130 LC meter
Bob Albert
Is there anyone familiar with this unit? I have one that isn't working and have traced the problem to the variable oscillator not oscillating.
It's plate voltage is too low but everything seems good otherwise. Too low for enough gain to oscillate. The tubes are good and the transformer resonates. There is less than one volt on the plate of V4A.
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Craig Sawyers <c.sawyers@...>
Have you checked the smoothing capacitor on the power supply? Mine was open circuit.
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Craig
Is there anyone familiar with this unit? I have one that isn't working and have traced the problem
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Jack Wills
My Tek 130 had several Sprague Black Beauty capacitors in the oscillator
circuit All were bad. I've had enough trouble (and been shocked enough) with these caps that I replace them on sight. On Sat, Oct 19, 2019 at 1:05 AM Craig Sawyers <c.sawyers@...> wrote: Have you checked the smoothing capacitor on the power supply? Mine was
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Bob Albert
I have spent lots of time and lost lots of sleep over this. I got up in the middle of the night last night and discovered that the problem appears to be a defective tube socket. Not the best news but I'll probably bite the bullet after a few more tests.
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I measure a variable resistance from one tube pin to ground, and see a spiky noise coming from the next stage. Bob
On Saturday, October 19, 2019, 01:05:56 AM PDT, Craig Sawyers <c.sawyers@...> wrote:
Have you checked the smoothing capacitor on the power supply? Mine was open circuit. Craig Is there anyone familiar with this unit? I have one that isn't working and have traced the problem
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John
Bob: have you sorted the problem? Was it the tube socket?
John
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Bob Albert
John, I replaced the socket and it's not working. However there seems to be a weak oscillation which I can adjust with the transformer. It's not much amplitude and insufficient for the unit to operate.
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I took it off the bench in frustration. Not sure what to do now. I saw this oscillator has a 1 meg resistoro not present in the other one so I shorted it but no difference. Maybe I made a wiring error. Bob
On Monday, November 4, 2019, 02:22:14 AM PST, John <john@...> wrote:
Bob: have you sorted the problem? Was it the tube socket? John
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Ed Breya
Did you see Jack's advice earlier, about the black-beauty or other brand paper caps? I'd recommend looking closely at the few that are in there. If they get leaky, it can upset the DC grid bias on a tube stage, forcing it away from proper level and action. I think your OP mentioned the oscillator plate DC was very low, which could be a symptom of bad grid bias.
I have a pristine 130, still sporting the original caps, and working fine, but have a set of modern plastic ones ready to go in for when the time comes. Ed
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Bob Albert
Thanks for the heads up Ed. I will take another look at the parts but as I recall there are no paper capacitors in that part of the circuit. A few ceramic discs and a couple of mica.
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When I get motivated I will measure the voltages again and see what's what. Bob
On Monday, November 4, 2019, 02:56:32 PM PST, Ed Breya via Groups.Io <edbreya=yahoo.com@groups.io> wrote:
Did you see Jack's advice earlier, about the black-beauty or other brand paper caps? I'd recommend looking closely at the few that are in there. If they get leaky, it can upset the DC grid bias on a tube stage, forcing it away from proper level and action. I think your OP mentioned the oscillator plate DC was very low, which could be a symptom of bad grid bias. I have a pristine 130, still sporting the original caps, and working fine, but have a set of modern plastic ones ready to go in for when the time comes. Ed
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Ed Breya
Ceramic caps and even micas can get leaky after many years - not often, but I've seen it. Ed
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Craig Sawyers <c.sawyers@...>
Mica caps are a known weak point in the Racal RA17 receiver. They are not a replace-on-sight because
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there are lots of them, but worth bearing in mind that they can occasionally be problematic. Craig
-----Original Message-----
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Bob Albert
Inthis part of the 130 therre are only two micas I believe. I will direct my attention there to see if they are causing a problem. Somehow I don't think that's it, but we shall see.
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I am wondering why the two oscillators are different. The variable one has a few extra components and I don't know why. Bob
On Tuesday, November 5, 2019, 12:11:04 AM PST, Craig Sawyers <c.sawyers@...> wrote:
Mica caps are a known weak point in the Racal RA17 receiver. They are not a replace-on-sight because there are lots of them, but worth bearing in mind that they can occasionally be problematic. Craig -----Original Message-----
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Richard Knoppow
There are two kinds of mica caps, silvered mica and plain mica. Silver mica of a certain period suffer from something called silver mica desease, probably due to moisture penetration through the Bakelite casings. More modern epoxy dipped caps do not seem to have this problem and it is only occasionally found on the Bakelite type. It results in the cap becoming unstable, sometimes jumping in value and sometimes having "scintillation" or rapid variations. One should be aware of it but mica caps do not have the predictable degradation with time of paper caps. The other type of mica caps, usually found a HV transmitting caps, are made of stacks of mica dielectric and foil pressed together and held under pressure. They seem to be nearly perfectly reliable but silver mica have better HF characteristics.
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Ceramic caps depend on the type of ceramic. Those made with high K material do age and change value. They also have considerable voltage coefficient of capacitance. The ones made with low K such as zero TC caps, are extremely stable. Some of these caps fail, particularly the disc type, because the leads have been stressed where they enter the body and have cracked it allowing some air and moisture to enter. Disc caps can fail so should never be left out of trouble shooting routine but neither mica or ceramic need to be shot gunned as do paper or electrolytic caps.
On 11/5/2019 12:10 AM, Craig Sawyers wrote:
Mica caps are a known weak point in the Racal RA17 receiver. They are not a replace-on-sight because --
Richard Knoppow dickburk@... WB6KBL
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Bob Albert
Yes I am aware of many of the failings of various capacitor types. However, I am not convinced that the problem in this circuit is caused by a poorly behaving capacitor. I will once again attempt to measure voltages and even a few components to see where something isn't right.
