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| Topward 6603A Linear Power Supply Teardown |
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| anton74:
The voltage is high and not stable, not responsive to the voltage adjust potentiometer. Also, one of the resistors (R12, 100 Ohm) is getting very hot. I thinks this resistor corresponds to R24 on 6000 series PS. There are 4 power transistors, 2N3772. Could it be the TL431 shunt regulator? Anton. |
| AlphZeta:
--- Quote from: anton74 on June 24, 2015, 11:46:40 am ---The voltage is high and not stable, not responsive to the voltage adjust potentiometer. Also, one of the resistors (R12, 100 Ohm) is getting very hot. I thinks this resistor corresponds to R24 on 6000 series PS. There are 4 power transistors, 2N3772. Could it be the TL431 shunt regulator? Anton. --- End quote --- I fixed one 6603A (not the one I did my original teardown with) a while ago and that one was showing similar symptom as yours did. The output of one channel would be at 70V all the time regardless of what the pot was set to. At first I thought it was power transistors as well, but it turned out it was one of the 2SC1384's (C-E shorted) and I changed it with a BD137 and everything worked great afterwards. Since it's fairly easy to remove the 2772's (they are connected via connectors) from the circuit, you can easily figure out whether it's the 2772 or something else. But that should be a good place to begin trouble shooting. |
| anton74:
Well, it seems that I found the problem. The previous owner probably did a modification: I found that a PCB trace that is going from the "Voltage" potentiometer is cut! There are 3 wires that soldered to the PCB contacts of the "Voltage" and "Fine" pots to the rear of the unit to the round DIN5 connector. I think the user did this mod in order to use an external potentiometer to control the voltage. The previous and the current users utilized this unit as a voltage source for the microscope lamp (in the research labs). However, the current user do not use any external potentiometer, so the unit was actually working but only the "Current" potentiometer was functional and was used for brightness control. It seems that in this mode one of the 100 Ohm resistors and one zener (5.1V) overheated, and finally damaged. After replacing the resistor, zener (I used 5.6V) and repairing the PCB trace that was cut, the unit works, both current and voltage are adjustable. The resistor and zener do not overheat anymore. I think the voltage reading of the display have some fluctuations when the load (24V 6Watt lamp) is connected to the output, should it be so? The 3000 series could produce up to 30V and up to 10A, so with the above load at 24-25V I am getting about 0.25-0.3A and the "Current" is fully cranked to the maximum. Theoretically, you cannot get more current (6W/24V = 0.25A; the resistance of the load is pretty high - 24V/0.25A = 96 Ohm), the voltage is nearly maximal. When the outputs are shorted, I can get about 10A at 30V. So, do you think these readings are normal? |
| AlphZeta:
--- Quote ---I think the voltage reading of the display have some fluctuations when the load (24V 6Watt lamp) is connected to the output, should it be so? --- End quote --- Depending on how much. Topward 6 series (and 3 series I believe) have very tight load regulation (<0.01%) so anything more than a few mV under that light load would be out of spec. Don't know if you can find a 5.1V Zener and give it a try as 5.6V is probably over what the circuit can properly compensate via the onboard pots. |
| anton74:
--- Quote from: AlphZeta on June 24, 2015, 11:01:30 pm --- --- Quote ---I think the voltage reading of the display have some fluctuations when the load (24V 6Watt lamp) is connected to the output, should it be so? --- End quote --- Depending on how much. Topward 6 series (and 3 series I believe) have very tight load regulation (<0.01%) so anything more than a few mV under that light load would be out of spec. Don't know if you can find a 5.1V Zener and give it a try as 5.6V is probably over what the circuit can properly compensate via the onboard pots. --- End quote --- Well, I changed the zener to 5.1V. The voltage display is fluctuating a lot. However, the actual voltage at the outputs is stable and reasonable, when measured with a voltmeter. Also, the voltage entering the display board is OK. Something wrong on the display circuit? There are two big chips, A/D converters, ICL7107SCPL and two HA17431's. The HA17431 are probably OK, I tried to swap for another ones (TL431) without any effect. The A/D converter is bad? Its power supply (two +5V)? Two 7805 regulators produce 5V when the display board is disconnected; when connected, one measures 4.6-4.7V, another one is 4.3-4.4V. Their corresponding diode bridges are somewhat warm. What else it could be? |
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