Author Topic: tek 453 no sweep (Read 3441 times)

m3vuv · « **on:** August 12, 2019, 03:47:51 pm »

hi all ive a tek 453 scope,been using it today ans was fine until i noticed the trace was just a dot,if i vary the input signal on ch 1 a verical line apears and gets taller with the voltage of the signal,its lost the horiz sweep,any idears where to look?,cheers paul m3vuv.

benj38 · « **Reply #1 on:** August 12, 2019, 04:28:20 pm »

I am probably stating the obvious, but better check the obvious before taking the unit apart

Did you make sure you did not turn the horizontal display knob to the X-Y position by mistake?

There is no standard failure of the 453 that I know of that causes no sweep. The 453 manual is easily available on-line, and it is highly repairable (though very cramped inside). When you start troubleshooting, we can help more.

m3vuv · « **Reply #2 on:** August 12, 2019, 04:34:49 pm »

no controls are in the wrong position,iusing the calibrator i can get a brief waveform for maybe .5 of a second then its back to a dot,ive tried tapping various boards but nothing.

JohnPen · « **Reply #3 on:** August 12, 2019, 06:07:46 pm »

First thought the 453 uses a nuvistor valve to generate the sweep. If that has failed , may be declining emission from the cathode, try replacing the nuvistor with the one associated with the B timebase. No change then a second likely failure is the the tunnel diode trigger. This latter generates a very short trigger pulse that sets the scan going. Hopefully you did check all the supply rails were correct as well. If it is the nuvistor failing a I modified my 453 to use the FET circuit from the 453A instead and that worked fine. If you need a tunnel diode, hard to find, I still have some I purchased from a russian source a few years ago.

John

edit: I have assumed you have located a copy of the 453 circuitry.

m3vuv · « **Reply #4 on:** August 12, 2019, 08:36:34 pm »

think about what your saying,! if it was a nuvistor would just be one chanell this is all!

barry14 · « **Reply #5 on:** August 12, 2019, 10:34:10 pm »

The early versions of the 453 used 6 identical nuvistors. 4 are used where high impedance was necessary: input to the vertical preamplifiers and input for the external ch.1 and ch.2 triggers. The other 2 tubes are used to generate the A and B sweeps. Thus a failure of the A sweep nuvistor will result in no A sweep for both channels. As already suggested, you could try swapping one of the other nuvistors to check this possibility.

jdragoset · « **Reply #6 on:** August 13, 2019, 01:21:01 am »

Set the controls for XY and feed the Cal signal to both.
If the X deflection amplifier is working, you should see two diagonally displaced dots on the screen with a faint square wave rise-time line connecting them.
Keep intensity down to avoid phosphor burn.

m3vuv · « **Reply #7 on:** August 13, 2019, 09:37:27 am »

any idea where the nuvistors are located,i cant find them!,so far ive checked all the transistors on the horiz amp,they are all ok.

bd139 · « **Reply #8 on:** August 13, 2019, 10:15:31 am »

Not all of them had nuvistors in them. If your SN is > 20000 it will have FETs instead. They are near the front of the instrument on the vertical board (can't remember if you have to turn it upside down to get to that bit)

m3vuv · « **Reply #9 on:** August 13, 2019, 10:20:09 am »

i just noticed mine has a serial number of 100919,from what ive read after s/n 20000 they were fets.

bd139 · « **Reply #10 on:** August 13, 2019, 10:33:05 am »

Yep that's correct.

Diagnostic steps for these:

1. Check X-Y mode. If it wont sweep with an X input then suspect the amplifier. I can't remember how you set up X-Y mode in these TBH. It was weird if I remember. Inject a signal into the front end and trace it through with a scope or DMM (you can use a power supply and DMM for signal tracing. Just wiggle the power supply voltage up and down and look for changes in the amp with a DMM - that's how I fixed my first one of these about 15 years ago)
2. If it still won't sweep in auto, then it's probably trigger. Common failures on these are the tunnel diodes. You can pull them out and test them with a curve tracer. As no one has one, you can use a "octopus" tester which is basically a 6V AC transformer and a series resistor (usually 1-10k) and another scope to do an X-Y plot.
3. If the trigger works then it's probably the sweep integrator. Main timebase switch gets a bit ganky.

Step 2 is the PITA bit. It uses a couple of tunnel diodes for triggering. I actually have some original Tek tunnel diodes that will work in these if it comes to it.

Also the "apollo era" design of these gets in the way. They are rammed tight inside. Difficult to get at front panel bits.

