diyAudio

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)

Rws62 31st January 2013 06:59 PM

Marantz 8B
 
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

boywonder 31st January 2013 10:20 PM

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.........

Rws62 31st January 2013 10:23 PM

Marantz 8b schematic
 
1 Attachment(s)
marantz 8b schematic

boywonder 31st January 2013 10:34 PM

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.......

Rws62 31st January 2013 10:43 PM

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

SY 31st January 2013 10:59 PM

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."

Rws62 31st January 2013 11:06 PM

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?

Rws62 31st January 2013 11:22 PM

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

boywonder 2nd February 2013 02:02 AM

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.

gingertube 18th February 2013 03:37 AM

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


All times are GMT. The time now is 06:29 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio


Content Relevant URLs by vBSEO 3.3.2