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Troubleshooting tips for a Tek 485 scope

Reed Dickinson
 

Hi Gang:

I want to pass on a few troubleshooting tips I have developed as an aid to keeping your 485 running perfectly. Follow these steps in sequence for best results if your 485 ceases to run properly.

1) Remove case and inspect all PCB's very carefully. Look for empty sockets or burned or charred components.
2) Before applying power measure the DC resistance, to ground, of each of the 13 low voltage power supplies in your 485. If you find one with low or 0 Ohms resistance start removing inter-board jumpers while watching the ohm meter. First remove the N and P jumper blocks. This isolated the A8, A10 and A11 PCB's. If the short persists then remove the X, Y and Z jumpers. If the short persists then remove connectors on P1500, P1580, P1590, P1560 and P1570. If the short still persists then the problem is on the A13, the power PCB. Before replacing the shorted cap you want to know if the short has taken any other components out. Find the number of the defective cap from the TANT_PCB.XXX file that I downloaded to the forum. Remove the associated capacitors while monitoring the DC resistance. When you find the shorted one check the schematic to see if any series component might be damaged. Don't overlook R1579, which may be on the back side of A13 if the +5.5V rail measures low or open. Once the first short is fixed re-install all the removed jumper blocks and repeat part 2 again. If no further shorts are found proceed to 3. If the short disappears when the N and P jumpers are pulled then leave N and P out, move your ohmmeter to the defective rail on the A10 PCB and pull jumper L to isolate the A8 PCB. pull jumpers E, F and K to isolate the A12 timing board and jumpers C, D and AA to isolate the A11 horizontal PCB. Go to the defective PCB, replace the defective cap and repeat part 2. Use similar logic to locate any defective caps on other boards.
3) Now apply power and is all is well the power should come on. If it does not then repeat part 2 again as another short has developed due to the shock of power being applied.
4) At this point you have determined that power should be on but it isn't. The +120, +25, +9, +5, -15, -9 and -5 Volt supplies are all monitored at pin 2 of U1910 on A13. This summing junction should be 0V for power to come up. With the power off jumper TP1951 at pin 2 of U1910 to ground. Put a Voltmeter on the +50 rail and momentarily apply power. If the +50 V rail does not come up replace U2042 and repeat 4. If the +9 rail is low replace U2052; U2062 if the +5 is low. U2072 if the -5 is low and, U2086 if the -9 is low. Do not leave the power applied any longer than necessary as a critical bias voltage might be missing.
5) If it still does not power up put a voltmeter on U1910 pin 13, if more than 0.5V is present when power is momentarily applied then something is drawing excessive current. Possible problems are shorted HV caps in the A11 PCB. With power off remove the brown wire shown on the lower left corner of schematic 13, be careful as there is -3KV on that wire, reapply power. If it still does not work check C1601, C1602, CR1601 or C1603. If these components are OK remove jumper U on A13, cut pin 3 free and re-install the U jumper with only 4 pins on it. This bypasses the X6 Voltage multiplier U1600, try power again. If none of these work check all the diodes and transistors on the A14 inverter PCB. If these are OK then you might have a bad transformer on the transformer PCB, replace the PCB.
6) I take out the A7 vertical PCB to replace all the 11 Tantalum high risk caps on it. To remove the PCB you must disconnect the high voltage lead going to the CRT. Do NOT, repeat NOT, pull the plug in the HV lead unless you have allowed at least 6 hours for the stored charge in the CRT to dissipate. The 18KV stored in the CRT will seek a path to ground and that path is through U660. If you pull the HV plug and there is a full charge on the CRT you will ruin U660, the vertical output IC. Please don't ask me how I found this out.

I have noted that the Tantalum caps that are in a hermetically sealed enclosure never fail while those in the dipped configuration, with a low Vc/Va ratio, frequently fail. So, I have concluded that the presence of contaminants has shortened the life of the dipped variety cap. It is either moisture entering via the exposed legs or components internal to the cap leaving via the exposed legs. I do not know which is the case but I do know it is happening. Tek used Tantalum capacitors in large quantity because they were thought reliable, cheap and plentiful. Over time Tek has found out that serious shortcomings exist that were not known early on. We are now paying the price for Tek's oversights.

