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Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Electronics Workbench

Electrical & Electronic gadgets

Ampeg B-12-XY Cap Autopsy

Before trashing (*) all those caps from the Ampeg, I marched them past a capacitance meter that gives the dissipation factor D. As D = tan δ = ESR / ¦X¦, we know ESR = D*¦X¦ at the meter’s 1 kHz test frequency. We don’t know the magnitude of the total reactance X (the meter doesn’t tell us that) and in this case we can’t assume the ESR will be small with respect to the capacitive reactance Xc = 1/2πfC.

Ampeg capacitors

The smaller green 0.022 µF Cornell-Dubilier caps all came in with D=0.05, so they’re marginal.

The larger green 0.15 µF Cornell-Dubilier caps had D=0.00 and the black 0.1 µF was D=0.01. Those are OK.

The small black caps had D=0.14. Yikes! The larger one and the yellow cap had D= 0.01 or 0.02.

The blue Ducati (!) electrolytics ranged from 0.06 to 0.48. That was without reforming, as the last time Phil turned it on, the finals about melted down: I wasn’t going to risk that again just to find out if you can reform all the electrolytic caps without the tubes in place.

So, yeah, some of the coupling caps were exceedingly bad. If you’d like to rub the values & data against the schematic to find out which one(s) were killing the finals, go ahead.

All of the measured capacitance values were within spitting distance of their nominal values.

[Update: Eks points out that I really should measure the leakage at operating voltage, so as to find the current that would drive the grids off their normal bias points. That’s a project for another day… ]

(*) They’re in the e-waste recycling box, of course.

The raw data, not that anybody cares:

2011-09-01
Stepper Dynamometer: First Light
As a quick test of the stepper dynamometer, I lashed the larger stepper to that Pololu driver hairball, connected one winding of the smaller stepper to the oscilloscope, and recorded open-circuit voltages as a function of rotational speed:

Output voltage vs rps – open circuit

Now, if that isn’t suspiciously linear, I don’t know what is!

The slope is 0.583 v/(rev/s).

I used the scope’s RMS trace calculator, which smushes out the non-sinusoidal nature of the lower speed waveforms. As expected, there are several nasty mechanical resonances that appear in the output waveform while they’re tormenting my ears:

Stepper Resonance – 4.82 rps

Top trace is the winding output voltage, bottom trace is the drive input current, plus a line of junk I forgot to turn off.

Useful conversions:
- Drive waveform frequency / 50 = rev/s
- Drive waveform frequency * 6/5 = rev/min
So it works. Now I must figure out how to connect load resistors with something more reliable than crappy alligator clips.
2011-08-30
Exposed Stepper Motor Windings
Got a stepper motor from halfway around the planet from the usual eBay source, intended for a direct-drive extruder (at some point). This one has integral wire leads, which is fine with me, but the opening in the rear endcap reveals a bit more of the innards than one usually sees:

ACT 17HS5425 stepper – exposed winding

Yup, that’s one winding peeking out. Although the wire insulation should take care of anything conductive, I’d expect the same casual attention to detail in the winding terminals.

I’d worry more if this were being used in a metal-cutting operation, but a snippet of heatshrink tubing and a blob of hot-melt glue seem in order.

For what it’s worth, the motor is an ACT 17HS5425:
- 1.8°/step
- 48 mm case length
- 3.1 V
- 2.5 A
- 1.25 Ω
- 1.8 mH
- 48 oz·in holding torque
- 2.8 oz·in detent torque
- 68 oz·in rotor torque
No torque curves and nothing more in the way of a datasheet.
2011-08-19
Ampeg B-12-XY: Recapping

Just for completeness, here’s the original underside:

Ampeg B-12-XY – Underside – old caps

And with the new caps, many from Eks’ stash and a few from mine:

Ampeg B-12-XY – Underside – new caps

With all those in place, the firebottles lit up properly, the power tube plates remained dark, and it sounded great. The edge-lit engraved acrylic panel in the middle is a wonderful custom mod!

Ampeg B-12-XY Firebottles

It’s in mint condition, with the original footswitch and a remote Echo speaker box with a pair of drivers:

Ampeg B-12-XY – ready to rock

It still has those original huge electrolytics, though. Eks says the best test comes after half an hour: if the cans remain cool, the leakage and ESR will be good enough.That’s the case, so we’re rolling with them. However, the amp has some residual hum that the Hum nulling pot can’t remove, plus a bit of noise, which means those ‘lytics probably hover at the bare minimum values required to keep it going.

I discovered (inadvertently, of course) that swapping the two identical 6D10 triple triode tubes killed the Vibrato oscillator. That triode would oscillate for a few seconds after the footswitch grounded the cathode, but one tube didn’t have enough gain to keep it going. More likely than not, the feedback resistors have increased in value, too. Swapping the 6D10s restored it to operating condition.

My Shop Assistant compared her tiny DSP Fender amp with this monster and concluded that DSP effects only sound good when you don’t have the original for comparison. Of course, you could lose that tiddly amp inside the Ampeg’s speaker case.

