The Bedini 150 MKII sounds good but has been running for 33 years and it's time for a refresh. I've read around the net and two things came up.
1. upgrade the transistors were from the MJ15024 to MJ21194 - NPN
2. change out the yellow caps hanging off the boards to something much better. I don't know what caps these are and can't find a schematic.
Is there anything else I should do? Thanks in advance for your help.
1. upgrade the transistors were from the MJ15024 to MJ21194 - NPN
2. change out the yellow caps hanging off the boards to something much better. I don't know what caps these are and can't find a schematic.
Is there anything else I should do? Thanks in advance for your help.
Attachments
Do not change the output transistors. These amps need closely matched devices to keep good thermal tracking and low offset.
That yellow cap could maybe be refreshed, but is still probably fine.
Quite frankly unless you know what you're doing, leave it alone.
The other problem is that board is pretty fragile, and easilly damaged.
By the way, wrong forum.
That yellow cap could maybe be refreshed, but is still probably fine.
Quite frankly unless you know what you're doing, leave it alone.
The other problem is that board is pretty fragile, and easilly damaged.
By the way, wrong forum.
Those yellow caps look like a mod already. You could replace them with some boutique audio brand. Same with the electrolytics on the two boards - and the two gigantic power supply caps. Pull the parts and test them; replace if anythings wonky measurement wise. Electrlytics dont last forever, the yellow film caps maybe longer than forever ;')
I second the advice to leave the output transistors alone, unless every other one blown or something, seems like opening an opportunity for trouble moreso than a fix that will improve the sound.
I second the advice to leave the output transistors alone, unless every other one blown or something, seems like opening an opportunity for trouble moreso than a fix that will improve the sound.
Just check the tightness of the mounting bolts for the TO-3s.
Also verify the values of each emitter resistor, if you have a good enough meter.
They are likely ok though.
Also verify the values of each emitter resistor, if you have a good enough meter.
They are likely ok though.
Last edited:
Relocated to solid state.. missing transistors
It looks like the amp is missing 4 transistors. There should be 2 on each board
From John Bedini on audiokarma in reply to a person with a 150/150 that was also missing driver transistors:
"Why not put the drivers in the board? You could do this yourself 4 Mj15024's and 4 36 ohm resistors 4 small heat sinks T03's. Then re-set the bias to 35Ma. That amp would really sing. Could you take a better picture of the boards so I can see what it is. My only interest is to make it sound the best for you.
Mod Parts
4 MJ15025's
4 PC Mount Heat Sinks TO3
4 36 Ohm 3 watt resistors
Re-Set Bias to 35 Ma across emitter resistor Positive Side
John Bedini"
It looks like the amp is missing 4 transistors. There should be 2 on each board
From John Bedini on audiokarma in reply to a person with a 150/150 that was also missing driver transistors:
"Why not put the drivers in the board? You could do this yourself 4 Mj15024's and 4 36 ohm resistors 4 small heat sinks T03's. Then re-set the bias to 35Ma. That amp would really sing. Could you take a better picture of the boards so I can see what it is. My only interest is to make it sound the best for you.
Mod Parts
4 MJ15025's
4 PC Mount Heat Sinks TO3
4 36 Ohm 3 watt resistors
Re-Set Bias to 35 Ma across emitter resistor Positive Side
John Bedini"