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Roberts 770 (Akai M8) mono block amp question.

dread31

dread31

Addicted Member
Okay, I'm just about done rebuilding these things. Just waiting on a new can for the left channel. Due in tomorrow. There is one issue left, however. Both of the VU meters on these guys are stuck, one just above the peg and the other about halfway. They light up, but they don't move. I like meters, but I don't have to have them. Looking over the schematic, it looks as though I can simply disconnect them and remove them from the chassis. And as long as the wire connecting the output transformer to the speaker output remains unbroken, it should be fine.

Is that correct?

Schematic attached.

Dave
 

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I just went and looked at my modified 770x amps and it looks like that's
exactly what I did, so your plan to remove the VU meters from circuit
should work fine.
 
Hey Dave,

Read your PM. The meters on my amps worked fine, so I don't know anything about how to fix your VU meter problem.

Scott
 
Sure you can disconnect the meters but how about trying to use them?

Here's what I would do..... temporarily remove R20, the 5K resistor to ground, and replace it with a 10K or 25K pot with long leads. Remove the wire from the "bottom" of the VU meter that runs to the switch. With the amp on and running, see if you can trim the functionality of the meter with the pot you just installed. If you can, and you get it to an appropriate range, shut off the amp, clip out the pot without touching its shaft and then measure the resistance. Install a fixed resistor (or two in series) of that same value where R20 was.

RWood
 
Did you try zeroing the meters using the small screws on front of the meters? Do this with the amps on and no source signal.
 
RWood said:
Sure you can disconnect the meters but how about trying to use them?

Here's what I would do..... temporarily remove R20, the 5K resistor to ground, and replace it with a 10K or 25K pot with long leads. Remove the wire from the "bottom" of the VU meter that runs to the switch. With the amp on and running, see if you can trim the functionality of the meter with the pot you just installed. If you can, and you get it to an appropriate range, shut off the amp, clip out the pot without touching its shaft and then measure the resistance. Install a fixed resistor (or two in series) of that same value where R20 was.

RWood
Click to expand...

Thanks, RW.

I'll stick a copy of this message in my folder for future reference, but this time I think I'll just take them out.

Dave
 
Cap terminals?

Okay, my new cans arrived this morning.

They are both 20uF + 20uF cans. But, the old ones have three terminals and the new ones have two.

What gives? Are the new ones a different type?

Can I use them, and if so, how?

Or do I need to send them back?

Dave
 

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The one on the right is using the case locking tab as (-).

You can certainly use them but you might have to think about how they are going to be secured and possibly insulated from the chassis if that is a consideration in the circuit. I don't know from what is seen so far.
 
So, the wire formerly connected to the tab with the black dot now goes to the case locking tab?

Dave
 
dread31 said:
So, the wire formerly connected to the tab with the black dot now goes to the case locking tab?

Dave
Click to expand...
Yes. I just looked a pic of an Akai M8 which I assume is similar.... My questions would be...

Will it fit securely into the previous can clamp?
Is it short enough so that it fits without getting in the way of anything else, case top or whatever?
Is chassis ground? ie, in the original implementation is the black dot terminal (-) wired to the chassis in common connection to the can clamp?

If all these are yes you should be ok.
 
I attached the schematic in the OP, if you want to have a look at it.
And yes, the Akai M8 is basically the same amp.

They do stick out further than the old cans, but I will be making new cases for them so this is not really an issue.
I can allow for the longer length. They appear to be the same diameter.

It looks like the ground wire connects to the 0v tap. (In the schematic) Also the old can has an insulating layer of paper and plastic between the clamp and the case, so I'm taking it that it was designed to not be grounded directly to the chassis.

There is a warning on the old can---"This can is not insulated"

Dave
 
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dread31 said:
Also the old can has an insulating layer of paper and plastic between the clamp and the case, so I'm taking it that it was designed to not be grounded directly to the chassis.
Click to expand...

Given that, if you maintain the insulation, then I think you should be good to go and will not have added any extra possibilities for ground loops, etc.
 
Looks like I used the exact same multi-cap cans you bought. Here's some pictures of how mine is wired up.
 
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