Keith's Electronics Blog

I do all of my mobile computing — and my blogging slouched on the couch in my family room — using a hand-me-down 500MHz PowerBook G4. It was given to me used, battered, and obsolete two and half years ago, and after upgrading the RAM to 1G, installing OS X Tiger from a family license, and installing an AirPort card, it has served me well.

It’s starting to feel a little slow when loading bloated web pages, but my main complaint at this point is its battery life. I was given two batteries with it, one of which never held a charge at all. The second battery’s usable lifespan has slowly decreased from about three hours to around one, even when I turn off power to the AirPort card.

Worse, the battery meter still shows upwards of 60% charge remaining when it suddenly powers off with no warning. I’ve run Apple’s battery recalibration procedure several times, but it doesn’t make any difference.

Replacement batteries seem to run upwards of $80, and it’s impossible to tell from the online sales pages whether they’re original stock every bit as old as my own failing battery. I’m not afraid to dig into things and I’ve rebuilt my original Roomba’s battery before, so I decided to tackle this one myself. And since I had a spare battery that was already worthless, I could rebuild the spare without risking damage to my main battery that still mostly worked.

But it turned out I couldn’t find replacement cells at my usual surplus sites — they’re a little larger than AA, which was all I could find. eBay to the rescue. One of my favorite sellers is a surplus recycling center in Topeka. I get up to Lawrence enough that I buy a trickle of stuff from them, then pick it up when I’m in the neighborhood anyway, and save a bunch of money not having to have things shipped.

I started watching their auctions for notebook batteries. Not PowerBook batteries — but other batteries at low enough prices that I could cannibalize them for cells. Within a matter of days, I had purchased a Compaq and a Dell battery for $5 each. I’m well aware that they’re not likely to be brand new, but for only $5, it’s still worth trying them for a rebuild.

Today I got around to doing it.

Inside the Batteries

Here’s my original battery with the two donors:

And the same batteries with the cases removed:

You can see that the Dell battery has only six cells, which aren’t enough to rebuild the PowerBook. They’re still available as donors; but as the Compaq has eight cells, the Dell’s won’t be needed.

The first step was making a careful diagram of the inside of the original battery, noting cell polarity and wiring connections. Here’s the top view:

And the resulting diagram:

The heavy black lines along the tops and bottoms of the cells are foil strips spot-welded to the cells, joining them together two-by-two in parallel, and the pairs into a series string. When I took apart the donor batteries, I looked for sets of cells welded together the same way, to minimize unnecessary soldering during reassembly.

The bottom side had only two wires:

So its diagram was trivial:

Happily, the cells in the Compaq donor were spot-welded together in parallel pairs like the PowerBook cells, making reuse easy.

This set of four can replace either the left and middle or middle and right cells.

Where a pair of batteries was spot-welded in parallel, both ends were connected rigidly and the two cells formed a solid assembly. But where only one end was connected in series to the next pair, the strip at that end acted as a hinge and the two pairs tended to flop out of alignment. The Compaq battery had clever clips sandwiched between parallel pairs to maintain alignment. At first I thought I’d be able to reuse the clips, but my PowerBook battery case had ridges sticking up between pairs, so I had to remove the clips.

Reassembly

After comparing the various spot-welded tabs on the new cells to those on the old, I decided which cells to put where and test-fitted everything into the case.

Looks good! Now I needed to rewire.

The original battery had fine wires from the battery-control PC boards to the different cells, but also a few heavier-gauge wires directly from cell to cell. Rather than desolder those wires from the original and move them to the new battery, I looked around for similar-gauge wire. The motherboard wiring harness from a dead PC power supply looked perfect — and it’s even a little heavier gauge (20 instead of 22).

First I fitted and soldered the black and white wires on the underside that connect the upper and lower single cells into a parallel pair. I also made the series connection from their right terminal to the upper terminal of the right pair.

