Incorrect voltages around Q1, Q2, and Q3

[SoundBase]

I finished assembling 8 N72 six months ago but am finally getting around to calibrating them and putting them in the rack. The calibration prcedure went fine on the first seven but I seem to be having a problem with the last one. Specifically I am getting incorrect DC voltages around Q1-Q3 before the final biasing. (The dc voltages on the first seven unit were all within spec.) The voltages are
q1 c-6.49 b-4.39 e-4.01
q2 c-5.83 b-6.48 e-5.80
q3 c-22.76 b-5.82 e- 5.21
The voltages for q4-q6 are all within spec.

I rechecked all of the resistors visually and also with a VOM (they are soldered in the board) to make sure they are correct, and that seems ok. I visually checked all of the caps (not sure how to test them with a VOM), and I'm not sure that they would be contributing to this error since this is DC voltages we are talking about. In any event, they seemed right, too. I also reheated all the solder joints to make sure I don't have a cold solder joint somewhere.
I have some extra BC184 transistors. Would they be the cause for the voltage errors; should I swap them out ? Or maybe it is not possible to reliably check the resistors now that they are soldered in the board ?What else could be the problem and what should I check ?
Any help would be greatly appreciated.

[tpryan]

You're absolutely sure that R25 is 5K1 and not 51K? What resistance do you measure between the C and E of Q2? (Make sure the power is off when you make the measurement) How does that compare to a working module? Let us know what you find.

[SoundBase]

Sorry for the delay in responding; I was at the Tape-Op conference over the weekend.

I remeasured R25 and it came out at 5.1k, not 51k.

I then went on to measure the resistances around all of q1-q3

First the problematic module (Number
Q1 C-E: 12.1 Mohms
B-E: 4.2 Mohms
C-B: 3.5 Mohms

Q2 C-E: 3.5Mohms
B-E: 3.6Mohms
C-B: 85kohms

Q3 C-E: 2.5Mohms
B-E: 2.5Mohms
C-B: 5kohms

Next I went on to read the same measurements on two other modules (#6 and 7) that have correct DC voltages but have not been biased, and lastly one module (#5) that has been biased.
These are their readings
#7 #6 #5

q1 C-E: 0.68M 0.59M 1.4M
B-E: 64k 65k 65k
C-B: 0.62M 1.8M 1.16M

q2 C-E: 0.74M 2.82M 1.04M
B-E: 0.83M 2.93M 1.12M
C-B: 85k 85k 85k

q3 C-E: 0.82M 1.68M 1.06M
B-E: 0.83M 1.79M 1.08M
C-B: 5k 5k 5k

There still seem like some wide variances in the resistance measurements in modules 5-7; I'm at a loss to explain them, since they all give (or gave) the correct Dc voltages prebiasing. When I was reading the high resistances they often took along time to stabilize on my Fluke VOM. Perhaps this is the nature of this measurement?
However there clearly is something wrong when I compare the base to emitter resistance of q1 (#5-7)-64k and compare it to number 8 which reads 4.2 Mohms. Would this suggest something is wrong with Q1? Also the other resistances in number 8 seem anywhere from two to (sometimes) ten-fold higher than the other three. Is there anything else that looks suspiscious?

Thanks for all the help.

[tpryan]

I really just wanted to know if you measured a short between the C and E of Q2, since those voltages are very close. Also, the B to E voltage of Q1 is just plain wrong. If your meter has a diode test feature, use it to check Q1 through Q3. If you don't know how, see: http://www.kpsec.freeuk.com/components/tran.htm Let us know what you find.

[SoundBase]

I tested the three transistors over the weekend according to the method on the web page reference you provided. All transistors were tested in both forward and reverse bias configuration. For the C-E tests, Forward bias denotes that the red probe was on the collector, and reverese bias was tested using the black probe on the collector. These tests all showed a voltage, rather than infinite resistance as the web page suggested should happen. I'm chalking this up to the fact that the transisitors were soldered in the board and not tested alone.
My meter shows infinte reading (>50Mohms) as OL. Also the (b) denotes that my meter beeped during this particular test (whereas otherwise it didn't). I read the manual, but could figure out what the beep meant. If you recall Nos. 6 & 7 gave correct DC voltages and No. 8 was problematic. I made a table in excel and tried to copy it to the post here. If this is hard to read let me know and perhaps I can repost it a different manner.

No. 6 No. 7 No. 8
Q1 For Bias Rev Bias For Bias Rev Bias For Bias Rev Bias
B-C 0.714 OL 0.714 OL 0.714 OL
B-E 0.716 OL 0.716 OL 0.715 OL
C-E 1.393 1.698 1.398 1.737 1.096 OL

Q2
B-C 0.712 2.15 0.714 2.16 0.711 OL
B-E 0.714 1.79 0.716 1.816 0.712 OL
C-E 1.241 1.253 1.241 1.261 1.237 1.89

Q3
B-C 0.705 1.437 0.704 1.433 0.703 1.433
B-E 0.706 1.966 0.706 1.969 0.704 2.141
C-E 1.115(b) 2.038 1.126(b) 2.037 1.11 2.093

Based on this data, it seems to be that both transistors Q1 and Q2 are defective, in light of the fact that the Q1 CE reads too low a voltage in forward bias and OL in reverse bias, and Q2 shows OL in reverese bias for both B-C and B-E as well as too high a voltage for C-E in rev bias.

Is this correct ? If so, I'll order more solder braid so that I can replace Q1 and Q2. I already have extra transistors that I had bought from Mouser.

