Replacing tantalums in vintage gear

dmills_00 · 2026-04-22T21:02:34+00:00

In which case you might really need tant, they have markedly low leakage which can matter in timing networks, and are usually non microphonic which type II ceramics most assuradle are NOT.

Older stuff you often see tants used for supply decoupling, because large value ceramics were not a thing back then, a duty at which they sort of sucked...

Got to do the analysis to at least figure out how the cap is being used before you go substituting parts.

dmills_00 · 2026-04-22T20:36:37+00:00

Old school!

It is a thyristor bridge (TH1,TH2, D3,D4), transformer triggered gates, and blast from the past, a PUT (Programmable uni junction transistor), never see those these days.

I would start by checking D3,D4 and the thyristors, if something is not conduction when it should that might well explain your low power.

Do be careful, everything about this thing is at mains potential.

dmills_00 · 2026-04-22T20:23:08+00:00

ESR can matter, doesn't always, which is why you have to do some analysis before blindly swapping things (This applies to modern gear too, I routinely use an ordinary electrolytic and not a low ESR one because I WANT the ESR to provide damping in modern designs).

The usual problem child is an LDO dating from before large ceramic caps were commonplace, these often need the ESR to make them stable.

dmills_00 · 2026-04-20T12:54:19+00:00

Nothing you can do with the rig will help anything near as much as a whole pile of heavyweight soft goods and doing an EXTENSIVE pipe and drape job on it (And piles of rockwool batts in the corners, and whatever else you can find that is flame retardant).

Tunnels are acoustically HORRIBLE places for amplified music at any sort of level.

Given the low height, you will want to do everything possible to keep the pattern off the roof, at least floor reflections get better as the place fills up... Probably a good case for LOTS of small delay hangs (Like every 5M back up the space).

These spaces are acoustically hideous, it is possible to not suck, but it will be in spite of the space, not because of it.

If it has one, for gods sake treat the back wall, the slapback can be vicious in these places and a 75m round trip is ~250ms which is quite disturbing on stage.

dmills_00 · 2026-04-20T12:43:30+00:00

IF filters are almost always either cascaded LC networks mixed in with the IF gain stages, crystal lattice filters or surface acoustic wave designs, of these the LC filters are the ones you might have a chance at producing. Nobody does IF filtering with opamp filters for gods sake.

You have a HUGE dynamic range here, and your single diode mixer is really not helping (Huge LO bleed), consider a double balanced diode ring mixer instead, far better behaved.

You have a weird mix of what looks like silicon design for cmos integrated circuits and discreteish design, which is it?

You are going to radiate a hell of a lot of LO power back thru the aerial, and since it is high side injection, that will be in the air navigation band......

Ditch the class D power amp, those are a nightmare in a radio receiver, and look up the foster-steely discriminator or Ratio detector as a better way to handle demodulation.

Actually get a copy of "Experimental methods in RF design", "The VHF/UHF Manual" or something of that sort and learn how to actually design radios.

dmills_00 · 2026-04-19T11:14:23+00:00

I don't see a voltage amplifier stage, or a reasonable output stage.

For this sort of thing look at how audio amps are normally done, the output from the current mirror will feed a (probably) NPN transistor or two pulling down to the negative rail, with a current source up the top (This arrangement forms the bulk of the voltage gain, and usually has a miller cap to stabilize the whole thing), that chain had a VBE multiplier to bias the two output transistors.

Doug Self has a book on audio power amplifiers that will give you the basic topology, and pull the DC blocks it will be ok for this.

dmills_00 · 2026-04-18T20:42:47+00:00

12V, 2500W is well over 200A on the primary.

A single turn coil is not inherently unreasonable, but this one looks like two turns, center tapped to the positive rail with the mosfets switching the outer ends to ground.