Next, I needed to connect the left pair in series with the center pair. The original battery had their upper terminals all spot-welded to the same strip, but the donor battery had a different cell configuration, so I needed to make that connection with wire. I chose yellow to match the yellow PCB fly wire that connects to the same junction. Now the inter-cell wiring was complete.

Reconnecting the PCBs

I pushed the PC boards approximately back into place and started reconnecting their wires. Leaving for last the upper left connection with heatshrink covering some inline mystery component, I started on the bottom set of wires. I hooked up the small black wire from the lower left PCB to the lower left battery terminal, then the orange wire to the center of the lower side of the center cells.

As I soldered the brown wire to the right end of the lower cells, I smelled something funny — and it didn’t smell just like melted insulation. I quickly desoldered it and sniffed around. Convincing myself that it was just the insulation, I resoldered it.

Then I noticed that the right two cells in the main row were warm — quite warm — and it had been a long time since I had soldered them. I desoldered every connection as fast as I could and sat back to think about what was happening. At that point, my infrared thermometer said that the rightmost cell was 100°F.

Connection Sequence Matters

There were only two good explanations: Either I had made an error in my rewiring, or it mattered in what order the cells were reconnected. After re-rechecking my connections, I redrew my diagram of the original battery to consider exactly how the cells were wired.

After seeing the diagram drawn out like this, it made sense to think that the order of attaching connections could matter. The black wire hidden in the lower left is the ground, and the red wire going to the connection I’d been saving for last is the positive output. The orange, brown, and yellow wires tap into the series chain between cell pairs, presumably to measure the health of each pair.

So . . . maybe the monitoring circuit didn’t like having the lower cells reconnected without the main V+ connected yet. By then, all the cells were back down to room temperature, and I cautiously reassembled them in the case. This time I made the red connection first, then the black, completing the overall circuit.

I checked the temperature of the cells again and it didn’t seem to be rising, so I reconnected the orange, brown, and yellow wires. After waiting a few minutes and checking the temperature yet again, I put insulating tape back over several of the connections and called it good.

Testing and Results

The battery case press-fits together well enough that it doesn’t need much help to stay closed, but I put a couple of pieces of clear tape on opposite sides to keep it from surprising me. If the rebuilt battery works well and seems worth using, I’ll probably replace them with thin, metallic foil tape.

I shut down my PowerBook and took it to the kitchen sink, where a hypothetical fire would be the easiest to contain with the least damage to the house. I cautiously put the rebuilt battery into the computer, and nothing blew up or got warm. I connected the power cable and its LEDs went orange, indicating that it was charging the battery. The battery’s lowest light flickered, also indicating a charge. So far, so good.

I booted OS X and watched the battery monitor on the menu bar. By the time the computer was up, the indicator showed 6% charge, climbing very quickly to 8% as I watched. Switching to time-remaining mode indicated 0:18 to a full charge. Hm, that doesn’t seem nearly long enough.

Well, after a few weeks of disuse, NiMH batteries often need several charge cycles before regaining their full capacity; maybe Li-Ion batteries are similar. I’ll run a few charge cycles and see.

The Trouble with the Old Battery

Out of curiosity, I put the meter across each cell pair in the old battery and measured the voltage.

It looks like just one cell pair — or maybe just one cell within that pair — was bad. Maybe the rest of the battery could reasonably have been salvaged — maybe I should still go back and replace just those two cells. If they were NiCd, I’d consider zapping them to burn out the fuzz crystals on the electrodes, but I don’t know enough about Li-Ion to know whether that’s safe (or useful) to do.

I’ll see how the rebuilt battery works before thinking too hard about this. Even if the cells I just installed aren’t the best, I’m guessing I could look around and get newer cells that’d be fresher and hold more of a charge than the better of the original cells, so it’s probably not worth trying to salvage the rest of the original battery.

As mentioned previously, I’m rather fond of SAE A502 amplifiers. Enough so that as I’m starting to think about biamping or triamping my system, I’ve started shopping for one or more A202 amps, the A502′s 100W little brother. And I found one on eBay, and it arrived this week.

