Need help with bias adjustment - troubleshooting C84

[oddur]

I´ve assembled two C84 rev 1.6. The 1st behaved correctly when adjusting the voltages, the other one however did not. Unfortunately I only have the rev. 1.9 assembly instructions and there seem to be quite a few differences.

After turning R63 and R67 fully couterclockwise (compromised step 34 of directions) I get the following readings with neg. probe J3 pin 2 and pos. probe on pin 8 of U1 +13 volts, pin 8 of U3 +14 volts.

Step 36 with negative probe still at J3, pin 2, measuring voltage at U1, pin 4 and U3 pin 4 I get a measurement of +12.7 volts! Here I should get about -14 volts according to the assembly instructions.

In step 37 there is a troubleshooting clause in case the readings are way off. They do not apply to rev. 1.6.

[tpryan]

You can find older versions of most documents on the site by changing the rev number in the link, but we haven't bothered to keep all versions of the assembly instructions.

C84 rev 1.6 bom: http://www.seventhcircleaudio.com/C84/C ... 84_bom.pdf
C84 rev 1.6 schematic: http://www.seventhcircleaudio.com/C84/C ... 84_sch.pdf
J99 rev 3.3 bom: http://www.seventhcircleaudio.com/J99/J ... 99_bom.pdf
J99 rev 3.3 schematic: http://www.seventhcircleaudio.com/J99/J ... 99_sch.pdf

Step 36 with negative probe still at J3, pin 2, measuring voltage at U1, pin 4 and U3 pin 4 I get a measurement of +12.7 volts! Here I should get about -14 volts according to the assembly instructions.

Do you measure the same voltage at pin 4 of all op-amps?

[oddur]

Many thanks for your reply and the links to the BOM and Schematic diagrams.

To answer your question:

Yes I get the same voltages on pin 4 of all three Op-amps.

To clarify my position further. When I started assembling the kits I downloaded the assembly instructions. When I was well on my way I came across parts of the instructions that didn´t make sense. I realized that the instructions were for a different version. I matched the BOM with the silk screens on the boards and put the components in place. I got carried away and installed the op-amps U1 and U3 without first adjusting the bias. I adjusted the voltage on both amps but neither of them passed audio.
I now wonder if I might have damaged some components?

After realizing the mistake I had done I desoldered and removed U1 and U3 and adjusted the bias voltage with the results in my previous post.

I have however not yet checked if the 1st preamp (the one that behaved correctly) passes audio!

One additional point: U5 LM337 becomes quite a bit hotter after powering up on the pre amp with the wrong voltage measurements.

[tpryan]

Reinstall the ICs and make absolutely sure that you have all of the transistors installed correctly. Get someone with some experience to verify the transistors are oriented correctly, or post some photos that clearly show the orientation and part numbers of the transistors. After that, measure the voltage at:

J3, pin 3
D16, Cathode (banded end)
D16, Anode
U5, pin 1
U5, pin 2
U5, pin 3

Let us know what you find.

[oddur]

Hi

I checked all transistors and found that I had swapped transistors Q6 and Q9. The other transistors were all correctly oriented and installed in the right positions. Is it o.k. to swap the transistors before I proceed further?

Regards
Oddur

[tpryan]

Is it o.k. to swap the transistors before I proceed further?

You should verify that the transistors are not shorted before attempting to re-use them. Also, if you have little experience desoldering it may be safer to simply snip them out and replace with new parts. Transistors are cheap compared to a damaged circuit board.

[oddur]

A friend of mine measured the Transistors and came to the conclusion that MJE 172 is shorted. The local suppliers do not have this transistor, but one has a substitute MJE 253. Is it o.k. to use that transistor in the circuit?

[tpryan]

MJE 253 may work, but the circuit is intended to use complementary PNP and NPN devices. I recommend ordering a full set of replacement transistors from Digi-Key, Mouser, Farnell, or whomever is most convenient.

[oddur]

Will do. Many thanks for your help.

[oddur]

Hi,
had to order the transistors from the US. It took a while but they came yesterday. I installed new transitors in both positions Q6 and Q9, correct ones this time!

I checked the voltages at pins 4 and 8 of U1 and U3. They were correct, however pin 4 voltage became unstable after a while and drifted and ended up giving a reading of +14 volts. Transistor Q9 became quite hot as did U4 and U5.

[tpryan]

Did you replace the BC550s and BC560s as well?

[oddur]

No I didn´t. I will check and replace them ASAP. I´ll report further.

Thanks