I'm working on my friends solid state amp, which hums really loud when you turn it on and then blows the fuse. The power tranny is fine, the filter caps are fine, and I am unsure how to test the bridge rectifier but I think it is fried. It has 4 terminals on it, clockwise are as follows [ "+", "~", "-" ,"~"]. All of the terminals have continuity to ground except the "-" lead. Does this mean the rectifier is bad, or should I keep looking for the short? Thanks.
The test of the bridge is to measure across each side of it for shorts. STart at one corner and measure resistance to the next corner. Then start at that second corner an measure to the third, etc. Don't want to see shorts. If possible, disconnect the power transformer wires so the transformer winding doesn't interfere with the measurements.
I would be you have some shorted output power transistors.
It would be a lot easier to help if you identified the amplifier.
It's an SWR Bass Twelve Combo.
Measuring the resistance in the same order as before ["+", "~", "-" ,"~"], I get these values:
a) 0.5ohm (+ to ~a)
b) 5.2M ohm (~ to -)
c) 4.5M ohm (- to ~b)
d) 14.9M ohm (~b to +)
I tested the tranny, and the secondary leads still have continuity to ground, even when disconnected. But it doesn't blow fuses when I power it up with the secondaries disconnected.
Also, how do I check the power transistors? I measured for continuity to ground, but there was none.
you have shorted output transistor(s)Originally Posted by fatboy01
Looks like your bridge is indeed shorted. You should not see half an ohm across any two terminals of a bridge. It may look different, but all a bridge is is four diodes in the same package. It is exactly the same as four individual diodes wired as a bridge like you've seen a million times.
You transformer winding has a grounded center tap, and the thing has a low resistance, so it appears like continuity to ground, but it is OK.
Continuity to ground is not usually much help testing things. Transistors are like control valves for current. if the in and out are shorted together, the valve is stuck open. The power transistors are the things mounted on the heat sink. None of the three legs should be shorted to any other.
you need to remove the bridge from the circuit to test the resistance. the circuit will cause incorrect readings. but the bridge might be shorted, take it out and test it to find out for sure.
Yes, but the winding will not show up as a short from AC to + terminal. Any side that shows short will be bad, even in circuit.
not advisable to check diodes or bridge in circuit, must be disconnected. capacitors and other components will cause inaccurate readings. better to be sure before replacing.
Alright, thanks for all the input guys, I appreciate it.
I was pricing out parts, and the rectifier and 2 transistors are all under $1 each, but the two big transistors are $6 each from mouser. I measured the ohms of each transistor (E-B, B-C, E-C) and I got 100ohms E-C on both big ones, with around 12M ohms E-B and B-C. Not sure what that means, perhaps I should pull them out of the circuit before measuring. Is there a good way to measure them to figure out which is shorted, instead of just replacing them all?
Ok, I think it must be the rectifier. I tested the power transistors using a diode test found here:
http://www.anatekcorp.com/qdmmvom.htm
Everything seems normal, with .479v.
Am I jumping to conclusions here, or should I just order a new rectifier and see what happens?
A shorted transistor will show up as shorted even in circuit. If it's open it may not. I've never had capacitors or resistors give false readings when testing rectifiers in circuit. If they are good they will test good in circuit and if they are bad they will test bad in circuit. Put your meter in diode test and go around the bridge. From leg to leg or Anode to Cathode red lead on anode black lead on Cathode(stripe) you should get your .479 or close to that. If your not sure which is which reverse the meter leads and if you don't get that .479 either way your bridge is bad. That's your call on the output transistors but I would change the drivers as well because if they are bad the whole thing could go again. A variac would be a good idea when bringing it back up but if you don't have one you'll have to wing it.
KB
did you take the parts out of the circuit to test?
testing in circuit is not reliable. must disconnect
before checking with ohm meter.
if you found only two resistive paths on transistors
and no reverse paths, then it might only be the rectifier.
I replaced the rectifier, and the amp works now, all seems well. Hopefully the output transistors are stable, time will tell. Thanks for the input guys
(Iain) I'd look up the part numbers, at least. If the PSU on your average SS amp has failed, there is a reason. Usually that something in the poweramp or driver stage is on it's way out.
Being perverse transistors, though, all of the little buggers may last another 10 years. Or 10 weeks. Only they know.
So B+ is the one that hurts when you touch it, yeah?
the transistors are probably ok if you do not hear any 60 cycle hum,
way 2 go fatboy.
Haha thanks a lot man. I may know enough about amps to attempt to fix them, but I started learning and asking questions here at this forum years ago and I still come back here when I get in over my head. Many thanks to you guys who help out.
60?
Consider the source.
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