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Thought I ought to introduce myself as I have a bunch of queries to resolve.

I'm an electronics engineer with 35yrs experience in the RF and analogue electronics industry. I first came across Tektronix equipment at Cambridge Consultants, where we had 7904's with the 6GHz sampling setup for use in designing impulse radars (before UWB was a twinkle in anyone's eyes). I then used them until about 2005 for various applications, particularly things like the differential amplifier plugins , TDR and sampling rigs and just for general 4 channel 'scopes, before moving to a company that only had new fangled digital scopes.

I acquired two 7904's, one in 'working' condition and the other provided as a spares mule by the kindly test gear dept leader when purchasing the 1st one in a testgear scrap sale. The 7904 was my principal homer scope for sometime, but after a house move ended up stored for about a decade. Now they are back in my new workshop, so its time to resurrect them.

I also have a TM504 4 slot frame with a PG502 pulse generator , FG504 function generator and a DC508 counter all of which need attention, so I'm going to be busy :-)

I'm also lucky to have a Tektronix TDS5054 DPO scope which is useful for debugging, but I much prefer the thin green line of the analogue 'scopes - call me nostalgic, but at least an analogue scope never lies!

Kevin.

Thanks for the responses. I'm familiar with restuffing cans,? ?but a bit wary as this a line voltage cap and the new capacitor would not be able to vent in case of catastrophic failure in its new confines.

I already replaced the secondary filter tants and a couple of dead rectifiers and 220uf electrolytic.

This unit was given to me as a spares unit by the lab test admin when I bought the other one in a scrap sale.? The rectifiers were unsoldered and caps removed, so had already been part diagnosed. I found arc marks on the faulty line cap, so I think it was accidentally shorted at some time. The inrush limiter was bust too - the coating had crumbled so probably moisture ingress for that one.

Kevin

Hello learned audience!
I have looked at the archives for the above topic. Wanted to ask if the D10/D11 were the only ones that would substitute into a 577 curve tracer. I have a couple of 577 which I have yet to test and wanted to NOT discard a potentially useful tube. I am scrapping a pile of O'scope frames (need space) so wanted to pull the CRTs if any use. I also have several PMI 1038s which were also mentioned so may save those as well. Any other recommendations? A shame as I have a pile of 5000 series scopes, real pretty, but too dated now! Thanks in advance. Regards, Jeff Kruth

Dave,

X types are for across the mains and Y is from mains to ground. If there is a 1 after the letter, it is available in a higher voltage which is fine for 120V types and even better for 230-240V. Another thing that helps with noise reduction is to use a shielded cord. The L and C of this reduces noise. If an ISOBAR is used, that further reduces noise and line spikes.

Mark

I don't know the specifics of their failure mechanisms and will take
Gerrit's word on that, but their "special purpose" is to help control
conducted emissions (EMI) that the instrument couples onto the power line.
Their reactance profile is such that they are open at low frequencies
(e.g., power line frequencies) but provide a conductive path from
line-to-neutral (120 VAC mains power) or line-to-line (240 VAC mains power)
at higher frequencies, increasing as the noise frequency increases and thus
keeping the noise (conducted emissions) from getting out the power line
and onto the external power infrastructure.

I believe that their use pre-dated the use of internal line filters made by
companies such as Corcom and Schaffner, but I could be mis-remembering.

The two types "X" and "Y" are for the different mains voltages (120 or
240). I forget which type is for which voltage.

If one fails, assuming it fails open (which they are designed to do), the
instrument will generally function as it did before the failure, although
with increased conducted emissions.

DaveD
KC0WJN


On Tue, May 27, 2025 at 19:57 evan foss via groups.io <evanfoss=
[email protected]> wrote:

I have older equipment with other less aesthetically pleasing
capacitors in the same functions and they are all fine. I had the
parts fail without ever having been installed. Kinda suggests it's not
wear related.

On Tue, May 27, 2025 at 6:18?PM Gerrit via groups.io
<amigaking@...> wrote:

Those Rifa caps in the line filter are safety caps. I was told this is

generally a failure mode of safety caps. Short voltage spikes on the power
line damage the caps. Safety caps are self healing. But with every damage
they lose a bit of capacity. When the capacity reaches a critical level,
the line filter may start oscillating at its resonance frequency. This
effect in combination with the parasitic resistance will heat up the
capacitor and destroy it finally.

Thats why it is also not a good idea placing a second line filter in

front of an existing line filter. The combination might get also resonant
and destroy al least one of the elements.

