CRT help, Telequipment D43 Oscilloscope. - Page 1
Forum section: electronics / repair

author: Tibs79
author_rank: Newbie
timestamp: July 11, 2023, 04:19:00 pm
content: Hello,10 days ago I started using a Telequipment D43 (1968) to troubleshoot an audio signal (see picture). Shortly after I took the picture, the trace disappeared. I went back through the manual for initial setup, etc. Currently the test or "cal" trace is only showing at the outer edges of the CRT, and only showing half of the wave: previously it would show a square wave, and now it looks like the ascending half of a sawtooth, with no descent.The first thing I did was a visual inspection, of course. The next day things smelled strange, which turned out to be one of the caps on the 3kv anode supply, part c403 (Fig 5.2, though labelled C416 on the schematic. Values of c403 and c406 from the component list coincide with the labelling on the components, and the schematic). I replaced this with the same value capacitance, 3kv instead of 2kv rating, ceramic. This made no difference.I have tested every canned capacitor in the power supply and trigger circuit, they are all within spec. I have cleaned all contacts with Deoxit D5, and used some mechanical persuasion on some that were not convincing. I have pulled all the tubes, cleaned them and cleaned their contacts. Nothing has changed. On Fig 4.1, the test voltages near the upside-down smiley fluorescent bulb were about 25v and 87v, which seems reasonable. One thing I have not checked is the Anode voltage, as I don't have a meter that can take 3kv.The scope had one Type A input amplifier in the upper bay, and a Type B in the lower, I was/am not getting any signal from the lower deck. I tried swapping the amps around, and tried inserting a spare Type A in the lower bay, concluding that I am not getting any trace from the lower bay, regardless of the toggle switch position. The two bays connect to the CRT directly with two pins each, continuity seems good.So, had the CRT not worked minutes before failing, I would assume that the CRT itself was at fault. Since the CRT was working well, I am guessing that the main fault is somewhere else in this lovely piece of equipment that was built with a massive amount of pride, and with clear intent for it to be serviced.Schematics and manual are herehttps://josepheoff.github.io/assets/2020-04-03-d43hv/d43.pdf

author: George Edmonds
author_rank: Frequent Contributor
timestamp: July 11, 2023, 04:49:14 pm
content: HiWhat a blast from the past. I have not used or repaired a D43 for over forty years.I hope that you realise that a D43 is almost unique in that it uses effectively two CRT’s in one envelope that share a common screen.Telequipment had a bad habit of using resistors around the CRT that changed value in time and with applied voltage , usually upwards or just simply going open circuit.G Edmonds

author: Tibs79
author_rank: Newbie
timestamp: July 11, 2023, 05:34:37 pm
content: Hello G,I did not realise that the CRT is unique in that regard, however this does explain why referencing other CRTs is confusing me.I am fascinated by how the case is designed to be serviced on a workbench, and how easy it is to access most components. Oh, and how beautiful the wiring is. Quite something when compared to a modern thing like a computer.I have not encountered resistors that change their values over time, oxidation within the ceramic? I shall look at resistor values, I am guessing I should start with the ones on Fig 4.1 as they are grouped together on the schematic around the CRT as CCT. Some of these components are on the PC4B board. The schamatic refers to it as PC4A, but the parts do seem to be the same and I can confirm by colour code.I like that PC4B has the neon beacon which appears to indicate the remaining charge in the power supply caps. I haven't taken the time to identify how the shunting works, but the whole circuit seems to be "safe" within about 20 seconds.Thank you for the input,T

author: George Edmonds
author_rank: Frequent Contributor
timestamp: July 12, 2023, 06:58:17 am
content: HiI have reread your original posting a number of times and can find no real clues as to what is happening. The upside-down smiley fluorescent bulb is actually a Neon voltage stabiliser with a nominal stabilisation voltage of 85V, so your measurement of 87V is indeed reasonable.Before I go on I must warn you that the D43 scope contains many voltages that could be lethal to you and that in the mid 1960’s when this scope was designed electrical safety standards were not what they are today with precautions to protect the safety of the person working on the equipment being almost none existent.When making any voltage measurement you must also into account the loading of the measuring device, as was normal in the 1960’s all quoted measurements were made with a 20K per Volt meter (AVOwhereas modern DVM’s normally have a constant input resistance of 10M Ohms, so at times mental adjustments have to be made to any measured voltages.Having earned my living for some years using Telequipment scopes carried around in a car boot I know that Valve/Tube failures were common, frequently I had to repair the scope before I could repair the equipment that I had been sent to repair.Most of the resistors used in a D43 are Carbon composition type ones, in time they all normally drift high in value, especially ones over say 100K due to long term chemical changes within the Carbon composition material used in making them. An even nastier characteristic is the they change value with applied voltage, this frequently shows itself with a scope following power up the trace slowly disappears over time as the voltages applied to the CRT slowly change, this can take up to half and hour to fully occur.Please keep in mind that your D43 is now some 55 years old and will need some serious TLC to bring it back to life. Although I have mainly concentrated on the Carbon resistors you will probably find that most if not all of the electrolytic capacitors need replacement along with some of the Valves/Tubes (for any American reading this)G Edmonds

