I recently aquired a Woodson SS guitar amp. It is basically a Kustom amp copy. Apparently Woodson was a split off from Kustom after a divorce. I could not find a schematic for a Woodson amp, but I found some Kustom amp schematics that are close. Here is a power amp board that is close.
PC5033 by capndenny, on Flickr
The amp runs fine for a while and then cuts out. It has a thermal cutoff mounted on the heatsink for the power transistors. When it trips it shuts off the incoming AC power. This happens at low volume after about 5 minutes of playing. After a few minutes it comes back on.
I have done the usual checks and haven't found any bad parts so far. It does work till it overheats. It doesn't sound terrible, so I don't think there is anything outright broken. Perhaps a tolerance issue.
Here is the pcb.
IMG_1965 by capndenny, on Flickr
There is some discoloration on a few parts. It also looks like someone has attempted to repair it already. It seems that the 0.51 ohm and a 100 ohm resistor in one side of the PP circuit is getting warm. But only the power transistors, shown off the pcb in the schematic would cause the thermal trip to activate.
Looking for ideas on how to proceed. All parts pass the DMM check of the diodes and transistors. Even the burned resistors ring out OK.
PC5033 by capndenny, on FlickrThe amp runs fine for a while and then cuts out. It has a thermal cutoff mounted on the heatsink for the power transistors. When it trips it shuts off the incoming AC power. This happens at low volume after about 5 minutes of playing. After a few minutes it comes back on.
I have done the usual checks and haven't found any bad parts so far. It does work till it overheats. It doesn't sound terrible, so I don't think there is anything outright broken. Perhaps a tolerance issue.
Here is the pcb.
IMG_1965 by capndenny, on FlickrThere is some discoloration on a few parts. It also looks like someone has attempted to repair it already. It seems that the 0.51 ohm and a 100 ohm resistor in one side of the PP circuit is getting warm. But only the power transistors, shown off the pcb in the schematic would cause the thermal trip to activate.
Looking for ideas on how to proceed. All parts pass the DMM check of the diodes and transistors. Even the burned resistors ring out OK.
I think I would begin by checking that the 0.51 ohms are correct in value and also that the two 100 ohms are good (across B and E of the output transistors).
If you monitor the voltage across the 0.51 ohm (either) you can calculate the current flowing. I would say anything over 50 milliamps when the amp is idling is probably to high.
The diagram shows a chain of three diodes that ultimately set the bias current. The photo seems to show a transistor ? bolted to the heatsink near the thermal switch. Perhaps there is a difference between the circuit and your amp in that department. If it is a transistor then it should be in close thermal contact with the heatsink to track the temperature and keep the bias current reasonably constant.
If you monitor the voltage across the 0.51 ohm (either) you can calculate the current flowing. I would say anything over 50 milliamps when the amp is idling is probably to high.
The diagram shows a chain of three diodes that ultimately set the bias current. The photo seems to show a transistor ? bolted to the heatsink near the thermal switch. Perhaps there is a difference between the circuit and your amp in that department. If it is a transistor then it should be in close thermal contact with the heatsink to track the temperature and keep the bias current reasonably constant.
The extra transistor in the photo is actually the third diode in the biasing diodes. It is in a metal can, and is supposed to be in the small gold clip you can barely see in the photo. It does bias adjustments due to temp rise of the power transistors, to keep them from becoming unstable at high temps.
I was trying to get it back in the holder and snapped the lead. It is pretty loose in the holder, but I just don't think at low volumes the parts are designed to get very hot, and need the temp compensation at idle.
I may sub something else to allow me to continue troubleshooting.
I was trying to get it back in the holder and snapped the lead. It is pretty loose in the holder, but I just don't think at low volumes the parts are designed to get very hot, and need the temp compensation at idle.
I may sub something else to allow me to continue troubleshooting.
It is very important the diode tracks the heatsink temperature, even at idle. Suppose the bias is 50 millimamps. That is 2 watts dissipation with a static voltage of 40v across each output transistor. That is more than enough to heat the transistor junction and see a rise in current... more current more heat... and so on.
So the output transistors could see increasing bias (thermal runaway) if the diode is fitted correctly.
Also a fixed bias design like this is very device dependent. If transistors have been swapped for more modern parts then the characteristics could differ and cause the initial current to be higher (or lower) than the original spec.
So the output transistors could see increasing bias (thermal runaway) if the diode is fitted correctly.
Also a fixed bias design like this is very device dependent. If transistors have been swapped for more modern parts then the characteristics could differ and cause the initial current to be higher (or lower) than the original spec.
