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577 step generator stopped working

DW
 

Upon replacing some various capacitors on the main board of the 577 I was careful to install the correct voltage and capacitance and tolerance capacitors as the manual described as well as make sure they were installed in the correct direction. I turn on the instrument, no pop or smoke at least but the step generator was definitely not working when I tested a transistor, it was showing nearly a short on all the transistors I was testing, I got some form of life as it slightly shows the step ramp but in very small increments. I probed test point 305 and test point 360 with an oscilliscope, I see the step pattern appear as it should and respond to the step increments speed and with step generator on off correctly. So I have a partially functional step generator

I noticed R300 step zero was looking a little brown looking but it always looked that way before, I decided to move it when it made a small spark and then I shut the instrument off. It seems that the step zero R300 pot has failed. Seeing the spark it appears some amount of current goes through R300 which I am not sure if that is normal.

DW
 

https://groups.io/g/TekScopes/photo/86536/0?p=Created,,,20,2,0,0

After some reviewing of the manual it appears R300 is a 100K pot that adjusts between the positive and negative 12 volts, from the spark as I turn it I believe it has failed. It goes to U305 which I believe is the pulse generator circuit. I have tried out the pulse generator on the step generator before and it appeared to have worked fine before from what I recall, R300 was also brown looking from before.

Here are the capacitors I replaced on the main board to give an overview what I have done
C391 C393 - 15uF 100V (Floating supply step generator)
C591 C596 6.8uF 35V (+30V Q594 sweep dimming)
C295 C297 15Uf 20v (+12V -12V DC filtering from P722)
C568 C569 4.7uF 50V (Display filter)
C268 C265 0.27uF 50V (U268 Pulse generator
C218 C215 0.47uF 35v (U268 Pulse generator)

Chuck Harris
 

When you were replacing capacitors, did you replace
any tantalum capacitors with electrolytic caps?

Particularly any low value caps.

The reason I ask is electrolytic caps are too leaky,
and to poor tolerance to substitute for tantalum in
timing or loop stability circuits.

As an example, the step amplifier has a pair of 6.8uf
tantalum caps that are part of a "T" lowpass filter,
that wouldn't be very good with aluminum electrolytic
capacitors.

The step generator has C265, C268, C215, C216, which
should not be replaced with electrolytic capacitors.

Most of the tantalum capacitors in the 577 should not
be replaced with electrolytic, only those that are filters
on the power supply leads are candidates.

-Chuck Harris

DW wrote:

Upon replacing some various capacitors on the main board of the 577 I was careful to install the correct voltage and capacitance and tolerance capacitors as the manual described as well as make sure they were installed in the correct direction. I turn on the instrument, no pop or smoke at least but the step generator was definitely not working when I tested a transistor, it was showing nearly a short on all the transistors I was testing, I got some form of life as it slightly shows the step ramp but in very small increments. I probed test point 305 and test point 360 with an oscilliscope, I see the step pattern appear as it should and respond to the step increments speed and with step generator on off correctly. So I have a partially functional step generator

I noticed R300 step zero was looking a little brown looking but it always looked that way before, I decided to move it when it made a small spark and then I shut the instrument off. It seems that the step zero R300 pot has failed. Seeing the spark it appears some amount of current goes through R300 which I am not sure if that is normal.



DW
 

I will go back and check for any solder bridges if there might be any on the soldered capacitors I installed, there shouldn't be any but I will make sure. Just to be clear I have not replaced any disk ceramic capacitors on the main board, I have entirely left those alone.

Chuck Harris
 

Ok, of that list, only C295 and C297 could be replaced with
electrolytic capacitors. Better put the good ones back.

-Chuck Harris

DW wrote:

https://groups.io/g/TekScopes/photo/86536/0?p=Created,,,20,2,0,0

After some reviewing of the manual it appears R300 is a 100K pot that adjusts between the positive and negative 12 volts, from the spark as I turn it I believe it has failed. It goes to U305 which I believe is the pulse generator circuit. I have tried out the pulse generator on the step generator before and it appeared to have worked fine before from what I recall, R300 was also brown looking from before.