My opinion:
If the scope is not in daily use and is unplugged most of the time, new

Rifa caps will last very long again. Maybe the scope should be kept in a
dry room too for various reasons.

Regards,
Gerrit

I have older equipment with other less aesthetically pleasing
capacitors in the same functions and they are all fine. I had the
parts fail without ever having been installed. Kinda suggests it's not
wear related.

On Tue, May 27, 2025 at 6:18?PM Gerrit via groups.io
<amigaking@...> wrote:

Those Rifa caps in the line filter are safety caps. I was told this is generally a failure mode of safety caps. Short voltage spikes on the power line damage the caps. Safety caps are self healing. But with every damage they lose a bit of capacity. When the capacity reaches a critical level, the line filter may start oscillating at its resonance frequency. This effect in combination with the parasitic resistance will heat up the capacitor and destroy it finally.

Thats why it is also not a good idea placing a second line filter in front of an existing line filter. The combination might get also resonant and destroy al least one of the elements.

My opinion:
If the scope is not in daily use and is unplugged most of the time, new Rifa caps will last very long again. Maybe the scope should be kept in a dry room too for various reasons.

Regards,
Gerrit

Kevin,

These cans I restuff with Nichicon LGR 1200mfd 200 or 250V types. I use small screws and ground lugs to mount the cans inside. Drill a hole at the bottom of where the screws go slightly smaller than the screw threads to allow for enough bite, not too much to twist the screw head off. You may have to move the drill bit around to increase the diameter of the hole enough. The additional capacitance is fine. To compensate for the additional capacitance and further reduce inrush current, I put in a CL-70 NTC from the mains filter to the wire. This is inside the cavity behind the filter cans. You may want to restuff the cans in both scopes.

While you have the power supply/ies out, check the 1mfd tantalums on the trigger board that faces the rear. Mine were open. I used 1mfd 63V film types. That helped, for me. I also did other work such as increasing resistor wattage and condenser replacement.

Mark

Those Rifa caps in the line filter are safety caps. I was told this is generally a failure mode of safety caps. Short voltage spikes on the power line damage the caps. Safety caps are self healing. But with every damage they lose a bit of capacity. When the capacity reaches a critical level, the line filter may start oscillating at its resonance frequency. This effect in combination with the parasitic resistance will heat up the capacitor and destroy it finally.

Thats why it is also not a good idea placing a second line filter in front of an existing line filter. The combination might get also resonant and destroy al least one of the elements.

My opinion:
If the scope is not in daily use and is unplugged most of the time, new Rifa caps will last very long again. Maybe the scope should be kept in a dry room too for various reasons.

Regards,
Gerrit

In the groups interested in resurrection of old(er) radios, this is a common problem. One solution is to, carefuly, open the capacitor base crimp and remove the innards. Replace with suitable leaded capacitor or combination to get the needed capacitance. Insulate leads, solder to the base contacts, insert, (test), recrimp. Usually the replacement caps are smaller and fit is not a problem.

Hello,

I have two 7904A scope frames. The first is now functioning after some power supply repairs, but the second is very sick.

Eventually I have determined the trouble is that one of the 950uF 200V capacitors is leaky, and probably very leaky at full voltage and needs to be replaced.

The problem is I cannot find a suitable part anywhere - does anyone have a supply or know where to get one? They have an oddball mechanical mounting and screw spacing (~19mm) and all the caps I can find that might work have 22mm spacings which wont fit.

Reluctant to consign the scope to the great breadboard in the sky, but also dont want to do a major change unless absolutely necessary.

Any help appreciated,

Kevin.

PS I also have a reluctant 7a26 plugin, but thats a matter for another thread :-)

On Sat, May 24, 2025 at 01:55 AM, Mark Vincent wrote:

is it best as PM to do a sweep of the sockets in the scopes and plugins? Is
there a point where the sockets were better so replacing would not be needed?

The bad C95 sockets I mentioned are easily recognized by the unique profile on the short end as
shown on the center picture at the bottom of the Wiki page. If you have those in your scope or
TM500 plugins the best would off course be to replace them with some modern sockets or even
skip the socket and solder the IC's directly to the board. Depending on instrument this could
require a lot of work and be quite time consuming so if not done and you experience intermittent
problems at least the sockets would be the first suspects. Other sockets could off course also
be bad sometimes but not by far as common as the C95's.

/H?kan

By "buzzing out" I mean using a meter with either a buzzer /continuity or set to ohms to find what wire goes where. The intensity pot wiper - white/yellow stripe wire I think - goes from the front of the scope, to the back/EHT section. It connects to R830, a 3M3 resistor & some caps, the other end of R830 goes to the grid of the CRT. So, find R830 by tracing the wiring, once confirmed pop your meter across it set to DC volts, power on, read meter.

