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475 with no Display and no HV
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Hello all,
We had several Tektronix scopes donate to our museum (Museum of Radio and Technology) late last year. One is a Tek 475 with no display and no HV at the -2450 volt test point. All of the low voltage rails are within specs. There is 20 vdc at TP1318 but the waveform is only about 700 mv p-p at 22.3Khz instead of 40 volts p-p at ~50Khz. The waveform at the base of Q1308 is 150 mv p-p also at 22.3Khz but the DC voltage measurement at that point is 3.4 volts instead of 4.4 volts. The rest of the voltage measurement compared to the manual are no where near correct. I have check all the tantalum caps on the driver side of the HV oscillator, the solid state devices, and even the resistors. All appear to be good. I have even taken a working 475 and did a side by resistance check of T1320 for open windings or shorts between windings of that transformer. A side by resistance check was done between the two scope for BCE to ground on transistors Q1306 thru Q1318. The only significant difference that I found was from pin 6 of transformer T1320 and the junction at the taped secondary of that transformer and C1320 (the .01 uf HV cap). One the scope that works I measure 9.6 Megohms. The 475 that doesn't, work shows a resistance of 1 Megohm. I am sitting here scratching my head. Do I have a bad transformer or is there something else that should be check. Also know that I took the cussed thing apart to get to the high voltage multiple and the input to GND resistance measured infinite, if I remember correctly. It certainly did not read a low resistance or a short. Anyway, I welcome any comments or tips Ripley |
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Tony G4NGV
I have a very similar fault on mine. I disconnected the loads from the
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secondary, and the crt itself, and I get bigger oscillations. Reconnect the crt again and they are gone. Tony On Thu, 28 Feb 2019, 15:44 , <bob.ripley@...> wrote:
Hello all, |
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Hi Tony,
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Early on I followed the Service Manual’s troubleshooting tree for no HV including disconnecting the connector on the back of the CRT but it made no difference on the waveform sizes back on the primary side of the high voltage transformer. There has been much time invested on this 475 since then. I will try disconnecting the CRT again including the output from the voltage multiplier and see if anything has changed. Ripley The words are mine but this iPad does what it will with them. On Feb 28, 2019, at 4:51 PM, Tony G4NGV <tonyvirago2@...> wrote: |
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On 2/27/2019 9:09 PM, bob.ripley@... wrote:
Hello all,I've had those HV caps go leaky - maybe that accounts for the resistance reading? Ed |
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Hmmm. That’s a thought. I will check it out and report back.
Thank you Ed. Ripley Sent from Mail for Windows 10 From: ehsjr Sent: Thursday, February 28, 2019 10:37 PM To: [email protected] Subject: Re: [TekScopes] 475 with no Display and no HV On 2/27/2019 9:09 PM, bob.ripley@... wrote: Hello all,I've had those HV caps go leaky - maybe that accounts for the resistance reading? Ed |
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Hi There;
New guy to the group. I'm repairing and 475A, had the same issue and was able to solve it. Maybe it will work for you. I believe the part numbers, at least in this area, are the same for the 475 and 475A. On my scope, after repairing the LV supplies, the oscillator for the HV supply would only barely oscillate if at all. Eventually I found that C1304 (2.7uf tantalum) that provides bias to Q1306 had failed. The interesting thing was the capacitor had a resistance of about 500 ohms, not the 0 ohms I was expecting. I replaced the capacitor, the HV fired right up and I now have nice clean lines on the CRT. I still have other problems I'm working through on the scope but at least progress. Give it a try on yours and let me know. All the best Don |
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New guy to the group, but I am repairing a 475A and had a similar problem that I solved.
