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Tek 4654M; No Trace, No +5V, No +95V; Help
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Greetings; I am more of a mechanical guy than electronic guy. That said I am trying to troubleshoot a 465M scope that has no trace, no trace with beam finder, no +5V nor +95V. I was testing for the +95V at the TP on the left side of the A11 Interface board forward of the power supply area and the +5V on the TPs on each side of the A11 board (right side in the rear and left side outboard of the +95V TP.. Unit does power up and CRT illumination is present. I am using a Air Force Service Manual (465M Operation and Maintenance) that I found on line. I have been tracing +5 voltage from the T700 transformer forward. Things seem good before the Q742 transistor. After the transistor I was looking for +4.9V and had -0.121V (basically zero. Q742 is a 2N222 NPN transistor. My thought was to replace it to see if that restored the +5V. My issue with the +95V is I at a loss to determine where the +95V originates. I would really appreciate some "adult supervision and guidance" on both the +5V and the +95V issues and any suggestions, advice and guidance in general. Thank you very much. Stay safe and healthy. Harrison N1FAM
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Harrison:
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See Schematic FO-10 +95V originates at T550 - 1/2 wave rectifier D582, filter C582, L582 - jumper - C584, CR584 - TP+95V Dick K9FFK On 8/6/2020 10:14 AM, buma7@... wrote:
Greetings; I am more of a mechanical guy than electronic guy. That said I am trying to troubleshoot a 465M scope that has no trace, no trace with beam finder, no +5V nor +95V. I was testing for the +95V at the TP on the left side of the A11 Interface board forward of the power supply area and the +5V on the TPs on each side of the A11 board (right side in the rear and left side outboard of the +95V TP.. Unit does power up and CRT illumination is present. I am using a Air Force Service Manual (465M Operation and Maintenance) that I found on line. I have been tracing +5 voltage from the T700 transformer forward. Things seem good before the Q742 transistor. After the transistor I was looking for +4.9V and had -0.121V (basically zero. Q742 is a 2N222 NPN transistor. My thought was to replace it to see if that restored the +5V. My issue with the +95V is I at a loss to determine where the +95V originates. I would really appreciate some "adult supervision and guidance" on both the +5V and the +95V issues and any suggestions, advice and guidance in general. Thank you very much. Stay safe and healthy. Harrison N1FAM |
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Thank you Dick. As happens, I stumbled onto the source of the 95V shortly after posting. Regardless, I am still struggling on getting this unit repaired. I thought I saw issues with diode CR582 (in the 95V supply) and transistor Q742 (transistor in the +5V circuit). Replaced both components but no measurable difference in results after doing so. At a loss of where to go from here. My basic electrical theory and understanding of this unit may just be too weak. With the +5V TPs reading -.749 and the 95V TP reading +5.71V it points to a component going to ground. Maybe a cap, resistor, or perhaps a faulty transistor or diode. I just am not seeing where the circuits seem to go bad. I've been wondering if I could have an issue in transformer T550 or in U550 the HV module. For instance, should I have +95 on pin 2 of T550? Currently I don't. Harrison N1FAM
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Have been concentrating on the +5V circuitry. Checked everything out left to right from Figure F0-10 R741 and R742 through U722B to Q742 and Q744. Tried to test everything in place but did lift a lead on some items to get a better reading and especially if the in circuit reading was off and/or was affected by other elements of the circuitry. The only things I found out of spec were Pin 6 of U722B read 0.660V Spec is +5.0V. Also the Pin 7 of U722A (32V circuitry) read 14.71V. Spec is 6.0V. The Q742, R746, R747 junction reading was -0.125V Spec is +4.9V. Not sure where to go from here. Considering pulling Q744 and Q746. 744 is on the A11 interface board. Problem is to pull it the legs are so short I can't test it with a transistor tester. 746 is the large transistors mounted on the back panel of the scope. I haven't done checking of the +5 unregulated circuit coming off of CR741. Am planning on picking up the circuit at R748 and C748 and try to check the components to the right on the schematic. Regardless, still struggling on this one to find a fault. Current plan was to try to solve the +5V issue before moving on to the +95V issue. I'd appreciate any advice and guidance on this one. Thank you. Stay safe and healthy. Harrison N1FAM