If you have a cathode/grid short you'll get a high reading, over a 10v say. A normal valve correctly biased has a grid current of a few uV if so, that which would give a reading of 6v ish. I could be wrong about this though, others please chip in & tell me if I'm wrong.

Andy.

Mike this is exactly what I just said.

On Mon, May 26, 2025 at 9:30?PM Mike Drum via groups.io
<mjdrum1958@...> wrote:

New Rifa caps have the same failure mode as old Rifa caps???

I sure hope that's not true! Does anyone know for sure. Hard to imagine
manufacturing steps have not been taken to eliminate that.

Mike

On Mon, May 26, 2025, 20:52 Mr. Fang via groups.io <thomas.fang=
[email protected]> wrote:

I had to purchase this value (.1uF + 22ohm) recently for a Tek 7912AD
repair, and if it helps anyone, the part number to order at Mouser is
80-PMR209MC6100M022. (Assuming you're ok with the risk of newer RIFA caps
having the same failure mode as old ones.)

-Thomas

New Rifa caps have the same failure mode as old Rifa caps???

I sure hope that's not true! Does anyone know for sure. Hard to imagine
manufacturing steps have not been taken to eliminate that.

Mike

On Mon, May 26, 2025, 20:52 Mr. Fang via groups.io <thomas.fang=
[email protected]> wrote:

I had to purchase this value (.1uF + 22ohm) recently for a Tek 7912AD
repair, and if it helps anyone, the part number to order at Mouser is
80-PMR209MC6100M022. (Assuming you're ok with the risk of newer RIFA caps
having the same failure mode as old ones.)

-Thomas

I had to purchase this value (.1uF + 22ohm) recently for a Tek 7912AD repair, and if it helps anyone, the part number to order at Mouser is 80-PMR209MC6100M022. (Assuming you're ok with the risk of newer RIFA caps having the same failure mode as old ones.)

-Thomas

Thanks Mark. Will contact my friend to see if he has the appropriate adapter.

Gordon

Thanks Andy, but a bit puzzled by your procedure. My DMM doesn't have a continuity beeper so not sure what you mean by "buzzing out" from the intensity pot. Are we trying to determine if the resistance across R830 changes when the intensity pot is rotated? Or are we looking for an excessive voltage drop (>1V?) across it when the scope is (presumably) powered on.

Thanks
Gordon

According to a later manual the change was made at B040000:
0.1uF + 22 ohm 250VAC
Albert

Bruce your on 240 or 120V mains?

THe 240V blows them often!

120V no so.

Jon

My advise is this: Look up the RIFA parts you are replacing and then
use parametric search to find another vendor. RIFAs problem isn't wear
out from heating & cooling or "dry out". I left a fresh order of them
in their original packaging in a box in a climate controlled closet
(away from UV light) for a few years and when I opened it the
encapsulation had cracked. I believe the resin mix used is going
through an overly aggressive cure to keep the production line running
fast or something.


On Sun, May 25, 2025 at 6:54?PM bandrsn via groups.io
<bandrsn@...> wrote:

All,
Well, it finally happened - POP! - sizzle, sizzle. My fine 7104 blew its RIFA capacitor in a loud manner, and I know, I should have replaced it sooner.

I found what's left of the RIFA in the shielded power supply, but there's not enough of it left to get info from its case.
The downloaded service manual and schematics don't exactly match the B053423 S/N. The A23 Inverter Board shows two ceramic 0.02UF, 1400VDCAC capacitors in parallel (C1205 and C1206), both in series with a 27 Ohm resistor, R1205. My A23 board has (had) a single RIFA where R1205 was and what appears to be a jumper across one of the places for the capacitors.
My question is, what should the replacement RIFA be? Since the exploded one is in bits and pieces I can't get enough info from it to know.
The schematic shows the RIFA directly across the hot and neutral lines after the power switch.

Any brand/device values, etc. recommendations for a commonly available 115VAC replacement that, hopefully, will not explode again?

Also, the power supply construction makes it difficult to gain access to the back side of the Inverter Board. I was able to carefully snip away the bits of the RIFA plastic and foil, etc. so there are about 1/4" or slightly more of the tinned leads sticking out on the component side. I was planning to cut, bend and quickly solder to these exposed leads, being careful not to melt the solder too much on the other side and cause a short.

Does anyone foresee a problem with this if care is also taken to avoid any shorts, etc. on the component side too?

Thanks,
Bruce
AB6BA