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Help! Marshall 4203 blowing fuses, not PT

1/30/2006 11:30 AM
John L.	Help! Marshall 4203 blowing fuses, not PT Hi, I have a Marshall 4203 Combo Amp I bought new in the '80's. What a great amp that has served me well! Solid state pre-amp and tube power-amp give a great sound. The schematic can be found here: http://www.drtube.com/schematics/marshall/3203pwr.gif The Power Transformer blew and I had a replacement made from Heyboer. I hook up the new PT and hit the power switch, the indicator light comes on and the tubes light up. But when I hit the standby switch, the fuse immediately blows. According to the schematics, the transformer feeds 347 vac to the stand-by switch, which then feeds to a grouping of 4 or 5 diodes, which I believe act as the rectifier portion of the amp. Do you think it is possible that some or all of these diodes may be bad and are allowing the current to backfeed or short out which is causing the fuse to blow? Maybe this is why the original transformer blew... I am not an electronics expert, but I do know the basics, and I am learning more and more. Is there a way to test these diodes, or possibly track down the problem that's causing the short and the fuse to blow? Any info or ideas you may have would be greatly appreciated. Thanks. Best Regards, John L

1/30/2006 11:30 AM

John L.

Help! Marshall 4203 blowing fuses, not PT

Hi, I have a Marshall 4203 Combo Amp I bought new in the '80's. What a great amp that has served me well! Solid state pre-amp and tube power-amp give a great sound.

The schematic can be found here:

http://www.drtube.com/schematics/marshall/3203pwr.gif

The Power Transformer blew and I had a replacement made from Heyboer. I hook up the new PT and hit the power switch, the indicator light comes on and the tubes light up. But when I hit the standby switch, the fuse immediately blows. According to the schematics, the transformer feeds 347 vac to the stand-by switch, which then feeds to a grouping of 4 or 5 diodes, which I believe act as the rectifier portion of the amp. Do you think it is possible that some or all of these diodes may be bad and are allowing the current to backfeed or short out which is causing the fuse to blow? Maybe this is why the original transformer blew...
I am not an electronics expert, but I do know the basics, and I am learning more and more. Is there a way to test these diodes, or possibly track down the problem that's causing the short and the fuse to blow? Any info or ideas you may have would be greatly appreciated. Thanks.
Best Regards,
John L

1/30/2006 2:48 PM
R.G.	There are several things you need to check. First, go to http://www.geofex.com and read the Tube Amp Debugging Page. We'll use some of that. Next, you need to read the cautions about working with AC line power and the power inside tube amps. If you can't do this safely, do not even proceed, and get a pro's help. We want you to keep playing for a long time. Get your hand on the wrong thing and you could be ...dead... IF you're confident you can do it safely, and take it on yourself to continue, you need to make up a light bulb limiter. This is an AC outlet in a box arranged so you can plug in a lamp in series with the outlet, and the lamp will be in series in the hot lead. This will limit current to the amp to less than the bulb's full-line current. Now mess with the amp. 1. turn on the amp as it is. See the lamp brightness. 2. Disconnect one lead of the 347Vac leads to the diodes. Turn it on. Is the lamp bright or dim? If dimmer or off, you have disconnected the problem by disconnecting the secondary, and it's somewhere in the diodes, capacitors, wiring or tubes. If it's the same brighness, the problem is before the diodes - that is, in the AC wiring or the new power transformer. I'm guessing it'll be dim, showing the problem is in the following circuits. turn it off, reconnect the transformer. When I say "turn it off", what I really mean is, turn off the AC power to it, unplug the whole amp, and wait for a full minute. 3. remove all the tubes, turn it on. Lamp still bright (=overload) or dim indicating the load is removed? Dim means you've removed a bad tube. Bright is still a problem. 4. turn it off. leave the tubes out, disconnect D3/D4 junction from the first filter cap. Turn it on. Still bright? One or more diodes are shot. Dim? keep looking. Are the diodes bad? You can check this to some degree with a multimeter set to the resistance range. If a diode is good, it looks open one way, and low resitance, under 100 ohms, the other way. If it's open both ways or shorted both ways, it's dead. 5. turn it off. Reconnect the diodes to the first cap. Disconnect one lead of R8. Bright or dim? if it's dim, , you have a bad first filter cap. Dim, keep looking. 6. Turn it off. Reconnect R8. Turn it on. Check the voltage on the grids of the output tube sockets. If it's not more negative than about -35V, turn it off and check D1, C3, and C6 for open/shorted. 7. If the grids are solidly negative, turn it off, disconnect R9 (22K 1w) turn it back on, and look for bright/dim. Dim? probably bad C(blob) on the phase inverter. This is quite unlikely. If it's bright, call us back. Call us back anyway with what you find.

1/30/2006 2:48 PM

R.G.

There are several things you need to check.

First, go to http://www.geofex.com and read the Tube Amp Debugging Page. We'll use some of that.

Next, you need to read the cautions about working with AC line power and the power inside tube amps. If you can't do this safely, do not even proceed, and get a pro's help. We want you to keep playing for a long time. Get your hand on the wrong thing and you could be ...dead...