author: Tibs79
author_rank: Newbie
timestamp: July 26, 2023, 04:57:03 pm
content: Hello George,I have replaced a few erratic resistors, without making much change to the results.Today I had a look at the board where I started troubleshooting, and found that I am unable to get any kind of reading from either of the K8/50 Selenium rectifiers: I took them out, and they give no reading with a multimeter on the diode setting, and actually seem to have failed open, according to the ohmmeter.This makes me question whether any of that doubler circuit was doing anything at all, and whether having no voltage at the anode would give the symptoms I have been chasing.I built two replacement rectifiers using 4x 1n4004 diodes in series, with 5x 2w 1kR resistors to avoid surges. Now, however, I can hear arcing, I can't see it. Generally I feel this is not good. I am afraid that it may be arcing on the board I used to make the parallel capacitor. I shall investigate a way to test high voltage boards for arcing, or just try to distance everything more than veroboard offers without modification (I had it powered for a fraction of a second).T

author: George Edmonds
author_rank: Frequent Contributor
timestamp: July 26, 2023, 06:46:21 pm
content: Hi TFirst may I say STOP STOP STOP, you stand a very real chance of either destroying parts of the scope, including the mains transformer, or possibly killing your self if you carry on this way.You CANNOT test K8/50 Selenium rectifiers with a modern DVM, its output voltage is by far too low to provide conduction.Selenium HV rectifiers such as the K8/50 consist of multiple diodes in series and they are also very inefficient compared to Silicon diodes. The K8/50 has a PIV of 3.4KV so you would need at least nine 4004’s in series as they have a max PIV of 400V, BUT much more importantly the output DC Voltage would be much higher, to a damaging extent, due to the lower turn on resistance of the 1N4004 hence the arcing.The best way of finding faulty K8/50’s is a Mk1 nose, once you have smelled one you will NEVER forget the smell.Regrettably you need someone of a certain (old) age who has a lot of experience with Valve/Tube circuits to repair a Teleqipment D43.All the best and stay alive.G Edmonds

author: Tibs79
author_rank: Newbie
timestamp: July 27, 2023, 05:58:19 pm
content: Hello G,I don't mean to give you "the fear". I believe I feel comfortable working around the CRT because most of my past experience repairing old electronics was with photographic flash systems. So my fear level is drastically reduced by the fact that the high voltage doesn't have a huge reservoir behind it and significant amperage. To be perfectly honest I tread delicately around the flash from a disposable camera, even though it is only charged by an AAA cell.That aside, I was hoping that I had a suitable meter for the old diodes, dug out a TSE4341, but alas it does old transistors, but not diodes. I haven't powered that thing up in a while.So, I removed some lines of trace on the veroboard, and cut out space where I needed more, to make sure I had a gap of over 1mm. I also added a couple more diodes, so each replacement now has 6, which should be more than sufficient. I also increased the gap in the capacitor, and increased the thickness of the connecting wire. The arcing has ceased, and the CRT is illuminated much more evenly again, which I am pleased about.Curiously the Calibration on the Amplifier A now just puts a vertical line on the display. I connected a probe and held it in my hand for a good 50Hz stimulus, which is now displayed vertically. I may set the machine aside for a while, as I have a couple of other things I wish to achieve over the next short while.Incidentally, the notion of replacing the diodes with series 1N4004 came from another couple of threads I found when looking details on the k8/50https://electronics.stackexchange.com/questions/248547/question-about-diode-strings-in-high-voltage-applicationshttps://www.vintage-radio.net/forum/showthread.php?t=34738https://www.vintage-radio.net/forum/showthread.php?t=143739Thank you for your help and input,T

