Hello All,
I have a NAD T752 A/V Receiver which I'm busy repairing.
I have found various parts blown in the PSU, and have repaired it and got it up and running, however...
Please see the attached diagram which is part of the PSU.
U702 (7812 regulator) and U703 (7912 regulator) run really, really hot.
I see that the Nad designer has tried to alleviate dissapation in these regulators, by inserting two 5.1V zeners in series with the voltage rails. These zeners went s/c which is what caused the original faults.
My feeling is, that this is a poor design. So... I replaced the zener diodes with two 15v pre regulators, on a large heatsink, but sadly I still have a lot of heat in all the regulators. The input voltage as measured, is 23v DC, so 23v is being stepped down to 15v, which is then being fed to the 12v regulators U702 / U703.
Can anybody offer any advice to reduce the dissapation in this circuit (apart from adding a lower voltage transformer - no space!!).
By the way, I have also replaced nearly all the capacitors in both the +-12v PSU and the 5v PSU.
Thanks,
Tony.
I have a NAD T752 A/V Receiver which I'm busy repairing.
I have found various parts blown in the PSU, and have repaired it and got it up and running, however...
Please see the attached diagram which is part of the PSU.
U702 (7812 regulator) and U703 (7912 regulator) run really, really hot.
I see that the Nad designer has tried to alleviate dissapation in these regulators, by inserting two 5.1V zeners in series with the voltage rails. These zeners went s/c which is what caused the original faults.
My feeling is, that this is a poor design. So... I replaced the zener diodes with two 15v pre regulators, on a large heatsink, but sadly I still have a lot of heat in all the regulators. The input voltage as measured, is 23v DC, so 23v is being stepped down to 15v, which is then being fed to the 12v regulators U702 / U703.
Can anybody offer any advice to reduce the dissapation in this circuit (apart from adding a lower voltage transformer - no space!!).
By the way, I have also replaced nearly all the capacitors in both the +-12v PSU and the 5v PSU.
Thanks,
Tony.
Attachments
It is oK for power supplies to run hot.
It is worth measuring the temp of the heatsink.
Those regs will upto 150 degrees C before dying.
Without changing to a SMPS then the only way to reduce heat is to increase the size of the heatsink.
It is worth measuring the temp of the heatsink.
Those regs will upto 150 degrees C before dying.
Without changing to a SMPS then the only way to reduce heat is to increase the size of the heatsink.
I can't keep my finger on the heatsinks for more than about 2 seconds, they are that hot... In fact they are uncomfortably hot...
But as you say, I should measure them - but I'll have to buy a temperature probe first!
Thanks.
But as you say, I should measure them - but I'll have to buy a temperature probe first!
Thanks.
Those regs should take up to an amp.
You could measure the current to see if they are in spec.
If the heatsink is gettting hot, then is there any ventilation in the enclosure ?
You could measure the current to see if they are in spec.
If the heatsink is gettting hot, then is there any ventilation in the enclosure ?
Will do thanks for your help.
Just been looking at your website - interesting.
And reasonably priced too.
Don't know how you do it so cheap really..!
Just been looking at your website - interesting.
And reasonably priced too.
Don't know how you do it so cheap really..!
Just a thought. How about feeding the regs from a half wave supply from the tranny. It may be possible to reduce the excess voltage that way.
Hi Tony. I'm a few years late to this thread but...
I have my T752 apart to replace the caps on this board (most leaked and tops popped). I can see the diodes you replaced have got very hot, discolouring the board, so thought I'd make your mod while I'm at it.
So wondering, did yours work OK? I presume you used 7815/7915 in place of this diodes? But given these would be doing most of the work to drop the voltage I presume they get hotter than the 7812/7912 pair with proper heat sinks. Did they survive? Did you do anything else to improve cooling?
Also, I just wondered if you had a bigger pic of your heatsink. Where did you put the second screw through? I presume one used the main PCB mount.
Thanks.
Nick.
I have my T752 apart to replace the caps on this board (most leaked and tops popped). I can see the diodes you replaced have got very hot, discolouring the board, so thought I'd make your mod while I'm at it.
So wondering, did yours work OK? I presume you used 7815/7915 in place of this diodes? But given these would be doing most of the work to drop the voltage I presume they get hotter than the 7812/7912 pair with proper heat sinks. Did they survive? Did you do anything else to improve cooling?
