• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Nad c326bee amplifier pre amp not working?

T

TrevorBarten

Well-Known Member
I already posted about this amp +-6 months ago, however I haven't really touched it since. I got a working c326 cheap so now I have been testing the components with the working amplifier. I ruled out the power supply board, the entire power amplifier section, the entire front panel, and all the other modular components. I also tested the relays and they all seem to be functional. If I turn up the volume I can hear some sound out of the left channel but the volume is very low and there is a lot of popping and crackling in the background. Sounds like an issue with the transistors but I measured them (in circuit as I cannot really figure out how to remove them as the screws are basically inaccessible) and none of them seem to be shorted. Does anyone know what the next step would be? I doubt it's a capacitor issue since both channels are dead but it could be possible? Or maybe some of the transistors are still defective even if they aren't shorted. I did redo some solder joints as well as I read online that they could cause issues, though this mainly seems to affect the output transistors and not the others as far as I can tell.
 
Register to hide this ad
Hello. Could you post a link to the schematic?

The usual approach is to test the supply voltages reaching each amplifier board, and trace the signal is reaching the boards too.
 
Did you check to be certain the pre-out to main-in jumpers are present?
If yes, did you remove these jumpers and try another variable audio source to drive the main-in (power amp)?

EDIT: do these jumper removal and external source connection with the power off
 
jhjove said:
Did you check to be certain the pre-out to main-in jumpers are present?
If yes, did you remove these jumpers and try another variable audio source to drive the main-in (power amp)?

EDIT: do these jumper removal and external source connection with the power off
Click to expand...
Yes that is also why I know that the power amplifier section is functional. Using a separate pre amp the amp does produce sound and even sounds quite good.
 
TrevorBarten said:
Yes that is also why I know that the power amplifier section is functional. Using a separate pre amp the amp does produce sound and even sounds quite good.
Click to expand...
OK.
I have a C325BEE that I had picked up on ebay. There were several issues with the unit, one being the riser boards. These had some SMD components (electrolytic caps) that failed. I replaced these with solid-polymer nichicon and along with the other fixes (power supply, ISC circuit) got the amp sounding wonderful.

So I would look at these riser boards, specifically the one(s) that produce the signals to the PRE-OUT jacks.
 
jhjove said:
OK.
I have a C325BEE that I had picked up on ebay. There were several issues with the unit, one being the riser boards. These had some SMD components (electrolytic caps) that failed. I replaced these with solid-polymer nichicon and along with the other fixes (power supply, ISC circuit) got the amp sounding wonderful.

So I would look at these riser boards, specifically the one(s) that produce the signals to the PRE-OUT jacks.
Click to expand...
I was also pretty suspect of the riser boards, especially since I have fixed a few amps in the past that have had issues with them. I did redo the solder joints on the riser boards but none of the joints looked bad to begin with. The way this amp is broken at the moment is just so reminiscent of bad transistors to me that I kinda got tunnel vision. I'll try swapping some of the riser boards from the known good amp tomorrow to see if one of those is indeed the issue. Thanks for the tip.
 
jhjove said:
OK.
I have a C325BEE that I had picked up on ebay. There were several issues with the unit, one being the riser boards. These had some SMD components (electrolytic caps) that failed. I replaced these with solid-polymer nichicon and along with the other fixes (power supply, ISC circuit) got the amp sounding wonderful.

So I would look at these riser boards, specifically the one(s) that produce the signals to the PRE-OUT jacks.
Click to expand...
I just spent two hours swapping the riser boards but that didn't fix anything sadly. If anything it seems the problem has gotten slightly worse as the channel that was previously somewhat working is now even more distorted (it went from like 5% functional to 3%) probably due to the pcb getting warm though because those grounding pins just did not want to come loose. I even put the supposedly defective riser boards in the working amplifier and that still works just fine. I could start taking caps off the board and measuring those but it seems pretty pointless for an amp that's at best 10 years old. I still have no clue as to how I would get the transistors off and or which would even be the ones responsible.
 
If you have an ESR meter you can check the capacitors in-circuit for failures. But at any rate working with SMD components can be a challenge.

From experience with NAD stuff I'd hedge my bets on failed capacitors but better to check components for failures instead of just replacing parts.
You've already check solder connections might want to check any e-caps associated the riser boards on the main PCB.
 
jhjove said:
If you have an ESR meter you can check the capacitors in-circuit for failures. But at any rate working with SMD components can be a challenge.

From experience with NAD stuff I'd hedge my bets on failed capacitors but better to check components for failures instead of just replacing parts.
You've already check solder connections might want to check any e-caps associated the riser boards on the main PCB.
Click to expand...
I do not have an esr meter. How do I know which ones to check, just the ones close to the riser boards? And at what point would the cap be considered an issue. Say it's rated at 100 uf and measure 50 uf how much of an issue is that?
 
TrevorBarten said:
I do not have an esr meter. How do I know which ones to check, just the ones close to the riser boards? And at what point would the cap be considered an issue. Say it's rated at 100 uf and measure 50 uf how much of an issue is that?
Click to expand...
If this is tested out of circuit, for sure that is reading low and should be replaced. Most e-cap specs are +/-20% of the rated value.

It's difficult to test in-circuit electrolytic caps without an ESR meter. Use the resistance mode on a DMM and check for low resistance if the e-cap is shorted. If the reading is OL immediately (no steady increasing resistance to OL) then the e-cap is open. But even these checks can be dubious.

Visual inspection can be helpful. Domed e-cap tops are one indication, the plastic wrapping shrinking off the top of the e-cap is another (that it's been subjected to overheating, external or internal).
 
jhjove said:
If this is tested out of circuit, for sure that is reading low and should be replaced. Most e-cap specs are +/-20% of the rated value.

It's difficult to test in-circuit electrolytic caps without an ESR meter. Use the resistance mode on a DMM and check for low resistance if the e-cap is shorted. If the reading is OL immediately (no steady increasing resistance to OL) then the e-cap is open. But even these checks can be dubious.

Visual inspection can be helpful. Domed e-cap tops are one indication, the plastic wrapping shrinking off the top of the e-cap is another (that it's been subjected to overheating, external or internal).
Click to expand...
yeah ofcourse out of circuit. But would a cap that's only slightly out of spec cause an entire channel to stop working? Obviously if the transistors are actually the issue I would still have to figure out why they broke and that would most likely be bad caps but I doubt it's caps considering that both channels broke at the same time according to the previous owner. Unless this is not a dual mono design? I am not that knowledgeable about amp repair I only do them from tiem to time on the side so I don;t really know how to follow signals around for example. Would doing that here be helpful?
 
Well since both channels failed at the same time the common item would be the power supply for the front end. Start measuring voltages for the preamp and compare to the schematic.
 
You must log in or register to reply here.
Back
Top Bottom