Announcement

Collapse

Forum Rules and Etiquette

Our mission ...

This forum is part of our mission to promote the preservation of vintage computers through education and outreach. (In real life we also run events and have a museum.) We encourage you to join us, participate, share your knowledge, and enjoy.

This forum has been around in this format for over 15 years. These rules and guidelines help us maintain a healthy and active community, and we moderate the forum to keep things on track. Please familiarize yourself with these rules and guidelines.


Rule 1: Remain civil and respectful

There are several hundred people who actively participate here. People come from all different backgrounds and will have different ways of seeing things. You will not agree with everything you read here. Back-and-forth discussions are fine but do not cross the line into rude or disrespectful behavior.

Conduct yourself as you would at any other place where people come together in person to discuss their hobby. If you wouldn't say something to somebody in person, then you probably should not be writing it here.

This should be obvious but, just in case: profanity, threats, slurs against any group (sexual, racial, gender, etc.) will not be tolerated.


Rule 2: Stay close to the original topic being discussed
  • If you are starting a new thread choose a reasonable sub-forum to start your thread. (If you choose incorrectly don't worry, we can fix that.)
  • If you are responding to a thread, stay on topic - the original poster was trying to achieve something. You can always start a new thread instead of potentially "hijacking" an existing thread.



Rule 3: Contribute something meaningful

To put things in engineering terms, we value a high signal to noise ratio. Coming here should not be a waste of time.
  • This is not a chat room. If you are taking less than 30 seconds to make a post then you are probably doing something wrong. A post should be on topic, clear, and contribute something meaningful to the discussion. If people read your posts and feel that their time as been wasted, they will stop reading your posts. Worse yet, they will stop visiting and we'll lose their experience and contributions.
  • Do not bump threads.
  • Do not "necro-post" unless you are following up to a specific person on a specific thread. And even then, that person may have moved on. Just start a new thread for your related topic.
  • Use the Private Message system for posts that are targeted at a specific person.


Rule 4: "PM Sent!" messages (or, how to use the Private Message system)

This forum has a private message feature that we want people to use for messages that are not of general interest to other members.

In short, if you are going to reply to a thread and that reply is targeted to a specific individual and not of interest to anybody else (either now or in the future) then send a private message instead.

Here are some obvious examples of when you should not reply to a thread and use the PM system instead:
  • "PM Sent!": Do not tell the rest of us that you sent a PM ... the forum software will tell the other person that they have a PM waiting.
  • "How much is shipping to ....": This is a very specific and directed question that is not of interest to anybody else.


Why do we have this policy? Sending a "PM Sent!" type message basically wastes everybody else's time by making them having to scroll past a post in a thread that looks to be updated, when the update is not meaningful. And the person you are sending the PM to will be notified by the forum software that they have a message waiting for them. Look up at the top near the right edge where it says 'Notifications' ... if you have a PM waiting, it will tell you there.

Rule 5: Copyright and other legal issues

We are here to discuss vintage computing, so discussing software, books, and other intellectual property that is on-topic is fine. We don't want people using these forums to discuss or enable copyright violations or other things that are against the law; whether you agree with the law or not is irrelevant. Do not use our resources for something that is legally or morally questionable.

Our discussions here generally fall under "fair use." Telling people how to pirate a software title is an example of something that is not allowable here.


Reporting problematic posts

If you see spam, a wildly off-topic post, or something abusive or illegal please report the thread by clicking on the "Report Post" icon. (It looks like an exclamation point in a triangle and it is available under every post.) This send a notification to all of the moderators, so somebody will see it and deal with it.

If you are unsure you may consider sending a private message to a moderator instead.


New user moderation

New users are directly moderated so that we can weed spammers out early. This means that for your first 10 posts you will have some delay before they are seen. We understand this can be disruptive to the flow of conversation and we try to keep up with our new user moderation duties to avoid undue inconvenience. Please do not make duplicate posts, extra posts to bump your post count, or ask the moderators to expedite this process; 10 moderated posts will go by quickly.

New users also have a smaller personal message inbox limit and are rate limited when sending PMs to other users.


Other suggestions
  • Use Google, books, or other definitive sources. There is a lot of information out there.
  • Don't make people guess at what you are trying to say; we are not mind readers. Be clear and concise.
  • Spelling and grammar are not rated, but they do make a post easier to read.
See more
See less

IBM 5154 Problems & Electrolytic Caps Question

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    #31
    Are the power supply output voltages stable ? Minor voltage fluctuations may not show up on a modern digital multimeter.....
    but would show up on an older analog meter (Simpson/Triplett) or an oscilloscope.

    Comment


      #32
      I'm noticing more weird things. First, the text sometimes starts moving in a wavy motion as shown here. I also looked up the part number on the green "25000V" part of the flyback transformer and found it on eBay here, is this exactly what I need to replace? Or does the "8000V" part need to be replaced too? Finally, I noticed that the pins on the board for the "8000V" transformer also have pins in a circular arrangement (like those be used by most flyback transformers) connected to the pins in the square arrangement. Could a more modern, easily obtainable flyback transformer be connected to these?
      IMG_0985.jpgIMG_0990.jpg

      Comment


        #33
        Great idea with the AM radio Chuck(G).

        Well now at least you know what you are seeing is changes in the geometry of the raster due to an electrical discharge altering the EHT, so where is it?

