tek 453a-1 repair

[m3vuv]

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?

[bd139]

Ah you won the one on eBay that I was asleep when it ended 

Manual here: http://w140.com/mmm/tek-453a.pdf

It'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=248

Good luck. I'm slightly sad I missed that one

[m3vuv]

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.

[m3vuv]

heres a pic.

[m3vuv]

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

[bd139]

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.

[m3vuv]

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.

[m3vuv]

meant to say bfy 50 transistors not bfy7!

[bd139]

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.

[m3vuv]

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.

[m3vuv]

the focus does work but doesent have the range.

[bd139]

Does it improve if you change focus and astig?

If not it'll be resistors somewhere.

[m3vuv]

yes if i change the astig or focus it alters the trace but its still way to thick and fuzzy.

[m3vuv]

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?

[bd139]

Not sure. Depends on the PDA voltage.

Have you checked the ripple coming off the power supply as well? The 453 I know wasn't that great in the power supply department.

[m3vuv]

what is pda voltage?

[bd139]

Voltage at anode of CRT (PDA = post deflection acceleration). Not sure what voltage it should be on those.

[m3vuv]

by the anode,do you mean the eht,on the front of the crt under the rubber cup/connector?

[bd139]

Yeah that's the one. "anode cap". Or on a bad day "the red snake of pain" 

[m3vuv]

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?

[bd139]

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

[m3vuv]

ive already asked there but nothing yet,cheers anyway m3vuv73

[JohnPen]

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

[m3vuv]

Well between pin 12 on the crt and chassis ground i have .5 volt.,should it be +120 or _120?

[m3vuv]

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.