tek 453a-1 repair - Page 1
Forum section: electronics / repair

author: m3vuv
author_rank: Super Contributor
timestamp: October 03, 2019, 07:13:12 pm
content: hi folks,ive got a tec 453a-1 to fix,it has no trace,ive looked at the voltages on the psu low votlages board and they seem fine as far as i can tell,all the manuals i seem to get are for a 453 but not the same model,ie mine has no beam finder button or scale illumination pot,can anyone help on this.eithrt eith a manual etc,cheers m3vuv?

author: bd139
author_rank: Super Contributor
timestamp: October 03, 2019, 07:17:35 pm
content: Ah you won the one on eBay that I was asleep when it endedManual here:http://w140.com/mmm/tek-453a.pdfIt's common for the HT to blow up on these as they're pretty old. I think the 453A has same HT section as the original 453. Rebuild info here:http://vondervotteimittiss.com/belfry/?p=248Good luck. I'm slightly sad I missed that one

author: m3vuv
author_rank: Super Contributor
timestamp: October 03, 2019, 08:59:53 pm
content: thanks for the link,but i already have that manual,its for a 453a yes but not the same model i have,mine has no cal loop on the side,also no beam finder or scale intenity pot,its a 453a-1.

author: m3vuv
author_rank: Super Contributor
timestamp: October 04, 2019, 05:21:24 am
content: heres a pic.

author: m3vuv
author_rank: Super Contributor
timestamp: October 04, 2019, 06:58:54 am
content: update,ive checked the hv diodes in the crt supply,all seem ok,disconected one leg of the eht smoothing cap.no difference,i checked the high voltage test point as per the manual i have,i used my x10 scope probe and my fluke 83v the manual says -1960v,i read -937,could this be a clue?,saying that im not sure as its right or wrong as the 453a manual doesent seem to be for this variant of it!. and my

author: bd139
author_rank: Super Contributor
timestamp: October 04, 2019, 08:09:28 am
content: Yeah that sounds like it might be the issue. Probably drifted HV resistors or pot.If there isn't a manual variant for this it's going to be difficult to work around.

author: m3vuv
author_rank: Super Contributor
timestamp: October 04, 2019, 10:39:00 am
content: well a quick update,i found q1034 and q 923 o/c on the z axis board,ive peplaced them with bf7 50 npns,,i now have a trace but it wont focus properly,see pics,any ideas what will cause this?,see pics,first pic is ch1 2nd is ch2,all settings the same.it looks if i could get it to focus properly it wont be far off,may look at resistor values around the focus circuit,see pics attached.

author: m3vuv
author_rank: Super Contributor
timestamp: October 04, 2019, 10:40:15 am
content: meant to say bfy 50 transistors not bfy7!

author: bd139
author_rank: Super Contributor
timestamp: October 04, 2019, 11:52:23 am
content: That’s a good start. Trace is big enough to cover screen which means anode voltage is up. Probably focus and intensity resistors drifted or dead. If the Z axis was dead then that might be a sign of something iffy in the dividers. I’d try and reference the CRT bias voltages to the 453A manual and see if that makes any sense.

author: m3vuv
author_rank: Super Contributor
timestamp: October 04, 2019, 12:29:20 pm
content: thats good but i cant find the right manual so sort of guesswork,got a feeling the eht smoothing doorknob cap is us,i have some in a hf 1kw homebrew amp i buils so my rob that to try it.

author: m3vuv
author_rank: Super Contributor
timestamp: October 04, 2019, 12:30:22 pm
content: the focus does work but doesent have the range.

author: bd139
author_rank: Super Contributor
timestamp: October 04, 2019, 01:27:17 pm
content: Does it improve if you change focus and astig?If not it'll be resistors somewhere.

author: m3vuv
author_rank: Super Contributor
timestamp: October 04, 2019, 05:33:31 pm
content: yes if i change the astig or focus it alters the trace but its still way to thick and fuzzy.

author: m3vuv
author_rank: Super Contributor
timestamp: October 04, 2019, 08:00:22 pm
content: just wondered about this so thought id ask,the hv compartment has a 500pf 20kv doorknob cap,if i pull one leg of it,ie take it out of circuit,it makes no difference whatso ever to the displayed trace,i assume it filters noise?,i wonder if its noise on the trace thats making it thick and blured,ive a 500pf cap dorknob type but its only rated at 10kv,do you think i can get away trying it?

author: bd139
author_rank: Super Contributor
timestamp: October 04, 2019, 08:15:25 pm
content: Not sure. Depends on the PDA voltage.Have you checked the ripple coming off the power supply as well? The 453 I know wasn'tthatgreat in the power supply department.

