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kcbooboo

Member Since 18 Nov 2009
Offline Last Active Oct 18 2014 07:36 AM
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Posts I've Made

In Topic: Strange diodes on D75 main board

18 October 2014 - 07:35 AM

I explained how the diodes are connected in my original post.

They're on the middle of the three socketed ICs. The component layout calls it one thing, the schematic calls it something else. Whatever, it's the IOC comparator IC that they're soldered to.

Bob M.

In Topic: Strange diodes on D75 main board

11 October 2014 - 05:01 AM

The older board has three ICs: the uA739 dual op-amp, an LM339 quad comparator for the IOC circuitry, and an 8-pin dual op-amp for the balanced-to-unbalanced inputs. The diodes are soldered to pads that are obviously joining two pins together, and they're on the middle IC, the LM339. I'd rather just remove them as the other D75 amps I've worked on don't have them.

I could post a photo but I'm not sure how to add one on this BBS.

Bob M.

In Topic: D75 Bias issue

01 October 2014 - 07:02 AM

I was initially only going to measure the voltages in the negative half of CH1 vs CH2, and that probably wouldn't have found the problem. I think that since the positive half resistor was so high, the bias circuit compensated and reduced the bias to maintain roughly the right value on the positive half, and the negative half was left to live with it. With equal resistors, the bias voltages were equal, although slightly low. I needed a 200 ohm resistor but that's not a standard value, so I just padded down the existing 240 ohm resistor. If that hadn't done it, I might have just put two pots in and made it infinitely adjustable, like the later D75 amps have.

I don't know why the resistor looked singed. It's not near anything that would get hot, and nothing else seemed out of sorts. I might replace it with the proper part next time I order something from Mouser or Digikey.

Unfortunately, no one will be able to find this topic because the board's search engine won't accept D75 as a search term; too short. Sorry, can't make it any longer. Fix the search engine.

Bob M.

In Topic: D75 Bias issue

29 September 2014 - 11:03 AM

I removed the main board, took a photo of the foil side, and marked where all the transistor leads actually were. The parts diagram isn't quite accurate. Never did measure the voltages though. Before putting the board back in, I measured all the resistors. Found R223, a 5.6 ohm 1/2w resistor in the positive half of the right channel was 28 ohms and didn't look very healthy. I replaced it with a pair of 11 ohm 1/4w resistors in parallel and that brought the bias on Q212, in the negative half of the right channel (the original problem), back up to 0.29V, matching Q209. I tweaked the bias resistor value a bit and got the right channel bias up to 0.34V. The amp still works just fine; makes 20Vrms into 8 ohms before clipping and still has very low distortion and noise. Now I'm happy. All fixed.

On to the next project. Thanks to all who added their comments.

Bob M.

In Topic: D75 Bias issue

24 September 2014 - 08:24 AM

I will have to do more checking. It's difficult to locate some of the parts from just the foil side of the board, although luckily, and for the first time in nearly a dozen Crown amps, there IS a board number that matches one in a manual. I will be replacing the half dozen electrolytic caps on the main board once I place an order with Mouser this weekend. While the board is out I'll check the resistors from the component side. After re-installation I'll look more closely at some of the voltages on the board. With only one quadrant affected it should be easy to check, and even easier with both covers off.

Bob M.