Passing light through AMP PCB reveals exposure to elevated heat

[Alex Santos]

The pics you have already posted have been helpful, and I might re-post one or more of your pics, but with some specific components highlighted so that I can ask you questions. After that we'll dive into the transistors.

Hi Roger

I have the front control panel off but otherwise I hope these pics are good. Still on an iPhone 4s so the pics could clearly be better. If necessary I can pull out a proper Digital SLR if you need more resolution.

Another telltale sign that this has been 'serviced', there should be am isolating shield over the transformer. For whatever reason I found it missing when I opened it. I should consider an alternative.



Closer ups of the amp side PCB

 

[gadget73]

the forum shrinks the pics anyway, not honestly sure there would be massive benefit to using a DSLR for this stuff.

 

[Alex Santos]

This is just a side note to our discussion:

@roger2 and anyone else. I just enrolled in this free course at EdX, https://learning.edx.org/course/cou...019+type@vertical+block@vertical_8da88d8b7846

I don't know if that link will work as I am logged in so if it doesn't you can search for "MITx 6.002.1xCircuits and Electronics 1: Basic Circuit Analysis." I was originally thinking of adding a course through Udemy but this looks way better than anything I previewed on Udemy. It's a few years old I think but it looks very good.

Also, there is a 1000 page book that as far as I can tell accompanies this course. It is available for purchase online but also freely available on archive.org https://archive.org/details/foundations-of-analog-and-digital-electronic-circuits/page/n15/mode/2up. If you buy the physical book it is delivered globally for free. https://www.elsevier.com/books/foun...electronic-circuits/agarwal/978-0-08-050681-4

Sorry for derailing this thread so badly off-topic with this post.

 

[Alex Santos]

the forum shrinks the pics anyway, not honestly sure there would be massive benefit to using a DSLR for this stuff.

I found that you can click on the thumbnails which opens the images in a new tab/window and from there a zoom feature is available. Unless the original uploads are all scaled down to some preset max size.

 

[gadget73]

Unless the original uploads are all scaled down to some preset max size.

I believe that is the case. Zoom in too much and you will see it gets very pixelated.

 

[roger2]

RE pics

Alex thanks for the new pics. Those are fine, don't worry about getting a DSLR just for this. I tried your zoom suggestion and could not get it to work on your new pics on my PC (I don't do internet stuff on my phone). I believe that AK reduces jpgs to a max size. Not certain, but I think the max used to be 800 pixels wide, although it could be 900, I am not sure. It allows (last time I tried, a while back) me to upload 900 pixels wide. But...you used to be able to right-click and see image info including pixels. But today I am not able to see that. Your iPhone pics are fine.

RE the EdX course. Learning, having the desire, and taking actions to learn, all very good things!

I tried looking at the course from your first link but it asked me to login or register. So I looked at the book Table of Contents from your second link. It appears to me that the course assumes its students already have a solid understanding of basic electronics. I did not see the typical "Electronics 101" type of chapter headings. The concepts shown there are beyond me to be honest.

 

[Goldie99]

Hi Alex - I've just had a good read through your thread, and although I've not worked on any of the PE models I am familiar with a few of the older NAD models. Just a couple of thoughts for now:

If you print out a copy of the schematic, and highlight any components you find as bad / out of spec. etc., it's a lot more useful than using the PCB layout diagrams (as in your post #9) - also use part references whenever possible, e.g., 'Q429', etc. The schematic views let you see the relationships between parts much better than the pc layout view does, and it helps others follow what's going on.

While I wouldn't have removed all the transistors (yet) from the main heatsink, now that they are out, it's worth testing them all and recording the results. based on your failed parts so far, I'd expect more than just Q430 to be faulty. The following link is a good ref. for quickly testing transistors:

https://audiokarma.org/forums/index...-transistor-testing-basics.43186/#post-432830

 

[Goldie99]

@roger2, I had already realised that you were developing a logical 'plan' for Alex to follow, and I had no intention of 'derailing' or redirecting that in any way - I simply suggested that it was better to 'map' failed components onto the schematic, rather than onto a pcb layout diagram, to make it easier for all to follow.... that changes nothing whatsoever with any troubleshooting plan or approach, but just adds to the clarity for many of us.

