Noob attempting to fix a blown NAD T-760 AV receiver

[mariushm]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

For 2SA1941, 2STA1943 may be compatible: http://uk.farnell.com/stmicroelectro...ditmatch=rel_3

but you may have to replace ALL 2SA1941 with these if you change the blown one.

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Thanks Agent and Mariushm.

I'd sooner change all the 2SA1941's for newer transistors then some cheap knock-offs. In fact I'd change all the power transistors if I could!

Question. When you can't find exactly what you're looking for do you always get something that's as close to or above the spec of the original? With caps I take it you have to get the same uF but the voltage can be (a lot) higher or as close as possible to the original. With transistors I wouldn't know what to look out for. Think I'll google to find out but if you'd like to chip in that would be well appreciated.

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Found and read this so I think, and hope, I can find a more modern replacement for the EOL transistors.

Title :CHOOSING A REPLACEMENT TRANSISTOR



And actually read the page google sent me to this time and clicked the link to find equivalent transistors for the 2SA1941 and co.

http://alltransistors.com/crsearch.p...fe=0&caps=TO3P

Will have to double check the specs but it's looking good for finding a replacement!

[Agent24]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

http://www.diyaudio.com/forums/solid...placement.html has a suggestion

Searching that forum for 2SA1941 comes up with quite a few results - you might find something useful there.

[Elysarian]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

This site seems to do a pretty good job of replacing all the old Farnell cross reference books I used to use back in my TV repair days:

http://alltransistors.com/crsearch.php

[Agent24]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

http://www.electronics-radio.com/art...ransistors.php also some good ideas

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Thanks guys ! :-)

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Update.

Okay after replacing a few of the caps and resistors I've managed to power the unit back up but it still switches over to "safe mode" after the display comes on for a few seconds. Just the fact that it hasn't turned into a blazing pile of components when I first switched it back on has made me happy.

Powering down after the first couple of tests I looked around a bit more and found that a caps jacket had melted. c141. The jacket hadn't come off fully but my suspicions had been raised. Hmmm what could this be? The cap was on the AC3/DTS board. First pic.

Pulling the board out I noticed some discoloration on the board. Second pic.
Fliping the board over. Third pic.
The discoloration is in three sections. Left hand under the melting cap, the center, under the bridge rectifier, and over in the very top and bottom right hand side of the board. A few of the tiny surface mount caps look odd. Ive been looking at youtube vids for soldering and these surface caps don't look like they had the concave soldering shape. The solder at the ends are more round balls than anything else. Maybe they were replaced back in 2003 when the unit was last serviced. Flux has bubbled but I'm not sure where it came from. It might be left over from a service.

Funny thing is when the unit is powered on the "digital " sign will flash on and off a couple of times. I wonder if that's how it tells the user or technician that there's a problem in that section of the amp. Or it could be saying hey I'm in digital mode. I just don't know.

Another thing. My soldering gear is way out of date and I'm not even going to post what I've been using it's so embarrassing. I've been reading this post https://www.badcaps.net/forum/showthread.php?t=31448 and yes the stuff at work that I've been using is A LOT worse than the posters gear. I might as well use a hot clothes iron :-(. Just using 2 mains soldering irons and a cup to as a holder :-). I've soldered loads of times before but maybe it time to buy some proper gear to last for a while.

So I've done some research and am thinking of buying this first.
http://cpc.farnell.com/jsp/search/pr...sp?sku=SD01738


And much later on.
http://www.amazon.co.uk/858D-Rework-...pr_product_top

Would the 858D station do all the jobs of the normal iron or would it be best used exclusively for SMDs?

Got my mesr-100 but not really getting the hang of it yet. Zeroed then tested the popped 470uF 63V cap and it says 0.038 ohm. Ref for 25v E cap: Good cap with low esr. So I'm very :-S confused right now when it comes to this meter. Worst case on the meters table is 0.09 for this cap. Then testing the replacement cap it measures 0.042 ohm. What????!!!! My A6013L cap meter reads the popped cap at 460uF.

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Past the 90 minute mark so can't edit the update post.

Just wanted to add.

The more I look at the board the more I see "new" solder points.

Pulled the rectifier off the board and tested it with the meter on diode setting.

After connection the negative clip to the positve point on the rectifier and going round and getting zero I reversed the clips. With the positive clip connected to the positive point at the bottom and went clockwise and read (left )535 then ( opposite)1280 and finally (right)560 at the last position.
A page for the RB152 http://www.datasheetdir.com/CDIL-RB152+download
That 1280 reading shouldn't be there should it? If I've got the message from the tube vids it should be between 400 and 600 shouldn't it?

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Past the 90 minute mark so can't edit the update post again.

Disregard my previous post about the rectifier. In the noob zone.

