Blown trace on PCB

[WholeToneMusic]

I am working on a PCB that blew a trace near the bridge rectifier and is causing a short with an adjacent trace. Is there any insulating material that can be used to keep the two traces from conducting?

[olddawg]

I am working on a PCB that blew a trace near the bridge rectifier and is causing a short with an adjacent trace. Is there any insulating material that can be used to keep the two traces from conducting?

Generally, if it is a paper-resin composition PCB, the burned board area turns into conductive carbon. So..the only choice you really have is to take a Dremel or other suitable tool and remove the conductive area. Then you can jumper the missing traces point to point with solid wire of an appropriate guage. Some people like to fill in the damaged area with epoxy. You can also glue a piece of perf or scrap board board over the removed area and remount components on that. It all depends on how big the damaged area is and how clean of a repair you want. It is more difficult on dual sided or multi-layer boards. The bottom line is that the conductive area has to be removed. If it is a fiberglass board, it may just have a conductive film built up on it and can be cleaned, but more than likely you have some surgery to perform. It isn't a big deal, just a pain.

[WholeToneMusic]

Trace ok now still drawing excessive current

The carbonized area was tight so I could not get a dremel in there. I was able to use a small file that I use on guitar work to remove it. The amp is a solid state Dean Markley DMC-80. It's funny because most people say Dean Markley made amps?

I put a new bridge rectifier in the amp because it was blowing fuses. I fixed the trace. The amp is still drawing massive current. I have a current limiter hooked up to it to test voltages. The + and - DC side of the rectifier measure +3.5 & - 3.5 volts which seems fine because the limiter is reducing the voltage, However when I switch the amp off and back on the + & - DC voltage is .008. I have checked the zener diodes out of circuit, the filter caps, IC's, transistors, etc nothing really looks suspect. The original Power transformer was replaced because it was blown.

[Don Moose]

So your problem is downstream from the power supply ... most likely.
Do the filter caps test OK?
Are there series components you can lift (like a tube amp's B+ dropping string)? to take sections in and out?

[olddawg]

The carbonized area was tight so I could not get a dremel in there. I was able to use a small file that I use on guitar work to remove it. The amp is a solid state Dean Markley DMC-80. It's funny because most people say Dean Markley made amps?

I put a new bridge rectifier in the amp because it was blowing fuses. I fixed the trace. The amp is still drawing massive current. I have a current limiter hooked up to it to test voltages. The + and - DC side of the rectifier measure +3.5 & - 3.5 volts which seems fine because the limiter is reducing the voltage, However when I switch the amp off and back on the + & - DC voltage is .008. I have checked the zener diodes out of circuit, the filter caps, IC's, transistors, etc nothing really looks suspect. The original Power transformer was replaced because it was blown.

Have you tested the original bridge rectifier with the diode scale on your multimeter? Was it shorted? Do you have it in correctly? (It is a very common mistake to install a bridge rectifier incorectly) Was the PT open? (You have no idea how many amps I have fixed because some shop said the PT was bad just to get out of the repair and the PT was OK) Have you floated the rectifier and checked the rails for a short? Are you sure you don't have a shorted filter cap? If you took out a PT, fried a bridge rectifier, and burned the board, something is drawing serious current and is probably fairly obvious like a direct short to ground off of the DC rails. Things to check would be, filter caps, output devices, regulators, etc. Can you post a schematic?

[Enzo]

Here is where I wager my lunch money.

Go down the row of transistors on the heat sink. How many of them are shorted? In particular between legs 2 and 3.

[WholeToneMusic]

When I received the amp it had been neglected. Some one thought that a rolled up piece of foil would be a good fuse replacement. I disconnected all the secondaries of the Power Transformer and the fuse still blew which meant the Power Transformer was shorted. There is a big carbonized spot from the bridge rectifier blowing.The bridge rectifier was definately blown. I installed the new bridge rectifier with the + to + DC side of the first filter cap and the - to the negative side of the second filter cap. The ~ goes to the PT secondaries. The filter caps and the zener diodes tested fine when removed from the circuit. I did a battery voltage drain test on the filter caps to see if they were holding charge and they were fine. There is a schematic of the DMC 80 on the Dean Markley website. The schematic isn't the gospel though, there's errors on it.

I had originally checked all the transistors on the heat sink and replaced two of them. I removed them all and checked them with the diode check function on the multimeter. I will double check them Enzo, When you say 2 and 3 are you referring to Emitter - Collector? I have gone through all the transistors, diodes etc on the output board, the sand block resistors on the output board all test good.