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2019, 08:12:25 am
content: what is pda voltage?

author: bd139
author_rank: Super Contributor
timestamp: October 05, 2019, 09:15:52 am
content: Voltage at anode of CRT (PDA = post deflection acceleration). Not sure what voltage it should be on those.

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2019, 09:25:00 am
content: by the anode,do you mean the eht,on the front of the crt under the rubber cup/connector?

author: bd139
author_rank: Super Contributor
timestamp: October 05, 2019, 09:45:07 am
content: Yeah that's the one. "anode cap". Or on a bad day "the red snake of pain"

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2019, 12:21:17 pm
content: well,ive swapped the 500pf dorknob cap in the high voltage compartment for a 500pf 30kv rated one,also changed a couple of resistors that were slightly off value,its made no difference,also noticed the grid bias pot and r900 pot on the z axis board makes no difference to the trace,any ideas?

author: bd139
author_rank: Super Contributor
timestamp: October 05, 2019, 02:46:39 pm
content: completely out of ideas unfortunately.Next step is the TekScopes group on groups.io I imagine. Someone will have had this exact problem I'm sure or have a solution. I'm out though

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2019, 03:41:10 pm
content: ive already asked there but nothing yet,cheers anyway m3vuv73

author: JohnPen
author_rank: Regular Contributor
timestamp: October 05, 2019, 06:24:15 pm
content: It would seem unlikely that CRT circuitry for your version would have changed much compared with the 453a. The revision from the original 453 was more involved losing the EHT valve rectifiers was a relatively major change. The EHT voltage for both 453 and 453a stays at +12Kv and -1960v across the CRT. It would seem very likely that it would be the same for your 453a-1. It is particularly important that the -1960v rail is correct as this really provides the correct voltages for the CRT to operate the 12Kv mostly just needs to be present. However it is well worth checking, as BD139 said, the values of the high Meg ohm resistor chains associated with CRT circuitry. In the CRT TV days it was a common problem with high Meg ohm resistor chains that individual resistors could develop large changes (even a Meg ohm out) in their value. If you have a genuine -1960v supply and you have the 453a circuit diagram a first check should perhaps be a voltage check on the VR963 120v Zener in the focus resistor chain pin 12 on the CRT. If Ok a good start then work back up the chain checking the value of each resistor to make sure they are the correct values (Or match what they say they are if actually different on your version 453a-1). If the -1960v supply is incorrect try checking the various high Meg ohm resistor chain connected to the High Voltage adjustment pot R903-10. Hopefully this may help.John

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2019, 07:16:15 pm
content: Well between pin 12 on the crt and chassis ground i have .5 volt.,should it be +120 or _120?

author: m3vuv
author_rank: Super Contributor
timestamp: October 05, 2019, 07:48:20 pm
content: i measure 75v across the 120v zenner,one thing strange tho,mine has a 200k resistor in paralell with it,its not on my schematic either!,i cant see any continuity with the diodes -ve leg anode and chassis ground either.

author: m3vuv
author_rank: Super Contributor
timestamp: October 06, 2019, 07:22:21 am
content: i tried grounding the anode of the 120v zenner vr 963 to ground via a 1/4 watt 1k resistor,it cooked the resistor and made no difference to the trace!.,ive another 453 working,its not the a model tho,i may delve into that and do some like for like checks.

author: JohnPen
author_rank: Regular Contributor
timestamp: October 06, 2019, 09:47:00 am
content: Rather puzzling. Since pin 12 of the CRT does not appear to relate the VR963 voltage perhaps the 453a-1 is more similar to the 453 circuitry. On that basis the VR963 anode may be connected to the -ve side of the unregulated 12v supply (C1142 1500ufd) and uses the earlier CRT where Pin 12 is not mentioned at all. However there is a 0.3 ohm current sense resistor, for supply protection, on the LV regulator board that connects the -12v unregulated to earth. Some of the online 453a info implies that the 453a-1 has a 50Mhz B/W rather than the normal 60Mhz B/W of the 453a so it could be they used the older crt with the later chassis.If you measure the voltage between Pin 1 heater and Pin 4, remembering that it is -1960v on Pin 1, you can check the focus voltage range. Calculating for the 453 it should be 47v to 547v and 150v to 480v for the 453a. However most voltmeters only have 10Meg ohm input resistance which will modify those values particularly the reading at the higher voltage end of the range. With a little luck, with the meter connected, it may even improve the focus. Best of luck.John

author: m3vuv
author_rank: Super Contributor
timestamp: October 06, 2019, 11:38:44 am
content: well ive nearly sorted it i think,i pulled my 453 apart do do some like for like checks (its not the a model tho) while checking resistors in the hv compartment i found the wire from vr911 had a nic in it and was touching a screw,i tried it and the trace was about 1/4 inch wide,i also noticed that my good scope was doing the same!,i then put the cover back on both hv compatments,this resulted in a nice trace that focuses well on both machines,the only issue i have now with the a model is the hv pot r900 has no afect on the trace,also r926 is running hot.,it seems ok but when i set the timebase to the same on both scopes so i get one peak per division horiz,on the 453 its spot on ,on the a its about 1.5 divisions,i assume this is down to the hv pot circuitry?

