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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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dead Philips PM3240 oscilloscope
I have a faulty Philips PM3240 oscilloscope. The problem is that the
spot is not showing up anymore. The spot is sometimes illuminating very rapidily for one 10th of a second as a one off and then disapears for 1 hour. I have checked the modulation signal applied to G1 and it seems fine. The CRT is a D14-125 GM/08 but i do not have its characteristics and it is difficult to find out if the voltage between the wehnelt and cathode are correct or not. There is a ZA1004 neon which is acting as a voltage stabilizer between the cathode and wehnelt. This ZA1004 seems to stabilise 230V but i have no idea if this is correct or not as i cannot find any information about this component. The voltage values of the CRT are as follows: G1: -1500V G2/G4: +33.4 G3: from -950 to -1120 (focus) G5/G6: ground G7: +45V G8: 8KV I've also checked the filament and it's current is constant even when the spot is not displayed. Any help would be greatly appreciated ! Cheers, Thierry. |
#2
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dead Philips PM3240 oscilloscope
Thierry Delaitre wrote in
: I have a faulty Philips PM3240 oscilloscope. The problem is that the spot is not showing up anymore. The spot is sometimes illuminating very rapidily for one 10th of a second as a one off and then disapears for 1 hour. I have checked the modulation signal applied to G1 and it seems fine. The CRT is a D14-125 GM/08 but i do not have its characteristics and it is difficult to find out if the voltage between the wehnelt and cathode are correct or not. There is a ZA1004 neon which is acting as a voltage stabilizer between the cathode and wehnelt. This ZA1004 seems to stabilise 230V but i have no idea if this is correct or not as i cannot find any information about this component. The voltage values of the CRT are as follows: G1: -1500V G2/G4: +33.4 G3: from -950 to -1120 (focus) G5/G6: ground G7: +45V G8: 8KV I've also checked the filament and it's current is constant even when the spot is not displayed. Any help would be greatly appreciated ! Cheers, Thierry. If the neon between grid and cathode is lighting,that means the DC restorer circuit/grid drive has problems.The neon is to LIMIT the G-K voltage in case something in the grid drive circuit(DC restorer and Z-axis amp)fails.The DC restorer shifts the Z-axis signal(unblanking signal) to CRT potentials(HV). Possibly a diode breaking down under HV,or a bad HV cap(less likely) (My experience is with TEK scopes,though) -- Jim Yanik jyanik-at-kua.net |
#3
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dead Philips PM3240 oscilloscope
Jim Yanik wrote:
Thierry Delaitre wrote in : I have a faulty Philips PM3240 oscilloscope. The problem is that the spot is not showing up anymore. The spot is sometimes illuminating very rapidily for one 10th of a second as a one off and then disapears for 1 hour. I have checked the modulation signal applied to G1 and it seems fine. The CRT is a D14-125 GM/08 but i do not have its characteristics and it is difficult to find out if the voltage between the wehnelt and cathode are correct or not. There is a ZA1004 neon which is acting as a voltage stabilizer between the cathode and wehnelt. This ZA1004 seems to stabilise 230V but i have no idea if this is correct or not as i cannot find any information about this component. The voltage values of the CRT are as follows: G1: -1500V G2/G4: +33.4 G3: from -950 to -1120 (focus) G5/G6: ground G7: +45V G8: 8KV I've also checked the filament and it's current is constant even when the spot is not displayed. Any help would be greatly appreciated ! Cheers, Thierry. If the neon between grid and cathode is lighting,that means the DC restorer circuit/grid drive has problems.The neon is to LIMIT the G-K voltage in case something in the grid drive circuit(DC restorer and Z-axis amp)fails.The DC restorer shifts the Z-axis signal(unblanking signal) to CRT potentials(HV). Possibly a diode breaking down under HV,or a bad HV cap(less likely) (My experience is with TEK scopes,though Hi, Thanks for this very useful