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Re: Hello - new to this list but not to Tek scope - Power supply trouble
Paul: good call - I hadn't realised that - thank-you! I looked on Ebay, and the ones there aren't quite what I need.
Ed: thanks for the thread sizes - saves me fiddling with the thread gauges! I'm erring towards the adapter board method. Will lay it out on KiCAD, and get the boards made by JLCPCB - likely I'll have some spares and also I can put the gerbers in the files area once I know they work. Will report back in due course - thanks for (a) verifying that they are unobtanium, and (b) helping me work out a plan of attack and a back up :-) Kevin. |
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Re: Tektronix 7104 - RIFA explosion and replacement
Maybe the correct answer is that these caps should have not been made or sold or bought or used since the problem was known and understood (probably quite a few years ago). I don't know the real answer either, but I think it's a pretty safe bet to avoid using any metalized paper dielectric cap of any brand for any power application, unless it's submerged in an appropriate (non-PCB) oil or saturated with some other material (remember the waxed-paper/lead foil caps from the old days?). There are plenty of modern plastic/metal film type caps with X ratings available.
My own experience is that these RIFAs and similar are a POS that should be replaced whenever encountered and possible. Even here in 120V-land, I've seen enough go bad to declare summary judgement on these things. It's not an emergency, but definitely do it when opportunity arises. I still have lots in my old gear collection, and I try to take care of it whenever practical. BTW if you need to replace an R/C snubber unit, it's easiest to just use a proper cap and tack in a separate series resistor. Ed |
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Re: Tektronix 7104 - RIFA explosion and replacement
Richard, have a look at a current RIFA manufactured capacitor of the
same type and value as you find in a 20 year old instrument. Does it look like they changed anything? Isn't it odd that they are the only ones on the market that are in that pretty transparent encapsulation? Would my sharing the pictures of the never even soldered in or clipped leads on the cracked parts make my test seem more believable? If there is a credible counter argument to what I am saying it's that I only have a sample size of like 5. For the record I don't believe that they all fail as quickly as the ones I was unintentionally testing. I find it odd that my simply suggesting : Get the same type and value but from another manufacturer is controversial. On Wed, May 28, 2025 at 10:24?PM Richard Knoppow via groups.io <dickburk@...> wrote:
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Re: Tektronix 7104 - RIFA explosion and replacement
Not sure. Was repeating what I read here. Rifa was/is a Swedish company. It sold its capacitor division some years ago. See Wikipedia for the history.? My understanding is that Rifa had? patented process which is no longer used. May have got it wrong. Now if the failure of 240V caps is notorious why continue to make them the same?Sent from my Galaxy
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-------- Original message --------From: "Mr. Fang via groups.io" <thomas.fang@...> Date: 5/28/25 6:59 PM (GMT-08:00) To: [email protected] Subject: Re: [TekScopes] Tektronix 7104 - RIFA explosion and replacement Richard: regarding your statement that "the caps that fail are of a design no longer made", are you sure? The Rifa ones I just bought new look exactly the same as the old ones and another person in this thread described the outer translucent encapsulation layer cracking open before the caps were ever installed in a circuit. -Thomas
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Re: Tektronix 7104 - RIFA explosion and replacement
Richard: regarding your statement that "the caps that fail are of a design no longer made", are you sure? The Rifa ones I just bought new look exactly the same as the old ones and another person in this thread described the outer translucent encapsulation layer cracking open before the caps were ever installed in a circuit.
-Thomas |
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Re: Hello - new to this list but not to Tek scope - Power supply trouble
Those 950 uF caps in the 7904 and some other models are very nice high quality (called computer grade in the old days) units that are very easy to rebuild. I use an un-crimp/re-crimp method which I have described before, to open and close the the cans. The bases are very stout Bakelite or other plastic, with threaded aluminum posts molded in for mounting and connection. The screw threads are blind 10-32, and it's easy to drill through the bottoms to the inside, then tap for 4-40 (or similar size metric) thread. The original hole bottoms are almost always conical because they're drilled, so this pilots the center for the tap drill, which will then easily clear the bigger threads. Then you can mount solder lugs on the inside of the base to attach the new caps, with no worry about electrical bonding to the aluminum. You just have to use short enough small screws on the inside, and be careful to not over-torque them since there's typically not a lot of thickness remaining for the small thread depth. I think Mark earlier described doing roughly the same thing.