author: m3vuv
author_rank: Super Contributor
timestamp: October 06, 2019, 02:56:51 pm
content: heres some pics of both scopes,the a seems to have a tb issue.

author: m3vuv
author_rank: Super Contributor
timestamp: October 06, 2019, 02:58:34 pm
content: trying to upload pics but struggling.

author: JohnPen
author_rank: Regular Contributor
timestamp: October 06, 2019, 06:02:07 pm
content: Glad you are making progress. It should now be whether Q913, Q914 and Q923 are working correctly as you have a visible trace. I note you replaced Q923 with a BFY50 rather than the recommended 2N3053. The latter has a higher maximum current rating but there should be no real problem as the circuit only runs with ~2.5 ma. Where is the R926 resistor located as I cannot find it on my 453a circuit diagrams? If you meant R925 (30K 0.5W) that will run fairly warm, just under 220 mW, in normal operation.If your focus controls are in relatively similar positions on both scopes that would imply that the -1960v supply may be about right. It might be worth making up a resistor chain to enable your voltmeter to read the actual value of the supply. The manual emphasises that this supply needs to be within +- 20v as it may have an impact on other circuits in the scope.

author: m3vuv
author_rank: Super Contributor
timestamp: October 06, 2019, 07:08:16 pm
content: just remembered ,last night i had pulled one leg of 3 .1 caps in the hv section,then the tb looked ok,i put them back in circuit ,maybe gonna pull them againe and look,noticed also last night the grid bias pot made a differenc when altered,i gues one of the caps is leaky bringing the hv to its knees? ,its a job for tomorow

author: m3vuv
author_rank: Super Contributor
timestamp: October 06, 2019, 07:10:34 pm
content: seems the fuzzy trace was down to the cover beeing off the hv section,got red herrings due to that!,suprised no one spotted it.

author: jdragoset
author_rank: Frequent Contributor
timestamp: October 06, 2019, 09:18:42 pm
content: These work great for focus voltage to 3 Kv/ rectified HF transsformer output.https://www.ebay.com/itm/SENSITIVE-RESEARCH-INSTRUMENT-CORP-ELECTROSTATIC-VOLTMETER-3-0-kV/352321837657?hash=item5208049a59:g:CV4AAOSwnCxaxGHE

author: m3vuv
author_rank: Super Contributor
timestamp: October 07, 2019, 04:31:04 pm
content: well im making sort of progress,when i got the scope q1034 on the z axis board was dead,,i replaced it and it brought up a trace,but something killed the replacement,ive replaced that too but its running well hot and still the r900 hv pot does nothing!,i pulled q1024 and q1034 still ran hot,then pulled q 1043,it then ran cool,so with either q 1024 or q 1043 in place q 1034 gets too hot,still the hv pot does nothing,any ideas where to look?,cheers Paul m3vuv.

author: m3vuv
author_rank: Super Contributor
timestamp: October 07, 2019, 10:25:34 pm
content: heres parts of the schematic,hope they come out ok.

author: m3vuv
author_rank: Super Contributor
timestamp: October 07, 2019, 10:28:25 pm
content: that didnt work,its page 267/268 on the manual

author: m3vuv
author_rank: Super Contributor
timestamp: October 09, 2019, 02:27:23 pm
content: well i finaly sorted it,found i had an npn transistor in place of a pnp,i dont know if that was me or someones previouse attemt at a repaire,it had been got at as various screws were missing from pc boards,that was the cause of the transistor getting hot,also a 30k resistor was open,it seems to be working fine now,thanks all.m3vuv

author: bd139
author_rank: Super Contributor
timestamp: October 09, 2019, 02:48:43 pm
content: Nice job. Glad to hear it's working

author: JohnPen
author_rank: Regular Contributor
timestamp: October 09, 2019, 03:59:41 pm
content: Glad to hear you have solved the problems. It is probably worth checking the -1960v supply is within spec to complete the Power and Calibrator Adjustment section before you move on to the rest of the adjustments. Have fun.John