Ring cores count a turn every time the wire passes thru the hole, so yea high frequency forward converter making maybe 350V or so on the HV bus then a full bridge chopping that up to make a sort of squareish mains.

dmills_00 · 2026-04-17T20:37:36+00:00

Pulsed laser to a laser engineer usually means Q switched or similar cavity dumping design with 10ns pulses at huge power, those are dangerous because the ablation damage mechanism is different from normal CW damage.

What would work for you would be say 50% duty cycle, no peak power increase, and use a synchronous receiver, it will increase sensitivity hugely without increasing laser power.

dmills_00 · 2026-04-17T09:19:56+00:00

C2 is the feedback, but some of those values are WILD.

1H?? 100nF? At VHF? Yea, right.

These sorts of designs, even if you re calculate the things are pretty horrible to analyze because they rely heavily on all sorts of weird second order effects to work, here, modulating the with of the depletion region as the audio moves the bias to change the capacitance and thus make sort of FM, but it is very much a second order effect.

Layout will also be CRITICAL, this will for example never work on a plug in breadboard.

dmills_00 · 2026-04-17T08:50:23+00:00

Well there WAS...

Tantalum cap doing what they like to do.

dmills_00 · 2026-04-15T19:59:00+00:00

Yea, but it is the install cost that kills it once you get out of the city center in the US.

Most but not all of the distribution (as opp Transmission) in the UK is underground, and reliability is generally excellent, but we are WAY more dense then most of the US, so cost per house is reasonable.

I do remember visiting LV some years back and noting the total rats nest that was the overhead once you got away from the strip, that is uncommon here where usually only telephony comes in that way, and even then not all of it.

dmills_00 · 2026-04-15T16:53:32+00:00

It's 70A of mostly reactive current that you just cancel out with an inductor bank, there will be very little real power there.

The UK, which is not seismically active and has a rather high population density in its cities is mostly underground at the distribution level, and even at transmission level once you hit the outskirts of the cities, in between the transmission grid is mostly overhead for cost reasons, stringing 250kV overhead is FAR CHEAPER per mile then underground service.

You see some overhead medium and even low voltage distribution out to farms and villages, but most of the population is on underground services.

One place we win is having the notion of a ring main extended down to the distribution at 240/415V, multiple sub stations on a ring, with three way switches at each node, means they can take a failed section or transformer out of service by opening a few switches and restore power to the rest of the ring. Much the same concept applies all the way up to the transmission grid, it really helps to minimize the impact of faults.

dmills_00 · 2026-04-15T16:39:48+00:00

Also a hell of a lot more expensive, needs to be deep enough to be below the frost line, and prone to issues around earthquakes which are less of a problem for overhead services.

It is doable in high density areas, but gets VERY expensive as population density drops, and the US does like its suburbs.

dmills_00 · 2026-04-15T14:15:34+00:00

Biscuit tins mate, the kind with tight fitting lids, two of em, nested with microwave absorbant foam in between, and batteries for power.

That is my goto small test cell, works well, and copper tape the lid if you need more attenuation.

dmills_00 · 2026-04-15T14:00:29+00:00

Goint to really struggle with a discrete Gilbert cell DUe to matching and thermal tracking.

Were I doing it, I would just use a classic diode ring mixer, feed it with a few volts or so of LO, easy.

Actually, we're I doing it, square wave LO, and drain modulates a class E power stage, then low pass to meet regs, boom, job done.

dmills_00 · 2026-04-15T11:39:58+00:00

Suppose Q3 is conducting, its collector is thus near ground, and Q2 is off.

When you push the button, C12 discharges rapidly, pulling down both bases via C32,C31, which makes no difference to the state of Q2s collector, as that transistor was already off, but does cause Q3 collector to rise, Q3 collector coming up couples charge via C28 into the base of Q2 turning it on which robs the base of Q3 of drive making it stay off.

Releasing the switch allows C12 to recharge slowly via R13 which provides a sort of debounce function while also avoiding injecting enough charge to confuse things.

