C44 couldn't take the heat

[tpryan]

Yes, but please send an e-mail to sales@seventhcircleaudio.com first.

[rmiller2101]

Tim, I got an e-mail from you on December 10th that you had received the C84, but haven't heard anything since. Can you tell me when you'll look at the preamp?

[tpryan]

Can you tell me when you'll look at the preamp?

No. We'll get to it as time permits. If you have further questions, please send them directly to sales@seventhcircleaudio.com

[rmiller2101]

Okay, just to get up to date, I had Tim send me the C84 back. I replaced the burned out parts, powered it up and set all the voltages per the build instructions. All went smoothly. I kept it on for a half an hour or so and no smoke. It passes signal. The only problem now is that it's only giving about half the gain of my other C84, and the N72's already in the rack. Any ideas on what would reduce the gain?

Thanks,
Bob

[tpryan]

Gain in the first stage is set by the ratio of R25/R28 and the switched resistors, so make absolutely sure the right values are installed.

Gain in the buffer is set primarily by R39, R40, R41, and R49 in the non-inverting half and the equivalent resistors R52, R53, R54, and R61 in the inverting half. Make absolutely sure you haven't installed a 10K where a 20K should be and vice versa.

If the output is exactly half what you're expecting at every gain step, the problem is probably in the buffer. Make sure you're getting output on both pins 2 and 3 of CONN2. Let us know what you find.

[rmiller2101]

Well, R41 and R53, which are marked correctly, are measuring low in circuit compared to the working C84. R41 shows .5k vs 4.7k, and R53 shows 1.5k vs 4.7k. I also noticed that Q3 and Q8 appear to be fried - there's a tiny crater on each on the flat. Today the circuit is producing very little gain at all. So obviously I need to replace the transistors and resistors. Anything else?

[tpryan]

I'd bet the resistors are fine unless they've smoked. I'd suspect shorted inputs on U2 first.

[rmiller2101]

Showing my ignorance here, I assume you mean a short between pins 2 & 3 or 5 & 6. I show about 4.4M in circuit on U2, the same as on the working circuit. However, from 2, 3, 5 and 6 of U3 to ground (pin 2 of the power connector) I get very different readings between the sick C84 and the working one. The good one reads 596K, 122K, 121K, 121K, and the bad one reads 13K, 17.8K, 13.4K, and 17.95K.

[rmiller2101]

Q3 and Q8 are out of the circuit.