Tektronix 454


erik erickson
 

Hey, I have a Tektronix 454 and Channel two works sometimes and other times there’s no trace at all. I have swapped V113 and that didn’t work. I asked this group about 6 months ago about this but I have been busy. I do have a manual I’ve been reading. Any help is appreciated.
Sent from my iPhone


Roger Evans
 

I found some more useful information in the previous thread which I assume is still valid.

You said that Ch2 would flash on when you pulled the Ch2 Invert switch and also that with the invert switch half way out the trace would stay on screen but not showing any signal.

With display set to Ch2 and Invert pushed in (normal) press the beam finder switch. Is there a line showing that the trace was originally off screen? Now pull the invert switch and press beam finder. Is the trace off centre in the same direction or the opposite direction? If the direction changes the Ch 2 imbalance is probably before SW181, if not then the fault is probably after SW181 (Q184, Q194 or the switching diodes, note that Q184, Q194 are PNP Germanium).

The manual gives the DC levels to be expected in the Ch2 preamp with no signal applied. Start at the collector of Q124 and then the emiiter of Q134. You should be able to get Q134 emitter close to zero with the Ch2 vertical position control.

Q164 and Q174 should have almost identical voltages on their emitters (-0.78V) and collectors (+2.0V) and their bases should be close to 0V, but since the observation implies that the balance is way off at SW181 you may have found something wrong by now. Since the preamp is DC coupled it may not be obvious whether the fault is where you first observe a 'wrong' voltage or due to the DC operating point being wrong because of a fault later on. The observation using the invert switch should help to narrow things down. Another test is to swap Q164 with Q174 and see if the off-screen direction changes and similarly with Q184 and Q194. You can also use the good Channel 1 on the 454 to probe Channel 2 preamp. Display Ch1, put your probe on Ch1 and connect Ch2 to the calibrator, Ch2 set to 1V/div. Then look for where the signal disappears.

Regards,

Roger


erik erickson
 

Thanks for all the help. For some reason channel two is working perfectly right know. Trace shows with invert switch on or not but it moves down from the center, one horizontal line. Also the beam finder does the same. I assume this is normal or maybe I didn’t let it warm up enough. I will check the voltages and probe it later but for now I’ll just see how it works.
Sent from my iPhone

On Jan 27, 2018, at 4:48 AM, Roger Evans via Groups.Io <very_fuzzy_logic=yahoo.com@groups.io> wrote:

I found some more useful information in the previous thread which I assume is still valid.

You said that Ch2 would flash on when you pulled the Ch2 Invert switch and also that with the invert switch half way out the trace would stay on screen but not showing any signal.

With display set to Ch2 and Invert pushed in (normal) press the beam finder switch. Is there a line showing that the trace was originally off screen? Now pull the invert switch and press beam finder. Is the trace off centre in the same direction or the opposite direction? If the direction changes the Ch 2 imbalance is probably before SW181, if not then the fault is probably after SW181 (Q184, Q194 or the switching diodes, note that Q184, Q194 are PNP Germanium).

The manual gives the DC levels to be expected in the Ch2 preamp with no signal applied. Start at the collector of Q124 and then the emiiter of Q134. You should be able to get Q134 emitter close to zero with the Ch2 vertical position control.

Q164 and Q174 should have almost identical voltages on their emitters (-0.78V) and collectors (+2.0V) and their bases should be close to 0V, but since the observation implies that the balance is way off at SW181 you may have found something wrong by now. Since the preamp is DC coupled it may not be obvious whether the fault is where you first observe a 'wrong' voltage or due to the DC operating point being wrong because of a fault later on. The observation using the invert switch should help to narrow things down. Another test is to swap Q164 with Q174 and see if the off-screen direction changes and similarly with Q184 and Q194. You can also use the good Channel 1 on the 454 to probe Channel 2 preamp. Display Ch1, put your probe on Ch1 and connect Ch2 to the calibrator, Ch2 set to 1V/div. Then look for where the signal disappears.

Regards,

Roger



erik erickson
 

I spoke to soon, when I switch to .1 volts/div, 2 volts/div, 10 mV volts/div, and 5 mV volts/div the trace disappears. I though maybe adjusting the step attenuator balance may work but when I do that the trace shows up for 5 mV but disappears on all others. Also for a trace to even appear you have to almost put the position switch all the way to the right. The beam finder does bring up the trace though when it’s gone. Any thoughts or should I use channel 1 as a reference and just see.

