Will lightbulb protect my amp from to high idle current at startup? - diyAudio
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Old 18th September 2009, 02:26 PM   #1
seafire is offline seafire  Mauritius
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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
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Old 18th September 2009, 02:42 PM   #2
jaycee is offline jaycee  United Kingdom
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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.
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Old 18th September 2009, 03:06 PM   #3
seafire is offline seafire  Mauritius
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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?
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Old 18th September 2009, 03:36 PM   #4
AndrewT is offline AndrewT  Scotland
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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).
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regards Andrew T.
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Old 18th September 2009, 04:10 PM   #5
seafire is offline seafire  Mauritius
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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!.
BUT I'M SMITTEN WITH THIS HOBBY!.
So much that I would love to build my own amp soon but will post for advice on that another day.
Regards Seafire
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Old 18th September 2009, 05:37 PM   #6
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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!
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