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For now, though, my attention is directed elsewhere. I have other means of making measurements that the 130 can do, so my interest is only to have some fun with the unit. If I never get it working, it still is a vehicle for education. The lowest range, 0 - 3 pF, intrigues me but I can make those measurements with the Q meter if need be. I have a few 2 pF, I think, capacitors that I want to characterize. They are the ceramic dogbone style. I also have a box of variable capacitors but those are easy. It pains me to have these, as I have no projected use for them and they are almost works of art, with the lovely bearings and shafts and other moving parts. Back in the day, one might build an amplifier or tuner, but I don't need either. So I will stuff them into the closet and let my heirs dispose of it all, as they will be obliged to do with all my precious possessions. It might be prudent for some local enthusiast to make friends with me and my family in anticipation of this event. Bob
On Tuesday, November 5, 2019, 10:26:50 AM PST, Richard Knoppow <dickburk@...> wrote:
There are two kinds of mica caps, silvered mica and plain mica. Silver mica of a certain period suffer from something called silver mica desease, probably due to moisture penetration through the Bakelite casings. More modern epoxy dipped caps do not seem to have this problem and it is only occasionally found on the Bakelite type. It results in the cap becoming unstable, sometimes jumping in value and sometimes having "scintillation" or rapid variations. One should be aware of it but mica caps do not have the predictable degradation with time of paper caps. The other type of mica caps, usually found a HV transmitting caps, are made of stacks of mica dielectric and foil pressed together and held under pressure. They seem to be nearly perfectly reliable but silver mica have better HF characteristics. Ceramic caps depend on the type of ceramic. Those made with high K material do age and change value. They also have considerable voltage coefficient of capacitance. The ones made with low K such as zero TC caps, are extremely stable. Some of these caps fail, particularly the disc type, because the leads have been stressed where they enter the body and have cracked it allowing some air and moisture to enter. Disc caps can fail so should never be left out of trouble shooting routine but neither mica or ceramic need to be shot gunned as do paper or electrolytic caps. On 11/5/2019 12:10 AM, Craig Sawyers wrote: Mica caps are a known weak point in the Racal RA17 receiver. They are not a replace-on-sight because-- Richard Knoppow dickburk@... WB6KBL
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Rajesh VS
Bob,in case it helps..my LC130 was misbehaving when I acquired it, and
the cause was a broken solder joint around the Zero adjust controls (front Panel) . Don't recollect if it was in the fine or coarse control. Location is top left in the pic - http://w140.com/tekwiki/wiki/130#/media/File:Tek_Type130_Inside-1.jpg On Tue, Nov 5, 2019 at 11:00 AM Bob Albert via Groups.Io <bob91343= yahoo.com@groups.io> wrote: Yes I am aware of many of the failings of various capacitor types.-- /Rajesh
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Bob Albert
Thanks but that's not it. The zero adjust controls work okay and the problem is simply that the variable oscillator doesn't oscillate. I have a tiny signal from it whose frequency can be varied a bit with the tuning slug. But it's nowhere near enough to drive the mixer.
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Bob
On Tuesday, November 5, 2019, 11:54:04 AM PST, Rajesh VS <rajeshvs@...> wrote:
Bob,in case it helps..my LC130 was misbehaving when I acquired it, and the cause was a broken solder joint around the Zero adjust controls (front Panel) . Don't recollect if it was in the fine or coarse control. Location is top left in the pic - http://w140.com/tekwiki/wiki/130#/media/File:Tek_Type130_Inside-1.jpg On Tue, Nov 5, 2019 at 11:00 AM Bob Albert via Groups.Io <bob91343= yahoo.com@groups.io> wrote: Yes I am aware of many of the failings of various capacitor types.-- /Rajesh
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Rajesh VS
if you lift one leg of C22 (towards V15A), it will isolate the variable
oscillator, and then you can troubleshoot it without involving other parts of the circuit. Same with C17 - the amplifier for variable oscillator - where you can disconnect it from mixer and check its input and output. Check V4 heater voltage, and I hope C402 was checked. I cant recollect precisely, but I had issues around these components as well. if you want I can check the DC bias voltages for V4 and update you over weekend. On Tue, Nov 5, 2019 at 1:48 PM Bob Albert via Groups.Io <bob91343= yahoo.com@groups.io> wrote: Thanks but that's not it. The zero adjust controls work okay and the-- /Rajesh
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The old silver mica caps were used in lots of antique AM broadcast radios (1930s -1960s), particularly in the IF stages. They were known to cause a phenomenon called "thunderstorming", which, even when receiving a strong station, caused a random loud static that sounded exactly like a nearby thunderstorm. This was caused by the silver deposits on the surfaces of the mica layers actually migrating toward the edges of the mica slabs. The high voltages used in those old tube radios drew the silver atoms toward the edges of the mica, and when enough silver migrated to the edges, there was enough current leakage in the capacitor to cause the "static" noise.
Of course, the only solution was to replace the old micas with new ones and retune the IF stages. Spoken from experience {:>) Dave M
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Craig Sawyers <c.sawyers@...>
Agreed - that was my experience with the RA17. Of course I have a 130 LC meter (no surprise there!)
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and the only thing I had to do was replace the smoothing electrolytic, which had gone down with the "Sprague twist-lock disconnection issue". Haven't fired it up in quite a while; must do so and make sure it is still alive. Craig
The old silver mica caps were used in lots of antique AM broadcast radios (1930s -1960s),
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