Low voltage at Q6 collector, does not adjust with R39

[pintsized]

Hi,

I've seen many posts similar to this on the board, but none of the solutions proposed have worked for me. I have two N72s which I built a few years ago which have no issue. I've just built two more (rev 3.3.1), and they both have this exact same problem.

R39 was completely counter-clockwise before powering on, and R40 has not become hot at any point. R49 (which is now 20ohm as of 3.3.1, not 10ohm) still measures correctly in the board. I see a -0.4v drop across it, whereas on my older N72s I see something like -0.9v.

I see in the region of 23 to 24v at pin 1 + 2 of U1, C26, and C21, but basically nothing (0.48v) at the collectors of Q5 and Q6. R39 has no impact. I see almost nothing (0.04v) at R40. Continuity in these ares seems fine.

I've double checked my work, reflowed joints, checked polarised capacitor and diode orientation - I don't see any issues anywhere. And the issue is exactly the same on both modules.

Whilst I'm here, I (like others) found inductors to install at L3-6, which are not mentioned in the assembly instructions anywhere. I installed them in one module, but not the other, but this doesn't make a difference to my current problem. I just want to confirm that they should definitely be installed?

Any suggestions would be much appreciated!

James.

[pintsized]

What kind of voltage drop should I expect over R35? I'm seeing 23.5v on one side and 1.1v on the other. This should be more like 4-5v right?

R35 measures at 68K in the board. I have two modules showing this exact same behaviour, so I'm reluctant to assume a faulty part - or is that naive?

Here are some more measurements:

C26: 23.5v
R35: 23.5
Q4: C 1.1v, B 0.8v, E 0.2v
Q5: C 0.4v, B 1.1v, E 0.4v
Q6: C 0.4v, B 0.4v, E 0v
R39: 0v
R40: 0v
C21: 23.5v

Any help much appreciated!

[mattjhuber]

I am having pretty much the exact same issue.

I just built 3 N72s. One of them works perfectly fine; the other 2 have the same issue. I cannot figure out what's up.

As I went to set R39, I noticed that two of them would consistently measure .4v regardless of the pot position. So, I went through and started testing the transistors. Q1, Q2 and Q3 were perfect. Then, on both of them, Q4 measured a very low voltage on the collector then practically nothing on the base & emitter. Q5 measured 0 on all.

I reflowed all of the solder, checked polarity on all applicable components...nothing. It just seems strange that 2 of them are having the same issue.

[mattjhuber]

I noticed that the unit that works has different transistors from the other 2 -- same value, but different screen printing; maybe a different lot or different manufacturer?

The ones that work have a stylized "F" and then the code: BA35 (followed by BC184 C). The ones on the N72s that are having issues have an "F" matching the rest of the characters, and the code 408 followed by BC184C.

Maybe a bad lot of transistors?

[mikebanta]

Hi, just built my first N72 and having same issues.

24 volt supply checks out fine. I have 23.8v coming into R35, .47 coming out.
I have 23.8 at the + side of C21.
Q4 measures C= 1.17, B and E are 0
Q5 measures C=.47, B=1.16, E=.35
Q6 measures C= .47, B=.47, and E=0.

0 at R36 and R40. Also 0 at R39.

Like others, have checked all joints and orientation, all seems fine. Looking forward to what solutions come out of this thread.

[pintsized]

Are you guys both building rev 3.3.1?

@mattjhuber is your working module an earlier revision by any chance? On the transistor front, I was sent (several) incorrect parts, so for one of my N72s I had to source the BC184C and MPSA56 via Ebay. I see the exact same results on both modules; one built with the correct transistors from SCA, and one built with substitutes from Ebay with different markings, but the same part numbers.

[mattjhuber]

It doesn't look like a different revision -- everything other than the markings on the transistors is the same. I want to stress that the transistor values are the same, but the seem to either be from a different manufacturer or batch.

[tpryan]

I see in the region of 23 to 24v at pin 1 + 2 of U1, C26, and C21, but basically nothing (0.48v) at the collectors of Q5 and Q6.

It sounds like an open, possibly a bad solder joint at one of the transformer leads. If you snipped off the tinned portion of the leads you must strip the enamel insulation as well as the vinyl sleeving. Use fine sandpaper to expose bright copper and re-solder.

[mikebanta]

So, going through and reflowing / checking solder joints solved the Q6 issue.

Not done yet though, as I cannot get voltage low enough at Q6 collector without overheating R40. When R40 is at about 3.75, Q6 is still over 24. Q4 and Q5 are measuring somewhat low as well.

I will try the transformer solution mentioned above, and continue checking....

Thanks much.

[mikebanta]

My N72 is fully functional and sounding great now.

Some more research on the forum and I found a post where you can adjust R39 to put R40 at 3.75 volts, and it's ok if Q6 is running a bit high.

So my lesson learned on this one is - check your work, check your work, check your work. Reflowing solder joints, particularly in the area of the board around Q4 and Q5 solved the issue.

Appreciate all the comments here, and best of luck getting your modules working.

[mattjhuber]

Reflowed everything...still have 2 with the same issue.

[tpryan]

Reflowed everything...still have 2 with the same issue.

If you trimmed the transformer leads, did you strip them down to bare copper? Re-flowing won't improve anything if the enamel insulation is not removed.

[pintsized]

Thanks Tim, that was it! Well, sort of. The output transformers I have are solid wire in a single insulation - it certainly looks like bare copper to me, no sign of enamel. But I think I just didn't get these joints hot enough, even when re-flowing. I re-soldered them and everything is now working great.

[Diyer]

N72 ver. 3.3.1
Same problem as OP, same result. 0.3v at Q6 on its casing as well. Good voltages at Q1-Q3, 30 volts at Conn3, 48 volts at Q7 and Q8, 24V at C26 + lead, but low to no voltage at Q4 and Q5 and no voltage at R40. I reflowed all joints, no shorts, and Very close scrutiny of of all joints. It seems others mentioned the transformer leads having enamel that needs to be removed. Is this the large Carnhill transformer with the multi color leads or the smaller one with the vertical white/bare leads?
Finished the B16, D11, A12, J99, and all tested perfectly, with successful biasing and voltage adjustments. So, I saved the N72 for last, but have these same issues posted. Any other successful corrections would be appreciated.
One more thing....there seems to be an open solder hole above and left of R38. Nothing seems to go there, but it looks to be connected to the neg side of C14 (cp board top) and to the outside joint of R19, the blue dial (bottom side of board). Could this have something to do with the issue?
Thanks.

[Diyer]

Solved. I de-soldered all of the colored leads on the large Carnhill transformer, (using a Chinese made S-993A solder vacuum [couldn't do these kits without it]), sanded them to bare copper, and resoldered them back into place. I also desoldered Q6 and resoldered, as well as reflowed Q4 and Q5. Then I checked the voltage at Q6 and, Yea, it worked. I adjusted R39 to bring Q6 to 22.2v, R40 to 3.75v, and checked all the transistors. Everything was right on target. Plugged it into a Tascam UH7000 and enjoyed the sweet fullness of my N72. What a relief. My CH02 now has one N72, one J99, one A12, one D11, and a B16 comp, all working and sounding great. I have also done several of the mic kits from microphone-parts dot com, which are awesome btw. Still don't know what that hole is by R38, but I saw it unfilled in the N72 pics on the website, so, I left it open. Not needed I guess.
Finally done. Now, on to make music....