A change of pace from the Bose equalizers and hi-fi I’ve been working on a lot of lately, I had the pleasure of working on a 1936 GE Model A-52 antique radio.
This is a nicely designed and straightforward table radio with 5 tubes, AM and one Shortwave band. Back in the ’30s, RCA and GE shared chassis and designs quite closely and it’s no surprise this one uses all RCA metal tubes, 6A8 6K7 6Q7 6F6 5Z4.
This radio had been serviced in the past but was due for another go-around. Most of the capacitors had been replaced in the ’70s or ’80s, although there were a few that still needed to be replaced. I swapped the 4 capacitors which were definitely in need of replacement, but the other units tested fine and are recent enough I’m not too worried about them.
The radio power switch, though, had been bypassed. The radio’s owner reported the switch was sparking in the back. I tracked one down after several weeks and was able to get it installed and it functioned perfectly after that.
The radio’s alignment was already spot-on so no adjustments needed there. I re-assembled the radio and let it play for several hours of burn-in testing before sending it back to it’s home where it will continue to play beautifully for years to come.
I recently had another Bose 901 Series I equalizer on my bench for repair. It was in for the standard service, all resistors and film/electrolytic capacitors included and evaluation of the transistors and other components for replacement as necessary.
This one is in pretty decent physical shape for its age.
This equalizer’s owner reports it has never been previously serviced, so Bose must have mixed capacitors from two different parts buys together in this one; typically the filter capacitors are all the same model. It’s not a functional problem, though, and it obviously worked this way for many years!
The early film capacitors have the dreaded dark center spot indicating they’ve degraded.
The old carbon composition resistors had badly drifted and were throwing off the curves produced by the equalizer, further adding to the distortion introduced by bad capacitors. This one, 47K 10%, has drifted by over 32% – well exceeding its tolerance.
Component replacement proceeded uneventfully.
I finally replaced the mail filter capacitors and boxed it back up after burn-in testing. I cleaned the switches, which were quite crackly and didn’t make good contact, and now they work nearly good as new. This equalizer is heading back to its owner where it will serve for many years to come. I just love the wide sound produced by the Bose 901 speakers, especially the early series. If you haven’t heard them, I’d encourage you to give it a try!
I’m happy to announce that starting soon, I’ll be offering professionally restuffed vintage capacitors for historically accurate repair and restoration of your antique radio! In my rebuild process, the old capacitor is carefully stripped of the old wax coating and the innards carefully removed. The body of the capacitor is lightly cleaned and a new high voltage axial lead film capacitor is installed in the cylinder. The ends are then filled with a medium density clay filler to provide stability, followed by a fresh coat of bee’s wax. The end product is nearly indistinguishable from one in original condition, perfect for performing a historically accurate repair of a valuable antique radio.
If you’re interested, leave a comment! Pricing is expected to be $3-5 per capacitor depending on type and value.
I was lucky enough to get to work on a beautiful example of this 1954 Philips 778-2 radio. This is a rare, very high end, excellent piece of history which deserved to be taken care of. Very little information is available for it that I’ve been able to find other than a schematic which was published in the Radio College of Canada service manual series; even the Radio Museum doesn’t seem to have an entry for it, which I’ll have to correct in the near future. It looks great, too, in a massive cabinet weighing at least a hundred pounds.
Interestingly enough, this radio is AM only whereas you’d find a similar radio from the U.S. with FM from the same year. As I was told, FM radio hadn’t yet made it into Canada due to licensing issues; that lagged a few years behind the United States. If anyone has more information on that topic, I’d love to hear about it. There’s also a name plate on the top indicating this particular cabinet was custom-built for a certain wealthy Toronto family. As you pull the front panel down to reveal the tuner and record player, the top is attached to a linkage and slides back similar to how a piano might open.
Definitely a very rare pierce here.
With 14 tubes, including four 6V6s in parallel push-pull, this is a great performer. The transformer (a 25-Cycle model) is absolutely massive, too, and the entire chassis itself is thick stamped steel. This is built like a tank and very serviceable.
It used a very interesting linkage to control the position of the dial band indicator:
This radio was serviced once before in the modern era in Toronto, and there was some evidence of previous repairs. It still had most of the paper, and primitive ceramic disc capacitors which were still mostly wax coated and are generally suspect at this point in time. The shop which did the repairs left a lot of old components in place, only replacing a few. This definitely isn’t the standard of workmanship I hold myself to, and in general it’s a poor practice – paper and foil capacitors and early disc capacitors are about 50 years past their working life at this point and even they “work”, they’re not working as well as they should. All it took was being moved to cause something left in place to fail.
This radio was serviced a few times back in the day, and then recently by that shop in Toronto.
The resistors were all specified as precision types, and by some miracle, only a handful of resistors were outside their marked tolerance. The most drifted were the cathode bias resistors on the 6V6s which I replaced with precision metal film resistors.
There are a lot of large-value capacitors in this radio, which are fed by a pair of 6AX5s wired in parallel. Plenty of B+ current to go around. Based on my experience with this radio, I’ve added 68 uF and 100 uF to my stock, but during this repair I had to create those high values by adding 10s, 22s and 47s in parallel.
Four filter capacitors and the output cathode bypass capacitor in total: 5 very large capacitors in 3 cans.