New radial-leaded caps of higher capacitance should easily fit inside, and you don't need to worry much about venting. Unless you glue or pot the base of the the new part its vent will still work if needed, but will blow out into the remaining can volume, then pop the can or blow out the original rubber plug in the base, but at much reduced pressure and velocity. You can remove or drill out the old plug to have easier clearing if you want. So, don't pot or glue it in - you want to be able to rebuild it at least one more time if needed. You can fill the void partially with a foam or fiberglass material to keep it from rattling around. I sometimes wrap layers of crumpled aluminum foil around the upper part and top of the new cap if I think a re-stuff may need some help with cooling - it depends on the application and environment. Ed |
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Re: Tektronix 7104 - RIFA explosion and replacement
It seems to me they (the rfi filters) are bi-directional: i.e. to prevent coupling of signals both in and out.??? Now RIFA actually refers to a manufacturer,? not a generic type of capacitor but the caps referred to are of a special type, designed to fail open circuit.? The caps that fail are of a design no longer made. Modern caps for this application do not have the same failure mode.? ?BTW. The X and Y refer to a crude diagram of how the caps are intended.to be used:? X means? across the line; Y means from line to ground.? The safety considerations are different.? If an X cap fails short it will blow a fuse or breaker (if there is one).? If a Y cap short circuits it will put the chassis at line potential, causing a severe shock hazard.? So, a Y cap can be used either place? but X caps should never be used in place of Y caps.??Sent from my Galaxy
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-------- Original message --------From: "EJP via groups.io" <esmond.pitt@...> Date: 5/28/25 2:55 PM (GMT-08:00) To: [email protected] Subject: Re: [TekScopes] Tektronix 7104 - RIFA explosion and replacement > their "special? purpose" is to help controlconducted emissions (EMI) that the instrument couples onto the power line.I question this. In audio I find them in preamps and not in power amps. That suggests that their purpose is to keep EMI out, not in.The invariable failure mode is leaving the equipment unconnected for years and then a ¡®poof¡¯ and smell on power-up.
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Re: Hello - new to this list but not to Tek scope - Power supply trouble
Hate to follow up my own reply, but if you make your own, be sure to connect all 3 of the
can grounding pins on your PCB. Tek used the can as a jumper. -- Paul Amaranth, GCIH | Manchester MI, USA Aurora Group of Michigan, LLC | Security, Systems & Software paul@... | Linux/Unix - We don't do windows |
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Re: Hello - new to this list but not to Tek scope - Power supply trouble
Search on ebay for Tektronix capacitor adapter and you'll find a bunch.
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If you want to bother etching your own, it's easy enough to make up a pattern in your favorite drawing program, put probably easier to just buy a few. I've done it both ways. Paul On Wed, May 28, 2025 at 06:04:43PM -0400, Harvey White wrote:
You might check around, the adaptor designs are available, and some have --
Paul Amaranth, GCIH | Manchester MI, USA Aurora Group of Michigan, LLC | Security, Systems & Software paul@... | Linux/Unix - We don't do windows |
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Re: Tektronix 7104 - RIFA explosion and replacement
Ok.
DaveD KC0WJN On Wed, May 28, 2025 at 17:55 EJP via groups.io <esmond.pitt= [email protected]> wrote: their "special purpose" is to help controlconducted emissions (EMI) that the instrument couples onto the power line. |
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Re: Hello - new to this list but not to Tek scope - Power supply trouble
You might check around, the adaptor designs are available, and some have made the adaptors.