Pop

I set it up in my stereo to drive the center channel speaker (for now), got everything connected, and turned it on. Pop. Turned it off. Pop. On. Pop. Speaker off. Pop. Speaker on. Pop. Amp off. Pop. No input signal at all, and pop pop pop.

SAE amplifiers don’t pop. They have relays specifically to keep startup and shutdown pops from making it to the speakers; so (I figured) I must have an unwanted DC component to my output. I ran this idea past Ron and he agreed that’s what was happening, so I measured the DC voltage at the speaker outputs with no input signal:

Huh. Looks like it’s time to adjust the DC offset.

Interestingly, I’m only using one amplifier channel for my center-channel speaker, and I happened to plug it into the left amp channel — the bad one. Had I plugged it into the right, I wouldn’t have noticed this problem for a long time, until I started using both channels. And that might have been when I set it up as a treble amp to drive tweeters, which might have blown out from the DC voltage. That would have made me cross.

DC Offset

Why is DC on a speaker output a bad thing? Well, each driver (the individual cone assemblies that mount in a box to form what we call a “speaker”) has an electromagnetic coil of wire in it. Current through the wire attracts and repels the back of the driver toward and away from a permanent magnet, causing the driver to move in and out, move air back and forth, and make sound.

The coil of wire also gets hot. The fine wire used in driver coils has a much higher resistance than the large wire between the amplifier and the speaker, so the coil is where the majority of the amplifier’s power is dissipated. Some of the amplifier’s electrical energy is turned into mechanical energy to move the driver, but much of it is turned into heat. Too much heat on a fine wire turns into a bad thing. And extra DC current doesn’t make sound; it just heats the wire.

Not much, to be sure. In my case,

.64V / 8Ω = .08A

.64V * .08A = .0512W

which isn’t much for my 150W speakers to handle. But it doesn’t do any good . . . and it pops when I turn the amp on and off.

This small DC output voltage comes about from an even smaller unwanted DC voltage in the preamplification stage, faithfully amplified through the power stage and delivered to the outputs. Because of imperfections in semiconductor manufacturing and the way preamplifiers are designed, this DC offset is virtually impossible to design out. Instead, preamp stages have a DC offset adjustment potentiometer, to adjust that particular preamp’s DC offset back to 0V (or very close).

Inside the A202

This morning I opened the A202 to make the adjustment. I was curious to see how much it looked like its big brother, and the answer is, not very much.

The power transformer is similar, and I recognize some other components and circuits, but the layout is completely different. This makes sense, given the smaller physical size of the A202, and its smaller power demands. The A202 is laid out asymmetrically, with power transistors on the amp’s right side (this picture is taken from the back, so our left) and the power supply on the amp’s left, rather than with the power supply sandwiched between two sets of power transistors.

The preamplification stage is on this smaller PCB located at the back of the amp between the large heatsinks. The DC offset pots are clearly visible and easy to get to at the back edge of the amp. (Thank you!)

The speaker terminals (barely visible in the upper picture) are an odd design, rather than standard binding posts, and my voltmeter probes aren’t quite long enough to make contact. So I inserted wires into the speaker terminals and used gator clips to connect to the voltmeter. I powered up the amp and carefully turned the pots until the DC output was as small as I was able to get it. I was disappointed that I couldn’t get the right channel all the way to 0V (on my meter), but 5mV isn’t bad.

I actually first made the adjustment with the amplifier cold, and then realized how silly that was. After letting it warm up for twenty minutes or so, one channel had already drifted up to .025V, so I was glad to have remembered to go back and fix it.

Cosmetics

Any time you perform service work, you should always clean whatever you’re servicing to the best of your ability; it makes a huge difference to the customer’s perception of your work (even if the customer is you). Ron cleans and polishes VCRs he repairs; good auto mechanics clean the entire area around whatever they’ve replaced (and run your car through a car wash if they’re really clever), and I wipe down cases of PCs I repair for friends and family.