[Agent24]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Okay after replacing a few of the caps and resistors I've managed to power the unit back up but it still switches over to "safe mode" after the display comes on for a few seconds. Just the fact that it hasn't turned into a blazing pile of components when I first switched it back on has made me happy.

Did you replace the blown transistor(s) yet?

Powering down after the first couple of tests I looked around a bit more and found that a caps jacket had melted. c141. The jacket hadn't come off fully but my suspicions had been raised. Hmmm what could this be? The cap was on the AC3/DTS board. First pic.

Could be overvoltage on the capacitor, capacitor is backwards, capacitor was just faulty etc. If if's cheap enough it might have come with that sleeve defect from the factory. Someone could also have swiped it with with a soldering iron and melted the sleeve.

Do you know for sure it was not melted before you powered it up?

Pulling the board out I noticed some discoloration on the board. Second pic.
Fliping the board over. Third pic.
The discoloration is in three sections. Left hand under the melting cap, the center, under the bridge rectifier, and over in the very top and bottom right hand side of the board.

Discolouration is not always evidence of a fault, any item that runs hot under normal operation will usually discolour the PCB over time. The center with the bridge rectifier and what I guess are regulators would normally run hot anyway. Of course overloaded regulators etc would run even hotter.

A few of the tiny surface mount caps look odd. Ive been looking at youtube vids for soldering and these surface caps don't look like they had the concave soldering shape. The solder at the ends are more round balls than anything else. Maybe they were replaced back in 2003 when the unit was last serviced. Flux has bubbled but I'm not sure where it came from. It might be left over from a service.

Most likely someone replaced them or removed and tested and reinstalled them before, big round solder joints are indicative of someone who doesn't know how to solder SMDs properly. That's not to say the joints are a problem, they just look ugly.

Funny thing is when the unit is powered on the "digital " sign will flash on and off a couple of times. I wonder if that's how it tells the user or technician that there's a problem in that section of the amp. Or it could be saying hey I'm in digital mode. I just don't know.

Check the user manual and\or service manual. It may be an error code, OR it might just be a side effect of a fault.

Another thing. My soldering gear is way out of date and I'm not even going to post what I've been using it's so embarrassing. I've been reading this post https://www.badcaps.net/forum/showthread.php?t=31448 and yes the stuff at work that I've been using is A LOT worse than the posters gear. I might as well use a hot clothes iron :-(. Just using 2 mains soldering irons and a cup to as a holder :-). I've soldered loads of times before but maybe it time to buy some proper gear to last for a while.

So I've done some research and am thinking of buying this first.
http://cpc.farnell.com/jsp/search/pr...sp?sku=SD01738

And much later on.
http://www.amazon.co.uk/858D-Rework-...pr_product_top

Would the 858D station do all the jobs of the normal iron or would it be best used exclusively for SMDs?

Low wattage basic mains soldering irons are OK - on single-sided boards - on most double-sided boards and all multilayer boards they are not great - although they can be used if you preheat the board, a hair dryer can work well... (I still have a couple of old 25 watt ones, higher wattage can be a problem as they have no temperature control and will overheat.)

A controlled soldering station like that Tenma one is good idea. I have a 'Duratech' Hakko 936 clone from Jaycar. It's not awesome but it's better than a basic iron.

The hot air 858D station is primarily for SMD parts, though they also work well on heatshrink. It does not replace a soldering iron for through hole components.

Got my mesr-100 but not really getting the hang of it yet. Zeroed then tested the popped 470uF 63V cap and it says 0.038 ohm. Ref for 25v E cap: Good cap with low esr. So I'm very :-S confused right now when it comes to this meter. Worst case on the meters table is 0.09 for this cap. Then testing the replacement cap it measures 0.042 ohm. What????!!!! My A6013L cap meter reads the popped cap at 460uF.

Low ESR is good. Problems arise when the ESR is too high (well, problems can also arise in some circuits (especially power supplies) if ESR is too low, but in general low ESR is a good thing)

To really know if the ESR of a capacitor is OK or not, you need to look up the capacitor datasheet and find out what it's supposed to be.

If the capacitor with the popped sleeve looks OK otherwise and tests with normal capacity, it may well be that it's fine, just has cosmetic damage to the sleeve. Of course, the question is why did the sleeve split?

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Thanks for the advice. Will be taken on-board and stored away for good measure.

Yes I replaced the blown transistor. When I took the solder off transistor one of the legs fell off onto the work bench. Turns out that the leg had been properly blown right off the transistor!! :-O!

I'll look back through my pictures to see if I can spot cap c141. Hopefully I've got it in the background and should be able to tell if I messed it up or not.

I had a copy of the manual, but yes I left it at work. Will try to download it again to see if the flashing digital sign means anything.

