M-3 Adjustable Bias Mod Question

[Guest]

How in the heck does the -22V get produced? This is the first tube rectifier power supply I've tinkered with, so I am completely in the dark here. Also, what does the -22v feed? I know the -19V goes to the 6V6's, but changing the 56K resistor with a larger value would starve the -22V supply. This is a later M-3 with the PM Speaker.

[geoelectro]

Re: M-3 Adjustable Bias Mod Question



The -22Vdc is created by the center tap of the power transformer being connected to R101 which goes to ground. Normally, these center taps are connected directly to ground where the -100Vdc would be added to the plus side. Since there is a resistor to ground, a negative voltage is made using the back side of the 5U4 rectifiers. This negative is filtered by C62. Then travels through R103 which drops it to -22Vdc. After being filtered again, it ends up at -19Vdc. </p>

The -22Vdc is used in the pedal amplifier circuit, V10. The -19Vdc is the output bias. Common failures are the filters causing the negative voltage to be too low.</p>

George
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[Guest]

Re: M-3 Adjustable Bias Mod Question



Thanks for the quick and very thorough reply, George. I've seen center tapped transformers in power supplies before, but I'm still uncertain how R101 creates a negative voltage. Wouldn't it be positive in relation to ground, or is my thinking all screwed up? Also I'm not really sure what the -22V does in the pedal amp circuit now that you've told me where it is. Looks like a bias for that pentode's control grid, but what's with all the massive resistors? And how would a higher/lower bias voltage control the pedal delay?
</p>

[geoelectro]

Re: M-3 Adjustable Bias Mod Question



The transformer alternates it's voltage 60CPS. Let's look at it in one cycle only. Make the top of the secondary winding positive, and the bottom of the winding negative. The positive goes into the rectifier tube and into the load to ground. The negative side does not go into the rectifier as it is blocked by the rectifier. Since the resistance from the negative part of the secondary to the center tap is lower than the 180 OHM resistor (R101) the center tap goes negative. Like a voltage divider to ground. The size of R101 determines how negative the voltage. Also, whatever negative voltage there is, that voltage is subtracted from the positive. So in effect, the ground is just moved so that if falls between the negative and positive voltages.</p>

In a normal full wave, the center tap would have been at ground. That would cause the lower half of the secondary to just turn off, and the full swing of the upper half would go into the rectifier. With the no ground, the center tap is able to swing negative with the sacrifice of full power going to the rectifier. It's a simple way to derive a negative voltage and was also a good way to create a voltage to power a field coil speaker. They no longer needed the field coil voltage so the resistor was put in because they still needed the negative volts.
</p>

As for how it's used in the pedal amp, the negative voltage cuts off the tube until a pedal is pressed. The various resistors are there to control the different sustains.</p>

George
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</p>

[Guest]

Re: M-3 Adjustable Bias Mod Question

O.K. that makes a lot of sense. So, to install an adjustable bias, I should replace R101 with something like a 500 Ohm pot? This would increase the negative voltage going to the output tubes whilst maintaining a large enough negative voltage to keep the pedal amp cutoff until activated.

[geoelectro]

Re: M-3 Adjustable Bias Mod Question



No.Changing R101 would also change the positive voltage as well. If you don't need more then -22Vdc., you could put a bias pot across the -22Vdc and ground. You can also look at an older field coil schematic. There, they have -100Vdc but in order for that you have to change some other resistors to keep the positive voltage right. Could be done.</p>

</p>

George
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[fredy2]

Re: M-3 Adjustable Bias Mod Question

When making a variable source, never use all of the resistance from a pot. It is too easy to go to extreme ends of a pot. Use some fixed resistance in series as well. ALSO connect one end of the pot to the wiper if using in rheostat mode. Then if wiper momentarily opens, it won't smoke the works it will just go to max setting.