This amp was no different. The inside was full of dust, so I took it to my brother’s house and used his air compressor to blow it out.

The outside was more interesting. I don’t know what the former owner did with this amp, but the case looks like it had a fight with a parking lot and lost. Lots of dings, nicks, and scrapes.

I felt around the edge of the front panel for any dings with sharp edges, and carefully filed them flush with a fine file. Then I use a black permanent marker to fill in every shiny spot. It’s not a perfect match for the original finish and you can easily tell the difference from up close — but you can’t tell the difference from across the room in a normal listening position, and that’s well worth the effort.

About half of the model number on the front panel is scraped off, and I’m not sure how to repair that. It’s white (or near-white) paint, probably silk-screened on, and I think I’m more likely to make a mess than an improvement. I’d welcome experienced suggestions on how to touch that up.

Putting It All Together

I reassembled the amp and reinstalled it, and the popping sound is gone. Even with my ear directly in front of the speaker, I don’t hear any noise there — just the relay clicking in the amp itself.

Here’s what my stack looks like now. It’s in an old PA cabinet on which I mounted soft-wheeled castors for convenience and to protect my softwood floor. The cabinet has rack rails, but the amps aren’t mounted — just stacked. I need to get nylon shoulder washers to protect the amps’ front-panel finish before I’m willing to rackmount them.

From top to bottom,

• my Sony surround processor / preamp sitting on a rack shelf
• a rackmount, surge-protected power strip (love this!)
• my original A502 amp, running the main speakers
• the A202, running the center speaker
• the A502 with replaced relay, running the rear speakers
• another A502 that I bought working, running the subwoofer

I have two more broken A502s waiting to be repaired, plus a couple of other related projects, so there should hopefully be more along these lines soon.

In the late ’80s, almost certainly 1988, I used some of the money from my summer job to order an SAE (Scientific Audio Electronics; no longer in business) A502 amplifier from the DAK (Drew Alan Kaplan; no longer in business) mail-order catalog. The A502 is a two-channel solid-state audio amp rated for 200W per channel — but it’s as heavy as a tank (40-50 lbs) and actually able to live up to the promised wattage, unlike many smaller amps and modern receivers. At a comfortable listening level, the volume meter LEDs don’t even flicker on.

I’ve kept the amp and used it on and off since then, using it continuously for casual listening and home theater movie-watching for the last thirteen years. It’s never given me any trouble, and it performs admirably.

So admirably, in fact, that I’ve been shopping for more on eBay. In late December 2005, I bought two advertised as partially working; and this May, I finally repaired the first of them.

Replacing the SAE A502 Speaker Relay

The first of the two amps was advertised as having both A and B speakers work on the right channel, but only the B speaker work on the left channel. When I received it, I hooked up speakers to confirm the symptoms, then opened the amp to look around. The problem was immediately apparent:

Transient currents can generate popping and other unpleasant noises when a power amplifier is turned on. SAE designed around that problem by inserting relays between the output transistors and the speaker jacks, with delayed activation. Power on the amp, 2, 3, CLICK, and the speakers are engaged. Plenty of the time for the transients to disappear. And it works — my speakers are dead silent when turning the amp on and off.

Well, it works until one of the relays gets a little, um, melty.

I have no idea what happened to vaporize the contact, but I bet it was impressive. [Ron Tozier, local TV and electronics repair wizard, suggests that the contact was probably slightly oxidized, and the extra resistance led to heat buildup and eventual catastrophic breakdown.]

Now, although the relay is DPST, the amplifier circuit uses both contacts in parallel. So in theory, I could have cleaned the slag off the melted contact to make sure it stayed out of the way, polished the remaining contact, and resumed operation.

