Puzzling Power Supply Voltages on Crate PA-B8360

[TwinBeech]

Hello everyone. I hope someone can help me fix this supply problem. I get =74 and -74 vdc on the rails unconnected to the power supply circuit but when I plug it in the voltages drop to around 13 vdc and the transistors (TIP29B and TIP30B get very hot. I have changed the 16V Zeners to no avail. Does anyone have any ideas what is happening?

[allante666]

without looking at the diagram (as i'm out shopping and using my phone), short or leaky transistors pulling it down/?

[TwinBeech]

without looking at the diagram (as i'm out shopping and using my phone), short or leaky transistors pulling it down/?

I have pulled the TIP s and they check good with my diode function on the meter. I have 2 new ones coming in tomorrow that I will change. When you get a chance to look at the schematic all the highlighted components I have either changed or pulled and tested.

Thanks for your reply.
Cheers

[nickb]

If the main power rails drop to 13V and the Q2 and Q3 run hot then something is loading down the +16 and -16V rails. There may also be a problem upstream of the 61V rails. Measure the current draw on the +16V and -16V rails.

Please post the entire schematic. We can't help much with just a snippet.

[Enzo]

Did you mount the zeners correctly? If you reverse them, they are just diodes, which will short the supply.

[TwinBeech]

Yes. I double checked them. I will look again. I have traced it down to one supply run but I cannot see or measure anything that will load up the rails. 8 ICs are fed from that line with a few caps and resistors. All op amps.

[Enzo]

So if you unplug that cable to the preamp, the power amp runs OK?

it seems unlikely both 16v rails are loaded down, my suspicion would be that they are shunted together. Are any of those op amps getting hot?

But wait, I see on the power amp side there are 1/4A fuses in series with the 61v feeds. Those would blow in a heart beat if the 16v supplies were loading the 61v supplies to death.

[TwinBeech]

Yeah. The more I find out the more confused I get. I have blown one fuse while troubleshooting and one was blown when I got the amp. I only leave it on for a minute or less just to look at the voltages.By lifting components I narrowed it down to the row of ICs that start with IC4 through IC12. When I lifted the resistors that were in series with the supply to the IC4 series, I got rail voltage out of the supply except I still only got around 13 volts not the 16 that is supposed to be set by the zeners.

[Enzo]

Look for one getting hot, really.

[TwinBeech]

Thanks for the tip. I just checked that and found that IC8 is getting hot. The TIPs get real hot. I will change that one and see. It might be a component loading it up though?

[Enzo]

Just remove the IC and power up. Do the power rails come back to life? If they do it was bad.

I can't think of any component that would make the IC conduct rail to rail within itself.

[J M Fahey]

Agree.
I *suspect* (you are not showing us the full schematic) that upstreams of TIPs Q2 and Q3 (by the way Q3 is drawn wrong, it's PNP, not NPN as shown) there are a couple power resistors to share the dropping voltage job.

Now the electronics you show are (presumably) fine, but something is looading then down big way, as already suggested many times, so those resistors do their job.

By the way, they must be heating like crazy.

You can't beat Physics; in a series circuit current is the same along its path, and dissipation is V*I so if the TIPs dropping a few volts are hot .... imagine the poor things dropping from 71V (the raw voltage you measured) to 13V.

But in a nutshell: the supply shown is (basically) fine, just find the bad load pulling it down.
Again as said before, ICs are big suspects, also follow what other points are fed those +/- 16V .

You might even have a sub-regulator hanging from them to get, say, +5V for some digital module.
Wouldn't be surprised at one feeding an USB socket, many amps now come fitted with a Flash socket or MP3 player connection.

In a nutshell: leave no stone unturned.

[Enzo]

The 61v rails are on the power amp board, they feed through 1/4A fuses and series 220 ohm 10w resistors to the preamp, which he has posted.

Power amp:
http://bmamps.com/Schematics/crate/C..._Schematic.pdf

Preamp, includes the part he posted:
http://bmamps.com/Schematics/crate/C..._Schematic.pdf

[nickb]

The OP said the 71V rails were dropping to 13V, not the 16V ones.

It's probably not a good idea to keep 'baking' these components. Ideally, connect to variac to limit the input voltage and thereby reduce the current to safer levels. I do see that there are 220 ohm resistors in series with the 61V feeds. These are helping big time. If you don't have a variac can you add a couple of beefy 1K 10W resistors in series with them while to debug the real issue.

[TwinBeech]

You are indeed correct. The large power resistors in the power supply get very hot. I changed them at first because they showed a lot of thermal stress. I only leave the amp on long enough to get a voltage reading. I isolated some of the +-16v rails and found one that had the fault on it. Measuring across the two lines I was getting 13.8 R. After reading another post I was reminded to look for a hot IC and found one.I just pulled it and the rails are proper again. The pulled IC showed 13.8 R across pins 4 and 8. Now to get another op amp and see what happens. Thanks so much for the tips.

I only posted the power supply schematic because I though that was the problem. Sorry. Also the complete one is large.