        Wherever it is, it is likely drawing current (intermittently) from the output of the EHT transformer secondary. So this could be anything in the HV tripler, focus control and its wiring to the CRT socket assembly and the CRT's anode cap (externally on the CRT) along the surface from the anode connection to the external aquadag. This can also happen if the CRT's external aquadag is not earthed properly or connected to the correct place. So which is more likely ? Probably the discharge (arc) is happening in air, not in a potted medium and likely after the EHT cable exits the tripler.

        Since the tripler assembly is potted as is the EHT transformer, they are probably less likely to be the culprits (could always be proved wrong).

        I would leave the set off for a while . Put a thin grounded screwdriver blade under the EHT cap, discharge the CRT. Fold back the anode cap a little, reach under with thin long nosed pliers, squeeze the anode clip pronge together to release the anode cap from the CRT bulb. Clean the surface of the CRT all around the anode connection, clean the cap (use CO contact cleaner or similar, nothing ionic like windex). Then coat the inside of the cap and CRT bulb around the anode connection with clear silicone grease and re-fit the cap.

        That will then help to exclude the anode cap/connection area.

        Check the wiring leading to and from the focus control and leading to the CRT's base clean the pcb around the focus connection and check for anything that looks suspicious. However, since the focus is staying good with the fault, likely its somewhere from the EHT output from the tripler, to the CRT anode.

        Check places where the EHT wiring gets anywhere near a metal object on the way to the CRT's anode.

        If everything else gets excluded, it would more likely be the tripler assembly than the EHT transformer.

        (I would still double check the main power supply voltages are stable as a similar effect could occur with unstable voltage to the scan stages if there was some intermittent arcing in the main power supply , but on balance it looks like the EHT system is the likely problem).
        Last edited by Hugo Holden; July 27, 2019, 04:14 PM.

        Comment


          #34
          I finally worked up the courage to check the voltages on the PSU. It looks like all of the lines work (-6.1V for -6.3V, 11.9V for 12V, 20.16V for 21.2V, 55.59 for 56.7V, and 150.3V for 154V). They all seem to be pretty stable. However, two pins also seem to be giving random voltages (one has 2-7V, the other has 15-25V). IMG_0999-2.jpg. The longer I measure them with the multimeter, the more they will decrease. I've also noticed now that the screen will now turn completely green again. The raster is green, and everything has a green tint. The green tint will turn on and off at seemingly random times. You said earlier that no capacitor deals individually with the red and blue colors, so what would cause this?
          Attached Files

          Comment


            #35
            Originally posted by willmurray461 View Post
            I've also noticed now that the screen will now turn completely green again. The raster is green, and everything has a green tint. The green tint will turn on and off at seemingly random times. You said earlier that no capacitor deals individually with the red and blue colors, so what would cause this?
            There will be a DC offset appearing at the cathode of the green CRT's gun (causing the cathode voltage to drop and the green beam to turn on) Less likely a simultaneous fault in the R & B channels increasing the cathode voltages on both there.

            The green channel amplifier is a direct coupled affair, right back to the Green input circuitry. It could be a poor solder connection to any component in the entire green channel, possibly a defective resistor transistor. The thing to do is to monitor voltages at various points along the green channel amplifier and try to find where the problem is.

            (there is a small chance of an intermittent heater to cathode short in the actual CRT that could lower the cathode voltage turning on the green gun, fortunately that is less likely than a problem in the green amplifier chain).

            Comment


              #36
              Could you give me the names of the parts which I would need to check, or perhaps where they are on the board?

              Comment


                #37
                I will look at the schematic when I am home.

                One thing you could do is disconnect the connection to the Green gun cathode of the CRT (at the video amplifier outputs), connect it onto one of the other cathode connections (either the R or B channel video amplifier outputs/ cathode connections) the colors will then be wrong because two of the guns will then be getting the same drive, but if the intermittent green changing effect goes away, you can then be definitely sure that it was an offset voltage coming out of the green channel amplifier, and not a green CRT gun heater cathode defect in the CRT or a bad CRT socket connection to the green gun's cathode. If you don't really get what I am saying here, and are confident to do it, don't attempt it yet.

                Comment


                  #38
                  Looking at the schematic, T707 is the green video output transistor, leading to itfrom the input connections, buffer IC 501 handles the green signals (MSB & LSB) these pass via buffer IC 502, buffer IC 503 output pins 6, 8 to T702 & T703 transistor to mix the LSB & MSB green signal into the base circuit of T706, green driver transistor, via T705 to the base of T707 the output transistor. So you would need to measure the outputs (preferably with a scope) of each of these stages to find where the signal has intermittent change. The hottest running parts of this circuit are the output transistor and its emitter load resistor R716, check the solder joins.

                  Possibly though it could be the pots. Mark the position of the adjustment potentiometers RT702 (green lsb) RT701 (green msb) and TR703 (green cutoff) and give them a little wiggle back and forth. Sometimes an intermittent connection of the pot wiper to the track can cause the problem you are seeing, just for example if RT703 went momentarily open circuit, then the base voltage of T706 would shoot up and its collector voltage drop. This would pass via T705 to T707, causing the emitter voltage to drop and the green gun to turn on.

                  Comment

                  Working...
                  X