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 |
You have to be careful here- when the amp is one taking any readings in it can end in disaster if a probe slips! These instructions make perfect sense to a qualified technician- which is where you should take it!
|
I turned up the bias so the transistors are about 125 f and the fins are about 105 f. I'm not sure how high I can go? I read that it should run hot enough where you can only hold your hand on the fins for about 5-10 seconds. I have a long way to go yet.
This is what I found by searching the web, "Re-Set Bias to 35 Ma across emitter resistor Positive Side". This was written by John Bedini. I always thought current flows through a resistor and so positive and negative shouldn't matter? I have no clue what an emitter resistor is. |
|
My tech repaired my 250/250, he said it was the input differential amps, whatever that means? He had to replace four transistors that were matched.
I've been using it for a while and it runs really cool, maybe 105 degrees fahrenheit on one side, and 100 on the other. Aren't they supposed to run a lot hotter? Should the bias be increased?
|
Are filter caps the same as power supply caps? Yes. It took a fair bit if disassembly to replace the larger caps. I suspect that varies from amp to amp though. |
Are filter caps the same as power supply caps? I only see eight capacitors on this amp, four 2" and four little ones on the boards. It looks pretty simple to me.
|
We had one of the 150s come through our shop recently- desperately in need of new filter caps. Swapping them out was a bit tricky due to the construction, but once done the amp came right around. It proved to be one of the better amps we've had in our system.
|
Thanks, hopefully my tech can fix it first. I hooked it up to an old woofer and it has a pretty good hum also.
I did a quick read at DIYaudio and they sell a speaker protection circuit along with the soft start. I know I bought something like that on eBay a couple of years ago and never used it. It's powered on by 120v has a few second delay. It also senses any DC over 100 mv, then disconnects the speakers to protect them. |
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 |
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.
I just bought a 250/250 MKII this week and there isn't that much information out there. It has RCA MJ15024 transistors. You upgraded to the MJ21194? I read a post from John Bedini where he recommended MJ15025? Mine has dual transformers and 19,000 MFD 75V Sangamd capacitors, hard to read, I'm assuming those are power supply caps? My amp has 75 mv DC offset on one channel, and it reads up to 22 v DC when you turn it on. Hopefully my tech can repair it without a schematic. If it can be repaired I will try to do some mods to it. Can you tell me anything else about your slow start circuit? I think they sell them at DIY audio?
|
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 |
Hi pmotz,
I do believe autospec is Will Vincent's handle on agon. As far as I know,he is one and the same. I remember "autospec"selling a couple of Bedini amps a while ago. I also seem to remember JB mentioning Mr.Vincent in a couple of posts here before he passed away. Best of luck,
Ray |
Rocray, based on your post above I would like to contact Will Vincent about a Bedini amp I have, but not sure of his username. Is he “autospec”? That user sells Will Vincent modified Dynaco amps so he may be the one, or maybe not. Any help appreciated.
|
Hi skeepowder,
You may want to try contacting Will Vincent. I believe he has worked with JB,and is handling some of JB's equipment from his estate. If anyone would have any insight on the differences of these amps,it may be Mr.Vincent. |
Thanks. I don't need anything but the power switch. I was just wondering why everything was so different inside. My buddy was surmising that those Mexican trannys might be an aftermarket repair. After my initial post I found some pics that confirm that my amp was probably a generation 2 design from the factory. There was also another one with 2 transformers (gen 3?) which also has Mex trannys.What's a Mikita?
|
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 |
