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Dead 7603
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On Mon, Jun 25, 2018 at 09:38 pm, lop pol wrote:
When dag was no longer ground in TVs we used a HV probe, usually 1090 meg which with the meter made 1100 meg. But the capacitance, (Leyden jar) is to ground, not the cathode. With it OFF you can just arc it. It is a problem when you do it when on. When you short the ON "top" (+) HV to ground the "bottom" of that transformer winding shifts all its voltage to the cathode circuit. It has a quadrupler, that means the 3 KV on the cathode end is simply rectified, but quadrupled into the + so0urce for the ultor. If the + shorts that will cause double the voltage to the cathode circuit. That is just about the right amount to blow a bunch of shit before the rectifier or filter shorts out. I am not looking right now but many Teks have a shunt regulator tube for the cathode end, if you burn that out, where do you get one ? I have worked on some damn expensive equipment and have learned to be careful. Had some sort of car diagnostic machine once, them people will work on anything, or actually have me do it. no red. the other guy, normally competent, decided to rejuv the CRT. I said no. they did it anyway and still no red. I had to go to the video subsystem of a computer and find a bad gate IC. then we got red, but not the formally pristine cathode has now been stripped so guess which gun is going to go first. I INVENTED the dim bulb test., I do not claim to have invented it first, but when I figured it out I had never heard of one. It was house wiring, a fuse kept blowing. I screwed a light bulb into the fuse socket and it lit and I started disconnecting things until it went out. Non destructive testing. You don't want to know my age but, I hadn't yet gotten my first hardon. Point is, destructive testing is only for when it is absolutely necessary. |
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On Mon, Jun 25, 2018 at 10:37 pm, Craig Sawyers wrote:
Sorry, I didn't take that as meaning the lead from the CRT, I thought you meant the lead out of the multiplier. Doing that after it is powered up and down, if it has an arc you have cleared the CRT and the multiplier, but don't yet know which one is bad if it doesn't. So you didn't say to turn it on and arc the anode lead (output of the multiplier) to ground. This appears to be my mistake. You probably agree that it is not a good idea to arc the source while running to ground, but you never said that it was. I'm sorry, it's my mistake. |
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On Tue, Jun 26, 2018 at 11:02 pm, Jeff Urban wrote:
I have read here that discharging the CRT to the frame on the 485 has been know to kill U660 so that is one reason I would like to learn of another way to do it. |
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Craig Sawyers
I have read here that discharging the CRT to the frame on the 485 has been know to kill U660 so thatAAAAGHHHH! Switch scope off. You unplug the darned connector first. Said it three times already!!!!!!! There is absolutely no way on God's green earth you are going to blow anything up. Craig. |
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On Wed, Jun 27, 2018 at 05:30 am, Craig Sawyers wrote:
Ok. I read a post on 485 troubleshooting recently that suggested to wait at least 6 hours to let the CRT charge fall off before pulling the lead to prevent killing that IC. It is more than possible that I misunderstood.I have read here that discharging the CRT to the frame on the 485 has beenknow to kill U660 so thatisAAAAGHHHH! Switch scope off. You unplug the darned connector first. Said it |
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Craig Sawyers
Ok. I read a post on 485 troubleshooting recently that suggested to wait at least 6 hours to let theU660 is the vertical output amplifier in a 485. You must mean U1600, which is the x6 voltage multiplier. Can't think of a mechanism by which unplugging the CRT could zap it. Internally it is protected by a series resistor in the tens of megohms range so that if the output is totally shorted out, the current is sufficiently low to not damage the internal components. But let's do some what-if sums. I'm not sure what the capacitance of the DAG coating is, but let's assume 100pF. Accelerating voltage is 18kV. Let's say we wait those 6 hours before we unplug the HT connector - what sort of leakage resistance will we need to reduce the voltage? 