Broken sub amp

[richie00boy]

OK so how did it break then? If you know what you did when it broke the diagnosis becomes easier.

 

[anatech]

Hi Tom,
Don't feel too bad then. We all get surprised from time to time. The power supply capacitors can store a charge for a while. The is worse when there are no bleeder resistors

richie00boy has made a couple good points. In answer, the emitter resistors may be white rectangles near the output transistors. There will be 4 of them. Do you know someone who can lend a hand, just to keep you out of trouble?

-Chris

 

[RiverGmblr]

Sorry about the slower posts. Work takes my time during the week days. I'll try to post a pic tonight so you can see where I shortted it out.

I checked the emitters and all foyr of them read -0.00. Could that be right? I also checked continuity between tha supply rails and the speaker output and got nothing. I'm not shure I did it right though. Do I put one probe on the either + or - speaker out and the other probe on either power in pins? (where you plug it in)? Tom

 

[anatech]

Hi Tom,
With some meters, the low value emitter resistor will read 0 ohms, they are fine.
You measure from the speaker output to each polarity supply and reverse the leads also. One way you may get a diode drop, the other should measure open, or a very high resistance. If you measure 0 ohms, then you have at least one shorted output transistor.

-Chris

 

[RiverGmblr]

Ok, they measure 0 ohms. So how do I find the blown ones? Tom

 

[amplifierguru]

Hi RiverGmblr,

What anatech is saying is that the resolution of many meters will not tell you the true value if it's small, but just read 0. If they're blown they will be high or open cct.

Cheers,
Greg

 

[RiverGmblr]

So, what next? Tom

 

[anatech]

Hi Tom,
The easiest way is to desolder the emitter resistors and measure from each collector (middle lead or supply lead) to the emitter (where the resistor was, closest to the transistor). The short (0 ohms) is blown. I normally recommend to replace the entire channel and driver transistors if one is blown.

If you need to replace the outputs and driver transistors, take pictures to make sure you get them in right, then measure all the low value resistors in the area. Measure also the bias transistor (smaller one on the heatsink).

Greg,
Thanks for explaining.

-Chris

 

[RiverGmblr]

I'll give that a try. Tom

 

[RiverGmblr]

I pulled the emitter resitor and measured. The reading alternates about every second from 0.00 to 417.00. Tom

 

[anatech]

Hi Tom,
Strange, bad connection or leads? See what happens when you short the leads together.

-Chris

 

[RiverGmblr]

The reading is 0.00 when I short the leads together. I might be doing something wrong. I'll try again. Tom

 

[RiverGmblr]

I replaced two of the out put transistors and checked all of the resistors. I couldn't get a reading on the emittor resistors other than 0.00 ohms, should I replace them also? Everything seams to check out good. Still no sub out put. Tom

 

[anatech]

Hi Tom,
The emitter resistors are fine. They are actually somewhere between 0.1 ~0.47 ohms normally. The value is marker on them. Your meter just shows 0 ohms.

You may have other parts blown, so don't power it up yet. Some other things to check. The fuse now, the new ouputs, the smaller transistors (drivers) and the bias transistor. The other resistors in the area. There should be a resistor around 100~470 ohms between the emitters of the driver transistors. There may be others in slightly different locations too. Make sure they are not open. They can be burnt and look just fine. Check everything. Do not assume anything.

-Chris

 

[RiverGmblr]

I found one resistor that is supposed to be 68 ohms but the meter is slow in responding. It reads low about 3 ohms and jumps up to over 140 ohms, then slowly drops down to 67.7 ohms. All other resistors read ok. Tom

 

[anatech]

Hi Tom,
There is most likely a capacitor across your resistor. The final reading shows it to be okay. If you want to make doubly sure about this, desolder one end of the resistor, pull it out of the PCB and measure it again.

-Chris

 

[RiverGmblr]

I rechecked the resistor and it's good. For a few bucks I can replace all the transistors. There is a place localy that stocks them. Should I go ahead and do this or wait to check some other things out? Tom

 

[anatech]

Hi Tom,
I guess you don't have any way of testing the transistors. It may then be cheaper to change the parts. Certianly you won't miss a leaky transistor then. Just make sure you have tested all the other resistors and diodes. Fire it up through a 60W light bulb first.

-Chris