All LV supplies were good (after repair), but oscillator for the HV would barely start, if at all. I finally traced it down to C1304 -a 2.7uF tantalum that is decoupling a bias point into Q1306 at the beginning of the oscillator. I don't know if your numbers on the 475 will differ from my 475A. Let know if you have a problem and I'll send an image (somehow?). What was surprising to me was that the tantalum hadn't failed to a full short, it was at about 500 ohms. I was looking for tantalums that were short circuits and missed this one. But after measuring bias voltages and, I admit a wild ass guess, I replaced the capacitor. The HV fired right up and I have nice clean traces on the CRT. Now I'm chasing other problems on the 475A, but at least I'm moving it forward. Hope this helps and let me know Don |
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Chuck Harris
I don't know that that is a common problem, but I have
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made that exact repair on a 475 myself. -Chuck Harris Don A wrote: New guy to the group, but I am repairing a 475A and had a similar problem that I solved. |
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Hello All,
Per Don’s advice I pulled C1304 and one of it’s leads fell off. Hmmm, that’s not good. I did not have another 2.7uf 160volt capacitor so I tried a 4uf cap but it made no difference. The waveform at TP1318 is still 670mv p-p at 22Khz. Next, I lifted one end of C1320 along with one lead of CR1329 and C1326. The connector on the back of the CRT was disconnected along with the heavy red HV from the voltage multiplier to the CRT. Powered up the 475 again. Still no change back at TP1318. 厂颈驳丑…… Back to you for ideas and suggestions Ripley Sent from Mail for Windows 10 From: Chuck Harris Sent: Saturday, March 2, 2019 10:12 AM To: [email protected] Subject: Re: [TekScopes] 475 with no Display and no HV I don't know that that is a common problem, but I have made that exact repair on a 475 myself. -Chuck Harris Don A wrote: New guy to the group, but I am repairing a 475A and had a similar problem that I solved. |
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That is a shame.
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I've also read that Fuse 1318, C1318 and Q1318 are vulnerable and worth checking. Beyond that I don't know. Don On Sat, Mar 2, 2019 at 2:41 PM SuddenLink <bob.ripley@...> wrote:
Hello All, |
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Yes. I think so too. Usually given enough time I can figure out what’s going on and fix the problem but this one got me twisting in the wind.
Ripley Sent from Mail for Windows 10 From: Don A Sent: Saturday, March 2, 2019 3:43 PM To: [email protected] Subject: Re: [TekScopes] 475 with no Display and no HV That is a shame. I've also read that Fuse 1318, C1318 and Q1318 are vulnerable and worth checking. Beyond that I don't know. Don On Sat, Mar 2, 2019 at 2:41 PM SuddenLink <bob.ripley@...> wrote: Hello All, |
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Hi Gang:
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C1304, a 2.7 uF, 50 V cap is used to provide an RC time constant with R1304 during power on.? This lets the the node at CR1304 think it is heading for +110V but is nailed at +50V.? By all means, replace CR1304 with a 1N4007 and C1304 with a 2.7 uF, 200V rated cap.? If you use a 4uF cap at C1304 change R1304 to a 160K unit to give you the same RC time constant.? Then to test the transformer circuit momentarily connect a 1K resistor between the +5 supply and the base of Q1318.? If the transformer and associated diodes and other components are OK Q1318 should conduct and give a trace.? If you find R1303A is not 24.5M Ohms you might have to scab one together using three 8.2M, 2W resistors.? Solder the three 8.2M resistors together with very short leads and no sharp points and put them in a thick wall plastic tube.? It will be necessary to carefully select the resistor at R1303B to give you the required -2450VDC at the test point if you replace R1303A. Good luckReed Dickinsonreed714@... On Saturday, March 2, 2019, 12:43:41 PM PST, Don A <don.asquin@...> wrote:
That is a shame. I've also read that Fuse 1318, C1318 and Q1318 are vulnerable and worth checking. Beyond that I don't know. Don On Sat, Mar 2, 2019 at 2:41 PM SuddenLink <bob.ripley@...> wrote: Hello All, |
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Page 3-35 of manual has a good functional description including point that ambient current supplied to base (of Q1318) determines frequency (ie not LC arrangement) and as such there isn’t a specific cap component for resonate frequency (a function oscillator still relies upon however to function).