Comments, corrections or complaints are welcome.
Reed Dickinson
reed714@...

Brendan
 

On Sat, Jun 2, 2018 at 10:11 pm, Reed Dickinson wrote:


Hi Gang:

I want to pass on a few troubleshooting tips I have developed as an aid
to keeping your 485 running perfectly. Follow these steps in sequence
for best results if your 485 ceases to run properly.
Wow! Thanks for great writeup. 485s are my favorite scopes. The attenuator boards are a work of art and amazingly complex just in themselves. Thank you again for sharing your experience and knowledge. I will follow these procedures next time my 485 breaks down.

Mlynch001
 

Reed,

I have worked on several scopes, including 465,465B, 458 and 475A. I have just found a beautiful 485, near pristine condition. Other than dirty/sticky switches, I see one problem, I can put a 5V square wave into either channel set the scope to 1V range, this gives a trace that is almost 5 divisions high, as long as the trace is centered. When I shift the position up or down, the trace shrinks until it almost disappears. If I have a single trace with no signal, that line cannot be moved off the screen, as it normally should do. Any ideas where to start looking?

--
Michael Lynch
Dardanelle, AR

Reed Dickinson
 

Hi Michael:
Remove the scope from the case and spray contact cleaner on the slider of the BEAM FINDER pushbutton switch and then immediately exercise it about 20 times.   Your note said you have dirty/sticky switches.  Use the contact cleaner (the best one I have found is the 10 ounce Craig Products, P/N DCC-V510) cans are available through Jameco as item 2094231 at $9.95.  Use it generously on all switches, exercise them, and put a drop of Kroil on the shafts of each front panel adjustable potentiometer.  If you find problems with the vertical preamps you will have to dis-assemble them and VERY CAREFULLY adjust the fingers on both the 1MEG AND 50ohm PCB's.  Use the spray on the 1Meg/50 Ohm switch contacts and Kroil on the variable V/DIV control.  I always replace the 43 Tantalum caps with low VR/VA ratios.  See my eBay ad, item 133191518597, for the number of caps to replace.
Regards
Reed

On Thursday, January 9, 2020, 11:27:17 AM PST, Mlynch001 <@mlynch001> wrote:

Reed,

I have worked on several scopes, including 465,465B, 458 and 475A.  I have just found a beautiful 485, near pristine condition.  Other than dirty/sticky switches, I see one problem,  I can put a 5V square wave into either channel set the scope to 1V range, this gives a trace that is almost 5 divisions high, as long as the trace is centered.  When I shift the position up or down, the trace shrinks until it almost disappears.  If I have a single trace with no signal, that line cannot be moved off the screen, as it normally should do.  Any ideas where to start looking?

--
Michael Lynch
Dardanelle, AR

Mlynch001
 

Reed,

Thanks for the advice! I just finished the repair of a very nice 465. I had wondered if this was a simple matter of cleaning switches on the 485. The 465 had a beam finder that seemed to be working, but it had uncontrollable intensity. It turned out that the beam finder was not actually releasing, but was making some sort of intermediate connections and causing that issue. I cleaned the switches and used some DeOxit on them. This fixed that intensity problem and everything else as well. I have fixed many scopes as simply as cleaning their filth or stuck switches. I trust that this 485 repair is as simple as you have suggested. Your work speaks for itself, and I value your generous advice greatly. I’m sure that the capacitor replacement will take much more time than cleaning the switches. This particular scope is a beauty, I have been looking for a nice one for some time, and I have almost nothing in it. Since I have little experience inside the 485, I was not wanting to go poking around inside and cause inadvertent and most unnecessary damage. Thanks again.

