Orion HCCA 250 power supply issues.

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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?
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...
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...).
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...
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.
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
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?"
Anonymous, I'm interested in those fets. PM me with price and quantity. Or email me at gherrera1 AT netzero DOT com.

I am having a difficult time finding mpsu07/57 at resonable prices. Most folks are asking $8USD each! I was thinking of replacing all 8, but now I may cheap out and just replace the shorted one. Then again, you go cheap, you get cheap.

Are there any alternatives for the mpsu's? I know Clipped uses 2n6488/91, but those seem like overkill to me.
ECM, just email me your address and I will mail all 20 of them out to you, I don't want anything for them. Let me know if you need any common cathode and common anode 200v 16A TO-220 rectifiers as I have extra of these as well.

I'm not sure if they are any cheaper or easy to find, but MPSU07/57 can be replaced with SK3199/SK3200, ECG188/189, or NTE188/189 .
In most cases I'd avoid using NTE type parts because you never really know what you're getting. Since there were no similar parts, the NTE replacements may actually be relabeled U07/57s.

There are at least a couple of options...

Central Semiconductor makes the CEN-U07/57 but I really don't trust Central parts. In every blown amp that I've seen that had Central parts, at least one Central part was blown. If you use them, I'd suggest using the MPS parts as the drivers and using the central parts as the pre-drivers.

Move the pre-driver U07/57 to the driver location and use MPSW42/W92s in the pre-driver location .

Use TO-220 components in the driver location. The base and collector legs will go directly into the board. The emitter leg will be hardwired to the emitter pad. I've used both 2N6488/91s and MJE15030/31s.

If you're going to try different op-amps, you should install sockets. Some op-amps will cause these amps to oscillate.
Perry, I measured across pins 2 and 3. Zero volts.

From pin 3 to rca shield - 0.03V and 0.04V

From ground to ANY pin on either 5534, zero volts.

I'm not sure if the 5534 aren't getting power or if they are toast. I suspect they are not getting power because I measured across a couple of tantalum caps and read zero volts as well. Of course, if the tant is shorted, then I will get a zero volt reading.

Are the mpsw42/92 combo capable of driving the mpsu07/57? I have extra bd911/912 on order just in case.
Pins 4 and 7 are the power pins on the 5534. If there is no voltage, measure the resistance from the power pins to the secondary ground (RCA shield). If there's a short, the caps may be shorted.

If the caps are shorted, The resistors that Clipped mentioned are likely open.

There are 2 sets of regulators. The ones nearest the switches are for the 5534s (if I'm not mistaken). Check the voltage on the input and output of the regs to determine if the regs are defective.

I've used the MPSW42/92s as pre-drivers in quite a few amps and they've held up.

When you restore voltage to the 5534s, the DC offset will likely return to an acceptable voltage.
If the regulators were INSTALLED, then the op amps should get power right? Doh...

With regulators installed, all is well. I was under the impression the regulators were for the JRC opamps used for eq, bass boost, mono, etc. Each opamp is getting +/-13VDC.

With 6 fets in the power supply and two outputs per channel, I'm reading -40mV and -36mV offset per channel. It's still a tad bit high, but better than what I was getting before. In bridged mode it's about 4mV.

I will order some mpsw42/92 drivers along with some extra BD parts just in case.

Thanks for all your help Mr. Babin.
The power amplifier section of these amps are DC coupled. Any DC on the non-inverting input of the 5534 will be amplified. DC offset error of the op-amp will likely be amplified also. To get the offset as low as possible, you'll likely have to try several 5534 op-amps and select the one that produces the lowest offset.

The regs on the other side of the amp are for the rest of the op-amps.
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