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kcbooboo

Member Since 18 Nov 2009
Offline Last Active Apr 25 2015 06:49 AM
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Posts I've Made

In Topic: Strange diodes on D75 main board

10 January 2015 - 08:35 AM

I pulled those two diodes and cleaned up the excess solder. The amp seems to be working just fine without them. They were definitely on the outputs of the LM339 comparator that drives the IOC LEDs, limiting their excursion to no lower than -10V. I can't see how they could go lower since that IC is powered by +/- 10V.

Bob M.

In Topic: Strange diodes on D75 main board

25 October 2014 - 06:31 AM

I checked all the schematics I have for the D45, D60, D75, D75A, D150, D150A, D150Aii, DC300, DC300A, DC300Aii and found no diodes in the IOC circuit like my D75 has. Not sure why they're even present as they serve no useful purpose. Next time I have that amp opened they're coming out.

Bob M.

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.