Orion HCCA 250 power supply issues. - diyAudio
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Old 20th March 2008, 10:48 AM   #1
ECM is offline ECM  United States
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Default Orion HCCA 250 power supply issues.

I finally got around to working on my 250.

One bank of power supply fets were blown as were a few output transistors.

I removed all power supply fets and replaced the 10 ohm resistors on the blown bank (they were all open).

All caps in the primary and secondary side were replaced as well.

With all the outputs removed and only two power supply FETs installed I was getting +/- 8V rails and one side of the power supply was getting really hot. A zener diode (12v zener) between the gate and ground in the power supply was shorted so I pulled both.

Now the amp produces +/-26.6V rails, but one side of the power supply still gets really hot. I only have one fet per side and they are not clamped to the heatsink. I swapped out a few fets from my parts bin, but it still gets hot. It's the one installed on the side that wasn't blown. The other fet stays cool to the touch (I replaced that one as well, no change).

MPSA56/06 were replaced as were the 10ohm resistors going to ground. I inspected for solder bridges and didn't see any. The toroid looks rather suspect, but all coils measure around 5 ohms and is consistent across all coils.

Should I install more fets per side and test it in the heatsink?
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Old 20th March 2008, 11:30 AM   #2
ECM is offline ECM  United States
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I swapped the "hot" fet with another one and it stayed cool, but then a fet on the OTHER side got hot. So I swapped it out as well and that fixed the problem. I guess I had some faulty fets in my junk box. They are now in the trash.

I then installed three fets per side and the amp turns on and produces 26.6v rails.

The toroid hisses, should I rewind it or coat it with some epoxy to stabilize the coils?

I'll install some outputs next...
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Old 20th March 2008, 02:52 PM   #3
Clipped is offline Clipped  Thailand
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every orion amp i come across that has blown the PS fets makes that hiss....i still cant figure it out.

the good news is, it doesnt seem to affect performance... i never had any on a scope so not 100% sure.

on one amp the power supply never blew and i changed the fets and there is no hissing.

whats the input voltage for the 26.6 volt rails?
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Old 20th March 2008, 05:56 PM   #4
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If you can touch the coils of the transformer and they noise stops, epoxy would help. If touching it doesn't make a difference, the noise isn't from loose windings. Generally, the older Orion transformers are coated with varnish or epoxy so it's unlikely that you have loose windings.

5 ohms is too high for the resistance of any individual winding. You should essentially read 0 ohms across any individual winding. Primary to secondary will read ~120 ohms with no RCAs plugged in. With RCAs plugged in, you will read something near 0 ohms (primary to secondary) if the signal source is a head unit that's grounded to the same power supply as the amplifier.

Although it probably won't make a difference, you should install all of the FETs before you do more troubleshooting.

The noise could be from a regulator that's not damped well enough. If the voltage on the collector of the MPSA06 driver has significant high frequency noise, the regulator may be your problem. The voltage will vary but the it should vary with demand (on music peaks, when the voltage drops...).
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Old 20th March 2008, 07:20 PM   #5
ECM is offline ECM  United States
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clipped, not sure what the hissing is all about, but the 275sx I repaired a while back also hissed. I threw some epoxy on it, but it still hissed.

+/-26.6V with a 12VDC input. The 225 has about +/-22V and the 275sx had +/-29V with a 12VDC input as well.

Perry, I threw some output transistors and pegged the current meter. Some troubleshooting revealed a shorted mpsu07 driver. I replaced it with an mpsa06 for testing and all is well. DC offset is a little high ~200mv, but all transistors are pulls from blown amps so I'm not too confident they are within spec, as I learned from the power supply "hot" fets.

I measured the coils again and they are very close to 0 ohms. My cheap meter had around 5 ohms resistance in the leads! doh. I'll try to poke around the regulators and look for the hiss next time.

I have irf3205's and 88/91's on hand, but I'm gonna put this sucker on the back-burner until I get some parts on order, specifically, some more 88/91's to replace the shorted mpsu07. I will replace all mpsu drivers at once.

In the meantime, I have three more 225's and a XTR5004 to fix. Also on the list is an ADSPH15.

Clipped, you get my email? I need that 225 heatsink brother...
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Old 20th March 2008, 07:39 PM   #6
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Did you measure the DC offset directly across the speaker wires?

If so, the DC offset is likely due to defective 5534 op-amps. Measure the DC voltage across pins 2 and 3. If it's 200mv, the op-amp needs to be replaced.
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Old 20th March 2008, 08:28 PM   #7
ECM is offline ECM  United States
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Yes, I measured across both wires, in bridged mode. I just went back and measured from each channel to ground and I get ~400 and ~600 mv per channel.

I'll take a look at those opamps and the tantalum caps next to them. I have plenty of BB opamps to replace the 5534's.
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Old 20th March 2008, 08:47 PM   #8
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What's the DC voltage across pins 2 and 3 of the 5534s?

What's the DC voltage from pin 3 of the 5534s to the RCA shield ground?
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Old 21st March 2008, 09:24 AM   #9
Clipped is offline Clipped  Thailand
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if the opamp is defective, you might have to change the resistor connected to the tantulum also...i think it is 3 ohms...not sure.

in one of my amps the resistor, tantulum and opamp were defective...it made it throw out rail voltage (or close to it)...after changing the tantulum i had low level offset at low volumes (scratchy) then it would go away when the i increased the volume....then i knew the opamp was bad.

i was getting 0.5XX offset
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Old 21st March 2008, 10:55 AM   #10
ECM is offline ECM  United States
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Yeah, i'm getting ~400mv and ~600mv on each channel. I'll measure the opamps after work today. I'll be ordering some tants from mouser as well.

Clipped, did you use NE opamps as replacements? I have a few BB chips I may use. Which opamp made your amp sound "hollow?"
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