tek 453a-1 repair

[m3vuv]

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.

[JohnPen]

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

[m3vuv]

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?

[m3vuv]

heres some pics of both scopes,the a seems to have a tb issue.

[m3vuv]

trying to upload pics but struggling.

[JohnPen]

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.

[m3vuv]

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

[m3vuv]

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.

[jdragoset]

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

[m3vuv]

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.

[m3vuv]

heres parts of the schematic,hope they come out ok.

[m3vuv]

that didnt work,its page 267/268 on the manual

[m3vuv]

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

[bd139]

Nice job. Glad to hear it's working 

[JohnPen]

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