250M DC offset question

[goldswimmerb]

I recently got a 250M that was allegedly serviced, I can tell, to an extent, that that was true as many if the capacitors have been replaced. However, I was testing today and figured I would check the DC offset, one channel was at 15mv and the other was around 40mv, should I worry about this offset at all? Is it worth popping open to adjust or should I just sit back and enjoy it as it is?

 

[mbz]

Depends if you are comfortable working on electronics, I would certainly adjust it. However I accept
that there is some risk that the trimmer "falls to pieces" and so now needs to be replaced. Also
one of my probes will slip shorting out a series of transistors. 40mV is a little high but still acceptable.

 

[wlhd1610]

Good advice from mbz.

It's tight quarters in there and a slip of the screwdriver could not end well.
Some of the 250M pots are mounted horizontally making it easy while others are vertical on the board forcing you to use a right angle screwdriver.

If you're comfortable working on live circuits then go for it otherwise enjoy it as is.


Bob

 

[catrafter]

@mbz and @wlhd1610 presented good advise!
IMHO, that small amount of DC offset won't really affect the way the amp operates. It is not high enough to engage the protection circuit and it is also too small to cause a 'pop' in the speakers when the relay transitions.

Tom

 

[goldswimmerb]

@mbz and @wlhd1610 presented good advise!
IMHO, that small amount of DC offset won't really affect the way the amp operates. It is not high enough to engage the protection circuit and it is also too small to cause a 'pop' in the speakers when the relay transitions.

Tom

That's why I figured, just seemed high to me since my Yamaha M-40 from Goodwill, never serviced, read 3.7mv...

Since the 250 service manual calls for +-50mv I'm not going to worry.

 

[N8Nagel]

yeah at this point unless you have replacement trimmers on deck I would not worry about 40mv

 

[Alex Haidar]

In this amplifier voltages between 0 and 50mV are acceptable. More critical for the sound (distortion) and temperature is BIAS ADJUSTMENT*

The procedure indicated on Marantz 250M service manual to adjust DC OFF SET :

DC BALANCE ADJUSTMENT (R502)

1 - Connect oscilloscope or voltmeter to L channel loudspeaker terminals.
2 - Set Line Switch to ON and advance auto transformer (VARIAC) to the voltage selected.
3 - After relay energizes, adjust L channel amplifier board potentiometer R502 for an indication of a “line” zero (0V +/- 50mV) as indicated on the oscilloscope/voltmeter.
4 - Repeat test for R channel.

* BIAS ADJUSTMENT (R526)(IDILING CURRENT)

1 - Set Line Switch to OFF. Preset the bias adjustment potentiometer R526 on each amplifier board fully CCW.
2 - Set Wattmeter to ON and set VARIAC to 0.
3 - Set line switch to ON and slowly advance VARIAC while observing the wattmeter. The wattmeter should indicate less than 55W at 120V. If the wattmeter indicates either 0 or more than 55W, unit is defective. Turn off and correct fault.
4 – Connect a low range voltmeter across series combination of R531 and R532 (voltmeter set to 0-100 mV range.
5 – Adjust the L Channel bias adjustment potentiometer R526 for voltmeter reading between 12 and 15mV.
6 - Repeat for R Channel. A 12mV drop across the series resistance (R531 and R532) represents 60mA idling current in the output stage. A 15mV drop represent 75mA idling current.

 

[dr*audio]

FWIW, if you tape up the shank of the screwdriver so only the very tip is exposed, it will reduced the possibility of shorting things if you slip. There are also kits of Delrin alignment tools (Aven makes a good one, sold by Digikey) that you can get.

 

[mbz]

FWIW, if you tape up the shank of the screwdriver so only the very tip is exposed, it will reduced the possibility of shorting things if you slip. There are also kits of Delrin alignment tools (Aven makes a good one, sold by Digikey) that you can get.

eg, Bourn trimmer adjuster tools. mouser 652-H91 USD2.04