Also, I just wondered if you had a bigger pic of your heatsink. Where did you put the second screw through? I presume one used the main PCB mount.
Thanks.
Nick.
The regulators can run really hot without dying and that's why manufacturers don't put on a bigger heatsink than absolutely necessary (to reduce cost).
The difference between a diode and a regulator is that the latter has several kinds of protection, including thermal.
I would just use the preregs on heatsinks that fit. The voltage drop will be shared, so the original regulators should run cooler without any extra effort on your part.
BTW, I'd use 7818/7918 instead of 7815/7915 to drop the 23 V to +/- 18 V first. This way the preregs need to drop 5 V and the regs 6 V, this results in much more equal dissipation than with a 8 V and then 3 V drop when the 7815/7915 are used.
The difference between a diode and a regulator is that the latter has several kinds of protection, including thermal.
I would just use the preregs on heatsinks that fit. The voltage drop will be shared, so the original regulators should run cooler without any extra effort on your part.
BTW, I'd use 7818/7918 instead of 7815/7915 to drop the 23 V to +/- 18 V first. This way the preregs need to drop 5 V and the regs 6 V, this results in much more equal dissipation than with a 8 V and then 3 V drop when the 7815/7915 are used.
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Thanks jitter! I didn't know there was a 18v variant of the pre-regs til now. Yes that looks like a much better share of the work.
Nick, I just opened my T752 case today. I can't tell if my caps have leaked or it's just glue used during assembly - which in some cases the glue or cap discharge has discolored in spots maybe due to heat. The tops all look good and the only caps in question are all on the horizontal board that sits on the bottom (the subboards sit vertically on top of this). The glue/discharge is always only on one side of the caps, so despite the discoloration, I'm thinking the pattern is a little too regular to be blown caps and perhaps it's just glue.
Do you know if NAD used glue on the caps on this board that may make the caps look like they've leaked?
How did you know your had leaked - was the SQ affected noticeably? Mine seems to play okay, but this is one I just bought used, so I don't know what it sounded like new.
Also, when I was trying to fix a bad volume knob (DeOx didn't fix it), I messed up the display a bit. The whole thing (all non active icons) glow/pulsate with the active icons brighter). If any ideas, would love to hear them. I already tried to make sure all connectors to the front panel were secure, but not much more than that.
Thanks!
Simple dumb 2W power resistors would probably work better than the zeners, though the big mistake here is that NAD have crowded everything so close together that there is a major hot spot. It is probably what reduces the lifespan of the capacitors so much.
Small heatsinks like that will run very hot. The real problem is the electrolytics next to them. The ICs should protect themselves.
Capacitor heat problem, mute problem.
Talk about a few years late... so am I LOL. I just purchased oen of those T752 receivers also. It ran OK for about a week, then it went...
The problem in 2 parts:
Pt 1 - Muting -
1st problem seemed to be with the muting circuit - I Googled my problem and sure enough, lots of people had this happen. There was the case of the intermittent ribbon cable, the bad software, etc. I left the unit off
for about 5 hours and it came back to normal. Ah, thermal problem I thought...
So I took the cover off and fired up the unit again. It came on but in Mute mode. Playing around with the volume control - as soon as it was all the way down (oodb) a really loud thump could be heard from the speakers and the "overload" led on my Rane crossover unit would come on. With each moving of the volume up a bit then back down, this loud thump could be heard again. Turned the unit off and back on once more - all hell broke loose - multiple crackling noises from speakers, then the protection kicked in.
Some days later, still with the cover off, I turn the unit on, it runs OK. About 30 minutes into listening, I decide to check out if there are any loose connections - there seem to be none but there is a lot of heat from the PSU's components.
Pt 2 - Heat -
I notice that C724 at the input of the 5v regulator seems to have part of its insulating plastic removed or melted. Touching that capacitor as well as the 2 others nearby for the 12v reg's (C704, C707); they are running really really hot! As Davidsrsb and others have pointed out, the close proximity of of the heat sinks, zeners and capacitors is a bad way to design and build PSU's. I ended un changing the 3 capacitors thinking this might be the problem for the misbehaving mute circuits (dirty power), not to mention that electrolytic caps running that hot are a bad sign to begin with. The caps had not bulged or leaked.While I was at it, I changed the 2 zeners and moved them as well as the new caps away a bit from the heat sinks.With the unit apart, may as well do some regular maintenance checks so I reflowed many solder joints on the main board as well as all the other boards and found many that were suspect... As a bonus, when I took the back panel off, the male AC mains plug just fell out of its pc board holes!! So I fixed that too. Talk about a cheap unit. I thought NAD was pretty hifi but after seeing how poorly designed this receiver is, I wonder where the "Hi-Fi" part ever came from - this ain't no hifi electronics.
