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Old 26th December 2012, 08:15 PM   #1
2dbit is offline 2dbit  United States
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Default NAD 7250PE no service manual

I am writing this forum in regards to a NAD 7250PE that I'm working on.

I'm familiar with amp repair, but not enough that I can fully understand a schematic to know which potentiometer to adjust for DC offset and Idle Current.

I spoke with Mooly who is quite knowledgeable, and received this response in regards to adjustment:

"Idle current adjustment is via preset R624 and checked via the voltage between test points TP601 and TP602 located on the emitter resistors of the output transistors. I would guess a safe value would be around 9 millivolts DC giving around 20 milliamps per output pair. The manual may say different but without seeing it play safe."

I guess from here, since I'm replacing all potentiometers (only one removed so far was R618 which metered as over 100K from end to end), I want to make sure that during initial power-up I have everything in the correct position....

Some other manuals suggest that DC offset pots (in this case R645/R695) be set to center position before adjustment, and Idle current pots (in this case R624/R674) be set to full counter-clock wise positions.

I just want to confirm that this would be appropriate for this amp before power-up.

Also, I haven't dealt with any amps with a pot for distortion. I saw in another thread somebody just replaced the pot with a 100 ohm, straight-value resistor. Without access to an oscilloscope, I'm guessing my situation, I'll set it to 100ohms and leave it be?

In the end, I'll make sure that during initial power-up I'll have the stereo limited by a 40watt light bulb
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Old 27th December 2012, 07:22 AM   #2
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If you are replacing preset (not front panel controls) the priority is to have exactly the same setting as the one you removed. You can measure and adjust that as best you can before installation. Simply remove the pot without moving the rotor and measure end to end and slider to one end or both, if the total resistance is not very close to the the same. The ratio of one resistance to the other will be the important factor.

Multi-turn pots can be utterly confusing. Check that cw increases in the same way as the pot removed (probably single turn) and the leads fit the PCB. The ratio of setting before fitting is still the same, of course.

If this is a new or rebuilt construction, the setting for idle current is minimum or ccw but some goofs get that wrong during design. Check documentation in that case. Adjust from minimum to the correct position by test as instructions describe. The offset adjust starting position does not really matter but midway is a sensible starting point.

Whenever replacing presets, always leave one good, untouched pot in place for reference. If you pull them all and hope to have no issues, you are just pressing your luck. If you lose your way with no reference or forget to record the setting conditions, you will have major problems. Replace one at a time and test one at a time. Write it down and paste to the inside of the amp. Idle current is a critical setting - don't leave it to chance or "oh I forgot!"
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Old 21st January 2013, 06:49 AM   #3
2dbit is offline 2dbit  United States
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So, I replaced the failed R618.

I pulled all the pots from the circuit, one at a time, and checked their values. They all read just fine, and after I took their readings, I doused the pots with deoxit, worked them back and forth, and set them back to the value they came out with.

When all pots were soldered back in, I set the volume to zero, placed a 40watt bulb in series with the power line, and fired it up.

Bulb came on bright, then shortly dimmed down to almost completely nothing. That's good....

However, I'm getting no relay clicks, and I'm getting no adjustment out of either channel for DC offset. Then again, I'm taking my reading from the speaker terminals, since without a full schematic, that's about the best I've been able to do...

I AM getting adjustment on Idle current on the left channel, between TP601 and TP602, and was able to successfully adjust it to 9mV. I am NOT getting any reading when I adjust R674 and take my reading between TP651 and TP652. It just stays at 4.1mV.

Not quite sure where to go from here....
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Old 21st January 2013, 06:55 AM   #4
Mooly is offline Mooly  United Kingdom
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If the relay isn't closing then you need to measure the DC offset before the relay.

Measure the DC voltage from ground to both those test points on the bad channel. I suspect it will be high. Start with the meter on a high DC volts range.
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Old 21st January 2013, 05:47 PM   #5
2dbit is offline 2dbit  United States
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Yeah, ideally I would have been doing that from the start, but I'm not positive about where to check that. I am probably going to have to back trace from the speaker terminals until I locate the relay, then perhaps a little further to where I can check it from the topside of the board.
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Old 21st January 2013, 06:28 PM   #6
Mooly is offline Mooly  United Kingdom
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Just measure from the chassis or any known ground point to each of those test points in turn
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Old 22nd January 2013, 05:25 AM   #7
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Yikes.... I traced out the circuit and found where the relay was at, and took a reading from the other side. LH channel was out by only a little and trimmed up just fine. RH channel had -66.1 Volts...not millivolts...volts. Not adjustable.
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Old 22nd January 2013, 07:09 AM   #8
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Just in case it helps, I have the service manual for the 7240. Obviously I don't know how much would be the same but the front and back panels appear identical. I could email it to you if you wish.
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Old 22nd January 2013, 07:18 AM   #9
Mooly is offline Mooly  United Kingdom
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That voltage is as I suspected and indicates a failed output stage.

I think you will find that the one or more output transistors and possibly the driver transistors too will be faulty together with other "collateral" damage such as burned out resistors. All pretty standard stuff really.

I couldn't find a 7250 manual either and what the differences are I wouldn't like to say.

If you are confident removing/testing/replacing transistors and checking the surrounding circuitry then it may be do-able. You would have to apply what we tell you to the 7250 if it were different
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Old 23rd January 2013, 07:27 AM   #10
2dbit is offline 2dbit  United States
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Well, I found two failed power transistors. Q757 and Q753.

So far, I have been testing all components that branch off from B, C, or E on Q757. I haven't found any one component (lifted legs or removed completely) that has been out of spec. I did find that the trace for Q758 was missing between its emitter and R759...but it was a solder bridge before I removed it...so it's basically restored to how it was....big fat bridge.

As for all components branching off of Q753, I am out of time for now. I will do the same for that transistor as I have for Q758...just on another day.
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