information. The neon is lighting up. Note that the neon is connected as follows: ------ -| 2.2M |-- | ------ | ------ ------ ------ | ------ | ------ -1600V ---| 10.5 |---| neon |-----| 1.2M |----| 2.2M |-----| 5.6M |--- 0V ------ | ------ | ------ ------ ------ | |-------- ^ | - | | | | |1| | --- | | |0| | /_\ |--||--0V | |0| | | | | |K| | | | | - K G1 G3 focus | | | filament | |---------|----- | --- | --- 100pF--- | | \ / | | |1M --- | | | | |\ | - ---||--------| |--------|----- 1M --- 511 ---- G1 470pF |/ | BAX17 --- 470pF --- I've checked the two BAX17 in the DC restorer unit. I've also checked the resistors which are in series with the neon. I've tried to disconnected the two 470pF interfacing with the DC restorer to no avail. I've even tried to disconnect the plug at the back of CRT with GT8 and the neon is still lighting up with 230V accross its pins. Could it be that the neon is faulty ? What is the normal voltage accross it ? What is meant to be the normal voltage accross it ? Where could i get one if it is faulty ? Cheers, Thierry. |
#4
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dead Philips PM3240 oscilloscope
Thierry Delaitre wrote in
: Jim Yanik wrote: Thierry Delaitre wrote in : I have a faulty Philips PM3240 oscilloscope. The problem is that the spot is not showing up anymore. The spot is sometimes illuminating very rapidily for one 10th of a second as a one off and then disapears for 1 hour. I have checked the modulation signal applied to G1 and it seems fine. The CRT is a D14-125 GM/08 but i do not have its characteristics and it is difficult to find out if the voltage between the wehnelt and cathode are correct or not. There is a ZA1004 neon which is acting as a voltage stabilizer between the cathode and wehnelt. This ZA1004 seems to stabilise 230V but i have no idea if this is correct or not as i cannot find any information about this component. The voltage values of the CRT are as follows: G1: -1500V G2/G4: +33.4 G3: from -950 to -1120 (focus) G5/G6: ground G7: +45V G8: 8KV I've also checked the filament and it's current is constant even when the spot is not displayed. Any help would be greatly appreciated ! Cheers, Thierry. If the neon between grid and cathode is lighting,that means the DC restorer circuit/grid drive has problems.The neon is to LIMIT the G-K voltage in case something in the grid drive circuit(DC restorer and Z-axis amp)fails.The DC restorer shifts the Z-axis signal(unblanking signal) to CRT potentials(HV). Possibly a diode breaking down under HV,or a bad HV cap(less likely) (My experience is with TEK scopes,though Hi, Thanks for this very useful information. The neon is lighting up. Note that the neon is connected as follows: ------ -| 2.2M |-- | ------ | ------ ------ ------ | ------ | ------ -1600V ---| 10.5 |---| neon |-----| 1.2M |----| 2.2M |-----| 5.6M |--- 0V ------ | ------ | ------ ------ ------ | |-------- ^ | - | | | | |1| | --- | | |0| | /_\ |--||--0V | |0| | | | | |K| | | | | - K G1 G3 focus | | | filament | |---------|----- | --- | --- 100pF--- | | \ / | | |1M --- | | | | |\ | - ---||--------| |--------|----- 1M --- 511 ---- G1 470pF |/ | BAX17 --- 470pF --- I've checked the two BAX17 in the DC restorer unit. I've also checked the resistors which are in series with the neon. I've tried to disconnected the two 470pF interfacing with the DC restorer to no avail. I've even tried to disconnect the plug at the back of CRT with GT8 and the neon is still lighting up with 230V accross its pins. Could it be that the neon is faulty ? What is the normal voltage accross it ? What is meant to be the normal voltage accross it ? Where could i get one if it is faulty ? Cheers, Thierry. To check DC restorer diodes,you need a curve tracer to check them at or near the circuit's operating voltages,at least 100V.They could break down under HV. If I read your ASCII circuit right,the neon being in series with the focus string looks like it's supposed to act as some sort of V regulator in the focus string. For the neon to be a protector against too large a G-K voltage difference,the neon would have to be across the G-K. In my experience,the neons fire at around 60V,TEK uses two of them in series across the G-K,to limit G-K voltage difference to no more than 120V. -- Jim Yanik jyanik-at-kua.net |