Sincerely,
--
Michael Lynch
Dardanelle, AR

Kevin Oconnor
 

Hi Reed,
Great text in ur eBay listing on the 485!
I have a theory on the low Vr/Va tantalum failures, but not much data to support it. I suspect that the 485 tantalum “system” follows some form of Arrhenius behavior that is linked directly to the “live on time” of the scope. As a portable unit, some users would install the 485 on a bench and power it through the work week and maybe for many hours when the lab lights were out. Others were used only on rare occasions when a high performance portable scope was needed. After 35-40 years, these units would fall in very different places on an Arrhenius curve.
My 485 is from the latter case. A few 1000’s of hours rather than 60,000 (8hrs/day x 20years service life). I have never done a full recap. I did replace one transistor and one tantalum at some past date.
But it is also true that every time I push that on/off button it is like a one-armed bandit. Sooner or later I will get three cherries!

Kevin KO3Y

Mlynch001
 

Reed,

Followup on my 485 repair. I got the instrument apart and cleaned all the switches. I pulled the attenuators out on both channels so that I could access those 6 switches below the CRT. After checking all the power supplies (all OK), then cleaning all the switches and pots then re-assembly, the instrument works like a charm. It was just as you stated and very similar to that 465 that I just finished, the Beam Finder was stuck in an intermediate position. The Ch#1 50Ohm/1MOhm switch was also similarly affected. Once those were cleaned and lubed, the scope worked almost like new. This scope is very clean, no dust, dirt, almost none of that black film on the HV Section and no sign of any repairs. Date codes on the ICs are for 1982 in the middle of the year, weeks 15-30 are most of what you see. This scope appears to have been used very little. All the primary functions appear normal and it shows a good 50 Ohm DC bandwidth response to above 300Mhz on both channels.

There is one small issue, but I do not know the best solution for this problem. The Channel #1 BNC center is very slightly loose inside the PTFE insulator and obviously this is a special Part, not a "standard" BNC connector. This "looseness" causes the trace to fluctuate slightly when you first plug in, since the BNC back terminal does not appear to be soldered to the board. Looks to me like there is a little tab that snaps over the center pin of the BNC and simply presses against the board through that little snap on tab. I hate to replace the entire Attenuator, so I was wondering if just finding a new BNC and replacing this would be possible and satisfactory?

Do you offer a BOM from a vendor or a kit that you have assembled which contains all the proper replacement tantalum capacitors for this scope? It would be nice to be able to buy all of the correct parts in one batch, instead of having to guesstimate which ones are correct and ordering piecemeal.

Your advice was spot on correct and I want to thank you for helping me find this problem.

Sincerely,

--
Michael Lynch
Dardanelle, AR

Reed Dickinson
 

Hi Kevin:
There are two types of Tantalum caps used by Tektronix in their 485's and other scopes.  The first is the hermetically sealed model and the second is the axial dipped model.  In refurbishing over 200 485's I have only found one bad hermetically sealed cap.  I have found untold numbers of dipped Tantalum caps so I must conclude that moisture seeps into the interior of the dipped model causing it to short out.  This appears to be aggravated by the Vr/Va ratio as caps with Vr/Va ratios near 2 are much more prone to shorting out.  And, as you have observed, age and operating time aggravate the situation.
Here is how I find the shorted cap.  Plug in the scope and power on, if it ticks then the chance of a shorted cap is high.  Power off, wait 10 minutes for all caps to discharge and measure resistance to ground of all 14 voltages.  Once you find a short leave the ohm meter on that voltage and start pulling out, one by one, the 5 pin inter PCB jumpers while watching the meter.  Once the reading goes up you have isolated the bad cap, trace it out and replace the bad one.
RegardsReed

On Friday, January 10, 2020, 07:39:59 AM PST, Kevin Oconnor <@KO3Y> wrote:

Hi Reed,
Great text in ur eBay listing on the 485!
I have a theory on the low Vr/Va tantalum failures, but not much data to support it. I suspect that the 485 tantalum “system” follows some form of Arrhenius behavior that is linked directly to the “live on time” of the scope. As a portable unit, some users would install the 485 on a bench and power it through the work week and maybe for many hours when the lab lights were out. Others were used only on rare occasions when a high performance portable scope was needed. After 35-40 years, these units would fall in very different places on an Arrhenius curve.
My 485 is from the latter case. A few 1000’s of hours rather than 60,000 (8hrs/day x 20years service life). I have never done a full recap. I did replace one transistor and one tantalum at some past date.
But it is also true that every time I push that on/off button it is like a one-armed bandit. Sooner or later I will get three cherries!

