Tektronix 555 partially missing beams - Page 1
Forum section: electronics / repair

author: sharbinger
author_rank: Newbie
timestamp: September 11, 2019, 12:58:49 am
content: New to me 555. Initial test to see if the seller was correct about it being non functional resulted in a 42 second delay for the thermal relay. The unit clicked on, a brief glimpse of a beam appeared flying off the screen, it clicked off, then clicked back on shortly after. Two wide vertical patches appeared, each a little over 1" tall. The one is stuck in the middle of the graticule horizontally and wiggles under sweep without moving. The second is on the graticule and will sort of sweep when the timebase is used. Changing the plug in units or using no plugins did not solve the issue.I cleaned the power supply with a brush after pulling all tubes. With the indicator disconnected, most are not glowing. With the indicator connected post cleaning the on-off-on of the thermal delay vanished and only V664, V694, V754, V609, V624, and V634 remain unlit.The unit also exhibited a new behavior for a while before the beams shot off the display and it resumed prior operation.I recorded video of it and the effects tweaking the knobs had with the plan being to try to get it to settle on the graticule and have the sweep start. Instead it did not and I attempted to take pictures of the power supply to view for dead tubes.What do I need to look for specifically in this unit? My understanding is one tube pair plays a part in the regulation, but my knowledge of the rest of the functionality of the power supply is lacking. I am waiting on some clip probes to arrive friday so I can break out the dmm and check all voltages.My question is thus a multipart thing: should all tubes be getting heater power with the indicator disconnected?Would it be worth it to clean all sockets with contact cleaner and check for any changes prior to probing around, or better to clean and probe without the indicator?Are there any major known things to look out for on this model? The beam appears to be off the graticule and some secondary emission is bouncing an image of it and some spots onto the display, should I be checking the amplifiers or any part of the signal path right away?Is there anything I am missing?

author: med6753
author_rank: Super Contributor
timestamp: September 11, 2019, 02:24:59 am
content: First things first. If you don't have a copy of the service manual get one. PDF copies should be readily available. Second, you have quite a tiger by the tail. The 555 uses about 100 vacuum tubes and is actually 2 scopes in one. The CRT has 2 distinct beams which can be used together or separately. It also utilizes a separate power supply. And that's where you need to start. If not all the supply voltages are present it's going to be no go. And the 555 also utilizes a very unique filament regulation circuit which uses a very rare tube that if defective may be unobtanium. But we'll assume now it's OK. And if you aren't familiar in working with vacuum tube circuits be aware that you could encounter B+ voltages as high as 500 volts or even more. And that's the LOW voltages. Once you get into the CRT circuit you could be dealing with voltages as high as 10kV. So be extremely careful.

author: Chris56000
author_rank: Frequent Contributor
timestamp: September 12, 2019, 12:40:29 pm
content: Hi!Is the negative -150V regulated H.T. correct? - you MUST get this right before you do any fault-finding on Tek valved 'scopes!There are a few of these 2AS-15 temperature limited sensing-diodes on n.o.s. on ebay at reasonable prices, so don't worry about it too much at the moment!First of all, you need to ensure the +350V unreg. H.T. line feeding the anode and screen of the 6CZ5 Transductor control amplifier V794 is present, and is being passed thro' the safety link between 3 and 4 of the 2AS-15 to the screen-grid-circuit of the 6CZ5 - if the 350V H.T. is missing, check R700/R701 and the diode modules D672, D702, D732 and the changeover contact of relay K601-2.If the 350V H.T. line feeding the 6CZ5 is present, you can use a 1M potentiometer temporarily across pins 2 and 5 of the 2AS-15 socket, (with the diode removed please link 3/4 to maintain the screen-grid h.t. supply) and see if you can control the a.c. voltage applied to the primary of the controlled heater supply transformer T750 - set the potentiometer to give a reading as close to 6.3V a.c. as possible across the valves fed from T750 that are lit - if the lot are out, trace the wiring to and from the primary of T750 and the heater control transductor L790 back to the mains-input connector and on/off switch for open-circuits.If you can get temporary control of the regulated voltage to T750 primary with a temp. 1M pot, you can then replace this with a fixed resistor of suitable value to mantain it at approx. the correct value whilst you concentrate on repairing the rest of the 'scope - the operation won't be adversely affected, and you can always get another 2AS-15 control diode towards the end of the repairs!If only the valves mentioned in your first post are out, and the others fed from T750 are lit, then you have a wiring fault between the 6.3V winding 11-12 and those valves, possibly part of the indicator-cable wiring, which could be o/c or wrongly connected!Try the suggestions here to begin with and let us know how you get on!Chris WilliamsPS!Get your manual from from the Tek w140 Wiki - it's a better quality scan and all the circuit-diagrams are in complete pages and not half-bits!