6 hours is ~20,000 seconds. Let's say that is ten time constants, so the voltage will decay to ~100V and let's say, for argument's sake, that is low enough. So one time constant has to be less than 2000 seconds, which is the product of the CRT capacitance and the leakage resistance. Which works out at a leakage resistance has to be less than 2e13 ohms, and an initial discharge current of 0.9 nA. I would say that is entirely possible. Reverse leakage of the multiplier diodes should be more than enough in itself, not counting other leakage paths. So waiting 6 hours should indeed discharge the CRT. But it makes fault finding the restorer circuits a bear - wait 6 hours, try something, turn on scope - oops it wasn't that, wait 6 hours etc etc. And whether or not it is necessary to wait that length is time is anyway moot. The other way to see if the multiplier is faulty is to disconnect the input. The main symptom of a dead multiplier is that it loads down the high voltage transformer, so the -3kV test point reads low - a few hundred volts typically. If disconnecting the multiplier input results in the -3kV coming back up and reading correctly, then the multiplier is dead. Craig |
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On Wed, 6/27/18, Jeff Urban <JURB6006@...> wrote:
Subject: Re: [TekScopes] Dead 7603 To: [email protected] Date: Wednesday, June 27, 2018, 11:24 AM Ten 10 meg resistors in series. The same stack can be used to multiply your voltmeter range. Should use resistors made for high voltage use. Normal Carbon Composition resistor may have ratings of 250-500V each. 10 in series maybe 5KV. Carl Hallberg W9CJH |
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hi all,
The above written description is quite good, and clear. Due to diversity in human nature, sometimes a picture or video supplementing information for a procedure can be helpful to some, including the uninitiated. I wonder if some Tektronix YouTube 'expert' videos might be linked to some discussions ? We all gain with additional clarity, helps prevent accidents, and it seems that added image/video teaching tools have proved themselves over text-only technology. Tekscopes List evolution ? |
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On 06/28/2018 09:07 PM, garp66 wrote:
hi all,Which "above written description" are you talking about?? Unless you quote from the email you are replying to, we have no idea what you are talking about.? Notice how I quoted your email reply?? Now folks know who and what I'm replying to, and have the context of the conversation. Top or bottom replying is a religious discussion for another day. Mark |
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Vintage Test
Hi Jim,
On a 7603 - and I've sorted a lot of them - the main culprits of no power-up, when you've checked all the usual suspects, is the power supply filter capacitors. To check these, you will have to remove the raw voltage power unit, situated at the bottom rear of the 'scope. There are three large cross-head screws on each side of the chassis, which allows you to carefully draw out the mains transformer and attendant rectifier/capacitor PCB. If the capacitors are all metal cased, it is likely that these are faulty now and need replaced. Looking at the rear of the 'scope, there is a rectifier attached to the bottom left corner , underneath the PCB. This is the +/- 15V rectifier and the two caps nearest the left edge are the reservoirs. If these don't have about 24V DC on them, then nothing else will work. Have a look at this and get back to us. -- you can never have enough oscilloscopes, DMMs, valve testers or soldering irons . . . |
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Thanks, everybody, for your help. Here's where I am now: Primary not
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open; it measures about 3 ohms with the on/off switch on (obviously open with it off!). Poked around P1171 and measured DC voltages. -50V, -15V, +15V, +50V, and +130V all within spec. +5V measured 4.46 V, when it's supposed to be 4.85V minimum. Hey, the power light is lit now! Must have been a bad connection that I reconnected when moving the power supply section back into place. Checking ripple and looking for shorted tantalum capacitor(s) on the +5V rail next. Jim ------ Original Message ------
From: "Vintage Test via Groups.Io" <mel.purcell@...> To: [email protected] Sent: 6/29/2018 7:31:36 AM Subject: Re: [TekScopes] Dead 7603 Hi Jim, --- This email has been checked for viruses by AVG. |
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Here's what the ripple voltages measured in volts peak to peak:
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-50V: 20m -15V: 20m +5V: 3 +15V: 20m +50V: 10m +130V: 0.2 So, all out of spec except for +130V, and +5V way out. +5V was a high duty ratio waveform with little dropouts down to about 2V synchronous with the AC mains. +5V rail caps to be checked next. Jim ------ Original Message ------
From: "Jim Ford" <james.ford@...> To: [email protected] Sent: 7/6/2018 4:29:35 PM Subject: Re: [TekScopes] Dead 7603 Thanks, everybody, for your help. Here's where I am now: Primary not |
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Uh, it would be nice if I could find where the caps on the +5V line are located! I don't see anything on the A11 Low Voltage Board schematic.
