TAC Scorpion V+ power supply issue

[beezer4]

Hi all,
I repaired a TAC Scorpion power supply for a friend. The fuse holders and cable had dried out and something eventually shorted inside. I replaced quite a few components and had it back up and running solid not under load in my shop. Once the power supply is connected to the board, the V+ rail drops out. I'm helping him troubleshoot remotely and we've determined the V+ drops after a certain amount of channel strips are added into the console. I'm guessing it's a power draw / amperage issue, but am wracking my brain on where to start. Anyone have any immediate suggestions? It's difficult as I cannot be in front the console. Schematic attached.

Thanks!

Context on what was replaced:
Replaced Q1 with TIP32AG: https://www.mouser.com/ProductDetail/863-TIP32AG
Replace Q2 with NTE 219 PNP Power Transistor: https://bit.ly/2G4FjhM
Replaced U1 and U2 with NTE 935 Positive Voltage Reg: https://bit.ly/2R604jp
Replaced U3 with NTE 970 Positive Voltage Reg: https://bit.ly/378XQp7
Replaced 3 fuse holders with connectors and all fuses
Replace R17 resistor
Replace 48v bridge rectifier
Replace C13
Replace C14
Replace any bad diodes

 

[Ike Zimbel]

Main reservoir (aka filter) caps? I have know power supplies to do everything that their supposed to under no-load conditions (as in put out the expected voltages) only to sag under load because the filter caps were toast.

 

[jensenmann]

I´ve had a Scorpion PSU on the bench with a similar issue. It had a broken wire inside a flat pluck connected to the rectifier.
Another PSU had a burnt fuse holder.

 

[beezer4]

Hey all,
Replaced the filter caps and basically all the other caps as well… still having the same issue. Any other thoughts?

It’s tough to find a full console schematic, but I don’t believe there is any additional power supply filtering inside the console.

The only other immediate thing I can think of is something in the meter bridge since that’s always connect with our without channels installed.

Thanks for the help as always!

 

[Rob Flinn]

Did you try testing the PSU with a dummy load to determine whether you have a console that is trying to draw too much current due to a fault or whether the PSU has the fault ? If you are positive ('scuse the pun) that the PSU is at fault my guess would be to look at the connections to the transistors & the high wattage resistors in that area too. WIth the resistors where they are in parallel check you have continuity from resistor leg to resistor leg.

 

[beezer4]

Did you try testing the PSU with a dummy load to determine whether you have a console that is trying to draw too much current due to a fault or whether the PSU has the fault ? If you are positive ('scuse the pun) that the PSU is at fault my guess would be to look at the connections to the transistors & the high wattage resistors in that area too. WIth the resistors where they are in parallel check you have continuity from resistor leg to resistor leg.

Thanks Rob! Good call. I need to start with a dummy load. I just came across this other forum post with a similar issue: https://gearspace.com/board/geekzone/34159-tac-scorpion-power-supply-problems.html

Aligns with what you are saying. I'll check out the resistors and the transistor connections. The regulators and power transistor are socketed. I cleaned those sockets , but didn't reflow any solder around the socket leads to the board.

Thanks all for the help.

 

[Rob Flinn]

The 2n2955 is just adding to the current capability of the + rail. so if it's taking a dive under larger loads then it may not be working properly or at all. If it's just the LM388 that is working on it's own they definitely like to shut down if they get too hot. I think those hi wattage resistors might provide some sort of mechanism to shut the rail down under over load, therefore I would check their resistance & their connections carefully as well. Do the checking with a meter on continuity, visual inspections are not safe.

 

[Matt Syson]

The schematic is a minor variation of an application note by National Semiconductor and is normally very reliable BUT the transistor sock.rets used for the reg and MJ2955 are pretty lousy as they are clinched connections on the 'solder tag' and tightening the screws can weaken that connection. They are a.slso brass so don't solder properly! The regulators only really drop out if they get wildly hot so if it misbehaves when 'cold' it could be a bit of a clue. The ratio of the resistor values around the MJ2955 determines that the transistor passes about the same as the LM reg alone. When replacing the large electrolytics have you trashed the through hole plating? The fuseholders burn because heating is I squared and AC current is about 1.5 times the DC (load) current.

 

[beezer4]

Thanks everyone for the input it’s super helpful. I did see there’s a pretty common mod folks do to remove those sockets since they are so lousy. Thinking I may just do that.

The unit was recapped before I worked on it and looked like the through-hole plating was trashed in a few spots. They had scratched open additional space on the traces. All of those have good continuity with the traces and next components after my recent cap replacement.

Good call on checking the high wattage resistors as I have not done that.

I’ll have time to work on it next week and will follow up here with the outcome.

Really appreciate the help.