Kevin KO3Y

Reed Dickinson
 

Considerable interest has been shown on this forum concerning repair of Tektronix 485 oscilloscopes.  They are wonderful instruments but they do present new challenges to a troubleshooter.  There are 14 low voltages present in the 485 and almost all of them must be functional for the 485 to turn on.  Loss of even one would put the 485 in the 'tick' mode.  The inverter is trying to turn on but the voltage sensing node feeding pin 2 of U1910 on sheet 14 is not at zero volts.  This supply sensing is incorporated to insure that no biasing voltages are missing which might cause further problems.  To find out which supply is missing or faulty connect a jumper between TP1951, shown adjacent to U1910 on sheet 14, and ground.  With scope power off connect a voltmeter to the first voltage, +50V, test point on the 485 SWEEP PCB.  Momentarily power on the scope, read the meter then immediately power OFF.  If all is well move to the next test point +5V.  Continue testing every low voltage supply in the scope.  The +13 V supply is not in the sensing node so measure that supply at the test point just above completion jumper E4.  
For high voltage problems disconnect and insulate the brown wire at the rear end of A11, the horizontal amp PCB, it is labeled BN on sheet 13.  If the problem still exists replace C1602 and C1603 on the A15 transformer PCB.  If there is still a problem check the caps shown to the right of the BN jumber on sheet 13.
Good luck!
Reed Dickinson

Yeun-Jung Wu
 

Hi Reed:

Thank you for sharing your well documented trouble shooting guide on 485. I got my broken 485 back in 2014. Luckily culprit was only one tantalum capacitor. On September last year when I pulled out this 485 to display wave form of a HP function/pulse generator, the screen flickered a moment then died. Then I turned on another long stored 2467B: beautiful display followed by an explosion a few minutes later. Those few nights I stayed up to get rid of 2467B's X2 Rifa and successive tantalum failure of 485, one after another, totally six of them!

Later I found the 50 Ohm input of both channels died. Special custom made IC no longer direct the relay to route signal through 50 Ohm's attenuation path.

Do you have any suggestion on fixing this problem?

Yeun-Jung Wu

Reed Dickinson
 

Tektronix had an inductive collapse transient problem from the coil of K151 which, over a period of time, tended to wipe out U80, the 155-0076 input protection IC.  After SN B155789 a PNP transistor was installed as the relay driver which stopped this problem.  Get two 155-0076 IC's from Sphere, this will probably clear up these problems,  If your serial number is B155789 or below it would be smart to add the relay driver circuit shown on sheet 1 in your manual.
Regards Reed

On Saturday, January 11, 2020, 07:36:47 PM PST, Yeun-Jung Wu <b0190@...> wrote:

Hi Reed:

Thank you for sharing your well documented trouble shooting guide on 485. I got my broken 485 back in 2014. Luckily culprit was only one tantalum capacitor. On September last year when I pulled out this 485 to display wave form of a HP function/pulse generator, the screen flickered a moment then died. Then I turned on another long stored 2467B: beautiful display followed by an explosion a few minutes later. Those few nights I stayed up to get rid of 2467B's X2 Rifa and successive tantalum failure of 485, one after another, totally six of them!

Later I found the 50 Ohm input of both channels died. Special custom made IC no longer direct the relay to route signal through 50 Ohm's attenuation path.

Do you have any suggestion on fixing this problem?

Yeun-Jung Wu

Yeun-Jung Wu
 

Hi Reed:

Thank you so much for directing me to Sphere. I will try my luck to get a pair of them.

Regards, Yeun-Jung Wu

Kevin Oconnor
 

On Sun, Jan 12, 2020 at 01:06 AM, Yeun-Jung Wu wrote:


Hi Reed:

Thank you so much for directing me to Sphere. I will try my luck to get a pair
of them.

Regards, Yeun-Jung Wu
“Get two 155-0076 IC's from Sphere“

Please post your response from Sphere.
I have one problematic 485 channel too.

Kevin KO3Y