author: George Edmonds
author_rank: Frequent Contributor
timestamp: July 28, 2023, 04:38:07 am
content: Hi TYour first paragraph is exactly why I am concerned, as you clearly do not understand the multiple personal risks and dangers of working on this type of scope power supply. The 3.2kVdc EHT supply is directly derived from a 1.2kVac winding on the mains supplied transformer. Should you come into contact with this low impedance high voltage supply even for a short time, hopefully before a supply fuse blows or a trip operates, can cause you a lethal electrical shock or at a minimum give you severe electrical burns. Don’t forget that the Nation Grid has a Giga Watt capacity to kill you, we are not talking about a couple of Joules from a photographic flash system.I have personally seen one person killed when accidentally coming into contact with a high voltage supply and a number badly injured. I am not trying to frighten you, just trying make you safe and aware of the risks involved when working on high voltage mains derived scope power supplies. Most scopes now use HV/EHT supplies that are not mains derived for safety reasons.Now days using multiple 1N400X diodes in series with balancing resistors is a stupid solution, a buy far simpler and by far safer one is to use a single microwave oven diode. These are in a single in line package and have a PIV figure of around 10kV or more. You probably would need some series resistance added to mitigate the lower forward drop voltage/resistance of them plus it is possible that the series resistance and the microwave diode will fit inside the K8-50 tube. Also be aware that you cannot test a microwave diode with a standard DVM, the forward break over voltage is around 20Vdc. To determine the minimum PIV of the diode required multiply the 1.2kV RMS transformer output voltage by 2.828, this should be 3.39kV, the K8-50 is actually a 3.4kV deviceI have some NOS K8-50’s and will in the next few days measure the forward break over voltage of them, if they still work after nearly fifty years. To do an equivalent DVM diode test on them you will need say three or more PP3 batteries and a 10K resistor plus a DVM. Connect the PP3 batteries and the 10K resistor in series. Then add the K8-50 in series to complete the circuit, measure the voltage across the K8-50 with the DVM and note it, then do the same with the K8-50 polarity reversed and note the voltage on the DVM. One voltage should be the same or close to the total battery voltage, the other should be the lower K8-50 forward break over voltage. The 10K resistor is required to limit the forward current through the K8-50 to its maximum limit of 5mA.Two other points.The CRT uses a P31 phosphor, this can easily be permanently burnt by a very bright trace.Given the age of this scope you will probably have to replace all of the power supply electrolytic capacitors. When the power supplies and the CRT supplies are good I will talk you through checking the vertical amplifiersG Edmonds

author: Tibs79
author_rank: Newbie
timestamp: July 28, 2023, 10:38:54 am
content: Hi G,The flashes I was working with were between 200 and 800 joules, designed to deliver that within ~100-125ms. They took a while to "make safe" after a charge cycle. I would personally not recommend that people work on studio flash equipment.Since this thread will possibly help someone else in the future, are there any precautions that you would recommend apart from keeping the power supply cover on (when possible), not probing the unit when powered, and making sure to not touch any circuit until you are sure it is discharged (by checking voltage across the PSU and HV caps)? As I stated before, there is some safety measure somewhere in the power supply which discharges the whole circuit quite quickly, but we both know that capacitors and transformers take some time before ceasing to be dangerous.The use of microwave diodes does make sense, though there is also some validity to testing things the way I have, with on-hand parts, which has proved that the CRT itself is in good order: continuing to work on the unit is viable. I gather that loosing one capacitor and the two diodes meant that the anode was just getting 1.2kV, which possibly explains its dimness. If you do end up being able to test a good K8/50 then I can plan to revisit the D43 at a later date. For now I have put the covers back on it, and tried to make sure that I will understand my notes.All of the power supply capacitors do measure as balanced and within tolerance, but I do understand that one normally has to consider them all to be scrap at this point.I have read of some older amateur radio equipment which didn't use a transformer to drive the heaters in the tubes (in a bid to save money) which--in the USA--dubbed them widowmakers as their plugs could be inserted either way round, with a good chance of putting live mains on the case. Modern electronics are certainly safer, in general, but we are also surrounded by so many more devices.Thank you very much,T