I really wasn’t comfortable with that idea, though. SAE put DPST relays in there for a reason, and I wasn’t really interested in changing their design. So I looked for a replacement relay, with no success. Google found me nothing to match the Matsushita part number, Digi-Key had no matches, and searching online surplus catalogs didn’t get me anything that matched the coil and current specifications with even close to the same form factor. So I set the amp aside until I could find a replacement relay, and kept searching occasionally.

At Last, a Relay

This April, after over a year of fruitless searching, I checked Digi-Key again, and they had the relay listed! Unfortunately, they had a minimum order of something like 108 pieces, and I didn’t need to fix 108 amps. Fortunately, one of their techs was able to cross-reference the relay, and in short order I had a bagful of perfect replacements. (I know, I don’t need to fix a bag of amps — but being prepared to replace another relay at some point in the future is worth a few extra bucks not to have to track them down again.)

The board is a pain to take out because it’s bolted down to greased heatsinks, it has huge power-filtering capacitors mounted on the back side, and its cables are bundled such that they’re too short to pull the board out. Bleah! But after dealing with the mechanical issues, it was only a few minutes work to remove the old relay and solder in the replacement.

The new relay (installed as RLY201, the right one) has a matte finish whereas the original was shiny, and its body sits a little lower with respect to its contacts, but those are insignificant issues. It fits, and that’s what counts.

The silk-screened circle to the left of the relays, BTW, is the footprint of the filter capacitor on the other side of the board.

Test Driving the New Relay

I threaded the board back into position in the chassis, bolted it back into place, and made sure the wires were all routed appropriately; but I didn’t close the case yet, as I wanted to test the new relay first. I plugged the amp into a power strip that was switched off, switched on the power strip, pushed the “On” button on the front panel, and POP! There was a moderately loud bang and a moderately bright flash, and I shut off the amp and the power strip quickly to have a look.

Strangely, the flash came from the right rear of the chassis — the opposite side from where I had replaced the relay. More strangely, I couldn’t find anything scorched or missing. I figured surely I’d see an electrolytic cap with its top blown off, or a resistor or diode vaporized, but nothing appeared wrong. I unmounted the board and checked the bottom side for brown scorch marks, but nothing wrong was visible there either.

My guess was that after the amp had been off for a year, some component got “tired” and the inrush current destroyed it; but even with repeated checking, I couldn’t find anything amiss. I found a copy of the schematic online and studied it in detail, but didn’t gain any new insight. I was reluctant to power up the amp and start poking around with a scope, for fear that the (hypothetical) broken component might lead to cascading, more expensive failures.

Testing the Repaired, Exploding Amp

Finally, I took the amp to visit Ron and ask his advice. I found his method of testing rather ingenious. This amp has 8A slow-blow fuses between the power supply and the power amplification section, not just a fuse on the line cord. We pulled all the 8A fuses and replaced them a pair at a time with 1A fuses, so we were powering up only one channel at a time. Anything seriously wrong would blow the 1A fuses pretty quickly, hopefully without doing further damage to the rest of the circuit.

After doing a quick check of the major capacitors and power transistors, Ron put an oscilloscope on the speaker outputs and I powered up the left channel — the one whose relay I had replaced. The scope stayed flat as it should — no unwanted DC component. Ron touched the line-level input, and we saw a low-amplitude 60Hz signal on the scope. All as expected.

Next, I pulled the fuses from the left channel and moved them to the right, where I had seen something blow up. Ron moved the scope and we powered up. The left channel output was flat as well — good so far. Ron touched the input, and we saw the same amplitude 60Hz signal on the scope.

Hey . . . what?! It works???

Well, yeah, it does. We couldn’t find anything wrong. I put the 8A fuses back in, took it home, reassembled the case and put it into my stereo, and it’s been running my surround speakers for a couple of weeks. Nary a problem since that day.

So what were the pop and the flash when I powered it back on? My best guess is that some piece of crud (dust bunny, cat fur, etc.) got shorted across a couple of high-voltage pads and lit up when the voltage said hello. Cort says he saw that happen a lot when he was repairing dusty arcade games back in the day.

One down, some more to go!