Reassembled the unit, powered it up and it works just fine. BUT those capacitors are just as hot as before...
So here's my question to you: what do you think of those caps running this hot? I read Audio_Tony and Jitter's posts and did not bother to implement that modification - I dunno if this unit is worth all the bother. Unfortunately because of the bad design, one cannot easily stick probes into the unit so it is hard to take safe measurements with the thing running, but I wonder if the FWBR D715 as well as the diodes D701 to 704 might have a problem, letting high frequencies or AC leak and screw up the regulator circuits? Also, there are no bypass small value ceramic caps at the inputs of the regs & other pwr circuits so this may have an adverse effect on the regs also?
Any thoughts? Too bad this model seems to be a real lemon judging by all the problems reported and the evidence before me as the unit was apart... It does sound good though!
Thanks 🙂
John
Talk about a few years late... so am I LOL. I just purchased oen of those T752 receivers also. It ran OK for about a week, then it went...
The problem in 2 parts:
Pt 1 - Muting -
1st problem seemed to be with the muting circuit - I Googled my problem and sure enough, lots of people had this happen. There was the case of the intermittent ribbon cable, the bad software, etc. I left the unit off
for about 5 hours and it came back to normal. Ah, thermal problem I thought...
So I took the cover off and fired up the unit again. It came on but in Mute mode. Playing around with the volume control - as soon as it was all the way down (oodb) a really loud thump could be heard from the speakers and the "overload" led on my Rane crossover unit would come on. With each moving of the volume up a bit then back down, this loud thump could be heard again. Turned the unit off and back on once more - all hell broke loose - multiple crackling noises from speakers, then the protection kicked in.
Some days later, still with the cover off, I turn the unit on, it runs OK. About 30 minutes into listening, I decide to check out if there are any loose connections - there seem to be none but there is a lot of heat from the PSU's components.
Pt 2 - Heat -
I notice that C724 at the input of the 5v regulator seems to have part of its insulating plastic removed or melted. Touching that capacitor as well as the 2 others nearby for the 12v reg's (C704, C707); they are running really really hot! As Davidsrsb and others have pointed out, the close proximity of of the heat sinks, zeners and capacitors is a bad way to design and build PSU's. I ended un changing the 3 capacitors thinking this might be the problem for the misbehaving mute circuits (dirty power), not to mention that electrolytic caps running that hot are a bad sign to begin with. The caps had not bulged or leaked.While I was at it, I changed the 2 zeners and moved them as well as the new caps away a bit from the heat sinks.With the unit apart, may as well do some regular maintenance checks so I reflowed many solder joints on the main board as well as all the other boards and found many that were suspect... As a bonus, when I took the back panel off, the male AC mains plug just fell out of its pc board holes!! So I fixed that too. Talk about a cheap unit. I thought NAD was pretty hifi but after seeing how poorly designed this receiver is, I wonder where the "Hi-Fi" part ever came from - this ain't no hifi electronics.
Reassembled the unit, powered it up and it works just fine. BUT those capacitors are just as hot as before...
So here's my question to you: what do you think of those caps running this hot? I read Audio_Tony and Jitter's posts and did not bother to implement that modification - I dunno if this unit is worth all the bother. Unfortunately because of the bad design, one cannot easily stick probes into the unit so it is hard to take safe measurements with the thing running, but I wonder if the FWBR D715 as well as the diodes D701 to 704 might have a problem, letting high frequencies or AC leak and screw up the regulator circuits? Also, there are no bypass small value ceramic caps at the inputs of the regs & other pwr circuits so this may have an adverse effect on the regs also?
Any thoughts? Too bad this model seems to be a real lemon judging by all the problems reported and the evidence before me as the unit was apart... It does sound good though!
Thanks 🙂
John
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