I should’a learned to play the guitar…

2011-08-03
Ampeg B-12-XY: Tin Whiskers!

After Eks set me straight on cleaning the contacts involved with the Ampeg’s Echo circuitry, we emerged from his shop into brilliant sunshine. He looked into the thing and shouted “Tin whiskers!”

It turns out the Hammond folks made the outer frame from tin- (or, shudder, cadmium-) plated steel that has grown a dense crop of whiskers on its interior surface. They glittered in the sunlight like a carpet of crushed glass, with the longest ones maybe 3 mm tall!

This view looks nearly parallel to the side of the channel (upward as it mounts on the speaker box), with the steel wall to the bottom of the image. I applied gruesome contrast stretching to make the whiskers more visible:

Ampeg Spring Echo Unit – Tin Whiskers

This is the first time I’d ever seen a tin whisker in person and there’s a bazillion of ’em in there!

If that Ampeg had transistorized components, it’d be dead as a doornail! Fortunately, a tin whisker doesn’t stand a chance in an analog vacuum-tube circuit. The power supply puts 400-ish V into 40 μF caps, providing plenty of energy to vaporize the errant whisker; all you’d hear is a pop.

2011-08-02
Ampeg B-12-XY: Echo Circuit

Mad Phil asked me to fix up his trusty Ampeg B-12-XY (*) bass guitar amp, having recently fired it up and discovered that the power output tube plates glowed red-hot. I’d planned to replace the electrolytic caps, but Eks, who does this sort of thing all the time, suggested that leaky interstage coupling caps can also cause that problem; the leakage wrecks the phase splitter bias and thus kills the drivers.

While poking around in the amp I found that the Echo hardware circuitry doesn’t match the schematic for either the B-12-X or B-12-XY. Mad Phil says that’s probably because he had the factory upgrade his original B-12-X to a B-12-XY for the munificent sum of $25, back in the day. It’s unlikely you’ll ever need this, but here’s what I found:

Ampeg B-12-XY – as-found Echo circuit

The topology resembles the -XY schematic, but with different tube sections and part values.

The Echo unit over there on the left consists of two springs with magnetic transducers on each end, evidently made by the Hammond Organ folks, who should know something about reverb. This is the bottom view, with the unit attached to the board that supports the amp chassis:

Ampeg Spring Echo Unit

The input transducer, just in case you forget to label the ends before you take it apart:

Ampeg Spring Echo – input end

And the output transducer:

Ampeg Spring Echo – output end

Getting the thing off the speaker box posed a bit of a problem. Remove the four big screws holding the chassis to the board, tilt it carefully forward, hold it in place while you remove the six nuts-and-washers from the vibration isolators, then transport the whole disjointed affair to the workbench. Turns out you (well, I) can’t get the RCA plugs out of the Echo unit’s sockets from the top of the board, but the unit’s mounting screws are on the bottom of the board, where you can’t get to them before you remove the board. Of course, the cables leading to the aforementioned RCA plugs tether the chassis to the Echo unit with pretty nearly no slack at all.

With everything apart, I rounded the ends of the RCA plug cutouts enough to get them out from the top the next time around, with the board screwed in place atop the speaker box:

Ampeg Spring Echo unit – top view

After putting the whole thing together with new caps, the Echo circuit didn’t work. I had cleaned the contacts and connectors, but Eks showed me how it’s really done. Apart from the rotted caps, all the other problems came from minor corrosion in switches, connectors, and tube sockets. Now I know better.

* Yes, the model numbers really end in X and XY.

2011-08-01
MGE Ellipse 1200 Battery Arrangement

The SLA batteries in the MGE Ellipse 1200 UPS finally gave out. This picture shows how they’re arranged inside the box:

MGE Ellipse 1200 UPS – battery arrangement

They’re 12 V 5 Ah batteries that are about 12 mm thinner than the garden variety 7 Ah batteries you can get everywhere; they’re not the same size as the generic 5 Ah batteries you might think would work. Of course, there’s not enough room inside the stylin’ case for the larger ones, either. I’m thinking of using fatter batteries anyway and putting a belly band around the gap. Maybe an external battery box with a chunky cable burrowing through a hole in the UPS case?

For what it’s worth, APC absorbed MGE a while ago (so the MGE website redirects to APC), got Borged by Schneider, then spat out MGE’s consumer grade UPS units to Eaton. You won’t find any of that documented anywhere, but here’s the response from APC after I didn’t find this UPS on their list:

I do apologize; when APC was acquired by Schneider Electric, the single phase UPS line that MGE once offered was sold to Eaton. Eaton now provides support for the MGE single-phase products. We do not sell batteries for these models. You will actually need to contact Eaton for further assistance regarding the MGE Ellipse units. You may click on the link below to go to Eaton’s website:

http://powerquality.eaton.com/Default.asp

The Eaton website does have a battery replacement for this one, but sporting the dreaded “Contact us for price” notation. Given that I got the UPS cheap-after-rebate, I’m thinking maybe this isn’t worth the effort.

2011-07-22