why they would use the different transistors.

[Enzo]

You have no C3264 in stock? Find the first thing that remotely fits. No one will ever know....

If Digikey is selling Sankens from stock, great. In the past when I needed a Sanken that was not generally in stock, I had good results from B&D Enterprises.

[Axtman]

Enzo,

My local electronics shop has NTE "equivalent" transistors that I can go pickup today. If I remember correctly you do not like NTE transistors. I think you said they are too ballpark in their values and kind of "one size fits all".

[Enzo]

I will restate my objections to NTE.

The are indeed one size fits all. Look at their catalog, their few thousand parts replace a million other parts. Clearly some compromises are being made. In many cases, transistors can be rather freely substituted, but in cases like power amp outputs with parallel devices, it is crucial they all be close to the same. If two parallel transistors are the same, they track. If they are not, then even with the same voltage and current specs, the two parts may not track thermally, meaning one could run away as it warms. Further, they need to turn on at the same time, if one conducts a little later than the other, the early one will hog current and the slow one won't come on.

A very practical consideration is cost. I buy small TO92 signal transistors for a few cents each. What does NTE get for one in a bag? $1.59? What does a power transistor cost? $12-15? instead of $4 for a real one from Mouser.

Cost aside, if I have four 2SC3264 and one blows, that last thing I want is three of them and an NTE. I would much rather see all four NTE, never mix.

My objection is over cost, and their approximation. I don't claim the parts themselves are poor quality, I just question how closely they match what they are to replace.

[Axtman]

Enzo,

Thank you for the clarification. I will stick with name brand (Sanken) parts.