KA-701 bias adjustment problem

brillcat

brillcat

Active Member
I've spent some time reading threads about problems with bias adjustment and especially where the pots are hidden for the offset adjustment on the KA-701. Got the offset to spec (0 Vdc) but the the bias pots are turned all the way down (ccw) and I still get 165 mV on the left and 55 mV on the right (spec is 26 mV). Will this cause problems? Thanks for reading this and commenting :)
 
Does adjusting the pots make any difference? That left channel should be unbarebly hot.
 
There is a serious problem with your amp. Don't run it like that, you will blow the outputs. Either there is a problem in the bias regulation circuit, or one channel may be oscillating.
 
I found the schematic online for free. the bias circuit is very simple, there is just a pot and a diode. Check the adjustment pot with your ohm meter. Make sure it can go down to below 33 Ohms. (there is a 33 ohm resistor across it). If you read over 33 ohms across the pot, the 33 ohm resistor is bad and possibly the pot too. D1 and D2 should read 4 diode drops across them. If you have trouble reading the diode with your meter, and the pot is ok, then the diode is bad. they are no longer available. Read the threads on how to replace a STV-4H diode.
 
Thanks for this info dr*audio... I got resistance readings of 95 ohms (left) and 78 ohms (right) across the bias pots when they were turned down to the lowest setting (and the volume control all the way down). The additional testing and/or replacement you suggested is a bit beyond my skill at this time so I am going to take it to my friendly audio tech down in town. Thanks again for the help :)
 
petesbween said:
Thanks for this info dr*audio... I got resistance readings of 95 ohms (left) and 78 ohms (right) across the bias pots when they were turned down to the lowest setting (and the volume control all the way down). The additional testing and/or replacement you suggested is a bit beyond my skill at this time so I am going to take it to my friendly audio tech down in town. Thanks again for the help :)
Click to expand...

Well, that indicates that both bias pots and the resistors across them need to be replaced. If you can solder you can do that yourself.
 
I am pretty handy with the soldering iron, how do I go about locating the resistors on the board? Also I've never ordered parts like this before, where should I buy them? Thanks!
 
The trimpots are VR1 and VR2 in the power amp. The power amp section will be near the heat sink. The resistors should be next to the trimpots, R5, and R6. The resistors are 33 Ohm 1/4 W Carbon Film and the trimpots are 100 Ohm. You will have to measure the pin to pin spacing and make sure you order the right size. Try to get Bourns sealed pots. Digikey has them.
 
go on,

:lurk:

I've been "fixin" on one of these for a while.
Came from a "no output" pile.
Been mostly re-capped (before I got to it).
The 2SK150A was blown on one side.
Got that working so it comes out of protection but she's stll pretty sick.
Maybe this thread will get me excited about probing her some more.
 
DR*, thanks, I just located the resistors on the board and ordered them and the trimpots, I'll let you know how it turns out -- I am learning new stuff all the time on AK, this is a great forum!
 
SUCCESS! Got the trimpots and resistors in the mail today, installed them in about an hour and fired her up. Right channel readily adjusted to the factory spec (26mv) and I can get the left channel down to 28, though it creeps up to 40 after a couple of minutes. Still it's MUCH better than the readings I was getting before (165L, 55R). Thanks dr*audio for the tips, info and encouragement, I learned how to do something new today :)
 
Good job, but you're not quite done yet. The bias creeping up is a bad thing and it means the STV-4 diode is bad. If you let it go, the idle current could continue to creep up and blow the output stage. The diode is no longer available so you are going to have to build a replacement. Search the forums for STV-4 and you should find a solution. It will involve soldering 3 or 4 diodes together.
 
Ahh good suggestion -- Now how do I locate the STV-4? I have a pdf of the wiring diagram but it is very hard to read...
 
Got the diodes in the mail from digikey, yeah :)

Now kind of a dumb question...

The leads are wrapped in red and white tape. I know to solder them in series, but how do I know which way the current flows (red to white? white to red?) on the board so that I don't solder my replacement STV-4H in backwards?

...OK found some info online with a diagram of a typical diode (cathode, anode) and found a diode image printed on the board where the leads solder to it. I think I can figure this out yet.
 
Last edited:
petesbween said:
Got the diodes in the mail from digikey, yeah :)

Now kind of a dumb question...

The leads are wrapped in red and white tape. I know to solder them in series, but how do I know which way the current flows (red to white? white to red?) on the board so that I don't solder my replacement STV-4H in backwards?

...OK found some info online with a diagram of a typical diode (cathode, anode) and found a diode image printed on the board where the leads solder to it. I think I can figure this out yet.
Click to expand...

There should be a line on one end of the body of the part. That's the negative end, same as the line in the diode symbol.
 
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