Okay will invest in the Tenma. Spares and tips are available so it's a go then. Building up a list of caps and components I can find from cpc's site and when I finish that I'll add them to the order, total it and see what it comes to. Okay, gotcha on the rework station. Will leave that out for now.

Will try to find the details of the caps and find out what the esr rating is supposed to be from now on.

Thank you once again for the tips and advice.

[Agent24]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Some capacitors you can't find datasheets for, in which case you just have to guess what would suit from what the circuit it's in is for, etc. In a lot of cases it doesn't matter much what you use.

But the unidentifiable ones are usually general-purpose capacitors and if the ESR is not higher than the 'worst case' value it should be OK, as long as the capacitance is correct also. (A shorted capacitor will show up as very low ESR!)

I assume you checked\replaced all the parts around the blown transistor? Probably worth seeing how the protect circuit works and finding out what might be tripping it, I'll have a look at the manual too, later, see if I can figure anything out, hopefully someone else will give more\better advice though

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Quote from the manual "The Digital Input 1 is linked to the Video 1 input
(marked Digital In on back panel; No. 6). Digital Input
1 allows for connection of a digital source with an
Optical digital output. Whenever Video 1 is selected,
Digital Input 1 will also be selected; in the display the
“DIGITAL 1” indicator will start to blink. If a digital
audio signal is detected, the “ DIGITAL 1 ” indicator will
stop blinking and remain lit. If no digital audio signal
is detected,“ DIGITAL 1 ” indicator will cease blinking
and extinguish".

I guess that answers that then!

I replaced a number of smaller transistors and a couple of resistors around the area of the main blown transistor. It looks like I'm going to have to get back under the hood.
Got all of the main caps that I could order from CPC and the new soldering gear came too! Just need to find a supply of better shaped tips and I'm good to go!

Will search for details of the amps protection circuit.

[mariushm]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

That particular Tenma soldering station doesn't actually have a temperature sensor in the tip, so the accuracy of the temperature is not very good.
It's a rebranded Atten 938D I think, and.. on the cheap side.
If you didn't buy it yet, don't rush to buy it.

This one should be better : http://cpc.farnell.com/tenma/at60d-u...lug/dp/SD01845
It's a rebranded Atten AT60D so you could search for it on eBay maybe it's cheaper.

This one is also good, and it's a rebranded China Zghondi ZD-937 (scroll down) soldering station so again you can find it on eBay or as other brands and you find parts for it easily.

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Your info came a couple of days too late mariushm..... :-I. Thanks anyway.

Would be too much of a hassle to send it back. I'm just glad I've got some better soldering gear and if it does break down or something at least I can return it to CPC instead of shipping it back to China or wherever. Maybe I'll donate it to somebody else in the future if and when I upgrade.

[Agent24]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Are you sure? If their website is accurate: http://www.atten.eu/atten-at938d-sol...150-450-c.html they say it has current temperature readout, I can't see how they'd do that without a sensor.

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

I need to find somebody with with a T760 who can take some pictures for me so I can compare what's under my hood to theirs. Will post some pictures of some things that are confusing me.

Pic1 : From the main motherboard to the AC3/DTS board is this cable with the ground cut.
Pic 2 : Ground cut. Who knows why?!
Pic 3: NJM5532D* operational amplifier. Some strange reason one of the legs has that thing soldered to it which connects it to the pcb.
Pic 4: One the surround board is this wire that goes from the between the two links to the front amplifier board J514b
Pic 5: J514b.
Pic 6 : The AC3/DTS board ( In case you're wondering I replaced the bridge rectifier, just above the main cap, after taking this pic). I'm not even going to talk about that thing up on the top left hand side of the picture. Can't find it in the service manual but it's there on the board. Go figure. It's been wired to J885A on the surround amp board.
Pic 7 : Powers up for a couple of seconds without exploding. Bonus!
And that's why I need those pictures of a T760.
If anyone has one on a shelf gathering dust, grab a cam and post a few pictures of under the hood please.

Tomorrow I'm going to repair the ground link and remove those "mods" one by one to see what happens.

* Turns out I might be able to update these with NJM2114D. http://groupdiy.com/index.php?topic=7566.0
For now let just get the thing working first :-).

[Agent24]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Those changes might be a legitimate modification, from the factory or as a service update to fix or prevent some known problem, but I suspect not.

The wire from J514B looks legit though.

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

I find the suspect thing to be the NJM5532s' extension, Agent. These mods look just to crude to be done by a official Nad tech. I could be wrong.

If I'm reading the service manual right then the person who soldered the U109 pin 5 ( non-inverting input ) extension on the NJM5532 IC, soldered it to the same boards U103 CS5331A IC pin 6( analogue ground). Does that mean anything to anyone?

http://www.cirrus.com/en/products/cs5330a-31a.html

Going to cut it off anyway just to see what happens.