author: sharbinger
author_rank: Newbie
timestamp: September 12, 2019, 11:31:30 pm
content: Thanks Chris! I do actually have the tekwiki manual, I used it to run my voltage tests.With the indicator connected and just following the probing in fig 7-2 and 7-4 I get the following:+225 Supply: +322VDC+100 Supply: +351.7VDC-150 Supply: +165.9VDC+500 Supply: 629.5VDC+350 Supply: 448.1VDCCal in the indicator unit: .7VACOn the power supply V757, V754 do not light up and V609 has a slow pronounced flicker.V757 seems to be a 12B4 and V774 a 6AU6 while V609 looks to be a 5651. Looking up the latter says it's a voltage regulator tube-should it have a slow flicker or should I consider it suspect and replace it along with 757 and 754 before retaking voltage measurements?The indicator connection cable checks out from continuity, all pins connect through with 9-10 seeming to be wired together internally (which seems to fit with the schematic).I'm operating on assumptions, but I assume my very first course of action is replace suspect tubes (I gave it some time to warm up before measuring voltages/checking tubes with the indicator connected), rechecking voltages and ensuring the -150 is correct before anything further goes?Is there any part I should look out for or be suspicious of that could have friend and taken out something with it, or be causing issues, or is the unit going to be fairly safe if I get the voltages dialed in correctly?

author: sharbinger
author_rank: Newbie
timestamp: September 14, 2019, 06:21:21 pm
content: A short update as I await getting paid so I can replace suspect tubes and get a probe that won't burn up or burn up my meter when I try to check the HV.I followed the factory calibration. All power supply primaries to ground are open (good).I could be measuring wrong, but with the indicator connected I get the following readings:-150 open, should be 1.5K+100 .0521K, should be 2K+225 1.15K, should be 6K+350 open, should be 5K+500 open, should be 15KI checked in the power supply section as the procedure specifies supplies, not T750 as in steps 4 and later.

author: sharbinger
author_rank: Newbie
timestamp: September 28, 2019, 07:23:46 pm
content: Update:The HV probe arrived as well as the clips.T750 was checked (regulated heater supply) to see what would be present.1-4 (the input) measured 80VAC and all secondaries measured at 6.2VAC.By my understanding of the regulator, that should mean the 2AS-15 (V799) and the 6CZ5 (V794) and the saturable reactor are working properly?I am beginning to wonder if I should try testing anything possibly suspect on a component level in the power supply, or just tweak the adjustments a little with the DMM connected and see if I can get them to move in a favorable direction?

author: sharbinger
author_rank: Newbie
timestamp: October 18, 2019, 05:09:12 am
content: I'd definitely like to get some feedback on my troubleshooting here.I ran through every node in the power supply unit with a voltage callout and put it onto a print of the schematic for the DC supplies.Red measured not to specification, yellow was unknown due to me not seeing a callout on the print for that specific node. Green measured correct, blue circles on the left where green turns to yellow or red.My suspicion is that one or more diodes failed for the -150V section, and by my understanding of the power supply, the -150V is used to regulate the other supplies and can be adjusted, unlike the other supplies.I checked R640 and R641 and didn't see anything that seemed exceptionally unusual for their resistance, and when I checked in circuit the +90 node after them measured at 3.9V and before them measured 4V, from what should be 208Vrms rectified.I could be wrong and overlooking something with my 0V reference as my understanding is all the DC voltages are measured relative to ground, but I think one or more of D642 has failed.First, am I way off on my narrowing down?Secondly, all I can find is it's a Tek part # 152-047 which seems to be a Fairchild FD700 as one replacement as they seem to share the same NSN, but is there a specific list of replacement semiconductors for Tek part numbers using more modern equivalents that I am just not finding?Or is it more of a non-special component where one of the same current and voltage characteristics would be good enough?

author: Martin.M
author_rank: Frequent Contributor
timestamp: October 19, 2019, 06:08:13 pm
content: in the PSU,-3,9V have to be +90V related to ground.checking rectifier and the 125µF please, secondary AC voltage there alsoMartin

author: Chris56000
author_rank: Frequent Contributor
timestamp: October 20, 2019, 03:09:45 pm
content: Hi!You MUST get the stabilised –150V negative h.t. supply correct before you attempt to fault–find any of the other rails, (apart from the 335V for the transductor control amplifier!), nearly all the other h.t. supply lines are referred to or setup by, the –150V line – your circuit sketch suggests you're getting –267V with respect to chassis–earth which is clearly not correct, the rectified output from the 208V winding at the top of your diagram (across C125) would be about 310V off–load, or about 260–270V loaded, hence the +90V given by Tek on the part of the drawing where you're measuring 4V!Chris Williams

author: Chris56000
author_rank: Frequent Contributor
timestamp: October 20, 2019, 03:22:12 pm
content: Hi!Looking at your circuit, the first query that springs to mind is the –178V you're reading across the neon–regulator V609, the 5651 is a high–stability regulator tube similar to the Mullard 85A2, so over twice the nominal maintaining voltage isn't right, so make sure the tube is glowing satisfactory, (replace R608 with a new one if it's out altogether) if it is glowing, then try a replacement or buy a Mullard 85A2 to begin with, then you need to check R623, which is the anode resistor for the 2nd half of the 12AX7/ECC83 V624, then also check the two resistors either side of the –150V set h.t. control R615 for correct value – these are R616 & R617.Look at these points first then let us know how you get on but please concentrate on the –150V line first!Chris Williams