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Harvey On 5/28/2025 5:12 PM, Kevin Cobley wrote:
I mispoke, btw - both of my scopes are 7904s not 7904A's. The sick one is older than the other - it has the fixed power cord, rather than an IEC socket for the power supply. |
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Re: Tektronix 7104 - RIFA explosion and replacement
their "special purpose" is to help controlconducted emissions (EMI) that the instrument couples onto the power line. I question this. In audio I find them in preamps and not in power amps. That suggests that their purpose is to keep EMI out, not in. The invariable failure mode is leaving the equipment unconnected for years and then a ¡®poof¡¯ and smell on power-up. |
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Re: Hello - new to this list but not to Tek scope - Power supply trouble
I mispoke, btw - both of my scopes are 7904s not 7904A's. The sick one is older than the other - it has the fixed power cord, rather than an IEC socket for the power supply.
I'm wondering about doing a PCB to mount some snap in caps to that then has hex pillars for the screws on the PCB and the metal frame they hang off. That way I can maintain safety performacne of the caps and keep at least a similar mechanical arrangement. So cheap to get a PCB done now, that it probably saves money as snap in caps are so cheap.. Kevin. |
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Re: Introduction to Tek Scopes
Pulse compression in radars is beneficial where the peak transmitter power
is limited, but the mean power is not - or is at least less limited. In a peak power limited system, a longer pulses means there is more energy transmitted into the aether, which translates to increased range. But a longer pulse means reduced resolution. Pulse compression enables the receive energy to be concentrated into a smaller time interval, which translates to increased resolution. FMCW is a simple version of pulse compression. "Spread spectrum" codes can achieve similar effects. For amusement only, I was at CCL in the early-mid 80s. Great fun, a zany company, e.g. there was a scattergram of age vs salary "published", and nobody complained about the outliers because they were obviously worth it! On Wednesday, 28 May 2025, Kevin Cobley via groups.io <kevin= [email protected]> wrote: Complicated :-) pulse compression not my main expertise, but it followsthe normal rules - resolution is about bandwidth but range is about energy on target. target is the same for the same pulse duration. So resolution is improved. Octave or Python with numpy etc. and play with it. Just build some signals, create some target reflection, compute the matched filter, and see what happens. Several good websites out there with explanations. [email protected]> wrote:
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Re: Introduction to Tek Scopes
Complicated :-) pulse compression not my main expertise,? but it follows the normal rules - resolution is about bandwidth but range is about energy on target.
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So a cw rect pulse has narrower bw than an fm chirp pulse, but energy on target is the same for the same pulse duration. So resolution is improved. That's my understanding at least. I can recommend making a sim using Octave or Python with numpy etc. and play with it. Just build some signals, create some target reflection, compute the matched filter, and see what happens. Several good websites out there with explanations. Kevin. On 28 May 2025 16:30, "Jean-Paul via groups.io" <jonpaul@...> wrote:
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Re: Introduction to Tek Scopes
On 5/28/25 03:17, Kevin Cobley wrote:
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.Welcome Kevin, I started doing engineering in 1981, test, IR detector amps, MCU and memory chips, then some power, HV, micropython more recently. Have lots of tek scopes 7844s 7904A 7603 7103 7104 2330s 2465 2467B TDS7104 |
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Re: Introduction to Tek Scopes
Welcome, Kevin! I¡¯m of similar vintage as you. I wouldn¡¯t say an analog scope never lies, just that it lies less often than a DSO.
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Jim Ford Laguna Hills, California, USA On May 28, 2025, at 1:18?AM, Kevin Cobley <kevin@...> wrote: |
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Re: CR Tubes for TEK 577?
The 5000 series is a real ¡°sleeper¡±. Low BW (my 5103N D10 tops out at 1 MHz) means low noise for audio and power supply work. Simple circuits and no ICs means easy to troubleshoot and fix. Come to think of it, the 5103N D10 is my only scope that is fully operational at the moment. My 7904 has a focus issue, my 5113 parts unit got mangled in transit to me, my 7104 needs some TLC with the front panel, and my HP 54120B has a blown power supply. Yes, I have a few ¡°weekend projects ¡°. Don¡¯t overlook the 5000 series scopes; they have their uses.
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HTH Jim Ford Laguna Hills, California, USA On May 27, 2025, at 10:29?PM, Jeff Kruth via groups.io <kmec@...> wrote: |
Jean-Paul
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