You said “The rest of the voltage measurement compared to the manual are no where near correct” so perhaps a closer look at the whole setup noting Q1308/10 push-pull, gain and phase reversal Q1306 and bias arrangement behind Q1310 (ie the Q1312 and associated components). I have an old Tek544 where the HT falls dramatically when ambient room temp too hot and was amazed in the frequency shift involved. It is a different circuit but 475 still has resonance involved and the output voltage is regulated by driving current harder into the base. Just a thought. Please excuse me/any error as nubie here. Interesting fault. |
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Hello Reed,
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Thank you sir for your input. All four sections of R1303 measure correctly. Placed an order with Mouser this afternoon so parts should be delivered about mid week. At this point I game to try just about any suggestions. I will report back with my findings. Ripley The words are mine but this iPad does what it will with them. On Mar 2, 2019, at 5:19 PM, Reed Dickinson <reed714@...> wrote: |
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Hi Steph,
At one point and even though I didn’t think this exercise would do any good Q1308, Q1310, Q1312, and Q1316 where switched from the working 475 to this one. The non working Tek 475 still exhibited the same symptoms. While the working 475 fired right up with the transistors from the non working unit. Those other voltage measurements that I mentioned where way off are as follows: The base of Q1316 should be +.6 volts instead it measures +4 volts Over at the junction of C1317, CR1317, and R1317 the manual shows -4 volts. I see +.9 volts at that point. The base of Q1312 on my non working scope measures +3 volts where the manual shows that it should be +.5 volts. Let’s see. What else. In the manual the collector of Q1308 is -3.2 volts and the collector of Q1310 should be -8 volts. I see +4 volts at the collector of Q1308 and +1.8 volts at the collector of Q1310. Lastly, Tektronix shows in their 475 manual that the collector of Q1316 should be 1.6 volts. On this 475 I am measuring +.6 volts at that point. The only DC voltage measurements that agree with the manual around this part of the circuit are the low voltage rails and the +21.6 volts at TP1318 (I see +20.9 volts but that’s close enough in my book). Ripley Sent from Mail for Windows 10 From: Steph L Sent: Saturday, March 2, 2019 5:43 PM To: [email protected] Subject: Re: [TekScopes] 475 with no Display and no HV Page 3-35 of manual has a good functional description including point that ambient current supplied to base (of Q1318) determines frequency (ie not LC arrangement) and as such there isn’t a specific cap component for resonate frequency (a function oscillator still relies upon however to function). You said “The rest of the voltage measurement compared to the manual are no where near correct” so perhaps a closer look at the whole setup noting Q1308/10 push-pull, gain and phase reversal Q1306 and bias arrangement behind Q1310 (ie the Q1312 and associated components). I have an old Tek544 where the HT falls dramatically when ambient room temp too hot and was amazed in the frequency shift involved. It is a different circuit but 475 still has resonance involved and the output voltage is regulated by driving current harder into the base. Just a thought. Please excuse me/any error as nubie here. Interesting fault. |
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Hi Ripley,
Hooked, but the answer is probably on the next sheet somewhere "if only you had looked" (my thinking sometimes being too narrow/not standing back for a minute on search). Based on what you have seen I'd suggest double checking resistance pin 9 of T1320 to same earth used by C1316 and compare? The +0.9v on the negative lead of C1317 could be +0.7v (diode forward bias) higher than cathode CR1317/negative C1316/pin 9 - hence meter out. . So does that indicate Q1312 is off, emitter of Q1312 limited to +.7v forward bias of CR1314. Then Q1312 not conducting and base Q1310 is dragged down so it is off. The +1.8v you measure on its collector (+1.8v), is this same as on CR1310 anode and simply represents Q1308 (on) collector current shunted through R1312 etc. Q1316 hard on, current supplied via R1316, in pin 9, out pin 8 to base of main transistor Q1318, noting its emitter is direct connected to ground. I gather the -4v on the negative end of C1317 is result of pulse appearing across C1316, rectified and filtered to make -4v. . So if it is not pulsing (I'm asking) shouldn't you see some evidence of Q1318 being pulled hard on? Is that where the + 0.9v measured anode of CR1317 is from?? Not sure if explained earlier point re dry joint/cracked track. I have been caught out before. [Where a voltage can be measured along any point of a track that does not mean it is actually at each end of track.. So the quick service method is to assume properly conducting. The second time around (double checking everything because it doesn't make sense) , requires confirming same voltage at both source and destination components. Sometimes the layout is really crowded and it's not your fault for taking a short cut :) Sorry if I've rabbited on. I'll leave it here as enough and will be very interested in the final result. Good luck. |
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Hi again Ripley,
Had another thought. Suggest current probe in line from +15v unreg supply going to pin 6 (or through wire to collector) just to prove Q1318 it is being hard switched on. A reason for not much pulse could be excessive load. Your comment re marked difference in resistance between good and bad 475's measured at pin 6, to tap going to C1320 is therefore most significant. If not already done, disconnect various loads on T1320 secondary to see if transistor starts pulsing correctly Steph |
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Hello Steph,
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Yes sir, I disconnected the load to T1320, no change. With the load to T1320 disconnected I then disconnected all the connections to the CRT and fired up the scope again. Still no change to the waveforms on the primary side of T1320. Ripley The words are mine but this iPad does what it will with them. On Mar 2, 2019, at 11:03 PM, Steph L <stephlan@...> wrote: |
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I missed identified C1304. Instead it was C1468 with a broken lead that I pulled from the circuit board. A perfect example of a mistake made when one works on a problem to long without stepping away for a while.
