Orion HCCA amp parts substitution question - diyAudio
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Old 11th December 2007, 05:18 AM   #1
ECM is offline ECM  United States
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Default Orion HCCA amp parts substitution question

I'm repairing a few orion amps. A 250 HCCA, 225 HCCA, and a 275 SX.

All three of them have shorted outputs and shorted power supplies. I have on order irf3205's to replace the smps60n06 fets in the power supply. Must any changes to the gate resistors be made?

For the outputs, I ordered original 2n6488/91 parts, so no changes made. I'll be replacing all mpsa06/56 drivers since I have plenty in my parts bin. I'll order mpsu07/57 if needed.

What is the rail voltage on these amps? I'm assuming the 275sx has the highest since it makes the most power at 4 ohms followed by the 250 and the 225. Can I replace the rail caps on the 225 with 35v versions of higher capacitance? I will keep the 50 volt versions on the 250 and 275.

Thanks,
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Old 11th December 2007, 06:52 AM   #2
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They all use 10 ohm gate resistors so no changes will be necessary. Don't forget to check the 10 ohm resistors between the A56 drivers and ground in the power supply.

As long as the voltage rating on the capacitor is equal or greater than the originals, they will be OK.

If you don't know the rail voltage, find the Zener connected to the base of the 2n6488 standing vertically in the power supply. The rail voltage is ~1v greater than the Zener voltage.

If you have a couple of good FETs, solder one in each side of the supply and power it up (after removing the shorted outputs). Then you can measure the actual rail voltage.
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Old 11th December 2007, 07:38 AM   #3
ECM is offline ECM  United States
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Thanks Mr. Babin. With the fets installed, each resistor was measuring right at 10 ohms. I was thinking I would need to replace them with 63 ohm resistors to use the irf3205. Maybe I was thinking MTX power supply.

My plan is to strip all the fets and outputs, wipe down the board and replace the drivers. These amps are relatively old and one has minor corrosion, hence the wipe down. This will make it easy to fire up the amp with minimal fets to measure the rail voltage.

The 275SX is actually in a 225 case and an attempt at converting it to a 225 is next, but before I start rewinding toroids and adjusting rail voltages, I want to get it working first.

I have most parts on order. In the meantime, I can start stripping the boards down.

What's a good replacement for the double sided tape used on the transistor claps? I was thinking something along the lines of weather strip instead of double sided tape. That stuff is difficult to remove. Some transistors cracked when I removed the aluminum bars, but those were the ones that were shorted because once I took them off, all the shorts were gone.
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Old 11th December 2007, 08:03 AM   #4
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I use double-sided tape but I don't remove the backing so it can't stick to the transistors. When you burn the amp in, you need to re-tighten the clamps several times as the amp heats up and the tape compresses.

You could try to find some high-temp silicone rubber. http://www.mcmaster.com/ part number 93755K51 (or something on that page) should work well but I've never tried it.

To clean the boards, you can put them in the diswasher (don't use heat to dry them). I spray the boards with 409 and use a low pressure, pressure washer. Use compressed air to force the water from under the caps and ICs. If it's a warm day, set them in the sun to dry.

You can remove the pots before washing or flush them with contact cleaner after washing.
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Old 11th December 2007, 09:39 AM   #5
ECM is offline ECM  United States
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I already have a couple rolls of double sided tape so I'll use that with the back on. I thought about doing that, but I figured the wax paper used for backing wouldn't hold up.

Dishwasher to clean boards? I never would have thought of that, much less actually done it. At work we have some really potent contact type cleaner which dries damn near instantly. It leaves the parts cold as it dries, which sometimes leads to condensation. I have used this stuff to clean smaller boards and it does a good job, but it tends to eat away at certain plastics and removes markings off some components.
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Old 12th December 2007, 07:04 AM   #6
ECM is offline ECM  United States
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I pulled all the parts off the circuit board of the 275sx and cleaned it up a bit. The input power inductor is toast and I have some magnet wire on order to rewire it.

There are 4 main caps in the power supply primary. Two 1000uf 35V from b+ to ground BEFORE the inductor and two 3300uf 50V from b+ to ground AFTER the inductor.

Is there a reason why orion chose 35v and 50v caps for these locations? I was under the assumption that these were the smoothing caps, but those are on the audio side of the board and consists of 4 1000uf 35v caps and a couple smaller ones.

I'm thinking I can replace the 1000uf primary caps with 2200uf 25V units and the 3300uf caps with about 5600uf 25v caps as well. Is there a reason I shouldn't do this?

The board is pretty much bare at the moment so these mods should be fairly easy to do. The mpsa06/56 drivers in the PS have been changed already and the 10ohm resistors to ground all check ok.
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Old 12th December 2007, 07:22 AM   #7
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All else being equal, higher voltage caps generally have lower ESR and will run cooler when there is significant ripple current. When you choose replacements, choose caps with the lowest ESR/impedance you can find. You should also use caps rated to operate at 105C (on the primary side of the power supply).These have worked well for me.
http://dkc3.digikey.com/PDF/T073/P1417.pdf

If you can't find a source for the 3300@50v axial caps, Radio Shack generally has them. I don't like buying replacement parts from RS but I haven't had any problems using these for repairs.

If you're going to reuse the old inductor core, you should scrape it smooth and dip/coat it in epoxy.
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Old 12th December 2007, 09:37 AM   #8
ECM is offline ECM  United States
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I already cleaned the inductor core up since it was all crusty from over heating. Should I wind it and dip it or dip it before winding?

For capacitors, I have a few on hand and a few on order of different ratings and voltages. If I can fit a higher uF cap, I'll give it a try. If they blow up, I'll use originals. I didn't pay much attention to the esr when I ordered the caps. I was having a hard enough time finding the right size since I couldn't source any locally (I now live in Cyprus).

FYI, I installed two fets and the rectifiers and the sucker powered up just fine without the input caps. I installed a jumper where the inductor would normally be and the PS buzzed a little, but I didn't have any caps installed in the primary side. It was a really faint whine. I measured the rails at +/- 29V. This is for the 275SX.

Also on order are a few strips of high temp silicone to replace the double sided tape.

This hobby gets expensive fast. A few fets here, some outputs there, it all adds up. Of course, I'm ordering enough crap for 3 amps. The 250 alone uses 18 fets in the PS.
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Old 12th December 2007, 12:59 PM   #9
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I would coat the core before winding it.
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Old 12th December 2007, 02:03 PM   #10
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Quote:
Originally posted by Perry Babin
I would coat the core before winding it.
then coat it again after.
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