author: George Edmonds
author_rank: Frequent Contributor
timestamp: July 29, 2023, 04:20:16 am
content: Hi TMy first design and commissioning job as a young man was for a custom capacitor discharge welder, it involved charging a one Farad capacitor (actually a room full of smaller capacitors) to 2.5kV and then discharging them in sub 2ms. To save you having to make the calculation the stored emery was 3,125,000 joules.I have now had chance to check the two NOS k8-50’s that I have. Using a bench power supply that was current limited by a 10K resistor in series I found that to start them conducting in a forward direction one took 29.19VDC and the other one took 29.29VDC. To get a current of 1mA passing took a voltage of 35.6VDC for the first and a voltage of 36.6VDC for the second.To do a conventional diode test you would need a meter, be it analogue or digital, with an open circuit test voltage of at least 40VDC. I know of no such a multi meter except for perhaps a low voltage 250VDC insulation Megger (not common). The good thing about microwave diodes is that they consist of multiple diode junctions in series on a common substrate so do not require balancing resistors, they are also easily available.Apart from working with one hand in a pocket and not handling meter probes I can offer no further advice. I have to keep reminding myself that some of the bench kit I have collected over the years most do not possess, for instance I have a 5KVDV variable voltage low current (20mA max) bench power supply, so temporarily providing the 3.2KVDC to test the CRT is a simple matter for me.You must also appreciate that in the early 1960’s when the D43 was designed the skill base and safety regime of the electronic industry was totally different to that existing today. By and large most of the technical work force was either ex military or ex national service where transistors were virtually unknown, but working with very dangerous high voltages on HF transmitters and radar kit was a normal everyday event for which they would have been fully trained.The type of American radio equipment you refer to was also very common in the UK, but in a domestic environment. It was know as dropper cord radios. The mains cable, which was normally cotton covered contained three cores, two for the mains supply and one which was a spirally wound restive core to act as a dropper resistor for the valve heaters. The HT supply was derived by rectifying the mains with the result that the metal chassis was directly connected to the mains, not so bad if the chassis was connected to the neutral, lethal if not. Prior to say 1960 most houses had a twin fuse box, an allegedly 5A one for lighting which had no earth circuit and a 15A one for appliances, normally one 15A round pin socket in the entire property, frequently in the kitchen.I have early memories of my grandmother ironing clothes using a electric iron powered by a tree of bayonet two pin bulb adaptors in the overhead light bulb holder, at the same time the radio would also be powered the same way and from the same light fitting. As all of this lighting circuit was not polarised the radio chassis could easily be at high voltage with only, at best, a flimsy cardboard cover on the rear of the radio for protection, also keep in mind that most of these radios required an external aerial, which could also be live at over mains voltage.I do hope that when you have time you do complete the rework of your D43, there cannot be many of them left by now. If you do use either silicon diodes or microwave diodes you will have to take steps to bring the EHT back down to 3.2kV measured with a 20k Ohm per volt meter or risk causing further damage to the scope and possibly to the CRT.All the bestG Edmonds

author: dietert1
author_rank: Super Contributor
timestamp: July 29, 2023, 05:53:56 am
content: It's plausible that the bad HV cap took the rectifier. Checking the CRT anode voltage after the repair requires a special HV probe. Sometimes one can find a voltage divider inside the scope. Without that it will remain guesswork, though: If the trace is back and the resistor dividers around the CRT including the pots have been checked/fixed, one could just proceed and find the next fault. Multiple faults may indicate a problem with the supplies.I remember burning the cathode heater of a small service scope long time ago. The heater had one transformer turn in the anode HV SMPS and when i disconnected the anode circuit for a test it took one second and the CRT was gone.Regards, Dieter

author: tautech
author_rank: Super Contributor
timestamp: July 29, 2023, 08:30:07 am
content: Quote from: George Edmonds on July 28, 2023, 04:38:07 amWith some experience on CRO's I take issue with your terminology.Typically there are 3 elevated voltages in a CRO, the CRT plate drive of 100-150V and the highest voltage apart from those directly associated with electron beam creation in the CRT.Plate drive is HT.EHT is for 2 main requirements and can range from -1 to -3kV and this negative EHT is connected to the CRT (edit)AnodeCathode and its positive leg to chassis GND and the last CRT grid.Unrectified EHT feeds the multiplier to generate the PDA*, a positive voltage 3x or 5x EHT voltage....for those scopes with PDA as early and low BW CRO's didn't use PDA.When a CRO produces a trace we know EHT and PDA are doing their job and PDA is best left well alone as you correctly mention, it's dangerous and EHT to some extent also however to setup a CRO correctly EHT must be checked to meet the specified voltage.....just don't forget it's a negative voltage so get your leads connected correctly !* PDA = Post Deflection AccelerationQuoteYes you can although shotgunning caps does little to teach us diagnostic technique when we could instead refer to the service manual for ripple specs and check each rail at least meets rail ripple spec.LV and HT rails are easy however checking EHT HV DC restore caps is another matter and it is wise to test them out of circuit or just replace them.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: July 29, 2023, 11:34:14 am
content: Hi AllThis is a case of stupid comments, The service manual, which I have taken the time to read, calls the 3.2kV supply throughout the EHT NOT the PDA. In fact the CRT used in the Telequipment D43 does not use or have PDA, just a final Anode, As to ripple measurements in a 1966 dated service manual, unheard of.Before commenting may I suggest that you RTFM,G Edmonds