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Chuck Harris
There is no point on working on the 95V supply until you have
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the 5V supply working properly, as 5V is used to drive the regulator circuitry, and it looks as though if it is missing, it may shut the HV oscillator down... preventing the +95V from being made. -Chuck Harris buma7@... wrote: Have been concentrating on the +5V circuitry. Checked everything out left to right from Figure F0-10 R741 and R742 through U722B to Q742 and Q744. Tried to test everything in place but did lift a lead on some items to get a better reading and especially if the in circuit reading was off and/or was affected by other elements of the circuitry. The only things I found out of spec were Pin 6 of U722B read 0.660V Spec is +5.0V. Also the Pin 7 of U722A (32V circuitry) read 14.71V. Spec is 6.0V. The Q742, R746, R747 junction reading was -0.125V Spec is +4.9V. Not sure where to go from here. Considering pulling Q744 and Q746. 744 is on the A11 interface board. Problem is to pull it the legs are so short I can't test it with a transistor tester. 746 is the large transistors mounted on the back panel of the scope. I haven't done checking of the +5 unregulated circuit coming off of CR741. Am planning on picking up the circuit at R748 and C748 and try to check the components to the right on the schematic. Regardless, still struggling on this one to find a fault. Current plan was to try to solve the +5V issue before moving on to the +95V issue. I'd appreciate any advice and guidance on this one. Thank you. Stay safe and healthy. Harrison N1FAM |
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Thanks Chuck. I have taken your advice and concentrated on the +5V circuit. Current conditions +32 TP reads 34.1, +5 TP 3.8, -5 TP -4.7 and 95 TP reads 8.8. I have read out all components in the +5 circuit from the +32 feed through R741 and R742 to the jumper before the +5V TP. If I got a good reading in circuit, I went with that. If I did not, I pulled the item from the circuit or lifted one lead and tested it that way. Everything seemed to check out OK except for pin 6 of U722B (FO-10) should read +5 but was reading 0.660, pin 7 should read 6.0 but read 14.71. Because the ¡°power on¡± LED is not lit when the unit is turned on, I decided to jump over there a bit and read out U2280 on the Trigger board (Figure FO-5). All seemed well there except pin 6 reads -.38 instead of -3.6 and pin & reads -.25 instead of +5V. Now trying to figure if I need to back up more in the system or is the issue coming from somewhere else in the unit. I have no background on this unit. There appears by magic marker markings that some work or alignment has taken place, which is not surprising given the age of the unit. However, I do not see any real hack jobs or obvious internal damage or signs of non pro lab work. I have been trying to raise my electronic theory knowledge and troubleshooting skill on this unit. It came from a Ham fest, was labeled ¡°working¡± which it clearly was not. Bottom line, while I found a couple of bad readings, I am no closer to solving the issue and getting this unit back online. I did replace CR 582 (Figure FO-9) and Q742 (FO-10) I am not convinced either was bad. My initial TP readings were +32 read +32, -5 read -5.07, +5 read -.752, and 95 read 5.74. My current set of readings is different. I do not know if that is because of the two part replacements, something else or just variation in collection. I did use the same Beckman HD110 meter to take the readings. Because the TP reading are now out of spec should adjustments be made for the respective pots for each circuit? At a loss as to where to go from here. Don¡¯t like throwing in the towel but not sure I am making any meaningful progress at this point. Any advice and ideas, anybody, would be appreciated? Harrison N1FAM
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Hello, Harrison.
