Help with blowing transistor NAD T752 receiver

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

well done

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Well, in the end got the amp working again on the surround R + L channels, so am using it as a stereo system with 2 speakers and sub woofer. Sounds pretty good. Not completely lost

Tks guys!

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

Question for you Petehall347: your idea of putting in 1K resistors instead of finals, is that to make the amp think there are finals so that it won't go into protection mode?

I could probably do this, and just use the surround L + R channels? Right now, I took out the finals that went short on the Front/Center amp out, and when I try to start the amp, it goes into protection mode after a few seconds.

the resistors keep the feedback intact so then the dc offset can stabilise along with other voltages .
without the resistors and outputs the protect shouldn't see any dc offset so not needed .

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Question for you Petehall347: your idea of putting in 1K resistors instead of finals, is that to make the amp think there are finals so that it won't go into protection mode?

I could probably do this, and just use the surround L + R channels? Right now, I took out the finals that went short on the Front/Center amp out, and when I try to start the amp, it goes into protection mode after a few seconds.

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

,,,,,

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Hello,

After a few weeks of trying pretty much everything, I was unsuccessful in bringing this receiver back alive, so throwing in the towel and will use it for parts recycling. Thanks again to those who have tried to help me! Regards!

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Ok, you might say he's nuts but what the heck I decided to give this thing another shot. Turns out the surround channels are not blown at all, since they weren't playing I assumed they were shot, but it was a setting and they are fine, they play very well.

On the Front Center board (which is my problem board) the left channel seems to be so so ok, as it plays but very distorted - this may be caused by the right channel which is not playing but finals are ok or the center channel for which both finals are blown and one side of R574 is also blown. I'm going to attempt another repair and then check what Eccerr0r said in the previous post on each channel:

"
- breaking or disconnecting R518
- breaking (disconnecting) of Q506 collector or base or otherwise shutting it off completely
- breaking R5143
- breaking R520 (but it was burned, so something upstream, i.e. one of the above has occurred)."

Will report back.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Probably not -- they designed it such that it's impossible to fry finals by setting the idle current wrong. Normally shorting or turning all the way down of the idle current pot - R522 - would result in final frying. However they put in R519 to ensure they survive even if you turned it down all the way.

I can't think of any possible way of destroying the finals by setting the idle current wrong, so only thing I can think of are things like (for the left channel, of course the other channels have their own corresponding devices):
- breaking or disconnecting R518
- breaking (disconnecting) of Q506 collector or base or otherwise shutting it off completely
- breaking R5143
- breaking R520 (but it was burned, so something upstream, i.e. one of the above has occurred).

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

reading your first post again things looked to go wrong pretty soon after you adjusted the idle currents ..maybe your meter isnt good enough . or you read it wrong .

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

No, I'm pretty sure it's simply disassembly-reassembly sloppiness.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

I actually used them at very low volume every time I tested, and also tested with Tannoy C6 speakers. So I guess there's no purpose in fixing this amp, it seems to be all over the map...first it's the left channel that goes, then replace everything, then it's the right channel not working..then the center channel burns finals and now the surround L & R that stopped working. Ouf, maybe there's something wrong with the 60V rail?

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

It depends on how many watts you're trying to pump into the speakers. At lower volumes, low impedance speakers won't do much more harm than running higher impedance speakers at higher volume. Ultimately you need to balance designs of the amp and the speaker.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

some speakers are hard to drive . the static ohms isnt the same as impedance .some can drop to very low impedance whilst being driven ,
hard to say really .
maybe go back to the point of it being blown up in the first place to look for clues .

Hey Petehall347, I think you might be on to something, as I remeasured mys Rock Solid speakers and they were below 4 ohms, down to 3.2 ohms. I know you mentionned there is a difference between resistance and impedance, but that is below 4 ohms which is not recommended by the manufacturer. Well gess what, I had them on the surround L and R channels, and both channels blew leaving only the subwoofer working now. Fudge..........

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

some speakers are hard to drive . the static ohms isnt the same as impedance .some can drop to very low impedance whilst being driven ,
hard to say really .
maybe go back to the point of it being blown up in the first place to look for clues .

[rddube]

Re: Help with blowing transistor NAD T752 receiver

its rated for 8 ohms so your 4 ohm speakers are likely the culprit . am sure i asked about speakers earlier on .

From the NAD website:

All NAD amplifiers/receivers are rated for a minimum speaker impedance of 4 Ohms.

Mine are Rock Solid B&W speakers and measure exactly 4.8 ohms, so I don't think they are the culprit but I may be wrong? I did use the same speakers on the surround channels (Right and Left) with quite a bit of output and they are still working (the channels and the speakers), so I think the problem is elswhere and probably what Eccer0r mentionned about stress on the board because I have had to retrace a number of broken traces created from removing finals, transistors, burnt resistors, etc.

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

its rated for 8 ohms so your 4 ohm speakers are likely the culprit . am sure i asked about speakers earlier on .

[rddube]

Re: Help with blowing transistor NAD T752 receiver

At first I was testing without speakers attached, but when the incident happened I had 2 speakers attached to the left and right channel, 4 ohms 100W speakers.

Unfortunately I don't think I can set it up outside the case, so I'll have to pass on that idea but it would have been ideal.

So I'll simply remove the shorted transistors reassemble and will use the surround channels - anyhow I wasn't intending to use this amp for all the channels, but I think I can use the left and right channels pretty much like the front right and left channels. I'll see what it gives and if not acceptable, I'll keep it for parts. Thanks Eccerr0r!

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

A removed Q528 would simply cause silence on the center channel.
BTW are you testing with the speaker attached or not? The protection system doesn't necessarily protect against all DC conditions.

What are the specs of the speakers you're using, anyway (impedance and max wattage)?

It's still up to you but I can't protect you from destroying stuff by disassembly and reassembly. It makes it that much harder to troubleshoot when you're causing more problems as now they too need to be fixed. Ideally you set it up such that you can run the guts without the case and never reassemble until you're confident it's working 100%...

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Can't believe this amp is so much bad news... Likewise other channels, R573 is also a symptom of frying finals, and I'm beginning to think reassembly/disassembly is putting a toll on the boards causing opens and then destruction..

I think you're right Eccerr0r, so do you think I should just install 1K resistors and use the surround channels? Everytime I dissassemble, I have to push tos 5W resistors down then up when I have screwed in the finals to the heatsink and I did discover a transistor Q528 that had been "ripped" out of the board that I rebuilt the connections.

So what do you think, we've played enough with this guy? Thanks again for your advice.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Can't believe this amp is so much bad news... Likewise other channels, R573 is also a symptom of frying finals, and I'm beginning to think reassembly/disassembly is putting a toll on the boards causing opens and then destruction..