View Full Version : Can a bad bias pot on the output blow fuses?


bfreemannh
04-01-2013, 06:31 AM
I just want to know if it is possible if a bad bias pot could blow the output fuse on a solid state amp? I checked all the components on the output board and the only thing I could find wrong was the 50 ohm bias pot was reading in the mega ohms range. After removing the pot, it was reading open.

I just want to be sure i covered all my bases before ordering new pots.

Any insight would be appreciated,

Thank you

ConradH
04-01-2013, 07:14 AM
Absolutely! When a bias pot goes open the output bias current usually goes sky high. If there are fuses they might blow. If not, the output devices usually become fuses. Blown ones. You should check your output devices. If these are actual output fuses, i.e., speaker fuses, that's a bad sign. They would only blow if an output device shorted and put DC on the speaker or the amp railed because is lost control due to high bias. A good design will use fixed resistors around the bias pot so if it does go open the current is still limited to some sane value. A PITA, but more reliable is to establish the right resistance values and then solder them in place. No problem with going open with age and no problem with people misadjusting them.:thmbsp:

bfreemannh
04-01-2013, 09:02 AM
Thanks Conrad.
I'm working on the output of a Pioneer SX-9000. It is blowing the Left channel fuse, fast. The only thing I could find was the failed pot. I have never adjusted the bias of an amp before and not sure what the setting should be. The right side was reading 36.2 ohms and I assume that should be close. I am going to purchase two new multi-turn pots and set them in that range. See what happens.
The service manual does not instruct on how to set the bias on this model, least not the one I have. My searching only comes up with tube amp bias info. Is there a good generic bias setting procedure for solid state that would get me pointed in the right direction?

ryuuoh
04-01-2013, 07:32 PM
Looking at the schematic for the power amp, it looks nearly identical to an Akai AS-970/980 amp. Cap coupled, pots in the same places. I'll bet you could use the alignment procedure from the Akai.
http://www.hifiengine.com/library/akai/as-980.shtml

daveyh
11-26-2013, 10:35 AM
I have blown output transistors from real dirty volume pots by plugging the unit in and turning those scratchy things back and forth before cleaning them thinking that might help. Not anymore. I always clean them now with Deoxit before turning on the power on an old receiver I just got it. Maybe I'm just lucky like that.

GP49
11-26-2013, 11:01 AM
YES, an open idle current adjust pot will cause excessive current in the output stage, as explained.

My procedure:

1. With power OFF, clean the pots with DeOxit, and work them back and forth several times.

2. Check with ohmmeter, that the resistance varies smoothly as you rotate it with no open spots. A good analog ohmmeter actually is better for this than a digital. Note that most bias adjust controls are actually connected not as potentiometers, but as RHEOSTATS. Once you have checked it, LEAVE AT THE MINIMUM BIAS CURRENT POSITION which in most is the minimum resistance (normally zero ohms).

3. Power up with VariAC and monitor the current the unit is drawing.

4. While carefully watching the current draw, slowly increase the bias current by turning the pot. When it starts to rise, STOP and back off to the original idling current. Repeat for the other channel(s).

5. Now you are "in the ballpark" but on the SAFE side. Follow the manufacturer's instructions from here in. If you have a distortion analyzer you can monitor the crossover distortion on an oscilloscope. Verify that it is GONE at the manufacturer's specified idle current, on a 1kHz sinewave at about 1/2 watt output. If not, you will need to research whether that is CORRECT (in which case you have a bad-sounding amplifier by design) or if something else is wrong.

The distortion analyzer and oscilloscope are not necessary; in most cases a properly fixed-up amp will be fine at the manufacturer's specified idle current. But I consider the ability to vary the line voltage and monitor current draw to be indispensable when working on amplifiers. If you are using a dim bulb tester you are out of luck. A VariAC costs what, fifty bucks or so? And a device to monitor the current draw can be built up from a meter and a shunt resistor. Or you can BUY one. The ones intended for test bench use are pretty expensive but the Kill-A-WattŪ, normally used to monitor current draw by appliances and such, works fine and costs about $30.