#5
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dead Philips PM3240 oscilloscope
Hi,
Is there anyway i could replace the BAX17 with some kind of more standard diodes for testing purposes ? Thierry. Jim Yanik wrote: Thierry Delaitre wrote in : Jim Yanik wrote: Thierry Delaitre wrote in : I have a faulty Philips PM3240 oscilloscope. The problem is that the spot is not showing up anymore. The spot is sometimes illuminating very rapidily for one 10th of a second as a one off and then disapears for 1 hour. I have checked the modulation signal applied to G1 and it seems fine. The CRT is a D14-125 GM/08 but i do not have its characteristics and it is difficult to find out if the voltage between the wehnelt and cathode are correct or not. There is a ZA1004 neon which is acting as a voltage stabilizer between the cathode and wehnelt. This ZA1004 seems to stabilise 230V but i have no idea if this is correct or not as i cannot find any information about this component. The voltage values of the CRT are as follows: G1: -1500V G2/G4: +33.4 G3: from -950 to -1120 (focus) G5/G6: ground G7: +45V G8: 8KV I've also checked the filament and it's current is constant even when the spot is not displayed. Any help would be greatly appreciated ! Cheers, Thierry. If the neon between grid and cathode is lighting,that means the DC restorer circuit/grid drive has problems.The neon is to LIMIT the G-K voltage in case something in the grid drive circuit(DC restorer and Z-axis amp)fails.The DC restorer shifts the Z-axis signal(unblanking signal) to CRT potentials(HV). Possibly a diode breaking down under HV,or a bad HV cap(less likely) (My experience is with TEK scopes,though Hi, Thanks for this very useful information. The neon is lighting up. Note that the neon is connected as follows: ------ -| 2.2M |-- | ------ | ------ ------ ------ | ------ | ------ -1600V ---| 10.5 |---| neon |-----| 1.2M |----| 2.2M |-----| 5.6M |--- 0V ------ | ------ | ------ ------ ------ | |-------- ^ | - | | | | |1| | --- | | |0| | /_\ |--||--0V | |0| | | | | |K| | | | | - K G1 G3 focus | | | filament | |---------|----- | --- | --- 100pF--- | | \ / | | |1M --- | | | | |\ | - ---||--------| |--------|----- 1M --- 511 ---- G1 470pF |/ | BAX17 --- 470pF --- I've checked the two BAX17 in the DC restorer unit. I've also checked the resistors which are in series with the neon. I've tried to disconnected the two 470pF interfacing with the DC restorer to no avail. I've even tried to disconnect the plug at the back of CRT with GT8 and the neon is still lighting up with 230V accross its pins. Could it be that the neon is faulty ? What is the normal voltage accross it ? What is meant to be the normal voltage accross it ? Where could i get one if it is faulty ? Cheers, Thierry. To check DC restorer diodes,you need a curve tracer to check them at or near the circuit's operating voltages,at least 100V.They could break down under HV. If I read your ASCII circuit right,the neon being in series with the focus string looks like it's supposed to act as some sort of V regulator in the focus string. For the neon to be a protector against too large a G-K voltage difference,the neon would have to be across the G-K. In my experience,the neons fire at around 60V,TEK uses two of them in series across the G-K,to limit G-K voltage difference to no more than 120V. |
#6
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dead Philips PM3240 oscilloscope
Thierry Delaitre wrote in
: Hi, Is there anyway i could replace the BAX17 with some kind of more standard diodes for testing purposes ? Sure,as long as the PIV is equal or better.Small-signal diodes have better freq. response characteristics,which would affect the risetime of the unblanking signal to the CRT grid. You could use the ECG recommended substitute for the BAX17. -- Jim Yanik jyanik-at-kua.net Thierry. Jim Yanik wrote: Thierry Delaitre wrote in : Jim Yanik wrote: Thierry Delaitre wrote in : I have a faulty Philips PM3240 oscilloscope. The problem is that the spot is not showing up anymore. The spot is sometimes illuminating very rapidily for one 10th of a second as a one off and then disapears for 1 hour. I have checked the