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bogen MO100a monoblocks overhaul

Jcricket

Jcricket

not someday, but today
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Okay folks I am starting on my Bogen MO100A's now.
I spent some time tonight studying the schematics and looking the unit over. It looks like a pretty straightforward schematic and circuit. I went ahead and removed all of the hi-z, low-z. etc stuff and hooked and rca directly to the inut of the 7247 tube. I removed several other "extra" sets of wires too.

When I was done I had the amplifier circuit as shown with only an rca input to the 7247. The standby controller and the other socket, along with all of the switches and volume pot have been disonected and are dead now.

The unit came up nicely with a variac and sounded farily good, but after about 90 seconds it blew the power supply fuse. I checked and didn't see anything wrong, so I replaced the fuse and tried again. It lasted about 15 seconds then blew.

I have done nothing to the components of the circuit yet. All the caps, diodes, an resistors are original.

I am assuming the reason the fuse blowing is due to faulty caps and diodes. Please, someone chime in if I am wrong. Once I am sure it is not a bad PS transformer, then I will continue withthe overhaul.

I have a few ideas on some mods too, but I'll save those until I get into the actual rebuild.

The schematic is attached but it is hard to read. Here is a link to a better version.
http://makearadio.com/schematics/images/bogen-mo100a-7.jpg

Thanks in advance folks!!!
 

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J -- Lift one of the HV winding leads, isolate it, and try it again. The amplifier won't operate of course, but the heaters should still light. If the fuse holds, it is likely power supply component related, but not the power transformer.

Good luck with it!

Dave
 
Hey Dave,
Thanks for the help.
I am assuming you mean one of the red wires, and ideally the one that is connected between C9 and C10 - would that be correct?

Mark
 
wow, how time-appropriate. I just picked one of these things up today and was in the process of info gathering.
 
Dave, I tried that and it worked well. I was able to power up the unit for nearly 5 minutes before I shut it down. The fuse did not blow, so that means the PS components are the culpirt, right? I am going to go ahead and replace all three diodes and all of the caps too, but would you have a guess which component is more likely the problem?

Thanks again,
Mark
 
J -- You did the test just right, and received very good news from it!

Normally, a defective rectifier would cause the fuse to blow rather quickly, and consistently relative to time as well. Since your operating time before the fuse blows has been all over the map, I would more suspect the caps in the V-doubler or the adjoining area.

In any event, your power transformer has certainly passed the test, so you can get on with the usual restorative measures now with some assurance of outcome.

Good luck with it!

Dave
 
Cool. I do have a question though, for you or anyone who can help.
The Power supply consists of three 100uf caps, and then a 500uf@50v(IIRC) for the bias supply.

I typically like to increase the B+ supply caps so that there is plenty oc current available. This unit is a solid state rectified unit. I know on the Tube rectified units youh ave to be very careful about the first stage of the power supply, to big of a cap will burn up the rectifier.

So with this being solid state, can I increase the B+ caps? I have a whole box of 470uf caps that I would love to use, but that is more than quadruppling the supply - is that going to kill my PS transformer or cause other problems?

Mark
 
Use your caps! Just put in a good inrush current limiter on the primary of the transformer and all will be good.

Dave
 
Just out of curiosity, are you staying with 8417 outputs or changing to something else? I took a closer look at the one I picked up and its got two 8417's and two 6L6G tubes. I hope it wasn't run like that because the 360v max plate rating on the 6L6G won't have survived long with B+ at 650v like the Bogen runs. I guess I'll have to check the transformers when I have a chance just to see if they melted down or something.
 
I am staying with the original circuit and tubes. 8417's is what I bought. I got two nos quads off the bay.

Pics of progress to follow soon.
Mark
 
I've got two of these myself. I looked at Bricktop's thread on what he did, modified, etc. Ultimately, I thought what he did to the circuit was excellent, in fact outstanding, because one could use different output tubes besides the 8417.
I'm certainly interested in what you end up with, and your thoughts of sound quality.
Mike
 
I think I will need to read bricktop's thread. I have heard of it but not actually read it.

I'll do that before I go any further.
Mark
 
One of the biggest considerations when using these amplifiers is the output connections. The 16 ohm tap is provided, yet rarely used today. The 25 volt tap equates to a 6.25 ohm tap based on 100 watts RMS being delivered. This tap can be used with 8 ohm speakers, understanding that power delivered will be reduced to about 78 watts. The 25 volt CT is basically unusable for routine home audio use, since it represents a 1.5 ohm tap.

These amplifiers have reportedly excellent specs, and Bogen is a quality manufacturer, although I've never seen any independent testing to verify what the actual performance is.

Keep us posted!

Dave
 
dcgillespie said:
One of the biggest considerations when using these amplifiers is the output connections. The 16 ohm tap is provided, yet rarely used today. The 25 volt tap equates to a 6.25 ohm tap based on 100 watts RMS being delivered. This tap can be used with 8 ohm speakers, understanding that power delivered will be reduced to about 78 watts. The 25 volt CT is basically unusable for routine home audio use, since it represents a 1.5 ohm tap.

These amplifiers have reportedly excellent specs, and Bogen is a quality manufacturer, although I've never seen any independent testing to verify what the actual performance is.

