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kcbooboo

Member Since 18 Nov 2009
Offline Last Active Today, 06:32 AM
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Topics I've Started

Strange diodes on D75 main board

08 October 2014 - 07:37 AM

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.

D60 Driver / 2N6175

15 September 2014 - 06:02 AM

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.

D75 Bias issue

03 September 2014 - 06:51 AM

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.