modulation signal applied to G1 and it seems fine. The CRT is a D14-125 GM/08 but i do not have its characteristics and it is difficult to find out if the voltage between the wehnelt and cathode are correct or not. There is a ZA1004 neon which is acting as a voltage stabilizer between the cathode and wehnelt. This ZA1004 seems to stabilise 230V but i have no idea if this is correct or not as i cannot find any information about this component. The voltage values of the CRT are as follows: G1: -1500V G2/G4: +33.4 G3: from -950 to -1120 (focus) G5/G6: ground G7: +45V G8: 8KV I've also checked the filament and it's current is constant even when the spot is not displayed. Any help would be greatly appreciated ! Cheers, Thierry. If the neon between grid and cathode is lighting,that means the DC restorer circuit/grid drive has problems.The neon is to LIMIT the G-K voltage in case something in the grid drive circuit(DC restorer and Z-axis amp)fails.The DC restorer shifts the Z-axis signal(unblanking signal) to CRT potentials(HV). Possibly a diode breaking down under HV,or a bad HV cap(less likely) (My experience is with TEK scopes,though Hi, Thanks for this very useful information. The neon is lighting up. Note that the neon is connected as follows: ------ -| 2.2M |-- | ------ | ------ ------ ------ | ------ | ------ -1600V ---| 10.5 |---| neon |-----| 1.2M |----| 2.2M |-----| 5.6M |--- 0V ------ | ------ | ------ ------ ------ | |-------- ^ | - | | | | |1| | --- | | |0| | /_\ |--||--0V | |0| | | | | |K| | | | | - K G1 G3 focus | | | filament | |---------|----- | --- | --- 100pF--- | | \ / | | |1M --- | | | | |\ | - ---||--------| |--------|----- 1M --- 511 ---- G1 470pF |/ | BAX17 --- 470pF --- I've checked the two BAX17 in the DC restorer unit. I've also checked the resistors which are in series with the neon. I've tried to disconnected the two 470pF interfacing with the DC restorer to no avail. I've even tried to disconnect the plug at the back of CRT with GT8 and the neon is still lighting up with 230V accross its pins. Could it be that the neon is faulty ? What is the normal voltage accross it ? What is meant to be the normal voltage accross it ? Where could i get one if it is faulty ? Cheers, Thierry. To check DC restorer diodes,you need a curve tracer to check them at or near the circuit's operating voltages,at least 100V.They could break down under HV. If I read your ASCII circuit right,the neon being in series with the focus string looks like it's supposed to act as some sort of V regulator in the focus string. For the neon to be a protector against too large a G-K voltage difference,the neon would have to be across the G-K. In my experience,the neons fire at around 60V,TEK uses two of them in series across the G-K,to limit G-K voltage difference to no more than 120V. |
#7
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dead Philips PM3240 oscilloscope
Hi,
I've replaced the 2 bax17 in the detection part by by281 but no success. I've posted the schematic at http://www.cpc.wmin.ac.uk/~delaitt/PM3240/circuit.jpg and the following pictures taken with an oscilloscope: 1) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010339.JPG collector of TS1308 when intensity is in maximum voltage goes from 7v to 75v (using 1/10 probe) 2) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010340.JPG collector of TS1308 when intensity is in minimum voltage goes to 50v 3) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010341.JPG emitter of TS1306 voltage goes from 8v to 28v when intensity is in maximum 4) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010343.JPG emitter of TS1306 voltage is 8V when intensity is in minimum Cheers, Thierry. Jim Yanik wrote: Thierry Delaitre wrote in : Hi, Is there anyway i could replace the BAX17 with some kind of more standard diodes for testing purposes ? Sure,as long as the PIV is equal or better.Small-signal diodes have better freq. response characteristics,which would affect the risetime of the unblanking signal to the CRT grid. You could use the ECG recommended substitute for the BAX17. -- Jim Yanik jyanik-at-kua.net |
#8
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dead Philips PM3240 oscilloscope
"Thierry Delaitre" wrote ...