author: tautech
author_rank: Super Contributor
timestamp: July 29, 2023, 02:23:32 pm
content: Terminology was later corrected and changed.I had a Telequipment D83 for some years when I purchased, fixed and sold CRO's for a hobby.D83 was from a time when Tektronix owned Telequipment and D83 used the great 7603 CRT.However way back then D83 manuals were near impossible to find so I uploaded mine here:https://www.ebaman.com/repository/ELECTRONICS/Test-Equipment/Telequipment/My D83 was the last to go as it was my first scope and by that time it was very plain DSO's were to be the future.

author: Tibs79
author_rank: Newbie
timestamp: July 29, 2023, 03:26:02 pm
content: G,When I was working with flash units I found it frustrating how difficult it was to find anyone with an understanding of high voltage electronics. Building something of the scale you worked on must have been quite something. Was that used to fuse materials without contact? I recall finding a supply of high capacity caps which came from a unit which I think was used for some kind of PCB manufacture which used flash tubes to fuse materials.A year or so ago I met someone who is a voluntary medical first responder on motor races, and he had just gone through training to assist on a Formula 1 race which, he said, was mostly a technical briefing on which parts of the capacitor banks were lethal if exposed. I don't know if they still use capacitors for the EV boost "attack mode".In defense of the manufacturers of radios etc. in the UK, they were probably "intended" to be used with a 3 prong, which cannot be plugged in with the wrong polarity, though a socket can be wired wrong. The problem must not have been too great, however, since most European plugs can still be inverted, and toasters still don't eject toast, and they still have live wires as heating elements.I have digressed again.Future me will build a voltage divider to measure the HV, and work with a pair of microwave diodes, referencing your measurements and the manual.As for how many D43s exist, there are two of them here, three Type A and one Type B amp. I have not applied power to the other one. I know it has been briefly powered recently, but I may (at a later date), use a rheostat and bring it up slowly in respect of the capacitors.T

author: dietert1
author_rank: Super Contributor
timestamp: July 29, 2023, 04:33:00 pm
content: If the CRT high voltage is from the mains transformer, it probably depends on mains voltage. So the vertical gain of the scope will vary +/- some percent. One way to test whether the high voltage is about right is to test vertical gain. As you have two vertical channels, there is even some redundancy.Microwave oven diodes are cheap but not ideal. Low current high voltage diodes do exist, e.g. 2CL77 (5mA 20kV) or DD1000 (20mA, 10KV).By the way the Tektronix 7844 also had a CRT with two guns. But it's a different beast with distributed vertical deflection plates.Regards, Dieter

author: George Edmonds
author_rank: Frequent Contributor
timestamp: July 30, 2023, 05:22:50 am
content: Hi TThe capacitor discharge welder was used to weld the Copper top hat of a Germanium TO3 power transistor to its Copper base, this had to be done in a very short time to avoid overheating and damaging the Germanium semiconductor structure.As far as I know F1 cars use very fast charge and discharge Lithium batteries to produce short term power boosts, all part of a very complex energy recovery system. Such batteries would be very dangerous, not only from the high voltage charge held but also from the fire risk.A lot of the dropper cord radios were of US manufacture, the problem was compounded by the American colour codes as in the US the black wire was the phase whereas in the UK it was the neutral.I would suggest that if you are going to make a high voltage probe you do it for say an AVO 8 so as to make measurements which will duplicate the Telequipment manual ones. At one time AVO manufactured a x2 range extender for the AVO 8 which already has a 2.5kV or 3kV DC range.I still have a couple of solid state Telequipment scopes, which I no longer use.I think it is now time to let this topic slumber unless you do start work on your D43’s again.All the bestG Edmonds