Bedini 100/100?? Need help to set bias

[sondguy]

Greetings, I'll do my best to shed lite on my dilemna-I have a Bedini thats gone MAD after I installed BG caps on driver boards. Amp gets hotter on lft ch. than rt. ch. Has a hum. We have disconnected both driver bds. & ckd. power supply- both caps power up evenly within a 1/2 volt. Hook up left driver bd. only & get a1.5volt diff. reading on rt . power cap. This uses asplit power supply. b+ & B- give same values on Ps-caps. only wire common toboth bds hooked up is floating grd from PS board. hope this helps!!! Tkx Ronnie

[Sugawi]

Ronnie,
Have you ever resolved this issue? I'm looking for any info on setting bias Bedini amp.

[sondguy]

I will get you some info on where to make hoolups on boards to set bias. Ronnie

[bear]

Some of the factory modules have a trimmer pot on the board.

Iirc, the circuit has a standard Vbe multiplier with fixed resistors.
Trace it out?
I did not record the positions when I repaired one of them and a Strellioff (same thing).

If you swap the left module with the right, does the problem follow?

_-_-bear

[longaniza]

I have one 100/100, And Voltage on speaker terminals is about 150mv. It is high? What could be the problem?
I notice that increasing bias, this voltage go down, but consumption go very high. How much power consume in idle state? This is reading about 220Ws.
I see power transistors are not the same brand.
Thanks

[AVWERK]

Bedini's can take increasing the bias " if " the heat sinks can get rid of the extra heat. So using the touch test start increasing until you feel it's within a safe range, measure across 1 emitter then just set the other channel to match it.

When Bedini shop was in Paramount, CA. I used to go down there from time to time and he mentioned this to me.

Bedini's don't have a zobel network and the amp will eventually self destruct if you don't have a load hooked up... BTDT

Nice with tough electrostatic loads though

(check to see if the power supply has a 10watt resister on one leg to forward bias? My 150 mkII had this along with all mj15024 for outputs (from memory)

Regards
David

[sondguy]

David your memory serves you correctly. All Bedini's use MJ15024 transisters. Do you recall how much the bias factory setting is & how much appox. over can it be raised. I have MKII 150 & it is class AB & 100/100 is class A. Quite a difference in temp when they run. Regards R

[longaniza]

This has a bunch of 2n6254. Tried the touch method, and dc still over 100mv.
Also notice that voltage readings between different .25 ohms resistors varies (47, 38, 55 mv)

[AVWERK]

SG

The 150 had puny stamped sheet metal heat sinks, hence John B.
set the bias accordingly (high bias class A/B). All Bedini's share very similar circuitry. You can run any bias level you want this side of too hot. According to John (at the time) bias level was not a ultra critical component on his amps. (kinda goes along with his personality... if you met him, you would understand)

I pulled those sinks off the 150 and installed huge sinks to run the bias up. When I mentioned to John what I was doing, (Now you can crank it up!) exact words That's what the 100/ 200 series accomplished with many more outputs and bigger sinks.

So do this, start running it up (bias) and set it until you can't touch the heat sinks for more than about 5 to 10 seconds.. Their TO3's and that should put you at what, maybe 55 to 60C. Set it lower if you want.
Really, don't worry yourself over this bias setting, but make sure everything else is in order. I think that series resistor was a 1ohm in one leg of the Power supply and measured differently in reference to ground. They should not be equal ( +/- sides) as I recall

I think he's in Utah and still refurbes them, Google it.

Regards
David

( LG At .25 emitters, those variations are decent enough )

[gearheaddruid]

Avwork, I can agree with your comment about John B. Used to do warranty work for him.
The biasing of the outputs may go a little easier if you mix and match transistors so each channel's emitter resistors have as close a voltage drop as possible. If say one channel you have 30, 45, 50, 28 mv, and the other channel has 25, 32, 55, 50 mv, then swap the transistors so one channel should now have 25, 28, 32, 30 and the other should be 45, 50, 55, 50. This will insure closer current sharing and even heat dissapation. John used to gain match all of his transistors as well as cut the top of the cans off and fill them with epoxy to reduce microphonoics.
Hope my explination made sense.
Regards, Steve