Keep us posted!

Dave
Click to expand...
Only 78 watts? Well that does it, they go in the trash.:D

I have been working on them today and was wondering if I could get some feedback for ideas. If you look at the chassis in the pictures below, you will see the "stuff" on the left side. This is where there was a standby controller and a hookup with an amphenol plug as well as an AC outlet and a few switches.

Since I am doing a complete overhaul, I would like this to look like it was built to look as it does when it is done, not a throw together project.

My first idea was to clean up and re-install the AC outlet(the square hole), then replace the two "sockets" with new 8 pin sockets and put in a pair of OD3 voltage regulators. The screens in the original circuit use 315 volts, I believe the two OD3's will deliver 300v - that should work well I think. Then I would cut a hole where the two switches are and put a can cap on the chassis hooked to the 0D3's. Finally, what looks like a nine pin shielded socket I would replace with a pilot lamp. This should make everything look like it was built that way..

If you folks have any other ideas, please share. Also, when I start the next monoblock, I will create a seperate thread that gives a simple, but detailed version of how to convert these to a stand alone amp - thus if a person waned to just grab one and use it they could.

More to come!!!!
 

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A pair of gas regulators might work out well, though based on recent conversations with Dave (dcgillespie) it seems that hard-setting the screens to some voltage that doesn't float with the B+ line may not be the best way to do things. His stuff is regulated, but it follows the B+, so the screens remain at some percentage of B+ rather than a fixed voltage.

Where the switches are, I'd consider making one a filament switch and the other a B+, so you could warm the heaters up before giving it plate voltage if you felt the need to do that. Solid state rectification on these, so no B+ delay. I don't personally get concerned with that but some do.

Maybe add some convenient means to check the bias balance? This is basically a rocking beam balance pot like Williamson amps used. On those, they fitted 1/4" phone jacks that you'd plug in an a milliammeter to in order to set things. basically the knob was adjusted until both jacks had the same current draw. You could do the same thing with this, or just hang a 10 ohm resistor in the cathode circuit between each end of the balance pot and the cathodes. Simply adjust the balance pot for equal voltage drop across each resistor and your tubes are balanced. It would be fairly easy to install test points topside to give easy connection to that resistor to monitor voltage drop.
 
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gadget73 said:
A pair of gas regulators might work out well, though based on recent conversations with Dave (dcgillespie) it seems that hard-setting the screens to some voltage that doesn't float with the B+ line may not be the best way to do things.

Where the switches are, I'd consider making one a filament switch and the other a B+, so you could warm the heaters up before giving it plate voltage if you felt the need to do that. Solid state rectification on these, so no B+ delay. I don't personally get concerned with that but some do.

Maybe add some convenient means to check the bias balance? This is basically a rocking beam balance pot like Williamson amps used. On those, they fitted 1/4" phone jacks that you'd plug in an a milliammeter to in order to set things. basically the knob was adjusted until both jacks had the same current draw. You could do the same thing with this, or just hang a 10 ohm resistor in the cathode circuit between each end of the balance pot and the cathodes. Simply adjust the balance pot for equal voltage drop across each resistor and your tubes are balanced. It would be fairly easy to install test points topside to give easy connection to that resistor to monitor voltage drop.
Click to expand...

Those are some interesting ideas, but they don't fill the two socket holes. Maybe I could find a few small meters to install there, a volt meter and a milliamp meter. Hmm.................
 
If you had inclinations on a B+ delay, you could probably use an octal plug-in relay to fill the one socket. You could leave the other as an octal socket to use as the test points, sort of like Dynaco did on the ST-70. Not sure if yours had it, but my MO-100a has a jumper plug sort of thing that bypasses the external standby unit. You could just empty that out and use it as a plug cover when not messing with the adjustments, and just pop it out of there to insert meter probes for testing.

How about a magic eye level meter in the 9 pinner? Maybe an EM84 or something.
 
gadget73 said:
A pair of gas regulators might work out well, though based on recent conversations with Dave (dcgillespie) it seems that hard-setting the screens to some voltage that doesn't float with the B+ line may not be the best way to do things. His stuff is regulated, but it follows the B+, so the screens remain at some percentage of B+ rather than a fixed voltage.
Click to expand...

Hard regulation for screen voltage is a good idea IF you also regulate the grid bias supply.
 
BinaryMike said:
Hard regulation for screen voltage is a good idea IF you also regulate the grid bias supply.
Click to expand...

Okay. I can build fairly well, but some of this stuff is over me head.
I can regulate the screens well enough - that is easy. How would you regulate the bias supply. I think I have something like -16.5 volts.

I really dig he magic eye tube idea - that would be too cool. The standby accessory I have as wll and I could just use ut as a static cover until I am adjusting bias or what not. That is another good idea.
 
Agreed Mike. But if you don't regulate the plate supply as well, you've only got two good links in a three link chain. On the other hand, if you arrange to lock the relationship of the grid bias and screen voltage to the plate voltage under quiescent conditions, so that both grid elements then float by the same amount relative to that which the plate supply does, then performance will be maximized with the only loss being power output versus that of a completely regulated design. Advantages include equally low distortion to that achieved from a fully regulated design, and much simpler circuitry.

Dave
 
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