Sent from my iPhone

On Jan 27, 2018, at 6:41 PM, erik erickson via Groups.Io <the_truth_is_out_there=yahoo.com@groups.io> wrote:

Thanks for all the help. For some reason channel two is working perfectly right know. Trace shows with invert switch on or not but it moves down from the center, one horizontal line. Also the beam finder does the same. I assume this is normal or maybe I didn’t let it warm up enough. I will check the voltages and probe it later but for now I’ll just see how it works.
Sent from my iPhone

On Jan 27, 2018, at 4:48 AM, Roger Evans via Groups.Io <very_fuzzy_logic=yahoo.com@groups.io> wrote:

I found some more useful information in the previous thread which I assume is still valid.

You said that Ch2 would flash on when you pulled the Ch2 Invert switch and also that with the invert switch half way out the trace would stay on screen but not showing any signal.

With display set to Ch2 and Invert pushed in (normal) press the beam finder switch. Is there a line showing that the trace was originally off screen? Now pull the invert switch and press beam finder. Is the trace off centre in the same direction or the opposite direction? If the direction changes the Ch 2 imbalance is probably before SW181, if not then the fault is probably after SW181 (Q184, Q194 or the switching diodes, note that Q184, Q194 are PNP Germanium).

The manual gives the DC levels to be expected in the Ch2 preamp with no signal applied. Start at the collector of Q124 and then the emiiter of Q134. You should be able to get Q134 emitter close to zero with the Ch2 vertical position control.

Q164 and Q174 should have almost identical voltages on their emitters (-0.78V) and collectors (+2.0V) and their bases should be close to 0V, but since the observation implies that the balance is way off at SW181 you may have found something wrong by now. Since the preamp is DC coupled it may not be obvious whether the fault is where you first observe a 'wrong' voltage or due to the DC operating point being wrong because of a fault later on. The observation using the invert switch should help to narrow things down. Another test is to swap Q164 with Q174 and see if the off-screen direction changes and similarly with Q184 and Q194. You can also use the good Channel 1 on the 454 to probe Channel 2 preamp. Display Ch1, put your probe on Ch1 and connect Ch2 to the calibrator, Ch2 set to 1V/div. Then look for where the signal disappears.

Regards,

Roger





Roger Evans
 

Erik,

I am about to go on holiday so here is something of a long list.

Make sure the Ch2 V/div switch is cleaned with IPA and rotated back and fore a few times.

It is just possible that one of the C149A - J trimmer capacitors has developed a short circuit, perhaps with some foreign object.

Read the theory of operation section of the manual, it shows how the V/div switch has three separate functions, input attenuator, and two sets of gain switching. There is a table showing which parts are switched in at each V/div setting so it helps to narrow down the source of the problem.

Make sure the CH1 preamp and the main Y amplifier are close to being correct. With the input set to GND the trace should be centred on the screen with the vertical position control around midway for all V/div setings. You should be able setup the Ch1 step attenuator balance according to the instructions in the manual. Also the V/div setiings should be close to correct. This is to make sure that the Ch2 preamp is the only source of major imbalance.

If you are following the schematics in a scanned version of the manual beware that the original has blue lines showing where components are located on particular circuit boards. These scan in as black lines and even have (oversized) dots where they cross real interconnections.

Set Ch2 input to GND and at least to begin with choose a V/div setting where the beam is on screen. If you can, centre the trace vertically else go as close as possible to centre. Measure the voltages at the grid of V113, cathode of V113 and collector of Q24 and compare with the manual.

Measure the voltage at R137 and see how it varies with the vertical position control.

Measure the following: collector Q124, collector Q134, emitter Q134, collector Q144, emitter Q144 and compare with the manual. How do they change when you rotate the V/div switch? Is there an obvious correlation of the voltages that disagree with the manual with which segments of gain switching are in use?

If you are suspicious of any of the transistors then swap them, one at a time with the corresponding part in Ch1. Maybe clean the legs while they are out. If you are not sure of the significance of any of the measurements then repeat them on Ch1.

If you get this far and all is well then refer to my previous e-mail for probing around Q164, Q174, Q184 and Q194.

Regards,

Roger