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

author: m3vuv
author_rank: Super Contributor
timestamp: November 01, 2019, 01:36:48 am
content: Hi all,ive the above scope,i brought it as faulty with no trace,after some poking around i found a couple of transistors dead on the z axis board along with a few resistors out of spec due to age,i now have a trace but cant adjust the high voltage to - 1950v,the most i get is -1093,the hv adjustment pot R900 makes no difference to the voltage whatsoever ive renewed the door knob in the eht comp but no change,the cal is all to cock due to the low hv,any ideas where to look and what to check?,ive checked all the electrolytics for high esr,they all test good,cheers m3vuv 73.

author: JohnPen
author_rank: Regular Contributor
timestamp: November 01, 2019, 09:44:32 am
content: Assuming you have tested the 3 transistors Q914 (particlarly), Q913 and Q923 are OK. Check R900 wiper connection to R901 (4.3M) voltage range +75v to 0v. Check R901 really is 4.3M and not o/c. You cannot check the Q914 base voltage correctly because the normal DMM will load the high resistance feeds too much. However connecting a meter at that point should affect the -1960v rail if the transistors are doing something. Then check Q914 emitter voltage (TP914) it should be -11.9v. Similarly the collector voltage of Q914 should be -0.67v. Q913 collector should be -4.1v. R925 (30K) should have -4.5v. Unfortunately all of these stages are low current ( ~2.5ma or less) and therefore the voltage readings can only give an approximate indication of correct operation. However I would suspect that Q914 is no longer acting as a transistor as you are actually getting a fixed -1093v rail. Hope this helps.

author: m3vuv
author_rank: Super Contributor
timestamp: November 01, 2019, 03:03:11 pm
content: Well here goes,r914 checks good,the wiper of r900 is ok,r901 is good,tp914 has -0.742v q914 collector has -0.907v q91 collector has 0.137v r 925 has 75v one one end and 0.397v on the other.cheers Paul m3vuv.

author: m3vuv
author_rank: Super Contributor
timestamp: November 01, 2019, 04:02:18 pm
content: well while doing checks i herd a pop and the trace vanished,found f937 blown,further chcks revaled q390 sc,could the lack of hv reglation be thecause of the q390 dying ?,would a 2n3055 do as a replacement?,cheers Paul.

author: m3vuv
author_rank: Super Contributor
timestamp: November 01, 2019, 05:01:09 pm
content: well on checking the hv transfo i get( assuming the top front terminal ia pin 1?,pin 1-2 i get 0.1 ohm,pin 3-4 is 0.2 ohm,is this right or the tranfo shorted?,i replaced the shorted q390 with a 3055,still no ht or trace!

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 01, 2019, 06:29:59 pm
content: Not to say this is your problem, but be aware that current 3055s are not the same as the ones made 40 years ago. I had a bad 3055 in a 454 and I had to sort through a number of them until I found one that worked. IIRC, current ones have higher gain and they are fabricated differently.To make matters even worse, I see in the 454 manual parts list that it says "selected from 2N3055", so who knows what they were looking for.If you had any high voltage, it's likely the transistor was good. You could have a leaky cap on the HV side. They are very hard to test and any leakage will screw up the HV supply. They may not exhibit leakage until at operating voltage, so just checking capacitance or esr is fairly useless.

author: m3vuv
author_rank: Super Contributor
timestamp: November 01, 2019, 07:08:22 pm
content: Tbh,im more inclined to think the hv transfo is stuffed,realy nead to find the winding resistance data,i suspect shorted windings leading to low hv and overloading the transistor,rewinding it could be fun!.

author: JohnPen
author_rank: Regular Contributor
timestamp: November 02, 2019, 02:16:29 pm
content: Q930 is also shown in the 453A manual as a selected 2N3055 one. I believe it is usually more a problem of voltage spikes exceeding the more average 2N3055s rating than gain variations. The primary windings on the T930 will read low resistance as there are not many turns involved low voltage high current. Your measured TP914 voltage was definitely too 'positive' as correct operation should have been -11.9v implying -11.6 on the base of Q913. Before going any further it would be worth checking the whole 3M Resistor chain (R903 to R910) to confirm they are all OK. Also Check R911, R902 are OK and that CR911 has not become s/c. As suggested HV voltage caps do become leaky so it would be worth checking C906, C966 and C976. As Q930 has failed it would be worth checking Q914, Q913 and Q923 are still Ok before powering everything up with a replacement 2N3055.

author: m3vuv
author_rank: Super Contributor
timestamp: November 03, 2019, 08:45:12 am
content: Hi john,the 3meg resistor chain r903-910 are good.what and where is cr11as i cant find it?,cheers Paul m3vuv.I swapped the 3055 for a modern one still no hv,ive some old motorola ones hrom an old hp spec analyser,may try one of those,q913-914 q923,havent been rechecked yet,im in bed its 8.44am,will check them in a bit.cheers paul.