author: Chris56000
author_rank: Frequent Contributor
timestamp: October 20, 2019, 10:18:22 pm
content: Hi!You mentioned flickering or pulsing of the light in the neon regulator tube V609 – this shouldn't occur in normal operation, and this suggests the tube is being starved of anode current – when a gas-filled diode regulator gets old it's anode characteristic tend to shift to the point where the originally intended anode current becomes insufficent to bias the tube away from the first negative–resistance portion of it's anode characteristic immediately after the firing–point, so the voltage across the tube falls as it tries to take more current than R608 can supply to keep it lit, so the result is that the tube goes out until the capacitor C610 recharges to the striking voltage of the tube, whereupon the tube fires and the cycle repeats, resulting in a large sawtooth voltage across C610 that plays havoc with the rest of the power supplies!The cure for this is to replace both R608 and the 5651 regulator tube V609. For a full explanation, please see "Neons in the Experimenter's Power Pack", written by Martin L. Michaelis, Practical Wireless, July 1962, downloadable from American Radio History!Chris Williams

author: Chris56000
author_rank: Frequent Contributor
timestamp: October 21, 2019, 05:33:09 pm
content: Hi!You mentioned two or three valves still out, these are the 12E4 500V h.t. regulators and the EF94/6AU6 control amplifier for the 12E4s, these are fed from isolated winding 12–13 on T602, so if T602 primary is missing it's supply, check F601 and the thermal cut–out contact TK601 – these valves aren't associated with the stabilised L.T. (heater) supply in any way!If T602 is being powered, then almost certainly the few valves left out have o/c heaters, but please go thro' all the heater wiring (blue/red & blue/orange wiring) from tags 12 and 13 first!(Please don't forget winding 12–13 is returned to the 350V h.t. line via R717 by the way)Chris Williams

author: sharbinger
author_rank: Newbie
timestamp: October 27, 2019, 01:28:38 am
content: -150V has had a little progress in repairing it.D642A,B,C,D did check on a handheld meter as dead. The NSN reference that said it was an FD700 is in hindsight less of a reference and more of a joke, and I should have used the part number in my paper copy of the 555 manual. 1N3194 silicon rectifier diode as mine is a serial higher than 11560.Small signal diodes do not live long as rectifiers, and a more proper replacement is on the way.I don't have a tool to truly check C640 so I am leaning towards replacement, once I can find a can type 350V 125uF capacitor that isn't old stock or the wrong lead type. Or something that will work with the mounting method in place, it's not a multi-capacitor can and I am not too worried about the aesthetics of having a more modern capacitor.I'll do the checking of R623 and R616/617 and I will look into getting that Mullard 85A2 if V609 won't live. I've not been able to find any video of what one of them should look like for comparison, unfortunately.As far as T602, it does have primary and all the outputs are fine. The dead tubes were solved with a replacement and testing, however I've yet to order a replacement for the dead 6DJ8(?) in the indicator unit. Every other tube in the indicator lights up and the power checked fine, so it seems it just has a total of two dead tubes overall, which isn't too bad.

author: Chris56000
author_rank: Frequent Contributor
timestamp: October 29, 2019, 01:37:16 pm
content: Hi!Good, we're getting somewhere!Remember that, providing the electrodes are correctly biassed, the control grid electrodes of thermionic valves are "dead–ends" to direct currents, so the only current that R608 feeding the cathode of the neon regulator V609 provides is the anode current of the tube itself, so if you decide to get a Mullard 85A2, make sure R608 can provide the minimum amount of cathode–current specified for the tube given in the data–sheet, or you might get relaxation–oscillations throughout the circuit as I explained above in a previous post!The value of R608 should be:–(H.T. voltage across C124 – 85) / (Ik max + Ik min /2)where Ik max and Ik min are given in mA from the tube datasheet, and the value of R608 is in kΩ!Chris Williams

author: sharbinger
author_rank: Newbie
timestamp: November 03, 2019, 12:37:28 am
content: Okay, replacing D642 A,B,C,D with actual rectifier diodes (Shindengen D1NK60-5070 seemed like a close match for 1N3194 while costing about half as much. worst case I'd be buying 1N3194s to replace these) has some effect. The various rails have all stopped following 'shoot for the moon' as life advice, however the -150V rail has jumped to 1V and stays firmly there. The 5651 tube refuses to light completely now, but even with power on the diodes are reading roughly the Vf expected so I don't think they are the cause anymore.I suspect I will have to go through everything again and check all voltages, but at this point tweaking the -150V adjust still does nothing.The only behavior is it shifts from 0.9V to 1.0V when the thermal delay clicks on.C640 I could not find a replacement for that wasn't a salvage part, nor could I find a close enough in value capacitor with a higher voltage rating. Motor start capacitors seem potentially available on the various ali- websites but not at 125uF and I suspect they are not the optimal choice for a continuous duty filter capacitor. I did find 120uF capacitors with a clamp mount that would replace the entire C640, but they were axial leads and I did not want to try wiring that up (in addition to the value difference).Is there a way I can pull and test V609 without needing a tube tester or some other specialty hardware?