Hi, I've replaced the 2 bax17 in the detection part by by281 but no success. I've posted the schematic at http://www.cpc.wmin.ac.uk/~delaitt/PM3240/circuit.jpg and the following pictures taken with an oscilloscope: 1) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010339.JPG collector of TS1308 when intensity is in maximum voltage goes from 7v to 75v (using 1/10 probe) 2) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010340.JPG collector of TS1308 when intensity is in minimum voltage goes to 50v 3) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010341.JPG emitter of TS1306 voltage goes from 8v to 28v when intensity is in maximum 4) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010343.JPG emitter of TS1306 voltage is 8V when intensity is in minimum Cheers, Thierry. What I don't trust is the oscillation that is on the collector of TS1308. The signals on TS1306 seems to be correct. Did you try to disconnect the fillament from its supply? It can be that there is a short between the fillament and the grid or at the supply of the fillament voltage, and then lighting up the neon lamp? Anyways, I dont know if the by281 (correct nr btw??) is a good replacement. The BAX17 cross reference shows up this diode: http://www.fairchildsemi.com/pf/FD/FDH400.html When you put back the original diodes how is the signal at the collector of TS1308? Still oscillating? Al |
#9
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dead Philips PM3240 oscilloscope
The Al Bundy wrote:
"Thierry Delaitre" wrote ... Hi, I've replaced the 2 bax17 in the detection part by by281 but no success. I've posted the schematic at http://www.cpc.wmin.ac.uk/~delaitt/PM3240/circuit.jpg and the following pictures taken with an oscilloscope: 1) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010339.JPG collector of TS1308 when intensity is in maximum voltage goes from 7v to 75v (using 1/10 probe) 2) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010340.JPG collector of TS1308 when intensity is in minimum voltage goes to 50v 3) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010341.JPG emitter of TS1306 voltage goes from 8v to 28v when intensity is in maximum 4) http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010343.JPG emitter of TS1306 voltage is 8V when intensity is in minimum Cheers, Thierry. Hi, What I don't trust is the oscillation that is on the collector of TS1308. The signals on TS1306 seems to be correct. Did you try to disconnect the fillament from its supply? It can be that there is a short between the fillament and the grid or at the supply of the fillament voltage, and then lighting up the neon lamp? I've disconnected the fillament, R1337, G1, G3, R21, K and neon is still blinking. I've checked R1341, R1339, R1338 and there are correct. Anyways, I dont know if the by281 (correct nr btw??) is a good replacement. The BAX17 cross reference shows up this diode: http://www.fairchildsemi.com/pf/FD/FDH400.html When you put back the original diodes how is the signal at the collector of TS1308? Still oscillating? I meant BY218! I've now put back the original BAX17 diodes and taken pictures. The following shows the signal at the collector of TS1308 when intensity is at its maximum (up to 75v). http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010350.JPG The following shows the oscillating signal (6us) at the collector of TS1308. http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010352.JPG The following picture shows the board with neon lighting up. http://www.cpc.wmin.ac.uk/~delaitt/PM3240/P1010353.JPG Should i believe that the crt is not displaying anything due to the fact that the voltage of the wehnelt is too negative relative to the cathode ? Could it be that the neon is faulty ? Do they normally light up above a particular current threshold ? In my case, it seems the neon is lighting up when the current is 0.16mA (1300/8Mohm). Cheers, Thierry. |
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