547 Recap


J
 

I have a 547 that I'm thinking about recapping. But I have a few questions before I start working on this, as I was having trouble finding info in the search function on the website.

I was attempting to recalibrate the scope when I got to step 9d. on the vertical amplifier: "Short the cases of Q1074 and Q1084 (see figure 5-4) together (not to ground) and note the trace deviation from electrical center. Deviation should not be more than 0.5cm maximum. Remove the short."

When I performed this step, I had about 7cm of deflection peak-to-peak (about 3 to 4cm from electrical center), and was asynchronous to the line. I started suspecting that it was a problem with the power supply, and I measured the peak-to-peak voltage to be about 200mV on the +100V rail when the collectors of these transistors were short. I reasoned that shorting the transistor collectors increased the load, and caused additional noise on the power supplies. And as a result, I found 200mV ripple on the power supply spec (exceeding the 15mV specification), so I need to at least find the culprit capacitors, and possibly recap the unit.

I then started looking at the locations of all of the power supply caps within the 547 and realized the mounted ones generally have more than one internal cap, and were mounted in ways that were very difficult (impossible?) to remove the mounted capacitors - under the parts on the ceramic terminal strops. The mounted capacitors were all 35mm in diameter.

Questions:
* If I order caps that are still 35mm in diameter, can I use the same mounting fixtures?
* How do I gain access to all these capacitors without disassembling the entire housing?
* If I recap the timing caps while I'm doing this - what voltage rating should I use? It's not specified in the parts list.
* I'm counting about 15 capacitors with lethal voltages on them - not sure I want them dangling around in the chassis. How have others gone about mounting them securely?
* I've thought about just leaving the original caps and adding the new caps where I can find good locations in the chassis. This isn't very clean though, but I'm still curious if others have had success with this approach.

Jerome


Brenda
 

Hello Jerome,

If you are thinking about recapping this, I would just focus on the electrolytics first but of coarse that is just my opinion. Tek 547 scopes are newer and I have never opened one up before but I am pretty sure that these use quite a but of disc caps which rarely goes bad. I could be very wrong, but I do know that they would not have the crappy bumblebee caps. As far as the timing caps goes, I would NOT recommend changing those out. I think there was an older thread that those rarely go bad and changing them generally created headaches. I have a 535A with the original timing caps and they are doing very well. I would also suggest to change 1 electrolytic capacitor at a time to make sure that things are going good. Changing them all at once and if something happens, you would be giving yourself more troubleshooting to do.

Brenda


Harvey White
 

I can't help you with some of these things, but perhaps I can tell you what you're doing in some of this.

The deflection plates of a CRT should (for various reasons) be driven in push pull.  That is, if you have a signal at the input of the amplifier, it should produce two signals, one going plus, one going minus.  This gives you effectively twice the sensitivity of the case where you hold one plate fixed, and feed a signal to the other.

Somewhere in the horizontal or vertical amplifier chain, you have a single signal.  There's a circuit that produces a positive going and a negative going signal from one signal.  The vertical or horizontal amplifier amplifies each signal and drives each plate from that resultant signal.  You have one chain doing a positive signal, one doing a negative signal.

If you were to short the deflection plates of the CRT, then if the geometry is right, the spot ought to be in the middle (either H or V) depending on what you shorted.

A useful technique is to go from the plates back and short the equivalent signal - to +.  If you don't get a display where the dot is exactly in the middle, then the amplifier is not giving you the same signal + and -, and you may have a bad component.

Anything you get where the signal is synchronous to the line generally indicates either ac line leakage to the amplifier, or bad power supply ripple.

Shorting two  transistors' collectors in this differential amplifier doesn't really increase the load, it just makes the + and - channel signals identical.

Proper working of a scope says that not only are the power supply voltages within tolerance, but the ripple is, as well.

Nasty things happen with ripple.

Harvey

On 5/9/2021 7:42 PM, J via groups.io wrote:
I have a 547 that I'm thinking about recapping. But I have a few questions before I start working on this, as I was having trouble finding info in the search function on the website.

I was attempting to recalibrate the scope when I got to step 9d. on the vertical amplifier: "Short the cases of Q1074 and Q1084 (see figure 5-4) together (not to ground) and note the trace deviation from electrical center. Deviation should not be more than 0.5cm maximum. Remove the short."

When I performed this step, I had about 7cm of deflection peak-to-peak (about 3 to 4cm from electrical center), and was asynchronous to the line. I started suspecting that it was a problem with the power supply, and I measured the peak-to-peak voltage to be about 200mV on the +100V rail when the collectors of these transistors were short. I reasoned that shorting the transistor collectors increased the load, and caused additional noise on the power supplies. And as a result, I found 200mV ripple on the power supply spec (exceeding the 15mV specification), so I need to at least find the culprit capacitors, and possibly recap the unit.

I then started looking at the locations of all of the power supply caps within the 547 and realized the mounted ones generally have more than one internal cap, and were mounted in ways that were very difficult (impossible?) to remove the mounted capacitors - under the parts on the ceramic terminal strops. The mounted capacitors were all 35mm in diameter.