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I assume the voltages from the LV board are distributed to most of the other boards, correct? Where should I look for failing power supply caps? (Or other suspects?) Thanks. Jim ------ Original Message ------
From: "Jim Ford" <james.ford@...> To: [email protected] Sent: 7/6/2018 4:47:30 PM Subject: Re: [TekScopes] Dead 7603 Here's what the ripple voltages measured in volts peak to peak: |
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OK, here's where I am now: Power light comes on, and all LV DC voltages
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are within tolerance except +5 V, which reads ~+4.4 V. Ripple is out of spec on all except for +130 V. +5 V rail looks like a high duty ratio dipping from 5 V down to about 2 V periodically, synchronous with the AC mains. I figured a tantalum capacitor on the 5 V rail was a nominal short, but I don't see any tantalums anywhere in this scope, a late model 7603, Serial Number B345179. Just ceramic discs and aluminum electrolytics. Any ideas on where to look? I figure the crap on the 5 V rail may be messing up the others - make sense? Thanks. Jim ------ Original Message ------
From: "Al Holt" <grovekid2@...> To: [email protected] Sent: 6/25/2018 4:21:40 AM Subject: Re: [TekScopes] Dead 7603 You've got a dead 7603 with no lights. Definitely look at the low --- This email has been checked for viruses by AVG. |
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tom jobe
Hi Jim,
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This is a long shot.. but I remember some problem capacitors on those little PCBs that are between the female connectors that the plugins plug into on the back plane. If the search function still works for the message archive there should be some discussion of these capacitors on a 7603. tom jobe... On 7/15/2018 4:32 PM, Jim Ford wrote:
OK, here's where I am now: Power light comes on, and all LV DC voltagesT |
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Thanks, Tom.
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I'll poke around between the plug-in connectors. Maybe something will jump out at me as faulty. Jim ------ Original Message ------
From: "tom jobe" <tomjobe@...> To: [email protected] Sent: 7/15/2018 4:43:08 PM Subject: Re: [TekScopes] Dead 7603 Hi Jim, |
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On Sun, 15 Jul 2018 23:32:06 +0000, you wrote:
OK, here's where I am now: Power light comes on, and all LV DC voltagessuspicious Ripple is out ofDING DING DING! Problem here. Possible that one of two things is at fault (maybe both, but not likely). FIRST: suspect the bulk capacitor. SECOND: suspect the bridge rectifier. (On a 5440, had the same thing, as well as a toasted resistor in the 15 volt supply. filter capacitor (10000 uf, 12 vols) read extremely high esr (couldn't measure), and did not tell me what the capacitance was.) Replacing main capacitor, will mention what the result was, bridge tested OK. I figured a tantalum capacitor on the 5 V rail was a nominalCould be an aluminum electrolytic, but high ripple on a supply points you at either the main filter capacitor *or* the rectifier. I'm betting on the capacitor. The ripple may be causing all sorts of problems. Regulators do expect voltages at their inputs that never drop below the output voltage. (most of the ones in TEK equipment being series regulators, effectively). Harvey
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Thanks, Harvey.
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So I checked out the 8 V supply feeding the 5 V supply - it looks really bad! Similar waveform to the 5 V rail but dips from a high of maybe 14 V down to about 2 V. That's at the junction of the cathodes of rectifiers CR820 and C821, C821, R821, and the collectors of Q829 and Q835 on the Rectifier Board. Then I checked at the anodes of CR820 and CR821; more convenient to look across C820. I saw about a 30 Vp-p near-sinewave there, with the anode of CR820 180 degrees out of phase with the anode of CR821. That seemed OK, but the output is just wrong! I tried to measure C821 (18,000 uF) in-circuit with my cheapo Chinese transistor/resistor/diode/capacitor checker, but it couldn't get a reading. Ditto for C820 (0.1 uF, 100 V). I assume the transformer effectively presents a short and throws off the checker. Also, R821, from the cap/diode cathodes/transistor collectors point to GND, reads 5.5k ohms, when it's supposed to be 4.7k +/-10%. I was surprised to see it read high; most times the sneak paths make resistors read low in-circuit. Not that 5.5k probably makes a difference. Anyway, should I suspect the cap C821 or one of the diodes CR820 or CR821? Or something else altogether? Thanks, everybody. Jim ------ Original Message ------
From: "Harvey White" <madyn@...> To: [email protected] Sent: 7/15/2018 6:16:20 PM Subject: Re: [TekScopes] Dead 7603 On Sun, 15 Jul 2018 23:32:06 +0000, you wrote:OK, here's where I am now: Power light comes on, and all LV DC voltagessuspicious |
Mlynch001
Jeff Urban
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