When the amp is on it makes a loud low frequency hum. I haven't tried to lift any thing yet but did bypass the preamp and the power amp with the problem still happening. Also tried plugging a 1/4" dummy plug into all the jacks to see if a jack was shorted.

[R.G.]

For overcurrent problems, divide and conquer.

1. Open the DC connections between the first filter cap(s) +V and -V and the rest of the amp. Power it up. Still overcurrent? If so, the problem is before the amps, in the power supply alone. If not, it's in the amp.
2. Reconnect the filter caps to the amp, but disconnect +V and -V from the power output transistors, generally on the heat sink. Power up. Still overcurrent? If so, the problem is localized in the amp. If not, it's on the heatsink.
3. Short the bias connection to the power output transistors so it's severely underbiased. It will still work this way, just sounds bad. Power it up. Still overcurrent? If not, the bias network is hosed. If yes, you have bad transistors or drivers, or shorts in wiring/heatsink.

Here's a subtle one. Is the heat sink supposed to be grounded, or insulated from ground? Most are grounded, but a few are supposed to be insulated from ground because there is no insulator under the power devices for better heat flow. Grounding these (rare) heatsinks produces clouds of smoke from the power supply.

[olddawg]

Make sure the insulators for the output transistors are good. You can probably run it up with the OTs removed. But I'm with Enzo. If you already had 2 shorted outputs it is likely that others were stressed. Also make sure you subbed them correctly and the NPNs And PNPs are in the right places.

[WholeToneMusic]

Output Transistors

RG, The heat sink is grounded and the Output transistors have insulators between the device and the heat sink. Thanks for the info, If the output transistors aren't grounding out then I will go down the list you provided.

Good point Old Dawg, I will double check to make sure the insulators are good on all of them. I was very careful with the placement of the PNP (TIP126) and the NPN (TIP 121) transistors. Also I used heat sink gunk when I replaced them. The one nice thing about the PCB is everything is clearly marked for the diodes and transistor placement.



Much appreciated,

Brian

[olddawg]

RG, The heat sink is grounded and the Output transistors have insulators between the device and the heat sink. Thanks for the info, If the output transistors aren't grounding out then I will go down the list you provided.
Good point Old Dawg, I will double check to make sure the insulators are good on all of them. I was very careful with the placement of the PNP (TIP126) and the NPN (TIP 121) transistors. Also I used heat sink gunk when I replaced them. The one nice thing about the PCB is everything is clearly marked for the diodes and transistor placement.
Much appreciated,
Brian

You should be able to verify all of this with an ohm meter.

[WholeToneMusic]

update

I tested all the output transistors on the heat sink and they are not grounding out. The insulators are working fine. Llifted the + side filter cap and the - side filter cap and the amp is still drawing excessive current. There is not much left in the power supply. The power transformer Sec is testing 48 v from end to end, 24 volts each side to ground. Took the bridge rectifier back out and re-tested each junction and it was fine. I noticed that there is a resistor on the schematic R201 going to LED 3 which is not on the PCB. This doesn't seem right. The + dc side of the bridge rectifier is where the char is from the original bridge rectifier blow out. There is also a cap on the AC board c99 that I wasn't able to find on the schematic that tested fine. Hoping to make some more progress tomorrow morning. Going to replace some of the original AC wiring. It tests fine with the continuity tester but did get smoked bad.

[WholeToneMusic]

For overcurrent problems, divide and conquer.

1. Open the DC connections between the first filter cap(s) +V and -V and the rest of the amp. Power it up. Still overcurrent? If so, the problem is before the amps, in the power supply alone. If not, it's in the amp.
2. Reconnect the filter caps to the amp, but disconnect +V and -V from the power output transistors, generally on the heat sink. Power up. Still overcurrent? If so, the problem is localized in the amp. If not, it's on the heatsink.
3. Short the bias connection to the power output transistors so it's severely underbiased. It will still work this way, just sounds bad. Power it up. Still overcurrent? If not, the bias network is hosed. If yes, you have bad transistors or drivers, or shorts in wiring/heatsink.

Here's a subtle one. Is the heat sink supposed to be grounded, or insulated from ground? Most are grounded, but a few are supposed to be insulated from ground because there is no insulator under the power devices for better heat flow. Grounding these (rare) heatsinks produces clouds of smoke from the power supply.

When I disconnected the + and - Filter caps the amp was still drawing excessive current. I proceeded to disconnect the + & - from the output transistors and the amp is not drawing excessive current. Where would the best place be to short the bias connection? Would shorting R32 & R33 underbias the amp?