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About kcbooboo

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    Power User
  1. DC300a biasing

    I had a similar hum problem on a D-150A-II amplifier. Turned out to be the high voltage (70VDC) power supply capacitors. This uses one of the AC windings and a voltage doubler. It is drawn in above the +60V supply rail on the amplifier schematic. The two caps are both 10uF but NOT equal in voltage. I replaced them both and the hum went away completely. C4 and C5 on the main board. I recall seeing that same amp on fleabay and figured it would be a simple fix, but I didn't need yet another DC-300A amp. Bob M.
  2. I've got several D-75 amps that have IOC LEDs. I have a DC-300A without IOC LEDs. I did a lot of investigating and designed an add-in board (2x2 inches) that should work in D-60, D-150, and DC-300 amplifiers that don't already have IOC LEDs. I built and installed one in my DC-300A and it works beautifully. It's not possible to add one to a D-150 with the accessory front panel (believe me, I really wanted to). I'm about to put one into my D-60. I have a few boards available but don't know if anyone else is interested. Post if so. Bob M.
  3. Strange diodes on D75 main board

    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.
  4. Strange diodes on D75 main board

    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.
  5. Strange diodes on D75 main board

    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.
  6. Strange diodes on D75 main board

    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.
  7. I've got an older D75 that had the usual smoke and fire caused by one of the two 10V zener diodes shorting and flaming its dropping resistor. While examining the board (9750), I discovered two diodes soldered onto the foil side. These are on the LM339 comparator that runs the IOC circuits. One diode goes from the -10V rail (pin 10) to pins 13+14 (pointing to pins 13+14), and the other goes from the -10V rail to pins 1+2 (pointing to pins 1+2). One of them has had its glass shattered when the cover was depressed, but the other has three bands: red, violet, and black, which tells me it's a 1N270. I can't find these on any D75 or D75A schematic nor on any of the other D75 amps I've worked on; from what I can tell they serve no useful purpose. It seems they are clamping the outputs of the comparator to keep it from going below -10V, but since the comparator is being powered by +/-10V, the outputs can't go any lower even without the diodes. I'm not sure if these were put on by Crown or someone else, but I'm considering removing them if no one has any objection or can show a use for them. Any hints or clues? Bob M.
  8. D75 Bias issue

    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.
  9. D75 Bias issue

    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.
  10. D75 Bias issue

    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.
  11. D60 Driver / 2N6175

    I did buy a 2N6176, carefully extracted the D40V2, installed the proper-size part, managed to not lose any mica insulators, and the amp still works great. That's why I posted the previous info, to help future repair efforts. Bob M.
  12. Last year I acquired a D60 amp. One channel had a problem which I traced to a bad driver transistor. I called Crown and of course the original part (2N6175) was no longer available. They sent a replacement, a D40V2. Installation was not trivial because the cases were totally different. The 2N6175 is a thick (0.150) insulated transistor with EBC lead orientation and those leads are round and the size of 1/4w resistor leads. The center mounting hole is insulated but there is an exposed collector at the back. The D60 has holes that the leads fit perfectly in. It also has four of these same transistors used as drivers and they mount between the main circuit board and the chassis, acting as standoffs. The D40V2 is a totally different animal, at least the one Crown sent me. It has a plastic body that's nearly the same thickness but there is a thin metal uninsulated mounting tab that sticks out from the body. To mount this part, one needs a nylon screw because a metal screw would ground the collector. Then you need some sort of spacer to make up the difference in thickness; I used a #8 nut and a #4 flat washer. The leads, which ARE oriented in EBC order, are rectangular and much bigger than the holes will accept. Rather than drill larger holes, I chopped the leads off and installed short wires into the circuit board and wired those to the D40V2, which had to be bent nearly vertical to fit into the available space. In the end, it worked but it looked ugly and took a lot more work than it should have. This must have been the only transistor on earth of that size factor that still had EBC lead order; almost every other part has BCE. I'd have been better off by using a TIP47 and crossing the leads. I asked Crown about the mounting differences but never got a reply. Perhaps if the amp already had D40V2 transistors, things would have been easier. I did some research into the original 2N6175 and found some had round / EBC leads while others had rectangular / BCE leads, and no one had any to sell. I then came across a substitution list that mentioned the 2N6176 and 2N6177, which are higher voltage versions of the 2N6175. I found someone selling 2N6176 parts with round leads and the same case thickness as the 2N6175. So I bought two and installed one into the D60. It still works great and I'm back to metal hardware and a repair that looks original. There are places selling 2N6176 transistors, so if you've had this same experience, it's worth it to spend a bit more money and buy a part that fits and works just like the original. Perhaps Crown should buy some of these and sell them instead of the D40V2, especially for the D60 amps. Bob M.
  13. D75 Bias issue

    Thanks to all the responders. The caps have been replaced. Remember, the amp is putting out 40w per channel into 8 ohms with under 0.01% distortion from 10 Hz to nearly 10 kHz. If anything was open or not working, it wouldn't have such good performance. I am measuring 5-6 ohms from B-E of each of the four TO-3 transistors. I even pulled Q212 out, cleaned it, checked it separately with an ohm-meter and got the same values as with a new MJ15015. I also measured the resistance across the B-E solder pads on the output board: 5-6 ohms. I also checked the voltages on the four TIP-47 transistors and the only thing I found was slightly lower base and emitter voltages on the one that drives Q212, which tells me the problem is on the main board. I tried moving backwards into it but it's nearly impossible to locate things on the old board because the parts are oriented in whichever way there was room for them. The newer board has all the parts nicely aligned. Bob M.
  14. D75 Bias issue

    I've got an older D75, s/n 016xxx. At the moment it is working fine. Distortion on both channels is at least -80dB when running at 40 watts into 8 ohms. It meets the frequency and noise specs too. I measured the bias voltage (Base-to-Emitter) on the four TO-3 amplifier transistors. Three are 0.33V, the fourth (Q212) is 0.07V. The voltages on this transistor to ground are: E: -35.05, B: -34.97, C: 0.0. The positive supply voltage is 35.1 and both speaker outputs are sitting at 0.00V. As i said, in all other respects it's working just fine but the low bias on the negative side of CH2 has me concerned. The voltages on J1 and J2 are nearly identical between the two channels. I'm thinking Q211 but all of the transistors checked good in-circuit with a Simpson 260VOM. Suggestions? Bob M.
  15. DC300A bad right channel

    Just a follow-up. I added 500 ohm bias adjust pots, with 90 ohm resistors in series (they were the first ones I picked up). Put them in place of the fixed bias resistors. turned the amp on, and adjusted them for about 300mVDC bias voltage. I left the amp running for a couple of hours and went back and tweaked the pots for 325mVDC bias voltage on each channel. Quick and easy to get within 5mVDC. While the amp was open I readjusted the offset pots and got both channels within +/- 0.1mVDC. Bob M.