New Build - Blew Q7 - SOLVED

[jamesnorth]

Hi Tim/Everyone,

First time builder here - just took delivery of 4 x N72, 2 x J99 and 2 x C84 - looking forward to adding them to my studio soon.

I've completed building the 2 (Version 2.0 2011) C84 modules and one of them seems to power up ok with the suggested tests giving favourable results (still yet to try with a mic in practise but will do so soon).

The second unit blew the transistor at Q7 after about 15-30 seconds of being powered on. I've compared the two modules and all the diodes and caps seem to all be the right way around. Looking at the schematic I see that this is a buffer section of the circuit ... so have double checked all my solders around that part and they all seem solid.

I'm hoping to get a new BC560 part for Q7 over the next few days from a local supplier and I wonder whether you might be able to offer some other points for me to check for continuity or for certain values with my DMM?

I could probably post a high res shot in case it helps?

Thanks in advance!

James

[tpryan]

Problems with the C84 are almost always the result of transistors being installed incorrectly. Make absolutely sure you haven't swapped NPN for PNP and the MJE172/182s aren't backwards.

[jamesnorth]

Problems with the C84 are almost always the result of transistors being installed incorrectly. Make absolutely sure you haven't swapped NPN for PNP and the MJE172/182s aren't backwards.

Hey Tim,

Thanks heaps for the reply!

Yeah I've double and triple checked the 172/182 and 560/550 are all in correctly. I've taken a shot just to see if you can pick anything visually. Everything that has a polar nature seems to match my other module that is working ok.

I've replaced the top BC560 with another, as the original blew. I tried another 560 and it blew as well - so something's going on other than having a faulty transistor at Q7.

I also replaced R56 and R57 as I thought they might've been the problem (they are now 4.7k rather than 4.75k - hence the brown colour).

When checking R56 *in circuit* it shows pretty much 0 resistance. R56 on my working C84 shows its regular 4.75k rating.

When checking R57 *in circuit* it shows 3.65k whereas the R57 in circuit in my working C84 shows its regular 4.75k rating.

Hope the pics might help?

James

[tpryan]

If the BC560 failed, there's a good chance the other transistors on that side were also damaged. Replace the BC550 and MJE172/182 as well and let us know what happens.

[jamesnorth]

If the BC560 failed, there's a good chance the other transistors on that side were also damaged. Replace the BC550 and MJE172/182 as well and let us know what happens.

Tim,

Thanks again for replying!

Yeah sounds like the next step - I've got some BC550's here already, so I'll grab the MJE's when everything opens up again and swap them out.

I shall report back!

James

[jamesnorth]

Still waiting on replacement transistors for this section of the board - will test and update as soon as they come in. Still quite puzzled about what could be going wrong!

James

[jamesnorth]

If the BC560 failed, there's a good chance the other transistors on that side were also damaged. Replace the BC550 and MJE172/182 as well and let us know what happens.

OK managed to replace the 172/182 area - that's helped a lot ... nothing is blowing up anymore.... but!

I do get weird voltage readings and one of them is the wrong polarity, but I can't for the life of me find anything in backwards. My working C84 is going really well and I've been using it as a ref for some tests ...

Anyway, the non-working one has the following values at the test points:

(R64 and R68 are totally counter-clockwise - clicking)

At the +B point, I get: -5.5V (it falls from -7V when I first turn it on ... yes minus 5.5V)
At the -B point, I get: -27.7V (steady)
At the R29/R30 common servo junction, I get: 1 or 2mV
At the R35/R38 differential servo, I get: 110mV
At the R50/R60 output buffer, I get: 275mV

And at the "Congratulations you have a working C84 preamp" page, I get a frowny face

Hope that helps you to help me?

Thanks again Tim - enjoying using the N72's / J99 and C84 at the studio already and I think I'm going to replace my another 8 pres with more SCA stuff too.

James

[tpryan]

You're sure your black meter probe is at PGND?

Are you using the latest version of the schematic (from the web site)?

[jamesnorth]

You're sure your black meter probe is at PGND?

Are you using the latest version of the schematic (from the web site)?

Hi Tim,

Yes - definitely got my test probes correct. My working C84 tests perfectly.

Just for interest, I tried adjusting the pots on R64 and R68 while keeping the probes on and the voltage does not change at all on either +B or -B test points on the capacitor point.

I guess they're not affecting the circuit there ... does that assist?

I still get the same results above at the test points.

James

[tpryan]

We'll need to see it.

[jamesnorth]

We'll need to see it.

Hey Tim,

When you said this to me I thought ... "sounds serious".

I decided to do some more digging and get to the schematic to sort it out. I traced the voltage values from the power input via the diodes and tantalum caps and the voltage regulators first. I compared it to my working C84 and found that values started getting messed up around C34.

Turns out, after all this time ... I had swapped the LM317 and the LM337. ARRGGHHHH. I sourced new ones of these from a local supplier and replaced them, but something was still up (still getting -5v on what was meant to be the +V area).

I took out the nearby 1k trim pot resistor, C34 and a few other resistors (I think 1k24 and 118).

Turns out, I had nuked C34 (confirmed this testing out of the circuit).

I sourced a new one, put in a fresh trim pot and resistors and BAM! It was showing +14V!

The bottom area was showing -14, so obviously having the wrong voltage regulator down there didn't cause an issue.

Powered it up - plugged a mic in and ... WE HAVE LIFT OFF! Big, loud, clear sound.

So, thank you once again for helping out (Tim also helped me diagnose a J99 that I'd also stuffed up a little) ... I'm stoked to say that my 8 channels are in my rack and ready to be used. I think I'll get rid of my other 500 series stuff and get another 8 channels too because I love it so much.

I have just one question however....

Even though everything is testing perfectly and working equally as well, the new Lm317 and LM337 are running hotter than the other C84. Are they just perhaps a little cheaper components and do I have to worry too much about this? They're not too hot to touch, but noticeably hotter than the other module.

Thanks again!

James