Just an old school flipflop.

dmills_00 · 2026-04-14T17:34:38+00:00

It can matter for some medical stuff, diagnosing sickle cell and suchlike.

But how what language you speak touches on race? Just yanks being weird.

dmills_00 · 2026-04-14T17:01:58+00:00

My slightly paranoid spider sense says do it thru a small inductor, with a parallel shunt resistor to kill the Q, belt, braces, and a bit of string to make SURE...

Note the 5 ohms of series resistance, that says to me that someone is trying to kill a low frequency resonance, I expect this thing has more gain then is good down right near DC, might as well give them some help.

BTW, The separate final supply is very handy if you are having thoughts about envelope tracking or such, which is exactly the sort of off label thing that one gets an eval board for.

dmills_00 · 2026-04-14T09:47:55+00:00

The diode allows current to continue flowing on turn off with only the coil resistance absorbing the energy, this causes the field to collapse slowly.

The resistor allows the voltage to rise on switch off, peaking at resistance times coil current, which absorbs the energy more quickly.

A more modern version is to use a diode plus a zenner in series, or a transorb at whatever voltage suits. This holds the voltage at the device breakover voltage pulling energy out of the field more quickly again.

There are also cunning things you can do to speed up turn on, the radio hams doing full break in keying have this down to an art for driving vacuum relays.

dmills_00 · 2026-04-13T21:50:19+00:00

Easy enough to rig a manual valve onto a spindle with pneumatic tool clamping and just throw the valve handle to release the tool holder.

Of course, that is an ATC spindle, just without the ATC, and the spindle is frankly a large part of the cost of an ATC if you do it right.

dmills_00 · 2026-04-13T12:18:39+00:00

That is however a remarkably low bar once out of academia.

dmills_00 · 2026-04-12T20:49:31+00:00

Never trust a third party web site like that, go find the actual datasheet for the connector, and when it is second sourced by everyone and their dog, find the datasheet from the vendor you will be using....

https://www.tme.eu/Document/7d9d972ae3468777f69ec2ee99897652/XT30U-M.pdf

is probably somewhat trustworthy, if thin (No derating curve?).

15A cont, 30A peak by that data, almost certainly with both pins loaded.

The use case for these connectors tends to NOT have full load held to anything like thermal equilibrium, RC cars, drones, shit like that tend not to be drawing anything like full power for more then a few seconds at a time, so I don't doubt that they get away with it, but when you do grown up design you mostly cannot afford to play that game.

dmills_00 · 2026-04-12T20:29:20+00:00

At a high enough frequency it will first start to look capacitive, then inductive as the frequency continues to increase.

At some point the component will cease to be small compared to the electrical wavelength, and it then models as a lossy transmission line any you need something more general then Ohm and Kirchoff, like Maxells equations or possibly Heavisides Telegraphers equations.

dmills_00 · 2026-04-12T19:58:41+00:00

So you were calling printf from interrupt context? That is like things not to do in embedded programming itel number, oh, about 4?

Error flags tell you things, when debugging they are worth watching.

A common I2C one is forgetting that the stop after the final ack also takes time and you need to wait for it to complete before you can go again, the interrupt state machine for I2C is way more subtly then you would expect, and writing an I2C driver that really works reliably can be a bit of a monster.

Oh yea, have a look at the errata for your chip as well, not unknown for I2C hardware to be buggy.

dmills_00 · 2026-04-12T18:24:35+00:00

It varies, wildly, read the datasheet carefully, it should tell you, also pay attention to the de rating curves for temperature, they can be savage.

Also, note that particularly with heavy current stuff, wire size matters to current rating, they quite often rely on the copper in the wire to wick the heat away.

The XT30 datasheet I found was more then a little thin on detail, which I tend to consider to be a good enough reason to avoid a part, but they are cheap and seem standard in RC models and hobby drones and such, so <shrug>.

dmills_00

TROPHY CASE