IF you're confident you can do it safely, and take it on yourself to continue, you need to make up a light bulb limiter. This is an AC outlet in a box arranged so you can plug in a lamp in series with the outlet, and the lamp will be in series in the hot lead. This will limit current to the amp to less than the bulb's full-line current.

Now mess with the amp.

1. turn on the amp as it is. See the lamp brightness.
2. Disconnect one lead of the 347Vac leads to the diodes. Turn it on. Is the lamp bright or dim? If dimmer or off, you have disconnected the problem by disconnecting the secondary, and it's somewhere in the diodes, capacitors, wiring or tubes. If it's the same brighness, the problem is before the diodes - that is, in the AC wiring or the new power transformer. I'm guessing it'll be dim, showing the problem is in the following circuits.

turn it off, reconnect the transformer. When I say "turn it off", what I really mean is, turn off the AC power to it, unplug the whole amp, and wait for a full minute.
3. remove all the tubes, turn it on. Lamp still bright (=overload) or dim indicating the load is removed? Dim means you've removed a bad tube. Bright is still a problem.
4. turn it off. leave the tubes out, disconnect D3/D4 junction from the first filter cap. Turn it on. Still bright? One or more diodes are shot. Dim? keep looking.

Are the diodes bad? You can check this to some degree with a multimeter set to the resistance range. If a diode is good, it looks open one way, and low resitance, under 100 ohms, the other way. If it's open both ways or shorted both ways, it's dead.

5. turn it off. Reconnect the diodes to the first cap. Disconnect one lead of R8. Bright or dim? if it's dim, , you have a bad first filter cap. Dim, keep looking.
6. Turn it off. Reconnect R8. Turn it on. Check the voltage on the grids of the output tube sockets. If it's not more negative than about -35V, turn it off and check D1, C3, and C6 for open/shorted.

7. If the grids are solidly negative, turn it off, disconnect R9 (22K 1w) turn it back on, and look for bright/dim. Dim? probably bad C(blob) on the phase inverter. This is quite unlikely. If it's bright, call us back.

Call us back anyway with what you find.

1/30/2006 4:29 PM
John L.	Wow! Thanks so much for your response. It's great there are those such as yourself that help others! I Thank you. I will try what you suggest and let you know. Thanks again! John L.

1/30/2006 4:44 PM
John L.	Well, I checked out the goefex.com and realized I had been there before. Thank you for the safety warning reminder. I pulled the the tubes from the amp and powered it up, hit the standby and this time the fuse did not blow. This leads me to believe the problem is in my tubes somewhere. Is it possible that a tube could have been responsible for my power tranny blowing in the first place? Hmmm... Any thoughts? Thanks. John L.

1/30/2006 4:44 PM

John L.

Well, I checked out the goefex.com and realized I had been there before. Thank you for the safety warning reminder. I pulled the the tubes from the amp and powered it up, hit the standby and this time the fuse did not blow. This leads me to believe the problem is in my tubes somewhere. Is it possible that a tube could have been responsible for my power tranny blowing in the first place? Hmmm... Any thoughts?
Thanks. John L.

1/30/2006 6:22 PM
Peter	Yep faulty valve and most likely wrong value fuse although it could have been the thermal cutout in the transformer (a curse in expensive transformers). A ht fuse would probably be a good investment for next time.... Btw has any one ever tested the current generation of fuses in a jig to check their rating ? There seemes to be a gradual influx of no brand fuses and a diminishing of branded ones!

1/31/2006 5:06 PM

R.G.

quote:
"I pulled the the tubes from the amp and powered it up, hit the standby and this time the fuse did not blow. This leads me to believe the problem is in my tubes somewhere. "

It's the tubes. That comes in three flavors:
1. A tube shorted internally on the operating electrodes - K to P, K to S, etc.
2. A tube shorted internally across the heater windings, putting a short on the heater winding of the PT.
3. A power tube with shorted, improper or otherwise missing negative bias on the grid.

Generally, the output tubes and perhaps the phase inverter tube is the only one that can pull enough current to kill a PT because of the power supply arrangements.

If you can afford one fuse per test, do it this way.

With all tubes pulled out, turn it on. Fuse holds (i.e. the power indicator stays lit)? If yes, plug in one power tube. Does the fuse pop immediately? Yes, that's (one of) the bad tube(s). No, let it heat up. Fuse pop? Yes, the bias is probably bad.
No, put in the other output tube. Pop immediately? *That one* is the bad tube. No - let it warm up and see if the fuse pops when it's warm or if the plates on either one get cherry red. If the fuse holds and the tubes don't red place, put in the PI tube. Keep checking until you find the tube(s) that make fuses pop. Then you'll have a much smaller place to look.

quote:
"Is it possible that a tube could have been responsible for my power tranny blowing in the first place? "

Yes. Shorted or otherwise overcurrent tubes will blow a power transformer especially if they're just a little over. Big overcurrents pop the mains fuse.

The worst thing you can get in a tube amp is a tube that's shorted across the heater voltage. That will eat huge currents from the heater winding and generally not blow the line fuse until the winding melts. One bad tube can kill the power tube, the single most expensive part in the amp.

1/31/2006 10:44 PM
John L.	Awesome! Thanks for the help. I will get some more fuses tomorrow and see if I can discern the culprit tube. I will let you know. Thanks again! John L.

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