What they said.
Absolutely: if the tracking diode/transistor is flopping in the cool breeze it is at great risk of run-away. And bouncing off the safety thermostat is NOT a good plan.
Meanwhile: jumper one diode. To a point, it should now run COOL, play OK at medium/loud levels, but a bit "hoarse" when playing at whisper. That might even be tolerable for many Kustom (or step-child) users.
Absolutely: if the tracking diode/transistor is flopping in the cool breeze it is at great risk of run-away. And bouncing off the safety thermostat is NOT a good plan.
Meanwhile: jumper one diode. To a point, it should now run COOL, play OK at medium/loud levels, but a bit "hoarse" when playing at whisper. That might even be tolerable for many Kustom (or step-child) users.
I got my diode yesterday and put it in. I measured voltages and everything seemed good. Nothing was getting hot.
So hooked up a speaker and a guitar and dead silence? I found power supply lead that had broken off from moving wires around to test the power board. Once I connected that it worked again!
I played a low to moderate volume with no dropouts for about 20 minutes. It was shutting down about every couple of minutes before. So I guess it is fixed.
The only thing I did was on the diode whose lead I busted, which is mounted to a clip attached to the power transistor heat sink. The clip was sprung and wasn't holding the diode in at all. When I tried putting the broken one in for a test, it just fell back out. So I pushed on the clip to bend it back and made sure the broken diode was held securely. I think I will put a drop of epoxy in there.
I guess the clip had loosened and was causing the transistor to do a thermal runaway. I assumed that was only needed when driving the transistor really hard? But it appears that just to nudge it in that direction and it just keeps going.
I will button it back up and run it a bit more but for now it seems fixed.
So hooked up a speaker and a guitar and dead silence? I found power supply lead that had broken off from moving wires around to test the power board. Once I connected that it worked again!
I played a low to moderate volume with no dropouts for about 20 minutes. It was shutting down about every couple of minutes before. So I guess it is fixed.
The only thing I did was on the diode whose lead I busted, which is mounted to a clip attached to the power transistor heat sink. The clip was sprung and wasn't holding the diode in at all. When I tried putting the broken one in for a test, it just fell back out. So I pushed on the clip to bend it back and made sure the broken diode was held securely. I think I will put a drop of epoxy in there.
I guess the clip had loosened and was causing the transistor to do a thermal runaway. I assumed that was only needed when driving the transistor really hard? But it appears that just to nudge it in that direction and it just keeps going.
I will button it back up and run it a bit more but for now it seems fixed.
The thermal feedback to the diode is a required feature of the design. It needs it throughout the whole operating temperature range, even if the amp is used in sub zero temperatures. The diode has to track the heatsink temperature at all times.
Its hard to advise what to use to repair it without seeing it for real. If the diode is in some can or casing then consider using heatsink compound to fill the can containing the diode.
Its hard to advise what to use to repair it without seeing it for real. If the diode is in some can or casing then consider using heatsink compound to fill the can containing the diode.
A transistor power amp sure will run-away at idle. Just sitting there the outputs get a little warm. Their Vbe drops. The bias diodes don't get hot, their Vbe stays the same. So the output device current rises. And gets hotter. Runaway.
Consider my heating furnace. With thermostat in the house the flame runs until 65 degrees then stops. With thermostat in the garage it is "always 20 degrees" and the furnace will run until the house burns down.
It is possible to compute heatsink size versus required thermal NFB. However leaving a bias diode OFF the output device heatsink ensures NO thermal NFB and almost assures eventual runaway. Getting "very near" is better. But usually they are designed ON the heatsink, firmly clamped and often greased.
Consider my heating furnace. With thermostat in the house the flame runs until 65 degrees then stops. With thermostat in the garage it is "always 20 degrees" and the furnace will run until the house burns down.
It is possible to compute heatsink size versus required thermal NFB. However leaving a bias diode OFF the output device heatsink ensures NO thermal NFB and almost assures eventual runaway. Getting "very near" is better. But usually they are designed ON the heatsink, firmly clamped and often greased.
I'm not sure but I don't think it did it at idle. But just a little power to the speaker and it started it on it's way.
I have an Ampeg B4R bass amp that is rated 1000w. It cut out on me once and there was no way i was anywhere over 50 or 100 watts. Perhaps it has a similar issue.
I have an Ampeg B4R bass amp that is rated 1000w. It cut out on me once and there was no way i was anywhere over 50 or 100 watts. Perhaps it has a similar issue.
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