Sumo andromeda II - Broken..

[Sponkii]

looks a lot like yours, beside the output transistors and the caps.

but, if I decide to buy the cheap transistors for repair, could you guide me ?..

[Midnight Rider]

Have a pair of Andromeda II's in great condition as well as Athena II & Aurora if you know of anyone interested? Have an excellent weekend!

[SQLGuy]

Just bought some AII parts - transformer and boards, no heatsink or chassis. One board is good, other has a blown phase.

These are later boards with the IR TO-247 outputs. What I can't understand is why Sumo went from 100V 20A devices to 150V 10A devices (on the P-channels - 16A for the N-channels). The extra voltage shouldn't be needed with 44V rails, but the extra current capacity, and lower Rds On, would seem more desirable.

Hmmm... I kind of answered my own question looking at the old IR data sheets. For some reason, the older IRFP9140 had very high Rds On compared to the IRFP9142 and 9143... though the 9142 still looks like it would have been a better choice. It looks like modern Vishay IRFP9240's and IFRP240's, or maybe better-yet, the 9140 and 140, should be very suitable drop-in replacements.

[llwhtt]

I have two dead IIs here, an older model with TO3 style outputs and a newer one with the TO247s. I too wondered about the use of the unmatched "pairs" of N and P channel devices. This one has Harris devices in it, IRFP253s and IRFP9243s. Haven't yet looked for different replacements, came up with nothing on the originals.

Craig

[SQLGuy]

How badly dead are they?

Do you know how many of which types outputs you need?

[llwhtt]

The TO3 model had one complete channel missing when I got it so the chassis got reincarnated into a PASS A40. The TO247 model is a repair for a friend, once I found a couple shorted I quit and started to search for replacements. After coming up empty I haven't even looked at it. Need to finish rebuilding his Son of Ampzilla and Thoebe first. Then it's an old oddball SAE mono amp. Why are all of the bipolar amps no problem when it comes to replacements and the MOSFETs such a PITA? Used to work on a lot of Carvin MOSFET amps until they figured out that a few years down the line there would be no replacements, they only do bipolar now. So to answer your question I don't know how many I need or what other problems it may have, one channel is good though.

Craig

[SQLGuy]

Well, I'll probably do some experimentation with the parts I have. I'll let you know what I find, but I have a pretty good feeling about the Vishay's I mentioned.

Maybe I can trade some information for a good price on the remaining TO-3 board? ;-)

It would be nice to have a spare....

[rubidium7]

Originally Posted by rubidium7

I have a SUMO And II that is a later production run. On these runs, they switched to the 1RFP9243 and 1RFP243R devices. My unit is perfect on one side and rebuilt on the other with one problem I can't see to track down. It breaks into oscillation about 1/2 power (you can see it on the sine waver, just at the peak, when you cross a threshold, it starts to oscillate on the back side of the sine wave, both polarities of the sine wave.
I have repaired hi-end hi-fi for years and this was a project I was working on 7 years ago and then stuck it on a shelf. I would like to fix it, it is a challenge, but also would entertain offers if someone is interested.
There are NO burn marks on the board, just this oscillation problem. It biases fine.
The reason the one side is rebuilt is that I didn't realize, when I hooked this up to my test set up, that it was a floating output, so I grounded one side via my scope and speaker load common ground.
I had purchased (if I recall) about 12 output devices per output half in order to match up 4. I built a matching set up and they are within 2% of being perfect (i got a run of devices from the same batch, so was pretty lucky).

This has LED's in the bias circuit and it may be that there is where the problem lies....as you increase the input, just as it starts it's unstable behaviour, they light.

This can be powered on full power and idle all day with no issues and even run at low volumes, it is just when you bring it up to a point, this behaviour starts.
It has really kicked my butt, as over a 30 year carreer in repair there are only one or two items I couldn't fix.
Email for pictures, they are too big to post.

I have decided to pass on this project to whomever may be interested in buying it. I just don't have time to play with it.

It has perfect condition outputs all around and is biased fine. Has the oscillation issue on the one side, which some smarter than I can probably fix in two minutes. I can send pictures if you are interested.