[fredy2]

Re: M-3 Adjustable Bias Mod Question



Ever run your Triton from your FX1 via MIDI ?</P>


I have about 20 synths tied to my FX20.</P>


The nice thing is that the solo Kbd is three note poly emitted to MIDI.</P>

[geoelectro]

Re: M-3 Adjustable Bias Mod Question



Hey Fred, nope, not until I get the MIDI interface from you Heh heh. Only had the FX-1 for two weeks. Plugged in the MDR-1 and discovered that I don't have any disks for it. So I bought some RP-1's off eBay. Let me know when I can get the MIDI adapter from you. In fact, please send me the brochure.</p>

[email protected]</p>

Thanks,</p>

George
</p>

[toasterDude]

Re: M-3 Adjustable Bias Mod Question

[quote user="geoelectro"]


please send me the brochure.</P>


george@<U>bentonelectronics.com</U></P>


Thanks,</P>


George
</P>


[/quote]</P>


... oh hey, you're the guy who put up the excellent site about Hammond/Leslie servicing! (right?) If so, may I just say THANK YOU for the extremely detailed and helpful guides on your site. I just picked up a '59 C3 in the spring, that wouldn't start (pinion gear was stuck on its shaft and couldn't engage the starter gear)... your site enabled me to have it up and running in 10 minutes! So again a big thanks for the info. cheers, Scott</P>

[fredy2]

Re: M-3 Adjustable Bias Mod Question



George, the CFD2 disks for the MDR1 are very hard to get. They were used on Amstrad computers that were in England and occassionally you can find them from England... They often go for $6 to $10.</P>


If the MDR case was bigger I could retrofit with a 3.5 inch floppy. I have externally connected a 3.5 inch floppy and also a 5.25 inch floppy to teh MDR1 unit and successfully recorded and played back from them. The cabling is just aa bit tricky due to the way the unit is "unit selected". The 3.5 drive was a mitsubishi brand... not sure if newer ones will work... the size horizontally precludes putting the 3.5 drive into the MDR1 cabinet.</P>


I did do a retrofit for one individual where he made a new top case (wasn't pretty) and I mounted the 3.5 drive above the rest of the internals and also had a switch to use either drive... the 3.5 or the 2 inch CFD.</P>


I had toyed with having metal work done to make a professional conversion but there isn't enough of them to warrant the production expense at this time.</P>


I found a source... they have reallygone up in price:</P>


http://www.clive.nl/detail/24336/</P>


Whew !!!!</P>

[Guest]

Re: M-3 Adjustable Bias Mod Question

<BLOCKQUOTE>


This is what happens when keyboard manufacturers liscense royalty rich format standards with companies whose executives are wining and dining away all the research and development funds :)</P></BLOCKQUOTE>

[geoelectro]

Re: M-3 Adjustable Bias Mod Question

[quote user="toasterDude"][quote user="geoelectro"]


please send me the brochure.</p>


george@<u>bentonelectronics.com</u></p>


Thanks,</p>


George
</p>


[/quote]</p>


... oh hey, you're the guy who put up the excellent site about Hammond/Leslie servicing! (right?) If so, may I just say THANK YOU for the extremely detailed and helpful guides on your site. I just picked up a '59 C3 in the spring, that wouldn't start (pinion gear was stuck on its shaft and couldn't engage the starter gear)... your site enabled me to have it up and running in 10 minutes! So again a big thanks for the info. cheers, Scott</p>

[/quote]</p>

Thanks Scott. Very glad it helped!</p>

George[:)]
</p>

[bluetantra]

Re: M-3 Adjustable Bias Mod Question



George,</P>


I'd also like to thank you for taking the time to write and put up that site. I've learned so much from reading your service notes! It's good to see you on this forum. Welcome! Out of curiosity, have you considered adding instructions for rebuilding an AO-29 to the guides you've written? I'm sure I'm not alone in wishing for something as thoughtful and insightful as you've put together on other topics.</P>


Best,</P>


John Phillips</P>

[geoelectro]

Re: M-3 Adjustable Bias Mod Question



Thanks for the kind words. Yes, I have thought about giving the M-3 a similar treatment. That certainly would be a logical step. There are many more models that I could do as well. L-series, M-100 series, H-series, E-series etc. Hmm... where to get the time. Since I work full time in the service business and play at church, this may have to wait until I retire! [;)] </p>

George
</p>