I would not adjust anything at this point, as any adjustment can't correct for this much out of spec. Take a look at the voltages across C741 and C761. Measure both DC and AC values. The DC voltages should be around 10v or more and the AC voltage much smaller. Please let us know what you find. Dave |
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Chuck Harris
Definitely, it is exceedingly rare that you can adjust your way
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out of total failure. -Chuck Harris Dave Hills wrote: Hello, Harrison. |
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Thanks gentlemen. I have read the voltages across both C741 and C761 as Dave has advised. I used the Beckman HD110 and read to ground. I.E. the black test lead was alligator cliped to the chassis and the readings were taken with the red test lead. The unit was powered through an isolation transformer and the unit was on. Only visual indication of being on is the "scale illumination". In the case of C741 DC volts on one pin was +8.5V. The other four were 0.00V. Regarding AC volts, the pin that read 8.5V DC read 4.2V AC. Again the other four were 0.00V. In the case of C761 DC volts; 4 of the 5 pins read -11.9V. The fifth pin read 0.00. Regarding AC volts; all five pins read 0.00V AC. In comparison it seems to indicate an issue with C741. If that is the case, is C741 a capacitor pack? Is something like that available or would one have to say cut it open and rebuild the internal circuit. Maybe I am getting ahead of myself here. I am interested in what the two of you, or anyone else for that matter, deduce from the above readings. Thanks again for your continued support and advice. I really appreciate it. There doesn't seem to be as much traffic or discussion/posts on something as old as the 465M. For me it is a good covid-19 time filler and training project. Once it is on line I will be passing it on to my son Michael NC1V or Brian KA1OMR or some other Ham needing a scope.Stay safe and healthy. Harrison N1FAM
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OK. I will say 90% chance C741 is bad, 5% chance CR741, the bridge rectifier, has a shorted diode in it and 5%
chance something else. C741 is a single unit, it can be replaced with a part of equal or slightly larger capacitance and voltage rating and it will work fine. Be aware the four tabs that read 0 volts may be conduction paths and likely will need to be re-connected with jumpers if replacing with a two leaded replacement part. You can verify this by checking for continuity between the four tab pads on the circuit board after the capacitor is removed. If there is no continuity between them, they will require jumpers connecting all four pads together. Also, a modern part with the same rating will be physically much smaller. Dave |
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Dave; thanks for the advice and guidance. You were right on. I found a OEM C741 cap at QService Electronics in Greece. Cap was reasonable at $9.85 US but shipping was a flat rate $9.00 US. I replaced the cap today. Post replacement results was that now the power on light illuminates. Voltages are 32V, +5V, and -5V. So perhaps the +5V circuit is now resolved. The 95V TP reads 7.58V. The best I had in the past was 5.41V. I have 7.58V at the junction between CR582 and C582. I read out the pins on the secondary of T550. Respectively they were pins 1 through 8; 0.0, 0.0, 1.44, 0.0, 0.577, 1.44, 0.0, 0.0. I didn't read out the primary thinking that it was HV and I didn't have a meter that read up to 3KV. Wondering now if the issue is with T550. Does that make sense?
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If I understand correctly, you see zero volts DC on T550, pin4? That should be
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around 50vdc. Check the fuse F558 for open and capacitor C558 for short. Dave On Thu, Aug 27, 2020 at 02:44 PM, Harrison wrote:
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I checked F558 and to my surprise it was blown. I know I had checked it numerous times and it was okay. It must have blown during one of my probing expeditions. I noted with or without F558 the unit powers up and CRT illumination and power on light work. I lifted the outer lead of C558 and it read 8.34 micro Farad (an in spec reading).
