Wacky 5150 schematic

[Valvehead]

Im trying to figure out why R77 is smoking when i power up my 5150--still in standby mode!

looks like its in the 24v supply--because its taking out the 2 1A fuses at that tap.

If you look at the scem it shows R76 and R77 out by themselves , connected to nothing but ground.

also they list R77 as 47 FP. What the $#@ is FP ?

[Valvehead]

another weird thing--that resistor-R77-- tests about 45 ohms , but the color code is Bown/black/black ....

the one next to it tests 47 ohms and is colored brown/ black/ yel (100k)

[Jag]

47FP = 47 ohm flameproof


It looks to me like those resistors are between chassis ground, earth ground and I'm not sure what the circled earth ground symbol means. Looking at the schematic I'd guess circled is AC ground and not circled is DC ground. I'm guessing you've got a short to one of those grounds somewhere and those resistors are taking alot of current, since they seem to tie the three different grounds together.

If those resistors are getting that hot, it's possible the colorbands have become discolored. I've seen this happen in power supplies from old computers.

If you're right about the 24v supply, I'd start looking for shorts there.

I don't really know the topology of those amps sorry I can't be of more help.

[Valvehead]

thanks Jag. Weird on the scem isnt it ?


im going to pull the board out as soon as i get time. I was thinking the same --something before those resistors shorting . I dont see what they are connected to though....

[Jag]

Yeah, I've never seen that before (the circled ground). I think those two resistors tie the three grounds together. I'm sure someone else will have a better explaination. I'm guessing that those resistors are supposed to act like fuses and burn out in the event of a short to chassis. Hopefully I'm in the ballpark.

[Enzo]

Lots of circuits use multiple grounds. They use the two ground symbols, and then some are circled, while others get a square. The circle and square are just ways to get around using more ground symbols or numbering them. In a Marshall where you might see GND, GND2, GNDA and whatever, it is the same deal, just not as obvious.

The reason one resistor is 47 ohms and the other 100k is that R77 is 47 ohms and R103 is 100k. R77 is the one closer to the fuses end of the board. R76 is NOT next to R77.

The fact that R77 measures 47 ohms means you have a problem. Those resistors are there to keep the ground sections wired together in case the main connections fail. Normally you measure about zero ohms across those two resistors, since the conections to chassis are in parallel.

Here's the deal: all the preamp circuit grounds are gathered together, and then they connect to chassis at the input jacks. Note the input jacks have both preamp and chassis gound symbols connected there? Other collected ground connect together in other places.

If R77 is burning, that means it has become the current return path between the circled ground and the power supply ground (the circled rake).

You could lift R77, then find out what is blowing the +/-24VDC rail fuses. The four preamp tube heaters are in series across that 48v. The +24 also lifts the 6VAC. That supply runs the PI tube and the outputs. What if an output tube is shorted to heater? That will stress things. Pull the power tubes or unplug the power tube circuit board. 24v fuses still blow?

VERY unlikely, but check CR21,22 for shorts.

The power supply and power amp ground are sorta unto themselves, with R77 connecting them to the rest of the system. So of undue current needs to get to chassis gtom there, it will burn R77.

Explore my power tube heater short lead.

[Valvehead]

As the amp sits now, i did find a bad power tube(a 4 week old Sovtek suprisingly) . And i did find an open 100 ohm5w screen resistor in the power section. now repaired.

BUT I can still power on the amp -in standby and the R77 smokes/ blows the 1A fuses.

So now im thinking its in the filimant supply somewhere? The amp powers up fine and acts nice with NO power tubes... as soon as i insert the power tubes---smoke..

[Steve Conner]

Did you replace both power tubes with known-good new ones, and the resistor still smokes? Because it sure does sound like a heater-to-cathode short.

[Jag]

Thanks for the clarification Enzo.

[Valvehead]

Did you replace both power tubes with known-good new ones, and the resistor still smokes? Because it sure does sound like a heater-to-cathode short.

I dont think its a tube--it smokes with the power tubes AND pre amp tube boards disconnected from the main board

[Valvehead]

enzo: "You could lift R77, then find out what is blowing the +/-24VDC rail fuses. The four preamp tube heaters are in series across that 48v. The +24 also lifts the 6VAC. That supply runs the PI tube and the outputs. What if an output tube is shorted to heater? That will stress things. Pull the power tubes or unplug the power tube circuit board. 24v fuses still blow?"


they are blowing/ r77 is smoking with both of the tube boards disconnected from the main board .

[Valvehead]

forgot to mention, neither the power LED or standby lights would work until i lifted R77 from ground. Now the Power LED lights up, but the standby does not (fuses dont blow either). i dont know if thats a good clue or not..

[Valvehead]

So far ive found that R77 is open, BUT the following all test good:

C11
C33
C36
C37
C53
C65

R26
R33
R60
R61
R85
R103

[Enzo]

Standby light only comes on when the screen voltageas are present.

With R77 out, what voltages - with respect to chassis - are at either end of where it was?

How about this: pull the B+ fuse F1. NOW does R77 still stress. This tells us if the B+ circuit is in any way involved.

Alright, brute force method. Check all four 24v rectifiers, CR14,15,16,18. Check their filters C44,46. Lift R69 to remove any contact with the heaters. Now what happens?

[Valvehead]

with R77 removed, i get 0 volts on one end and 27v at the other.


Pulled the B+ fuse-- R77 still smokes


lifted R69, R77 still smokes

all 24v diodes check good out of circuit

I dont have anyway of checking c44 and c46 so i just tested them with the ohmeter,
c46 tests open--meter doesnt do anything,
c44 tests 0.0 ohms

i dont have a cap meter that goes that high for testing

edit--i did have a good 2200 mf cap i swapped with c46---r77 still smokes