As to testing the various already removed transistors, it would only have added (what will be) important information a little soner, and maybe even have distracted Alex from removing anything else in the mean time , no offense Alex...
I have no problem whatsoever to take a back seat as it were, and will gladly refrain from adding to any confusion. I'll maybe just pm any thoughts I may have to you instead.

Alan

 

[Alex Santos]

@roger2 You're right about the course. I took another look at Udemy but this time closely examined a few courses on electronics, I purchased three. I am determined to learn.

@Goldie99 Interesting that you point out marking the schematic with references to faulty parts. You know, that dawned on my when I took a look at the schematic. The reason I used the PCB layout was to highlight which parts were found to be faulty (out of circuit). Thanks for the link on the transistor basics, I will check it out

I appreciate the input from the two of you.

@roger2 for the time being, I will be focusing on redoing a deck here at the house. I have to take advantage of the dry pause (we had rain lately). I will then be shuffling things around upstairs to make room for this NAD project. I will continue to check this thread in case but you might have something to instruct me on or ask. Once I settle some proper space upstairs for all things electronics I will be shifting most of my attention to learning.

At the moment I feel confident that no real harm was done to the receiver, I have meticulous notes and photographic documentation on every step I took. I feel confident that nothing destructive took place but for two points.
One traces was lifted but not torn (my bad for sure) - it's 100% intact but peeled slightly.
I did reflow and although my dwell time was short, @roger2 pointed out that I may have caused some thermal stress to the more sensitive components layer on the PCB, possibly damaging them. Hopefully not.

 

[Alex Santos]

Guys, I for one can only say this … the quality of people and considerations made to otherwise is truly special.

@Goldie99 nono, absolutely. When I can, probably a few days, I will map out the bad parts to the schematic and I will measure each of the removed transistors (those along the heat sync) and document their expected and actual values here with their Q reference. I will do that for @roger2 and I am sure he appreciates the reasonable approach around these two points. It does make sense to me. Consider it done, I know that I will need a couple of days though.

 

[roger2]

Alex, before moving forward to other topics, can we more thoroughly explore any work that might possibly have been done to the amp prior to your ownership?

There have been a few references scattered among other posts. Q430 and R307 have already been mentioned.

Post #1, pic #6

In the area circled in red are empty spaces for two diodes, a transistor, and a resistor. The top diode is D412.
(1) What are the part numbers of the other three components? The PCB is clearly darkened by the through holes of the missing resistor. (2) Are the through holes of the missing transistor and/or the missing diodes also darkened? (3) Did you remove any of these parts yourself? (4) On the solder side of the PCB, does there appear to be evidence of non-factory soldering on any of these component locations?

 

[roger2]

Post #1 pic #7 is attached below.

Similarly to the pic above, this one shows many empty component locations in the two red circled areas. I can't make out most of the part numbers. In some I can clearly see PCB discoloration but am not certain about others. If there is PCB discoloration, either the through holes or the area directly under where the component would have been, then I think it is safe to assume that a component had to have been there.

But wondering if all empty locations held components previously, or if perhaps some were empty from the factory but not-jumpered. Once we have a comprehensive list of locations where we believe that parts were removed, we may be able to see some logic at to why this might have been done. But let's hold off on speculating for now, and first try to simply determine all parts that were removed by someone other than you.

Same questions for empty locations as in the post above:
(1) what are the part numbers of the empty locations? (please list them)
(2) is there PCB discoloration that might be more apparent in person than in the pics? Also note that some of the locations are not fully visible in the pic.
(3) did you remove any of these parts yourself?
(4) on the solder side, is there evidence of non-factory soldering work? ( here I am thinking of any locations that might not show PCB discoloring on the component side, as the discoloration is enough evidence IMO)
and additionally, (5) are there any other similar empty component locations elsewhere that are not shown in any of the pics you posted so far?

 

[roger2]

Alex. Attached below are post 1 pic 4 & pic 8, each with an empty transistor and resistor location circled in red. Same questions as above. These two pics might show the same two missing components, I can't tell for sure.

 

[Alex Santos]

Roger, I will reply to the last three posts.

Every component documented below were removed by me.

Note: I can't evaluate the solder side of the PCB in terms of any work that might have been done because I removed the components.