C1436 on the -8 volt rail was replaced with a 33uf capacitor. Reed, I checked C1304 and it is fine, measures 2.7uf on the dot. CR1304 was replaced with a IN4007 per your recommendation and I tried your test to the base of Q1318. The first time I taped the base of Q1318 with a 1K resistor in series with the +5 volt rail the waveform at TP1318 more than doubled in amplitude from 760mv p-p to 2.2 volts p-p and has stayed there. The frequency however, is now down to 14.4Khz Now when I touch the lead of the 1K resistor from the +5 volt rail to the base of Q1318 the Oscillator waveform at TP1318 jumps up to 7.2 volts p-p still at 14.4 Khz. When I take the 1K resistor away the waveform drops back to 2.2 volts. BREAKING NEWS: While typing this report up I turned back to the scope, taped the 1K resistor from the +5 volt rail to the base of Q1318 again. This time the waveform at TP1318 jumped up to 30.8 volts p-p and now I have a trace on the CRT. Take the 1K resistor away and the waveform at TP1318 keeps running at 30.8 volts p-p and there is still a trace on the CRT. Note: The trace is only covering about a two-thirds of the screen and the horizontal position is behaving strange. Instead of sliding the whole trace left or right. It erases part of trace moving towards the middle of the screen if the Horizontal position is turn left or right. I suspect that is because the HV oscillator is only running at 13.8Khz. I shut the scope off then turn it back on. The waveform at TP1318 is down to 2.2 volts p-p. Tap the base of Q1318 with the lead from the 1k resistor going to +5 volt rail and the oscillator waveform is goosed to 30.8 volts p-p again. The trace on the CRT is present as before and stays there until I shut the scope off and turn it back on. How weird is that! So now what? Is Q1318 flaky? I kind of doubt that. C1316 was replaced with a new 1uf tantalum capacitor. C1317 was pulled and checked. The LCR meter says it’s a 10uf capacitor. I don’t remember what the ESR was but I don’t remember it raising an eyebrow with me. CR1317 was check with the diode function on my HP 3466 DMM but it could be leaky…maybe? What the Heck? Ripley Sent from Mail for Windows 10 From: Steph L Sent: Saturday, March 2, 2019 5:43 PM To: [email protected] Subject: Re: [TekScopes] 475 with no Display and no HV Page 3-35 of manual has a good functional description including point that ambient current supplied to base (of Q1318) determines frequency (ie not LC arrangement) and as such there isn’t a specific cap component for resonate frequency (a function oscillator still relies upon however to function). You said “The rest of the voltage measurement compared to the manual are no where near correct” so perhaps a closer look at the whole setup noting Q1308/10 push-pull, gain and phase reversal Q1306 and bias arrangement behind Q1310 (ie the Q1312 and associated components). I have an old Tek544 where the HT falls dramatically when ambient room temp too hot and was amazed in the frequency shift involved. It is a different circuit but 475 still has resonance involved and the output voltage is regulated by driving current harder into the base. Just a thought. Please excuse me/any error as nubie here. Interesting fault. |
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