NAD 2200 current draw rises

[pwdiya12]

I have a NAD 2200 PE that I am repairing/upgrading. When I got it the right channel had a number of fried resistors, and three output transistors (Q320,Q318,Q314) and R368- the emitter resistors were fried and the main PS caps were bulging. I have recapped the unit, and replaced the output transistors (I had some spare originals that I had pulled form another 2200, replaced all the fried transistors and any that looked that they have heat damage, and also replaced Q302/Q304 and Q306/Q308 as although they were of on the transistor tester I thought it better to replace them. I have also replaced SR 302. The rest of the transistors test ok on the right channel and the unit fires up and comes out of protection, and both channels pass signal. However, these units on idle (no input no speakers connected) should draw 0.4A approx. This unit starts there and then creeps (in small jumps to towards 0.75A- which is when I noticed a "warm" smell- and turned it off. I use a Senore PR57- which does not have current limiting. I tested the NAD with cold bias (Collector Q302 to collector Q304) to see if it was the output transistors, and the result is the same- the current creeps up. So although I have "fixed" the unit, I don not think I have found the underlying cause as to why the right channel self destructed. any thoughts or guidance would be greatly appreciated Peter

 

[Mark Tillotson]

No that's utterly out of spec. No wonder it burnt.

The manual says adjust for 7.5mV across each 0.22ohm emitter resistor, about 35mA per output pair, 70mA per channel.

Check the Vbe multipliers, Q333/Q334, and adjust the idle correctly - perhaps it won't adjust? Check all the resistor values in the Vbe sections if not.

 

[rayma]

You mean 0.4A AC drawn from the AC power line, whch increases to 0.75A AC with time?

 

[pwdiya12]

Hey Mark, thanks


well my assumption was that is was the right channel and whilst I was adjusting the bias to around 7.0mv- (the current draw was about 0.55amp)- C301 just went bang- so I will investigate and report back


On the right channel Q334 tests fine and i have tested all the resistors- I think my assumption that the left channel (which worked) is OK- just proved to be false


thanks


Peter

 

[pwdiya12]

yes @rayma exactly

 

[rayma]

yes @rayma exactly

Were you monitoring the output bias voltage while the line current increased? Did it also increase?
Disconnect the power supply from all the audio circuits, and see if the line current still is a problem.
If not, then connect the (repaired) channels back, one at a time.

 

[pwdiya12]

So replaced the cap on the left channel and R329, left channel is fine.


I removed the power to the right channel (via the cable) and amp fires up, comes out of protection and stays stable at about 0.4A AC draw- no creep- so the problem is on the right channel.



Another observation is that it appears heat driven, as when the amp warms up, the current draw increases and the idling comes up form about 1mv towards 7.5mv- which I think is normal


so something on the right channel is causing the overall current draw to increase.


any thoughts?

 

[pwdiya12]

@rayma- yes the bias voltage did increase with the AC current draw, though even if I dropped the bias a little (to 6mv) the AC current kept increasing

 

[rayma]

Have you checked the HV switching, is it on the 62VDC, not the 95VDC? Are D330 and D332 ok?
Check the voltage on the other three emitter resistors also.

 

[pwdiya12]

Q332, Q328 and Q324 all test good


Q332 E -90.4/B -90 /C-58
Q328 E -90 / B-90 /C-58
Q432 E -58 / B-58.4 /C-90


Q330 tests ok


so I think the switching is working


I forgot to mention that ZD304 and D312 were also fried and replaced.

 

[pwdiya12]

will do

 

[pwdiya12]

D330 and D332 have been checkd and are fine- both R368 and R366 have been replaced

 

[rayma]

D330 and D332 have been checkd and are fine- both R368 and R366 have been replaced

The increased line current draw must be in the output stage. It's too much power (40W) to be elsewhere.
It's gotta go somewhere, what gets hot? The bias voltage emitter resistor is only one of four, check the others.

 

[pwdiya12]

all the outputs have been checked- Q320/Q316 E=0v, B=-0.5v C=58v- same for the positive voltage rail. I tried "finger test" on the transistors, and nothing is getting hot.
I agree that it has to be going somewhere- I will check Q326 and Q322 on the +ve side (i have focused on the negative side as this was where all the damage was).


The cold bias test, which should turn off the outputs, still saw the current rise.


I think it has to be a transistor-I will try the finger test and let the current rise a bit (but dont want to fry things again)- now I see why I want a current limiting AC supply.


@rayma- thanks for helping with this- it has me puzzled


I will report back on Q326/Q322- I tested Q330 and it tests good

 

[rayma]

Hook up a 10R load resistor and make sure it is not hot from HF oscillation.
Is the Zobel resistor hot (or open)? Do you have a thermal gun to check for hot spots?
Rule out as much as you can.

 

[pwdiya12]

Q326/Q322 test fine

 

[pwdiya12]

Zobel resistor is correct (4.7ohm) and not getting hot. I dont have a thermal gun, but will buy one, and will connect a scope and load to see if there is any oscillation.


any suggestions for a not too expensive thermal gun?

 

[rayma]

any suggestions for a not too expensive thermal gun?

Local hardware stores (or Amazon, etc.) have cheap Chinese types, which is good enough for this purpose.
Random examples:
Amazon.com: Kizen LaserPro LP300 Infrared Thermometer Non-Contact Digital Laser Temperature Gun with LCD Display -58℉～1112℉(-50℃～600℃) Adjustable Emissivity: Kitchen & Dining
Amazon.com: Fluke 62 Max Thermometer, Non Contact, -22 to +932 Degree F Range: Industrial & Scientific

 

[pwdiya12]

thanks- ordered, will be here tuesday, so will update when I use it

 

[rayma]

thanks- ordered, will be here tuesday, so will update when I use it

You can compare the two channels, there's gotta be a difference somewhere.