Here are the capacitors I replaced on the main board to give an overview what I have done
C391 C393 - 15uF 100V (Floating supply step generator)
C591 C596 6.8uF 35V (+30V Q594 sweep dimming)
C295 C297 15Uf 20v (+12V -12V DC filtering from P722)
C568 C569 4.7uF 50V (Display filter)
C268 C265 0.27uF 50V (U268 Pulse generator
C218 C215 0.47uF 35v (U268 Pulse generator)



Mlynch001
 

Chuck;

Very important information. The designers used tantalum caps in certain circuits for specific reasons. We all know the failings of these caps, however, I try to always replace like for like, unless a knowledgeable person can give specific and well founded reasons to do so. Increasing rated voltage of a capacitor is one thing, but completely changing types of caps is something entirely different. Making recommendations for Such changes is far above my knowledge and understanding. I defer to those who are in the know.

Thanks for your insights.
--
Michael Lynch
Dardanelle, AR

DW
 

If I understood correctly I didn't substitute any electrolytic capacitors for tantalum capacitors, the tantalum capacitors previously installed I replaced with tantalum

I did however replace some electrolytic cap's with other types as I will describe

C218 and C215 there were very small 0.47uF 35V electrolytic capacitors I replaced with tantalum

C391 393 was 15uF 100V electrolytic capacitors replaced with film capacitors, though larger in sized they managed to fit, they didn't appear polarized but if you would like to check yourself click the link below

https://www.digikey.com/product-detail/en/illinois-capacitor/156MWR100K/1572-1074-ND/5344004

Thank you for your response

Craig Sawyers <c.sawyers@...>
 

Chuck;

Very important information. The designers used tantalum caps in certain circuits for specific
reasons.
We all know the failings of these caps, however, I try to always replace like for like, unless a
knowledgeable person can give specific and well founded reasons to do so. Increasing rated voltage
of a capacitor is one thing, but completely changing types of caps is something entirely different.
Making recommendations for Such changes is far above my knowledge and understanding. I defer to
those who are in the know.

Thanks for your insights.
--
Michael Lynch
Dardanelle, AR
In my experience the only tantalums that should be replaced are those used for supply bypassing -
generally 16V on 15V lines, 6V on 5V lines or (believe it or not) 50V ones on 50V lines. Those fail
short circuit, and in later instruments Tek replaced them with aluminium electrolytic.

Any other tantalum bead capacitors as part of signal level circuits are not stressed and (with almost
zero exception) do not fail and should not be shotgun replaced. Tek used them in those locations for a
reason.

I think the OP has adopted the shotgun approach and rendered a functional 577 non-functional.

Craig

DW
 

Sorry for the delayed reply, I was compiling information in hopes that would help assist in this situation.

Below is the list of the configuration of the 577 as I received it before I I did any work to it. If some components appear incorrect below then it is a result of the previous owner who performed nonstandard modification. This seems to be the case of a previous owner modification as the manual calls for electrolytic capacitors in various locations on the board where I found tantalum capacitors. I found a install error on C299 which was supposed to be a 4.7uF 50V capacitor and I found a 6.8uF 35V tantalum, that was found as I was in the process of replacing capacitors.
C391 C393 - 15uF 100V (Floating supply step generator) *Electrolytic*
C591 C596 6.8uF 35V (+30V Q594 sweep dimming) *Tantalum*
C295 C297 15Uf 20v (+12V -12V DC filtering from P722) *Tantalum*
C568 C569 4.7uF 50V (Display filter) *Tantalum*
C268 C265 0.27uF 50V (U268 Pulse generator *Tantalum*
C218 C215 0.47uF 35v (U268 Pulse generator) *Electrolytic*
###((C299 C293 4.7uF 50V)) *Tantalum* *C299 was discovered to have a 6.8uF 35V tantalum inappropriately installed here (+30 -30V DC filtering from P2722)

Thanks for the reply Chuck about C295 and C297, please check out the link below showing a schematic that gives more information of their specific purposes. I said DC filtering but I think I take that back after looking at this, It appears to be coupling to ground to reduce noise. Perhaps if I have this right there is AC rectification ripple filtering capacitors which I think is what you meant that is specifically for electrolytic capacitors when you previously mentioned C295 and 297. From my observation of the schematic with the link listed below these are noise filtering capacitors which tantalum or even ceramic would be better suited for. Please correct me if I am wrong..

