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previous: John L. Hi, I have a Marshall 4203 Combo Am... -- 1138645858

Re: Help! Marshall 4203 blowing fuses, not PT

1/30/2006 2:48 PM
R.G.	Re: Help! Marshall 4203 blowing fuses, not PT 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.

Re: Help! Marshall 4203 blowing fuses, not PT

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.

Replies:

John L. Wow! Thanks so much for your respon... -- 1/30/2006 4:29 PM
John L. Well, I checked out the goefex.com ... -- 1/30/2006 4:44 PM