JohnPen · « **Reply #11 on:** August 13, 2019, 12:29:23 pm »

If it helps the 2 Tunnel diodes referred to by bd139 are labelled D475 (A Trigger Amp) and D505 (A Sweep generator) in the early 453s. In the 453A they become CR475 and CR505 just a relabel of the same devices.

m3vuv · « **Reply #12 on:** August 14, 2019, 07:59:08 am »

heres a pic of my top board also a pic showing 2 fuses,i was told if it has 2 fuses on the rear right like this has then it has nuvistors,all seems very odd!.

m3vuv · « **Reply #13 on:** August 14, 2019, 08:02:12 am »

seems the pics dont want to upload,says there to big but there under the 5mb limit.

pbarton · « **Reply #14 on:** August 14, 2019, 09:55:25 am »

You mention that "brief waveform for maybe .5 of a second then its back to a dot"
Well that implies that the horrizontal sweep function is working. Which implies that the initiation of the horrizontal sweep is the potential problem.

You could confirm this by placing the front panel A SWEEP MODE switch into the SINGLE SWEEP position. Then press the green reset indicator to 'arm' the single sweep function. With A TRIGGERING in the + SLOPE position (up), turning the A TRIGGERING knob CCW should cause the sweep to trigger (once). If this is repeatable then the horrizontal sweep function is intact (including the two tunnel diodes) and you probabally have a problem with the initiation of the horrizontal sweep. (You don't need anything connected to Ch1 or Ch2 inputs).

Do you have another scope? If you had, you could check for the presence of A GATE, which is available on a BNC Connector on the small side panel.

Your photos are of the top boards, B Sweep Generator and Horiz Amp. You need to look at the A Sweep generator board on the bottom!

JohnPen · « **Reply #15 on:** August 14, 2019, 12:44:47 pm »

Attached are 453A scans for the A-TB and A_trigger labelled PCB layouts that use the FETs rather than Nuvistors. As the PCBs still refer to 453 there is probably little difference compared with your late version of the 453. Also I have included a scan of the A-TB schematic albeit in 2 bits due to a plastic scroll binding on the manual. Hope this will help you fix the problem.

m3vuv · « **Reply #16 on:** August 14, 2019, 04:27:31 pm »

thanks for that but mine is not the A version,its a late model with fets,ive checked the board underneath,all transistors check ok.

JohnPen · « **Reply #17 on:** August 14, 2019, 05:35:13 pm »

Comparing the Nuvistor version PCB layout with the later version PCB using a FET TB most of the component layout is very similar just removing the Nuvistor and placing a FET there plus minor PCB track changes, adding mostly resistors, diodes and links to -12v, and it uses the same transistors plus 1. If the transistors are all OK, I have not had any fail yet in my 453, the most likely failure will be the tunnel diode D505/CR505. You could try borrowing tunnel diode D675/CR675 from the B trigger board if you feel courageous.

m3vuv · « **Reply #18 on:** August 16, 2019, 06:40:00 am »

ive been told to check r805 and r536,ive found the later on the A sweep board but cant find r805 anyone know where to look?

JohnPen · « **Reply #19 on:** August 16, 2019, 07:51:30 am »

r805 is the horizontal position dual potentiometer. r805A 'horizontal position' has a value of 10K and is connected between +12v and -12v. r805B 'position fine' has a value of 50K and is also connected between +12v and -12v.

m3vuv · « **Reply #20 on:** August 16, 2019, 08:16:16 am »

ahh ok,dont see that beeing the issue,none of the pots or switches were noisy.

m3vuv · « **Reply #21 on:** August 19, 2019, 01:05:46 pm »

Well just an update,ive changed the cap c561 on the A sweep board as it was open for a 120pf as it was the nearest i had to hand,good news is its working fine now!,many thanks for all your help,glad i stuck with it,im fond of this scope,cheers all paul m3vuv 73.

bd139 · « **Reply #22 on:** August 19, 2019, 01:25:03 pm »

Congratulations!

(not go buy another one on ebay for spares quick

)

JohnPen · « **Reply #23 on:** August 19, 2019, 04:50:27 pm »

Glad you have fixed it. It seems a strange one as c561 is part of the Single Sweep Reset Amplifier. According to the manual it's function is to shut off Q575 allowing Q585 to conduct and enable the Sweep Gate tunnel diode. Well done for finding that.

m3vuv · « **Reply #24 on:** August 20, 2019, 10:20:45 pm »

i wa at a los to know what direction to procede with it,i thought all the tranistors seem ok bar1i thought next step is to check the caps for high esr,i went thru them all,i assumed an esr meter was no good for low value disk ceramics but i was getting a small amount of meter deflection with all of them except the aformentioned cap,i removed it from the board and tested it with a chineese component teter i built as a kit,that said it was dead to,after replacing that and the sick transistor that i replaced with a bc108 i had in the junkbox,it started working fine,just how you can ue an esr meter a a go no go on disk ceramics,for what its worth the esr meter was the p diddy 5 tranistor design on here,word of warning tho ,the pcb layout from mauro i wrong,both of his versions wont work as the routings wrong on both,go with pdiddys artwork for the pcb,cheers all paul m3vuv.


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Author Topic: tek 453 no sweep (Read 3441 times)

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