Questions:
* If I order caps that are still 35mm in diameter, can I use the same mounting fixtures?
* How do I gain access to all these capacitors without disassembling the entire housing?
* If I recap the timing caps while I'm doing this - what voltage rating should I use? It's not specified in the parts list.
* I'm counting about 15 capacitors with lethal voltages on them - not sure I want them dangling around in the chassis. How have others gone about mounting them securely?
* I've thought about just leaving the original caps and adding the new caps where I can find good locations in the chassis. This isn't very clean though, but I'm still curious if others have had success with this approach.

Jerome





J
 

I was going to focus on the electrolytics for sure. I was also thinking about replacing them in sections rather than one at a time. Do the power supply inputs first, The power supply output second, then the plug-in caps. Based on your recommendation I may wait for the timing caps after working with them a bit. Thank you.


Brenda
 

On Sun, May 9, 2021 at 07:31 PM, J wrote:


I was going to focus on the electrolytics for sure. I was also thinking about
replacing them in sections rather than one at a time. Do the power supply
inputs first, The power supply output second, then the plug-in caps. Based on
your recommendation I may wait for the timing caps after working with them a
bit. Thank you.
Hi Jerome,

Harvey is right that Tektronix scopes do not like ripple. In fact, a lot of problems is due to the power supply issues. Just my 2 cents, replace one can electrolytic at a time. Some can be a single value while others will have multiple values. Doing 1 can at a time may seem like that it will take longer, but if there's a problem, will be much easier to troubleshoot. But yeah, go through the manual and the manual will tell you the max allowed ripple. If it's close to max, I would replace so you don't have to go back in a later time.

As far as timing capacitors goes, these are made by Tektronix in house and these are still superior compared to new capacitors. I am still learning about electronics and I have a few timing capacitors from Tektronix scopes that were beyond repair that still so accurate, it puts the new capacitors to shame as tolerance goes.

I do hope that this is helpful. And I would love to see that 547 come back to life again!

Brenda


J
 

Harvey,

I was looking at the schematic and noticed that when I shorted the cases of (Q1074 and Q1084) it was just testing the common mode of everything to the vertical deflection plates. I do get a trace nominally, so I wasn't convinced that there was anything wrong with the parts in the vertical amp. Once I short the cases, that is when I get all of the deflection. Hence my doubling back to focus on the power supplies since the 100V rail exhibited 200mV of noise during the short. Your point is well taken however, that the load really shouldn't change much. Problem is, I can't think of what could be wrong in the downstream circuit - particularly because I'm getting a reasonable scope trace when I'm just using a plugin to drive the input.

Perhaps more testing is in order before moving forward.

Jerome


Joe
 

Hi Jerome,

having given a 2nd life to a very early 575 Curve Tracer that had been resting in an attic for 25+ years I can tell you that all the electrolytics were still in useable condition and could be reformed, very slowly as Dennis T. had suggested but successful. No ripple problem! Please read my posts on the proceedings. I have also documented some photos showing the suspect capacitors and noted the ESR readings. In short, most caps of the HV generation were faulty as well as many bumble bees and some Spragues.

From my personal experience I can tell you when restoring some pretty old tube radio sets (e.g. Packard Bell from 1935) I found really dead electrolytics as expected. What I did was to remove them from the chassis, remove the innards just to have an empty can and then I put modern electrolytics in there. that was easy as new components tend to be much smaller than the old ones. Never choose the voltage ratings too close to the operating voltage. And I greatly recommend to just do one component at one time to be able to trace the results!

Regards, Joe


greenboxmaven
 

Tektronix power supplies often use one supply to reference the regulators on the others. In the 547 it's the -160 volt supply. If it is incorrect or has ripple, the other power supplies will be affected. In some models, like the 561 solid state version, the reference supply needs initially unregulated  voltages from the supplies it regulates to start up. If these voltages are not clean, all of them can be affected. Different people have their own ideas about reforming and keeping original filter condensers, I replace them all, usually at once. Most of the equipment I restore is 50-60 years old, and the heat inside has at least partially dried out the electrolytic condensers. If you like to restore boatanchors and have questions about keeping old condensers, consider getting a Sprague TelOhMike condenser tester or one of it's clones. It does a good job reforming old condensers and gives a good test for power factor and leakage.

    Bruce Gentry, KA2IVY

On 5/9/21 23:51, Brenda via groups.io wrote:
On Sun, May 9, 2021 at 07:31 PM, J wrote:

I was going to focus on the electrolytics for sure. I was also thinking about
replacing them in sections rather than one at a time. Do the power supply
inputs first, The power supply output second, then the plug-in caps. Based on
your recommendation I may wait for the timing caps after working with them a
bit. Thank you.
Hi Jerome,

Harvey is right that Tektronix scopes do not like ripple. In fact, a lot of problems is due to the power supply issues. Just my 2 cents, replace one can electrolytic at a time. Some can be a single value while others will have multiple values. Doing 1 can at a time may seem like that it will take longer, but if there's a problem, will be much easier to troubleshoot. But yeah, go through the manual and the manual will tell you the max allowed ripple. If it's close to max, I would replace so you don't have to go back in a later time.