I was thinking of staying out of the scope for a bit and spending more time in the manual rereading the circuit theory and operation theory. I say that because I am confused and going in circles. After replacing C741 I had a solid 32V, +5V and -5V at the TPs. The 95 TP read +7.58. As time has passed those readings are now 30V, +4.7V and -4.9V. The 95V TP reads +7.5V. The junction between CR582 and C582 reads +7.4V. These readings were all taken with the same meter. Nothing else was done nor any other components replaced between the two sets of readings. Monitoring the readings now for a bit it almost seems like they discharge a bit. I.E. like a capacitor discharging. I also noticed that immediately after replacing C741 the power on light would illuminate steady when the power switch was pulled. Now when I pull the power switch on it seems to blink/flicker once or twice before remaining on steady. Granted there is a linkage and several mechanical connections that might just be suffering the ravages of time and this might be expected. Regardless, I am just Easter egging now and need to get smarter on the circuits and relationship between them. Don't know where else to go from here. Plus like I said in my first post I am a mechanical guy with limited electrical/electronic theory schooling or background. While I read a lot, not everything sticks. Anyway, any and all ideas past and future are appreciated. The group has gotten me this far. I don't know if I am close to the goal line or still off in left field. Stay safe and healthy. Harrison N1FAM |
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On 8/29/2020 10:07 AM, Harrison wrote:
I checked F558 and to my surprise it was blown. I know I had checked it numerous times and it was okay. It must have blown during one of my probing expeditions. I noted with or without F558 the unit powers up and CRT illumination and power on light work. I lifted the outer lead of C558 and it read 8.34 micro Farad (an in spec reading). |
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With the 32v supply at 30v, the +/-5v readings make perfect sense as the reference
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for those two regulators is the 32v supply. [30/32x 5] = 4.68v Check the DC and AC voltages across C721. You should see around 50vdc there. With equipment this old it is not uncommon to find multiple failures of aluminum electrolytic capacitors which often dry out with age. This is not true for tantalum types, however, they do not have the same age related failure mechanism. There won't be any +95 volts until you have 50v at C558. Dave On Sat, Aug 29, 2020 at 07:07 AM, Harrison wrote:
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I will check out voltages across C721. Since my last posting I did some reading after which I decided to look closer at the HV Oscillator and HV Regulator circuits. I did an in circuit check of Q556, Q552, Q548, Q544, C548, and L554. I used my beckman DMM in the diode mode or the SZBJ BM4070 digital LCR tester. All seemed to check out okay except the Q552 (part of the HV Oscillator circuit). I plan on removing it, doing an out of circuit test and replace it regardless. But if I understand the manual correctly that circuit has a direct impact on the oscillation and output of T550.
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Hi Harrison,
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Back in 2017, I worked on my 465M. The following is from an email I sent outlining what I did. Perhaps that will help since it sounds very similar to what you're seeing with yours. ------------------------------------------------------------------------------------------------ And, so, to recap - after getting the scope on the cheap via a Craglist buy and finding it inoperative: 1. Filter capacitors were bad and restuffed them with new capacitors 2. Accidentally bumped L582 (in the +95V filter) damaging it beyond repaid so replaced it ("repaid" should have been "repair" - Barry) 3. Scope then worked for about 15 minutes and then pfffffft - F558 blew 4. Found Q552 was bad and replaced it but the HV oscillator was now drawing excessive current blowing F558 again 5. Multiplier was bad and replaced it and scope appears to be working again ------------------------------------------------------------------------------------------------ BTW, the 465M still works fine - at least the last time I used it several months ago. Thanks, Barry - N4BUQ ----- Original Message -----
From: "Harrison" <buma7@...> |
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Read out the voltages across C721. Also read out C741 and C761. Used the Beckman HD110 MM with the common to ground. C721; three of four pins read zero, both AC and DC. The fourth pin read 55.5V DC and 0.240V AC. C741; four of five pins read zero, both AC and DC. The fifth pin read 11.4V DC and 0.068V AC. C761; four of five pins read 11.3V DC and 0.078V AC. The fifth pin read zero, both AC and DC.
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In circuit tests of these components will be quite meaningless because of
the low impedance components connected to them and the other semiconductors connected across them, such as CR556 across Q556 b-e junction. The only accurate way to test is to remove them and test out of circuit. Before you start removing parts, have you replaced the fuse and checked for 55v across C558? Q552 and Q556 comprise a blocking oscillator which provides primary excitation for T550. This oscillator cannot function without the 50 volt bias at T550 pin 4 (or across C558). Q554 and Q548 are control circuits which regulate the blocking oscillator output to provide a stable +95volts. The voltages across C721, C741 and C761 all look good, so they are not the problem. Dave On Sat, Aug 29, 2020 at 07:38 PM, Harrison wrote: On Sat, Aug 29, 2020 at 10:28 PM, Harrison wrote:
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