Will lightbulb protect my amp from to high idle current at startup? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 18th September 2009, 02:26 PM   #1
seafire is offline seafire  South Africa
diyAudio Member
Join Date: Feb 2009
Default Will lightbulb protect my amp from to high idle current at startup?

Hi everybody

I am just about done repairing my amp once again that I re-fried whilst adjusting idle current on old sticky pods.Problem is that as a newbie I am a little confused.The pods seem to work in reverse to each other on the left and right channels.(amp is a dual mono construction amp).RH channel pod resistance measures 2.94 k ohm as set originally.LH channel measures 3.67 k ohm std setting.Both driver boards have the pods facing upwards so that they can be adjusted from the top (cover removed).
The confusion I have is that from what I understand the pods should reduce current anti clockwise and increase current clockwise.This however doesn't seem to be the case here.R ch pod measures 17.5 ohm fully closed and 10 kilo ohm fully open.The L ch pod if faced from the top of the amp looking down is just the opposite!..ie. 17.5 fully open (clockwise) and 10 k ohm fully closed.So this newbie's conlusion is that the gain is set in reverse on the LH channel.My other question is as follows:I will be using a 40w light bulb in series on start up on the mains.Will this protect the amp (MOSFETS) from over current frying on startup if I have the idle current settings wrong again?.The amp is a Marantz pm 94
HELP Seafire
  Reply With Quote
Old 18th September 2009, 02:42 PM   #2
jaycee is offline jaycee  United Kingdom
diyAudio Member
Join Date: Apr 2004
Location: Norwich, UK
Yes, but you should also be monitoring the idle current rather than simply checking resistance of the bias pot. If you can get the PM-94 service manual, it will tell you exactly how to adjust the idle current - IIRC it is more complicated than usual due to the "Quarter A" circuit in this amp.
  Reply With Quote
Old 18th September 2009, 03:06 PM   #3
seafire is offline seafire  South Africa
diyAudio Member
Join Date: Feb 2009
Thank you Jaycee.I do have the service manual but nowhere does it state in what direction the pods work and what a starting setup would be.It merely gives the procedure and the idle currents.anymore suggestions guys?
  Reply With Quote
Old 18th September 2009, 03:36 PM   #4
AndrewT is online now AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
I doubt a 40W bulb will allow setting the bias, the PSU voltage will drop rapidly as the bias is turned up, even a 150W bulb will probably not allow bias setting anywhere near accurately. But once started and you understand what the two adjustment pots are doing then increase the bulb rating to get you closer to what you need. Remember, when you finally bypass the bulb, that the bias current will increase massively compared to using the bulb in-line. Maybe even double or worse.

The lower the bulb rating, the less current that can be passed to the load (equipment under test).
regards Andrew T.
Sent from my desktop computer using a keyboard
  Reply With Quote
Old 18th September 2009, 04:10 PM   #5
seafire is offline seafire  South Africa
diyAudio Member
Join Date: Feb 2009
Thank you AndrewT.
That is what I was thinking.I will fire up the amp with a 40w to first check all the parts replaced.First the comparator without the power sections.Then add the output drives and figure out exactly what the pots do.Then (As per your advice) I will set the current way down before attempting adjustment without the lightbulb.
Hopefully somebody can post that owns one of these Marantz pm 94's or has worked on some and give me the run down.
Hold thumbs for this newbie as my baptism in fire has cost me dearly!.
So much that I would love to build my own amp soon but will post for advice on that another day.
Regards Seafire
  Reply With Quote
Old 18th September 2009, 05:37 PM   #6
diyAudio Member
EchoWars's Avatar
Join Date: Dec 2002
Location: Left of the Dial
Use the bulb to confirm that you don't have the bias cranked way up, and that you can change the adjustment pot and see that the bias is also changing. Once that is accomplished, turn the bias down to near-zero before you power down and remove the bulb. You can then power up directly from the AC mains and set the bias normally.

Never attempt a final adjustment of the bias current with a light bulb in series with the amp!
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
What's the Idle Current of the F5 Power Amp NV&H Pass Labs 56 3rd February 2010 10:56 AM
nad 3020b idle current reddish75 Solid State 4 4th July 2008 02:46 PM
High idle current on crown-BCA output? hjelp! mzzj Class D 6 28th November 2005 12:01 PM
idle current for bryston 4B Danielll Solid State 2 3rd October 2005 05:25 PM
SMPS Idle Current cm961 Parts 2 20th February 2004 05:56 AM

New To Site? Need Help?

All times are GMT. The time now is 10:00 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2