power amp advice sought

Hi group

I have a 240w single channel solid state transformer coupled power amp that isn't working. There is not a whole lot to the power amp board, but I do not have a matching schematic. I did find a schematic to a more recent version of this amp and its close enough so I was able to figure out the main power path, but not the front end, feedback, and protection circuitry. It looks like the company re-designed the front end, feedback, and protection circuitry on this later model. The power amp board had 2 open transistors, 3 shorted transistors, and showed signs of burns. The amp blew fuses when turned on.

I replaced all the bad transistors and an open resistor (burned). I reinstalled the large heat sink that connects to the two power transistors, the two pre-driver transistors, and the bridge rectifier. I verified that all transistors connected to the heat sink were electrically isolated from the heat sink and powered the amp up. It blew a fuse.

I removed the heat sink and the power transistor pair and powered things up. I found very high voltages throughout the output stages (like around 60 vdc) with a large amplitude distorted signal riding on it that was the input frequency I was feeding into the amp. The amplitude of this distorted signal followed the volume control. I assumed that this particular amp cannot be run w/o the power transistors, so I put them back in. I powered the amp back on and it worked. The only difference being I was running without a heat sink now.

There is a fan in this amp. One fan wire is connected to a ground wire that is connected to the power supply. These two wires are connected via a screw that screws into the heat sink. I found that as soon as this ground from the power supply is connected to chassis ground (via the heat sink originally) I lost all audio (but did not blow a fuse or apparently damage anything from the momentary contact). Anyway, I was running this way...no heat sink, fan wire not grounded, just audible output...thinking about what I should do next when snap, crackle, pop I had flames shooting out of the board. Both pre-driver transistors are physically cracked, I presume the power transistors are shorted, a couple of resistors have burned up, and the pc board really shows burn signs now. So, here's my question:

Could this failure be due to just running for 3-4 minutes at extreme low volume without the heat sink? I would guess not, but I don't have a lot of experience with class B power amps. If it could be, it might be worth replacing the transistors and trying again, otherwise I think the garbage is the best place for this amp.

Thanks

Dan

Reply to
Dan K
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Refailure of this kind is common. You really need to be able to run the amp up on a variac to see what's going on without risk of re-frying all the parts that you have replaced. It is possible that the transistors could have failed as a result of having no heatsink on them, but probably only as a result of something else still being wrong. With the amp running at just above idle, the output stage dissipation should be minimal, but not if there is a problem with the bias setting, if this is variable, or bias supply if fixed. Once the transistors start to get hot, they will quickly go into thermal runaway without adequate heatsinking, and rapidly fail.

If you can lay hands on a variac, it might be worth having one more go, but after that, without proper schematics, you will probably struggle to get to the bottom of it, and finish up throwing good money after bad

Arfa

Reply to
Arfa Daily

It might also be useful to post the brand and model # of the amp. Someone might have experience with--or be able to supply a schematic for--it.

jak

Reply to
jakdedert

Its a CSI P-240A, which is from Speco.

Reply to
Dan K

Like someone already suggested - a variac is necessary when fixing solid state amps. Monitor current and keep an eye on the output stage to see that it comes up with the voltage you expect. I lack experience with transformer coupled amps (except a few old Sansui amps from the 60's)-

If the bias is adjustable set it to minimum until the full voltage is present then set it to some reasonable value. (like 20-30 watts for a

250 watt amp)

Class B or Class AB? I have a pair of 500 watt amps - class AB and they work class A up to about 100 watts and above that start working class B. (24 output transistors and massive fan cooled heatsink)

It definitely needs a heatsink if it is class AB. Without the sink - the output stage gain increases dramatically when the transistors warm up - the bias current tracks the gain of the output stage - unless there's some thermal feedback to control it. (old designs usually had a pair of diodes, thermister, or bias transistor that would mount to the same heatsink as the output transistors)

I've seen amps with two pairs of Darlington transistors in the output

- and a small thing like too little heat sink compound on one will cause it to overheat.

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PS - old transformer coupled Sansui amps had a pair of series connected bias diodes in a molded plastic case with a bolt hole for mounting. That was the thermal feedback system for that amp. The diodes would develop an intermittent open connection and an otherwise perfectly working amp would crackle the speakers and start blowing fuses and transistors.

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Perhaps the output transistors weren't insulated from the heat sink?

Mark Z.

Reply to
Mark D. Zacharias

That was my first thought also, Mark, but if you go up 4 posts, you will see that he makes a point of saying that he checked that the devices were isolated from the heatsink. It's a mystery ... |:-{

Arfa

Reply to
Arfa Daily

Could have a damaged insulator...

mz

Reply to
Mark D. Zacharias

Could be. Might just be the way we are interpreting " checked ". When I check that devices are isolated from a heatsink, I prove it with a meter reading. Perhaps he was just visually inspecting ...

Arfa

Reply to
Arfa Daily

Nope, meter reading. Isolated. It wouldn't have worked for a while if they wasn't isolated, Vee would be shorted to Vcc. Could always be a crack in the insulator I suppose that only shorts at certain times...They look ok though

Dan

Reply to
Dan K

Yeah, it kinda sounded like you had a done a proper test when I first read it, and you are of course right that it wouldn't have worked ok for a while - I'd forgotten that bit ...

Arfa

Reply to
Arfa Daily

Sometimes you'll get a little 'burr' of sharp metal around the holes in the heatsink. It reads fine when cold, but when it heats up the burr expands enough to short through the insulator.

Clean off the sink, and feel for any sharp points.

jak

Reply to
jakdedert

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