I will list components now

• R456 is a Res. Metal 1K +/- 5% 1W. I removed it because it was visibly burned
• D412 / D414. Through holes are not darkened. Zener Diode RD4, 7ESB2. They read 0.731 on my DMM
• Q420 is a transistor, 2SC 3423-Y, no matter which way I read it I get a value (concerning?)
• R424, R414, Res Carb 8.2K +/-5% 1/2W, DMM reads both as 8.06K
• D402, D403 Diode 1N4148
• D405, D406 Zener Diode, RD15EESB3

Roger, perhaps it might be more fruitful if I list all the components I removed. I have every component and their locations referenced. I can take measurements with my DMM and share them with you. Just let me know.

 

[roger2]

In each of the four pics I posted above there is one empty transistor location visible. In all four pics is that Q420?
If not Q420, what are the other missing transistors' part numbers?

In the red circled areas of the four pics posted above, are there any empty component locations where you did not remove the part yourself? Anywhere else in the amp not in any of your pictures?

Right now no, please don't list all parts removed by you. Trying to get a clear picture of parts removed by someone else only.

But please do make a list of all empty (not factory jumpered) component locations for parts that you did not remove, with a notation of whether or not the PCB or through areas are darkened.

 

[Alex Santos]

Roger, in all 4 pics that is Q420. Correct.

In the red circled areas all the removed components are my doing.

Just in case, R454, and Q428, as per your last post were removed by me. They weren't viewable in the photos and your red circle though.

Could you rephrase the last instruction in bold please? Are you asking me to find missing components from the PCB that I might not have removed myself and to list those? If so, I carefully looked over the PCB about 5 times. I can't find anything that was removed other than by myself.

FYI: I found a BD502 component. It is surrounded by 8 through holes, 2 per side. There are no components in these through holes and no reference to components silk screened on the PCB, only the through holes. I see a few other spots along the PCB with 2 through holes but no component or silk screen references so I guess this is expected.

Done editing this answer

 

[roger2]

Good, that clarifies a few things.

Could you please post a pic of the PCB showing the empty location for BD502?

FYI to Alex and anyone who may be reading, I have searched threads here and at DIYaudio on this receiver (NAD 7240PE) and its sibling integrated (NAD 3240PE). In at least one of those threads (perhaps more than one, I don't recall at the moment), there was discussion of disabling the soft clipping circuitry, and also disabling the 71V power supply rails, in both cases by removing components. I was/am trying to determine if someone in the past might have executed one of those modifications, or had possibly removed components to prevent some type of fault.

 

[Alex Santos]

Pics of BD502 and surrounding through holes, sorry one side was not photographed as it was very difficult to get in there.


I did hear of folks switching off the soft clipping, not disabling it entirely on circuit but hey. FYI, actually there is a switch on the back of the amp to switch soft clipping on/off.

 

[roger2]

FUSES

Alex please inspect the fuses circled in red in your pic that I re-posted below. If there are more fuses elsewhere, please include them in your inspection. Please report back for each fuse, by part number, your findings for each of the numbered points below.

Be careful not to get them mixed up!

1) remove each fuse and test for continuity. if a fuse fails the continuity test, please take a close-up pic of it
2) compare Amp and Voltage ratings on each fuse with Service Manual values*
3) look at any other markings that might be a clue as to whether it is original or a replacement, for example, if 5 fuses are one brand but the 6th shows a different brand name


* Note: in my copy of the Service Manual, the Parts List fuse entries are confusing i.e. seem to show some part numbers multiple times. So for purposes of this fuse inspection, please look at and confirm that the actual PCB fuse labeling matches what is shown in the "Amplifier PCB Parts Location" map, and if so, use those as your reference values.

 

[Alex Santos]

please inspect the fuses

Continuity for all fuses PASS out of circuit
Fuse Location - value identified on fuse
Location F501 - T4AL 250V
Location F502 - T4AL 250V
Location F503 - F5AL 250V
Location F504 - F5AL 250V
Location F505 - T1A 250V
Location F506 - T1A 250V
Pic of fuses on amp PCB


J62 and J63 not circled in your image (they are not on the amp PCB)
Location J62 - T315mA 250V
Location J63 - T315mA 250V
Oddly, the fuses are silkscreened as J62 and J63 but the service manual references these as "F 801, 802"
Pic of fuses on non-amp PCB


Why is F501, 502, 503, 504, 505 and 506 marked as a 125V on every odd line of the image below? The Yellow arrow on the image is to highlight the non-amp PCB fuses