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2 different Bedini 150/150 Mk II Amps

I was comparing the insides of another Mk II with mine and they are really different. The first one is like my old one with no markings on the transistors. John Bedini was supposed to do some kinda voodoo inside them to work his magic. I remember this detail because my ex-girlfriends daughter cooked that amp 30 years ago by playing it all day at max volume when she was staying home from school one day. My repairman was never able to get any details on those trannys so the amp was never repaired. I guess we can't post pics so I'll just have to describe it.I purchased a different new to me B150MkII recently and the insides are really different. All the trannys are labeled Made in Mexico and there are 4 new ones on the circuit boards along with other different components. The other amp didn't have these 4 extra ones or slots to install them. Everything in my amp looks very fresh and new although it could be just well kept. It sounds fantastic.
I figure there are 3 possible scenarios with my new amp:1.I had read that John Bedini had modified these amps during the run so perhaps I have a later version.2.I had read that JB also would do mods for you when you sent it in for service.3. A really skilled tech modified this with a new design and new circuit boards which I doubt.
Is there anyone out there who could help me out.Also the only thing wrong with mine is the little light in the power switch doesn't work or will sometimes flicker. I can't find the direct replacement so if anyone knows a source could you please let me know. ThanksBobber


skeepowder

Showing 4 responses by minorl

minorl
1,015 posts
 
I’m confused.  What do you need? 

The transistors were MJ 15024 NPN transistors on the output.  Both rails used NPNs in A unique circuit configuration.

you can upgrade them to MJ 21194 NPNs.  The have the same specs but the 21194 is much better and more linear.  I have a 250/250 Mikita that I seriously upgraded.  I spoke with John Bedini long before he passed and he liked the updrafts.  With the newer transistors the amp really sings.

i also upgraded the pre drivers 
minorl
1,015 posts
 

Well first, since John Bedini erased the transistor designations from the transistors (many others did that as well) to try to protect his design, a good technician can reverse engineer the amp and provide the schematic.

When the amp failed  and the owner wanted it repaired, the tech did just that and determined which transistor was used and simply replaced the bad ones with new ones.  Since Motorola transistors were made in Mexico after Motorola sold their rights, that is where the new ones came from.  no big deal.

as far as the power switch is concerned, look on the parts express website or digikey for a replacement.  They are pretty stock/standard type.

my 250/250 mkii looks entirely different inside from the standard model.  I upgraded the power supply to  dual separate power supplies.  All the transistors were upgraded to much better newer more linear transistors.  And I mean all. 

The internal wiring is all Cardas now.  The speaker fuses are gone.  Power cord seriously upgraded.  Threshold style heat sinks added.  Bias increased.  Slow start circuit added.

This amp is really nice.

enjoy

minorl
1,015 posts
 
I believe the upgrade path on the transistors were:
1.  MJ15024 to,
2.  MJ21194.

Those were NPN transistors.  Verify on your amp if they were NPN or PNP.  I believe he used the same design on his amps with both rails using NPN output drivers in a nice circuit configuration.

The DC offset could be from a number of causes.  But, check the output/pre-drivers and bias transistors.  A standard inexpensive transistor checker and you are good to go.

You can upgrade the power supply caps to 42,000 microfarad caps with the soft start circuit implemented.  with bypass caps also attached.

my amp actually had enough internal room that I could separate the power supply and have one for each channel.  with added rectifiers, caps, etc.


I decided that I didn't want to spend the time designing a soft start circuit since there are many already out there, so I (ahem) borrowed a design from DIYAudio.

it is the DIYAudio soft start circuit  2012.  Very simple and inexpensive.

I got my own board and parts and it works very well.

If you can't find it on-line, message me and I'll send you the schematic

Most solid state amps have push/pull complementary NPN/PNP output drivers.  the Bedini amps I've encountered used NPN output drivers on both rails.  Easy to do if your pre-pre drivers are reversed.  I guess he didn't want to try to match NPN/PNP transistors.

enjoy
minorl
1,015 posts
 
The instructions make very good sense and are correct for that amp. You either need the schematic or if you understand circuit design and layout, you can see clearly the emitter resistors.  However, since, you don't know an emitter resistor from a carburetor, I would suggest that a qualified technician is the way to go. 

that is a nice amp.

enjoy 
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