If I'm incorrect in this assumption or there is something else I need to check please let me know.

Thanks again for the help.

[tpryan]

What those numbers suggest to me is that Q2 on the problem module isn't fully connected to the rest of the circuit. Try reflowing the solder on Q1 - Q3 and see what happens. Make sure you're getting solder to flow through the hole and not just piling it up around the lead.

[SoundBase]

I reflowed the solder to Q1 through Q3. The solder at the base of Q2 was originally a little thin on the component side of the board prior to the reflowing, but all of them seemed ok otherwise. After reflowing, I rechecked both the DC voltages and the transistor tests. Both measurements had dropped about 0.1 to 0.01 V (I can post the exact numbers if needed) but there were no appreciable changes.

For my education, why do those measurements suggest that Q2 isn't fully connected to the circuit?

Anything else I should try? I did order desolderbraid yesterday expecting to replace Q1 abnd Q2, and will get it in another 2-3 days. While I would like to try to solve the problem myself (I'll learn more that way), would you prefer that I just send the board to you to troubleshoot ?

Thanks again.

[tpryan]

why do those measurements suggest that Q2 isn't fully connected to the circuit?

Because the forward bias voltages looks perfectly normal, but the reverse bias read "OL" as if the transistor were out of the circuit. When you reverse bias the BC and BE junctions of Q2 in-circuit, current will still flow through leakage paths associated with Q1. The BC leakage path is primarily R23, R24, and R25. The BE path is through R26, R22, and the forward-biased BC junction of Q1. That's why the reverse bias voltages for the other modules aren't 0, or "OL". Before you swap Q1 or Q2, double check the value of the surrounding resistors, and re-flow all the joints.

[SoundBase]

Last thrusday I replaced Q1 and Q2 and reflowed all the solder to all the transistors, resistors, and capacitors associated with Q1 through Q3; however there were no changes in the transistor tests, so I didn't even bother with the DC voltages. The problem was clearly somewhere else.

So then over the weekend I removed R22 (67.3k), R23(32.6k), R24(47.2k), R25(5.07k), and R26(469) and measured them outside the circuit (results indicated). The resistors were all within spec. I also checked the resistance of the approparate circuit traces and they were all <=1 ohm. I haven't put the resistors back yet, since I was wondering if I should replace them anyway? At this point, is there anything left to check? The only thing I can think of is that there must have been a problem with one of the solder joints somewhere, but thus far in my trouble shooting they have all been reheated at least three times but that didn't seem to solve the problem.

Maybe I need to remove R27 and measure it since this seemed to be related to Q1 qnd Q2, and that resistor is common to both? Or do I just replace the resistors (or use the original parts) and try to really heat the joints when re-installing them? Can i install the resistors associated with q1 and then test and then the resistors associated with q2 and test, or do I need to do everything at once ?

[SoundBase]

Hi again
I was hoping someone else who had tackled a more difficult problem would chime in and provide some insight, but perhpas no one else has had a problem like this one ?

I removed and measured R21and R27 (which were both within spec.) but I haven't done anything further from my last post on July 3rd.
But I have been doing more reading and the bit that I have found would make me think that either the circuit board or one of the components was dirty with some sort of oil or grease when it was soldered in since flux wouldn't necessarily remove that. Hence I was going to reuse the resistors that I have after cleaning their leads with steel wool, and then I was going to clean the board and the components with 100% isopropyl alcohol (or is there something else I should use?), before soldering them in.

Does this sound like a good plan of attack ? Or should I just replace the resistors anyway?

[tpryan]

Defluxing the board is a good idea, just make sure you do a thorough job. It's very unlikely the problem is contamination of some kind, I'd suspect a PCB defect first. If you don't get any improvement, I'd send it in. I'm curious to see it.

[SoundBase]

So after several years here complete with frustration, and then ignoring the problem, and then lastly cancer, I finally have now tried to address this problematic module, and think I almost have it solved.
After rereading our previous posts, I took the module and checked all the circuit board traces and lo and behold found an open circuit that didn't exist in the other boards. To make a long story short, I added a jumper between C8 (where it connects to the emitter of Q3) and the positive side of C15. Now the voltages read:

q1: c-3.884, b-2.45, e-1.885
q2: c-12.71, b-3.86, e-3.25
q3: c-22.39, b-12.70, e-12.07

So these look pretty good but when I checked the remaining voltages on q4-q6 this is what I got (with r39 all the way off):
q4: c-2.098, b-0.96, e-0.39
q5: c-23.65, b-2.09, e-1.552
q6: c-23.65, b-1.551, e-0.994

So it seems as though the voltage on q6 was too high, 23.6 instead of 21.4 volts. So I started to open up r39 and eventually turning down q6 to 21.4. Now however the voltage across r40 is 4.885, which is exactly 0.5 watts, and yes it is warm, but not smoking or discolored.
So I reheated all the solder joints in q4-q6, and double check r18 (13.8 ohms), as well as the other resistors, but the voltages remained the same. I also then checked all of the circuit traces with my ohmeter but didn't find any shorts or open circuits.

So where do we go from here? I now have the dc voltages about right, but r40 is right at 0.5 watt, and I haven't done the balanced clipping tests yet, so it may change further. Does this point to a problem with q4 through q6?? Should I remove them and test them?

Or do I need to replace r40 with 1 watt resistor and try to move on with the fine biasing?

Thanks for the extended help,
Chuck