author: m3vuv
author_rank: Super Contributor
timestamp: November 03, 2019, 01:48:17 pm
content: Just an update,q914-q914 and q923,all test good.

author: JohnPen
author_rank: Regular Contributor
timestamp: November 03, 2019, 04:58:08 pm
content: On the 453a cct R903 (3M) is connected to R902 (1M) which is connected to R911 (200K) and C911 (2ufd). R911 is connected to +75v and CR911 is connected across R911. These components are somewhere on the chassis. If you can find R911 you should be able to check the volts on each end +75v and at a guess about +60v on the other end unless the reverse biased diode CR911 has failed.

author: nonkapo
author_rank: Contributor
timestamp: November 03, 2019, 05:18:32 pm
content: I have the older version of this scope and i think some documents including repair manual or at least calibrating one are available on the net if that can be of any help to you.

author: m3vuv
author_rank: Super Contributor
timestamp: November 04, 2019, 03:24:08 am
content: i have an older 453 as well,its my fav scope,ive got manuals for both but cant locate one for my variantie the a-1 model.

author: m3vuv
author_rank: Super Contributor
timestamp: November 04, 2019, 03:29:15 am
content: well,looks like the root cause of the problem is burnt hv windings,there is a spot about 2mm long of the hv windings and the covering tape black and burnt,im just building a coil winder to have a pop at rewinding it.

author: JohnPen
author_rank: Regular Contributor
timestamp: November 04, 2019, 09:06:25 am
content: The following information might be useful to you. I converted my 453 EHT set up from the valve rectifiers to the later solid state version and investigated the TFR winding data a lot of years ago.EHT winding 9-7 1100 turns 38 swgCathode (-1960v) 5-9 542 turns 38 swgGrid 6-10 582 turns 38 swgCRT Heater 3 turnsQ930 Collector 1-2 3 turns 26 swgQ930 Base 3-4 2 turns 26 swgHappy windingJohn

author: m3vuv
author_rank: Super Contributor
timestamp: November 04, 2019, 02:29:35 pm
content: cheers john,thats a big help,just got to work out how to drive my bipolar steppers with the code written for unipolars to finish my coil winder!,do you think the shorted hv transfo was the cause of my issues?

author: JohnPen
author_rank: Regular Contributor
timestamp: November 04, 2019, 04:22:35 pm
content: It does seem very likely. As you had -1032v originally after fixing the transistors on the Zaxis it must have been leaking current in one of the HV windings or it had a semi shorted turn somewhere. The EHT adjust circuit presumably tried to compensate by upping the drive to the transformer further cooking the winding to eventually kill Q930 by overloading it. Don't forget to use a suitable insulation layer between the separate windings when you rewind the transformer. Best of luck.John

author: PaulAm
author_rank: Frequent Contributor
timestamp: November 04, 2019, 11:22:08 pm
content: There was a discussion on tekscopes recently on rewinding the HT transformer for a 7834/7934. There was a recommendation for using 3M 1350 tape, 1 mil film (2.5um) with a total thickness of 2.5 mil (6.3um) including adhesive. It's a polyester tape with acrylic adhesive rated at 5500V as insulation between layers.I would probably use that stuff.There's also a photo album for rewinding a 453 transformer in the group photo section.Considering some, it's not too terrible a project.

author: 0culus
author_rank: Super Contributor
timestamp: November 05, 2019, 01:07:27 am
content: OP: may I respectfully request that you put spaces between your sentences and clauses? Iknowyou must have a working spacebar since you have spaces between words within each clause. Your posts would be considerably more readable for everyone else if you wouldalsoinsert spaces between sentences and clauses. Thank you.

author: tggzzz
author_rank: Super Contributor
timestamp: November 05, 2019, 07:51:05 am
content: Quote from: 0culus on November 05, 2019, 01:07:27 amYou aren't the first person to note that. A couple of his responses:https://www.eevblog.com/forum/projects/hv-probe/msg2762272/#msg2762272https://www.eevblog.com/forum/projects/hv-probe/msg2762670/#msg2762670

author: m3vuv
author_rank: Super Contributor
timestamp: November 05, 2019, 12:15:25 pm
content: didnt knowtiswasapost for grammernazi's! ! ! !

author: tautech
author_rank: Super Contributor
timestamp: November 05, 2019, 08:39:34 pm
content: Quote from: m3vuv on November 05, 2019, 12:15:25 pmOh c'mon !You've been pulled up on your lazy posting before and still you choose to ignore the wishes of those trying to follow your threads .....sometimes I wonder why they do.Carefully constructed posts with acceptable grammarandpronunciation are a basic courtesy to those reading on forums.Use a basic spelling checker that most browsers have (turn it ON) and most will have no issue with anything you post.I ain't hard.