comparison: content/hp8662a.rst
content/hp8662a.rst
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| 109 | 109 |
| 110 So I gave a close look at thes boards, beginning by the main board, the I found | 110 So I gave a close look at thes boards, beginning by the main board, the I found |
| 111 the culprit for the smoke: the input protection thermistor. Strangely, it still | 111 the culprit for the smoke: the input protection thermistor. Strangely, it still |
| 112 measures a decent resistance value, but hey. | 112 measures a decent resistance value, but hey. |
| 113 | 113 |
| 114 The thermisor is descrived the old fashion way: it's a 5ohm@25°C with a | 114 The thermistor is described the old fashion way: it's a 5Ω @25°C with a |
| 115 temperature coefficient of -3.8%/°C. I did not find an exact replacement in my | 115 temperature coefficient of -3.8%/°C. I did not find an exact replacement in my |
| 116 junk parts; only a slighty highter value one (a XXX, which is more like | 116 junk parts; only a slighty highter value one (a XXX, which is more like |
| 117 20ohms@25°C). Not ideal, but for now, it seems to work fine. | 117 20Ω @25°C). Not ideal, but for now, it seems to work fine. |
| 118 | 118 |
| 119 .. image:: {filename}images/hp8662a/smoking_thermistor.jpg | 119 .. image:: {filename}images/hp8662a/smoking_thermistor.jpg |
| 120 :class: image-process-large-photo | |
| 120 | 121 |
| 121 After this first step forward, I was not confident enought to plug the boards | 122 After this first step forward, I was not confident enought to plug the boards |
| 122 back in place and try again (I am always worried when I repair switching | 123 back in place and try again (I am always worried when I repair switching |
| 123 PSUs... they can fail in flame and glory sometimes). So I checked all the | 124 PSUs... they can fail in flame and glory sometimes). So I checked all the |
| 124 diodes and transistors of the PSU, but none looked short. | 125 diodes and transistors of the PSU, but none looked short. |
| 134 | 135 |
| 135 But when I tried to turn power on, the voltege OK green LEDs flickered once but | 136 But when I tried to turn power on, the voltege OK green LEDs flickered once but |
| 136 did not stay on, and the error LED did turn on. All the voltages produced by | 137 did not stay on, and the error LED did turn on. All the voltages produced by |
| 137 the inverter board were very low. Somethin was wrong. | 138 the inverter board were very low. Somethin was wrong. |
| 138 | 139 |
| 140 .. image:: {filename}images/hp8662a/PSU_inverter_schematic.jpeg | |
| 141 | |
| 139 Looking at the schematics and the boards, I did find several failure points: | 142 Looking at the schematics and the boards, I did find several failure points: |
| 140 | 143 |
| 141 - one of the 2 150µH inductors on the 160VDC rails (L6 anf L7 on A7A3) ) looked | 144 - one of the 2 150µH inductors on the 160VDC rails (L6 anf L7 on A7A3) ) looked |
| 142 a bit odd to me: it was not sitting flat on the PCB, which it should for this | 145 a bit odd to me: it was not sitting flat on the PCB, which it should for this |
| 143 kind of inductor; and when I gently tried to wiggle it, it appeared a leg was | 146 kind of inductor; and when I gently tried to wiggle it, it appeared a leg was |
| 149 | 152 |
| 150 - there are 2 small 400mA fuses on the inverter board to protect the base of | 153 - there are 2 small 400mA fuses on the inverter board to protect the base of |
| 151 the 2 switching (bipolar!) transistors (first time I see the base of an NPN | 154 the 2 switching (bipolar!) transistors (first time I see the base of an NPN |
| 152 transistor protected by a fuse, but it's also the first time I repair a | 155 transistor protected by a fuse, but it's also the first time I repair a |
| 153 switching PSU which the main switching transistors are bipolar ones and not | 156 switching PSU which the main switching transistors are bipolar ones and not |
| 154 MOSFET or so). These are small XXX fuses, they look nice and so but are very | 157 MOSFET or so). These are small BUSS GMW 4/10 (400mA) fuses, they look nice |
| 155 expensive (and a bit hard to find): something like 15€ each. So for now, I've | 158 and so but are very expensive (and a bit hard to find): something like 15€ |
| 156 replaced it with a simple 5x20 glass fuse: not as pretty but does the job for | 159 each. So for now, I've replaced it with a simple 5x20 glass fuse: not as |
| 157 a few cents. | 160 skookum but does the job for a few cents. |
| 158 | 161 |
| 159 .. image:: {filename}images/hp8662a/broken_choke.jpg | 162 .. image:: {filename}images/hp8662a/broken_choke.jpg |
| 160 | 163 |
| 161 After these fixes, I reassembled the PSU and tried again: the result was quite | 164 After these fixes, I reassembled the PSU and tried again: the result was quite |
| 162 promising, but not a complete success: the PSU seemed to start, but very | 165 promising, but not a complete success: the PSU seemed to start, but very |
| 163 quickly stops itself with an error LED on: the overvoltage protection was | 166 quickly stops itself with an error LED on: the overvoltage protection was |
| 164 activated. | 167 shuting down the power. |
| 165 | 168 |
| 166 I tried to check several things on the error detection circuits with no succes, | 169 I tried to check several things on the error detection circuits with no succes, |
| 167 and followed the service manual instruction, until I found this in the service | 170 and when (at last) I began to follow the service manual instructions, I found |
| 168 manual, about this overvoltage error: | 171 this paragraph about this overvoltage error: |
| 169 | 172 |
| 170 If red LED in the upper left hand corner of the board is lit indicating the | 173 If red LED in the upper left hand corner of the board is lit indicating the |
| 171 input voltagr from the A7A3 Inverter assembly was too high and the supply was | 174 input voltagr from the A7A3 Inverter assembly was too high and the supply was |
| 172 shut down, it is most likely a problem with the regulator not drawing enough | 175 shut down, it is most likely a problem with the regulator not drawing enough |
| 173 current. | 176 current. |