As far as timing capacitors goes, these are made by Tektronix in house and these are still superior compared to new capacitors. I am still learning about electronics and I have a few timing capacitors from Tektronix scopes that were beyond repair that still so accurate, it puts the new capacitors to shame as tolerance goes.

I do hope that this is helpful. And I would love to see that 547 come back to life again!

Brenda




Jim Adney
 

On Sun, May 9, 2021 at 06:42 PM, J wrote:

* If I recap the timing caps while I'm doing this - what voltage rating should I use? It's not specified in the parts list.
I'll second the recommendation to NOT replace any of the timing capacitors. In the first place, I doubt if any of those are electrolytics, so they don't have that tendency to dry out and fail over time. Secondly, I believe Tek generally supplied these in matched sets, to assure the correct 1-2-5 ratio steps.

I have a couple such Tek sets of matched timing caps, bought decades ago via ebay. I bought them because they were cheap, they were Tek, and they were local (no shipping.) I've never needed them and I probably never will. If anyone needs a set, the part number I have is 295-0134-00, dated Dec 7, 1984. There's a Tek note attached to the bag that says, "DO NOT MIX SUBPARTS BETWEEN SETS." Each set contains 4 of the special silver metal cased Tek caps plus one brown silver mica. I'd be willing to sell them if anyone needs a set.

Anyone know what scope these were used on?

I also discovered that I have a LARGE random bag of those very nice Tek silver metal caps. I don't remember where these came from, but each appears to be marked with a Tek part number. So if anyone needs some such thing, please send me the approx length and diameter, to help pick it out of the haystack, and the part number, and I'll see if I have one.


J
 

Harvey - I think you're onto something, but I'm not sure how to proceed. I did some more testing on the power supplies - loading the 100V as described in the TU-7 datasheet - I unfortunately don't have a load bank or power resistors that will work at the higher voltages. But all power supplies worked just fine with the 100V load. This gets me circling back to the potential of a bad part in the vertical amplifier - but I'm not quite sure how to test it to narrow it down. I've thought about shorting the collectors of the Q1114/Q1134 and Q1104/Q1124 pairs. I've done a DC voltage check and things looked okay. I was a little surprised to see Q1109 was in saturation and the base diodes were off - I'm not sure if that is the intended operation or not.


stevenhorii
 

I saw a set of Tek timing caps on the Sphere Research (Walter Shawlee’s)
site. You likely have some messages in your in-box about their “Stuff Day”
sale. If you open the link and scroll down through the items, I think you
will find them.

Steve H.

On Tue, May 11, 2021 at 08:52 Jim Adney <jadney@vwtype3.org> wrote:

On Sun, May 9, 2021 at 06:42 PM, J wrote:

* If I recap the timing caps while I'm doing this - what voltage rating
should I use? It's not specified in the parts list.

I'll second the recommendation to NOT replace any of the timing
capacitors. In the first place, I doubt if any of those are electrolytics,
so they don't have that tendency to dry out and fail over time. Secondly, I
believe Tek generally supplied these in matched sets, to assure the correct
1-2-5 ratio steps.

I have a couple such Tek sets of matched timing caps, bought decades ago
via ebay. I bought them because they were cheap, they were Tek, and they
were local (no shipping.) I've never needed them and I probably never will.
If anyone needs a set, the part number I have is 295-0134-00, dated Dec 7,
1984. There's a Tek note attached to the bag that says, "DO NOT MIX
SUBPARTS BETWEEN SETS." Each set contains 4 of the special silver metal
cased Tek caps plus one brown silver mica. I'd be willing to sell them if
anyone needs a set.

Anyone know what scope these were used on?

I also discovered that I have a LARGE random bag of those very nice Tek
silver metal caps. I don't remember where these came from, but each appears
to be marked with a Tek part number. So if anyone needs some such thing,
please send me the approx length and diameter, to help pick it out of the
haystack, and the part number, and I'll see if I have one.






Dave Wise
 

Q1109 saturated is normal. It was added at S/N 2340 to protect Q1114 and Q1134 from excessive voltage when the amp is processing a common-mode transient from the plugin. It limits Vcc to 210V instead of 225V.

Dave Wise

From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of J via groups.io
Sent: Tuesday, May 11, 2021 10:33 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 547 Recap

Harvey - I think you're onto something, but I'm not sure how to proceed. I did some more testing on the power supplies - loading the 100V as described in the TU-7 datasheet - I unfortunately don't have a load bank or power resistors that will work at the higher voltages. But all power supplies worked just fine with the 100V load. This gets me circling back to the potential of a bad part in the vertical amplifier - but I'm not quite sure how to test it to narrow it down. I've thought about shorting the collectors of the Q1114/Q1134 and Q1104/Q1124 pairs. I've done a DC voltage check and things looked okay. I was a little surprised to see Q1109 was in saturation and the base diodes were off - I'm not sure if that is the intended operation or not.