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TEK 468 no power
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I have a Tek 468 scope my son in law bought for me for fathers day last year. It has been working fine until 2 day ago when I pushed the power button and nothing happened. Both fuses are fine I have 121.5 volts on the line side of the IEC connector, the same on both sides of the fuse and J108. No voltage anywhere else. I'm not a scope guy and have never tried to repair one. I would like to know if there is a known common point of failure I should check for first. Looking at the schematic shouldn't I see some voltage on J110 with the power switch closed? Hope this makes sense. I will try to post a picture of circuit.
any thoughts are appreciated Michael |
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Just a couple of thoughts from also 'not really a scope guy' also but 'experienced' all the same...I don't know your experience but be careful with the voltages.
Is the mains voltage, which you measured, to Earth or to Neutral? If the chassis is earthed there may still be a neutral breakdown. . Presuming a diode-rectified supply, have the power supply diodes checked, Sometimes diodes blow on shut-down spikes. The rest I'll leave to those experienced contributors. My regards -- Jack |
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The measurements were taken neutral to line . I mostly work on tube amplifiers so I have a healthy respect for high voltage. I also use a isolation transformer for anything I am testing. Haven't checked the diodes yet as there seems to be no voltage getting to the primary of the transformer.
Thanks for the reply, Michael |
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Step one - the switch
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Is it getting there and is it getting out? On 16 Nov 2018, at 19:48, Michael Wyatt via Groups.Io <microwyatt@...> wrote: |
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Bob Albert
Step 2 - the fuse.? Step 3 - the power cord.? Step 3 - the line voltage selector.
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You should measure contnuity across the prongs of the power plug, maybe 10 - 15 Ohms. Bob On Friday, November 16, 2018, 11:56:43 AM PST, Robin_Birch <robinb@...> wrote:
Step one - the switch Is it getting there and is it getting out? On 16 Nov 2018, at 19:48, Michael Wyatt via Groups.Io <microwyatt@...> wrote: |
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Hi..ok and ok....then that should be a relatively straight forward chase(?), same as with a valve-amp....no voltage at transformer = break. Are their any 'conditioner' inductors on the line side of the transformer?
Is the main switch also switching .the neutral? Is the neutral actually only a 'phantom' A broken neutral can show continuity but fail under load. Check active and neutral at the outlet....I realise this sounds 'derr' but it can happen that the neutral is supplanted by a long path to earth. In the two cases I have experienced it over 50 years, it was a loose neutral connection at the incoming mains and a loose wire at the MEN link ....however if the rest of the house is all ok...it could be the GPO (outlet) a check is only 30 seconds.for a certainty and a firm starting point . I think another chap sent you circuits. -- Jack |
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On Fri, Nov 16, 2018 at 2:22 PM Michael Wyatt via Groups.Io <microwyatt=
[email protected]> wrote: Posted pics of the circuitAlbum for posterity: /g/TekScopes/album?id=78665. |
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tom jobe
Just follow the AC in and check for it at each obvious point, if it gets to the main transformer, follow it on the secondary side to the full wave bridges, and so on.
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This should be a simple fix... tom jobe... On 11/16/2018 11:06 AM, Michael Wyatt via Groups.Io wrote:
I have a Tek 468 scope my son in law bought for me for fathers day last year. It has been working fine until 2 day ago when I pushed the power button and nothing happened. Both fuses are fine I have 121.5 volts on the line side of the IEC connector, the same on both sides of the fuse and J108. No voltage anywhere else. I'm not a scope guy and have never tried to repair one. I would like to know if there is a known common point of failure I should check for first. Looking at the schematic shouldn't I see some voltage on J110 with the power switch closed? Hope this makes sense. I will try to post a picture of circuit. |
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Just an update on what I have found so far.
1.The main switch was burnt. 2. Electrolytic cap C603 3uf 150v shorted 3. 10k resistor R328 in crt HV section burnt. I checked all components I could be lifting one leg from the board. Everything checks, but without power it still just guessing. Does this sound like a shorted transformer? Michael Sorry for the slow response I've had recent leg surgery and have been off my feet for a few weeks. |
John Sykes
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