diyAudio - Marantz 8B

diyAudio (http://www.diyaudio.com/forums/)

- Tubes / Valves (http://www.diyaudio.com/forums/tubes-valves/)

- - Marantz 8B (http://www.diyaudio.com/forums/tubes-valves/229084-marantz-8b.html)

Hi, While recapping and replacing resistors, Inoticed the left channel R29 4.7k res was missing,clearly cut out. was there a reason for this, or should I put one back?And Can anyone one explain what the C12 170-780pf trimmer pot is for.Thanks...Rob

Quote:

Originally Posted by Rws62 (Post 3350506)

.........R29 4.7k res was missing,clearly cut out. was there a reason for this..........Can anyone one explain what the C12 170-780pf trimmer pot is for.Thanks...Rob

Not too easily without a schematic.........

Marantz 8b schematic

marantz 8b schematic

I'm no expert, but it appears that someone was playing around with the global feedback/open loop gain/input sensitivity.

My guess is that Variable C is to tune the feedback for optimal square wave response.

There are plenty of folks here that can give you a definitive answer, like Eli D, SY, KevinKr, DF96 and many others.......

Do I need special equipment or can it be done with a VTM?

Quote:

Originally Posted by boywonder (Post 3350803)

My guess is that Variable C is to tune the feedback for optimal square wave response.

Exactly right. I don't know why the 4k7 was clipped out, but I'd put it back in- it's part of the feedback network.

To adjust feedback, you need a scope and a square wave generator- a voltmeter won't tell you much. Details on how to do it can be found (among other places) in "Building Valve Amplifiers."

Thanks for the help. I guess I'll finish capping ,replace the resistors needed hook it up and listen to it and go from there.If it's not right ,will it be obvious?

Any suggestions on what scope and square wave generator to shoot for?

Quote:

Originally Posted by Rws62 (Post 3350846)

Thanks for the help. I guess I'll finish capping ,replace the resistors needed hook it up and listen to it and go from there.If it's not right ,will it be obvious?

Maybe, maybe not. Sub-optimal square wave performance can mean ringing/overshoot, loss of low freq/high freq, etc. Search for square wave testing, I'm sure you will find something.

Quote:

Originally Posted by Rws62 (Post 3350865)

Any suggestions on what scope and square wave generator to shoot for?

Well, any decent scope and func generator will work. The test is quite straightforward; you input a 10Khz square wave into the amp and connect the scope probe to the speakers outputs, and adjust that cap for the cleanest, sharpest square wave you can get, and it may not end up as clean and sharp as you hope.

Some basic results are:

Rounding of the leading edge of the wave means loss of high freq response

Ringing/overshoot means underdamped circuit and can manifest itself in poor transient response, harsh high freq response, etc.

tilted top of the sq wave means loss of low freq response IIRC.

Look at posts 79-96 in this thread for an idea of scope waveform.

http://www.diyaudio.com/forums/pass-...-gt-pcb-2.html

Although this is a solid state amp, dialing in the feedback compensation is the same idea. I started with a lot of ringing/overshoot, and by tweaking the feedback comp cap value (In you case a variable C, although you could just solder in different values upon test). My final waveform has some rounding on the leading edge but sounds fine, at least to my ears.

Try the official Service Manual. "*" components are described as is being fitted to suit particular output transformers. The resistors with a line through are described as "Fit for different damping factor".
Get your Service Manual here:
http://zilla.li/Resources/PDF/Marant...ice_Manual.pdf
Cheers,.
Ian