Tek 577 Step amplifier problem

[rodd]

Hi folks,
This is the current problem I am trying to solve in this fix.
Please refer to the attached part of the schematics
 Step Amplifier Sch 3.pdf (124.65 kB - downloaded 66 times.)

There signal at TP360 (voltage set resistors input) is ok, on both polarities.

However, the is no signal at P429 pin1 ( Step Amplifier Out to test fixture (tek 177).

I have disconnected and tested VR380/VR382 and Q384/Q386 and Q450A/b (dual NJFet, tek#151-1054-00 or DN1114)

All the he semiconductors were tested ok using a Peak Atlas DC75Pro.

Also, replace U380 (by MC1456) Is this a suitable replacement?


However, the voltage at pin 5 of U380B pin 5 is around -11V (referred to ground). Is this the correct reference?
This voltage should be 0V according to the schematics
This is a voltage follower, but the U380B output is at -2.7 V ! (Why?)

Also, I could not locate R450 in the part placement diagram in both the Artek Media scan and paper copy of the service manual.

Any ideas?
Thanks

[Testtech]

Check the relays, also, these instruments have the terrible TI IC sockets, these should be replaced. Be wary of the dual op amps, I have replaced many of them. I use the oe part number.

[rodd]

Thank you for the reply.

Two questions:
 1- When you say check the relays, do you mean the coil continuity or the operation of each contact?
Do the contacts develop contact resistance? Visual inspection shows no sign of thermal damage.

 2- Do you recommend to remove each board and replace the TI IC sockets?

PS. I found that one of the series resistors, R140 (22k 25W ceramic type) was open.
I could not find a suitable replacement up to now (only wirewound resistors).
Any suggestion?

Thannk you

[Testtech]

Thank you for the reply.

Two questions:
 1- When you say check the relays, do you mean the coil continuity or the operation of each contact?
Do the contacts develop contact resistance? Visual inspection shows no sign of thermal damage.

 2- Do you recommend to remove each board and replace the TI IC sockets?

PS. I found that one of the series resistors, R140 (22k 25W ceramic type) was open.
I could not find a suitable replacement up to now (only wirewound resistors).
Any suggestion?

Thannk you

I remove the ice cube relays and very lightly burnish the contacts with Deoxit on a piece of card stock. I don't ever recall an open coil.
The best way is to replace all the TI sockets, but it is labor intensive to remove the boards. You can apply a small amount of Deoxit to the IC pins and repeatedly inset/remove them to hopefully restore contact.

The 22K 25 W will be a wire wound resistor. I would replace this first and check function.

[rodd]

Thank you for your reply.
I have already applied deoxit to all IC`s and transistors after washing and drying the unit.
Did the same with the relays
Also replaced the 22k 25W resistor.
After replacing U380 another time, I am able to get the staircase signal at the bases of Q384/Q386, but no outputs.
Both semiconductors tested good on my Peak Atlas Semiconductor tester, but the instrument was damaged during the test.
It now shows code 3 at startup. But this is another problem.
I am waiting for some TIP142/TIP147 to arrive to use as replacements for Q384/Q386. (Are they adequate?)

Two question:
1 - Is it possible to remove the relays' plastic cover? Are they glued or just press fitted?
2 - I have read that people clean the fingernail contacts of TEK rotary switches using the same method you have suggested for the relays.
     The problem is that the screws that fix the rotary drum to the PCB are located in the bottom side of it.
    a- do you think I should try to clean them?
    b- is it possible to do this without removing the drum? ( I am afraid of damaging on finger since there is very little room to slide the paper).

Regards,
Roger

[Testtech]

Thank you for your reply.
I have already applied deoxit to all IC`s and transistors after washing and drying the unit.
Did the same with the relays
Also replaced the 22k 25W resistor.
After replacing U380 another time, I am able to get the staircase signal at the bases of Q384/Q386, but no outputs.
Both semiconductors tested good on my Peak Atlas Semiconductor tester, but the instrument was damaged during the test.
It now shows code 3 at startup. But this is another problem.
I am waiting for some TIP142/TIP147 to arrive to use as replacements for Q384/Q386. (Are they adequate?)

Two question:
1 - Is it possible to remove the relays' plastic cover? Are they glued or just press fitted?
2 - I have read that people clean the fingernail contacts of TEK rotary switches using the same method you have suggested for the relays.
     The problem is that the screws that fix the rotary drum to the PCB are located in the bottom side of it.
    a- do you think I should try to clean them?
    b- is it possible to do this without removing the drum? ( I am afraid of damaging on finger since there is very little room to slide the paper).

Regards,
Roger

The relay covers will usually just pull off by grasping the screw stud on the relay bottom with pliers, and pulling the cover off. Sometimes relasing the cover where the metal nub on the relay frame locks in is required.

Don't clean the attenuator finger at this point, probably fine, at least for now.

Are the darlington pair bad? If they tested good I would leave them.

Do you have the floating supply voltage? Check fuse F391.

[rodd]

Thank you for your reply.
I will try to open the relay and clean the contacts.
Two questions:
1-What is the knob setting to get the voltage readings that are shown in the schematic diagram?
   My readings are ok at some point and very far off at others, specially at U380, Q450 and Q384/Q386.
2- Is it possible to replace Q450 (151-1081-00) by a 2N3958?

Roger

[Testtech]

See page 7-2 in the manual for the setup.
By now you may have noticed that the 577 manual was not Tektronix' finest hour in manual production. The perf test is particularly tedious.   

[rodd]

Thank you VERY MUCH testtech!

I would never find it without your help!

Roger

[rodd]

Hi folks,
Thing got worse since my last visit.

After replacing the darlingtons (Q384/Q386) I was following the signal in the step amplifier (diag 3) when, suddenly, the staircase vanished.

About one minute later, the dot on the crt (which I always keep visible with low intensity and out of focus) vanished.

No "magic smoke" visible.

Now, something is draining the -30V supply if I press the STEP/OFFSET switch to normal.

I could not find any component warm to the touch.
I disconnected Q384/386 but they were not the cause.

The fault should be related to one of the circuits that are changed when K436 is energized/deenergized, but I can't think of any suspect.

Any help will be very welcome

[rodd]

Found the culprit of the -30V problem.
I bent one of the K436 contacts while cleaning as suggested here, in a way that a NO contact was NC.
I will proceed to troubleshoot the staircase generator.

[rodd]

I fixed the relay contacts, and no more problems with the +30V/-30V supplies.
Now there is a nice staircase signal at TP360 that change direction as expected when the polarity switch is pressed.
I am testing the unit in Voltage Steps mode.
The cam switch contacts for the low range voltages are not working as expected, but I will address this problem later.
I can get a good "clean" signal from .5V up at pin 2 of U380A, but NO SIGNAL at all at the output, pin 1.
I have tried to disconnect VR380/VR382 and Q384/386 and also replaced U380 without success.
Any help will be greatly welcome.
Thanks