https://groups.io/g/TekScopes/photo/86536/0?p=Created,,,20,2,0,0

DW
 

When I say "AC rectification" I noticed those two words don't go together. I should say the ripple after the rectification of AC which is filtered by the capacitor to smooth out the DC voltage, that makes more sense

DW
 

From the problem I encountered with the 577 curve tracer to the replies I noted on this topic, it has become clear to me of what not to do when servicing equipment. I value the 577 and appreciate its useful features it provides, thus I am the one who takes the most burden here as well as setting a example to others. I can't be so careless when servicing equipment. I have other working equipment which I value and want to avoid situations like this with them. It seems the concept I apply to the other instruments of if it isn't broke then don't fix it is working out quite well for them. Unfortunately I didn't leave well enough alone with the 577 and it is to my regret.

My hopes are to salvage the 577 thought I must admit my knowledge with electronics are limited (rather basic) as we can obviously tell by now and how I created a real mess on my hands to undo.

One thought I have at least to start is to check carefully for solder bridges as I have done a fair amount of soldering, at least that will assure a shorted connection isn't the issue.

Craig Sawyers <c.sawyers@...>
 

From the problem I encountered with the 577 curve tracer to the replies I noted on this topic, it
has
become clear to me of what not to do when servicing equipment. I value the 577 and appreciate its
useful features it provides, thus I am the one who takes the most burden here as well as setting a
example to others. I can't be so careless when servicing equipment. I have other working equipment
which I value and want to avoid situations like this with them. It seems the concept I apply to the
other
instruments of if it isn't broke then don't fix it is working out quite well for them. Unfortunately
I didn't
leave well enough alone with the 577 and it is to my regret.

My hopes are to salvage the 577 thought I must admit my knowledge with electronics are limited
(rather basic) as we can obviously tell by now and how I created a real mess on my hands to undo.

One thought I have at least to start is to check carefully for solder bridges as I have done a fair
amount
of soldering, at least that will assure a shorted connection isn't the issue.
At least you did not do what I did. To find where a shorted tant was, I unplugged the multiway
connectors at the power supply. And then reconnected one of them one pin out. And blew up lots and
lots of silicon. Magic smoke came out and would not go back in again.

I've mainly fixed it now, but there are still gremlins.

Craig

Mlynch001
 

We have all made dumb mistakes. We learn our lessons, fix the damage and move on. The person who has not made one of these mistakes has never tried to fix much. I’ve plugged IC’s in backwards, probed the wrong pins, shorted out power rails, plugged ribbon cables one pin off, and the list goes on. I certainly hope you can get your instrument working again.

--
Michael Lynch
Dardanelle, AR

DW
 

Well actually I did, it was a HP 8642B generator. Good thing it was a junk parts unit as I plugged the power cable ribbon cable in 1 pin out of alignment. First thing it did when I plugged it in I never recall the fan immediately tuning on and then I started seeing smoke come out of the A4 latch board and that is when I realized something was very wrong. I powered it off and connected the rectifier board to the regular board on wrong. It was a honest mistake and nothing was there to let you know that you are putting it in wrong. At that point I am particularly careful about connecting ribbon pin cables.

DW
 

"The person who has not made one of these mistakes has never tried to fix much", very true statement

DW
 

* * * *
I am glad I took the main board out. Concerning Q384 and Q386, the two transistors fastened to the chassis of the curve tracer, I believe both of these are supposed to have a thermal insulator at the mating surfaces? Q384 has one but Q387 doesn't appear to. Q387 appears to have made a dead short with the frame which could definitely explain my symptoms.

John Griessen
 

On 9/8/19 6:21 PM, DW wrote:
Q387 appears to have made a dead short with the frame which could definitely explain my symptoms.
The bright side of that is it protected everything else!

DW
 

"Floating power supply" I believe I see what you mean!

The transistor is a TO-127, I believe the collector was shorted to the chassis of the instrument

DW
 

Well maybe the collector was not shorted but was electrically connected to the chassis of the 577

Michael W. Lynch
 

I almost left that little nylon insulator off the screw on the back side of one of those transistors, which would have had a similar effect.

Michael Lynch

From My I-Phone

mlynch003@...

479-477-1115

On Sep 8, 2019, at 7:28 PM, DW <wilson2115@...> wrote:

Well maybe the collector was not shorted but was electrically connected to the chassis of the 577