In-progress shot while mounting everything up. The negative tabs of the old can make great mounting points, and in this radio all filter negatives connect directly to chassis.
I’ve mounted a couple of terminal strips to hold replacement filter capacitors.
I use red alligator clips to identify where to clip the wires to the right length and solder while relocating the capacitors to the new terminal strips. This was good for helping figure out lead dress of all the wires at once, without losing my place.
The coils and tubes all checked out during earlier testing, so with component replacement complete, I replaced the power cord with a new polarized model switching the hot side, and substituted a bench speaker on the output transformer. For this radio, I made a house call to remove the chassis assembly from the cabinet, since otherwise it would have been impossible to repair.
Finally, it was time for the first power-up! No smoke! I run the first power-up without the rectifier to guard against any obvious shorts, but the second power-up with the radio fully energized worked perfectly!
With component replacement settled, it was time to reinstall the radio onto the chassis assembly and do some listening tests to make sure everything was operating normally. The shop in Toronto added a line input/output across the volume control; I hooked this up to my phone playing Pandora to exercise that functionality. The sound was very, very good when hooked to my test speaker. Very warm and rich tone, and the separate treble and bass tone controls provide a good range of adjustment. The low-end isn’t quite up to modern standards, but as this amplifier predates “true” hi-fi designs by just a little bit, there’s a little weakness on the low end. This is almost entirely due to the output transformer’s size. You simply need a lot of iron to have good low-frequency bass response, and that gets large, heavy, and expensive quickly.
For comparison, an Edcor transformer for the same power rating flat 20~20K Hz weighs 4.5 lbs. and costs nearly $60 by itself. That’s what I’d spec if the original transformer was bad.
When mounting the chassis back to the board with the dial, it was important to get everything to line up.
I’m using a test adapter on the first alignment step, which involves injecting a signal into the IF amplifier grid.
The 455 kHz IF signal is coupled through a 0.05 uF capacitor, which runs very close to the IF transformer itself and so I insulated it with a sheet of paper.
I’m using my scope to watch the RF input (yellow, top) and demodulated audio at the speaker (blue, bottom). My EICO 324 signal generator is pretty unstable when measured with such precision, but it’s similar to what was used at the time and so is entirely suitable for this kind of work.
In this case, the generator’s internal modulation on this setting looks to be nominally 400 Hz. That’s reasonable. The top is the AM RF envelope; both are synchronized and it’s easy to see how the shape of the two waves corresponds.
Zooming in to verify the frequency of an RF alignment point and the level before switching back to watch the audio. That 400 Hz tone is encoded on the 570 kHz AM carrier.
The alignment was positive! Some components inevitably drift with this much age so it’s tough to get spot-on perfect (not to mention, rarely being that good when new anyway). This one is pretty accurate, though, with the dial tracking within one division of the scale (20ish KHz generally). The offset is slightly varied across the dial. This is often caused by permanent changes to coils – coil forms may change size and the coil’s inductance; temperature-compensated capacitors may be subject to drift. That sort of thing. It’s normal for a radio to have a bit of variability in it these days, although when new they were a little bit tighter. Modern radios use self-calibrating phase locked loops in place of L-C tank circuits.
With a 15′ foot wire antenna strung up, it has good tone on the loud music and talk stations. There’s just a few problems with hum that are resisting efforts to take them out, though. Below a certain volume, there’s a loud 120Hz hum and also a bit of buzzing. In a low-interference environment it’s not too bad (nearly normal-sounding, even) but in a more electrically noisy environment it turned out to be a major problem! Back to the shop for more investigations. One important lesson is that my bench speaker is much less efficient than the speaker this radio came with. The hum was much louder when installed with the original speaker.
By looking at the schematic, this is really a nice-but-pretty-well-settled-technology radio receiver coupled to a very high-end mono amplifier. There are 16 tubes total; 2 are the rectifier and 1 the tuning eye, leaving 13 working tubes. The radio receiving tubes (6SG7, 6SA7, 6SK7 and 6AL5) are the RF amplifier, converter, IF amplifier and detector. That leaves a full 9 for the audio amplifier: Five voltage amplifiers and driver tubes driving a full set of four 6V6 tubes. On a hunch, I started pulling AF tubes. At the time this was to check for issues with the shielding, but one stopped me: the schematic calls out five 6AT6 tubes, but I ended up pulling four 6AT6 tubes and one single 6AV6 in the first AF amplifier position.
They’re fairly similar tubes, with a key difference: the 6AT6 has a gain of 70, while the 6AV6 has a gain of 100. In practice, the 6AV6 is 30x more sensitive than the tube the circuit was designed for – and as a result, it was picking up interference the circuit as designed wasn’t sensitive to. This would have had follow-on effects, too: with the first position introducing the interference, every tube afterwards would amplify the bad signal with the good. Luckily enough I happened to have a single 6AT6 in stock to replace the incorrect tube and this radio began playing perfectly hum free as soon as it warmed up. Problem solved!
I’d speculate tube was replaced with an incorrect substitute last service, but we’ll never really know how that happened.
Now time to deliver it for real – reinstallation back in the cabinet:
Fully serviced, this radio will continue to play faithfully for many years to come! It’ll be able to keep up with the times, too, since it’s been retrofit with a standard audio connector – it would be perfect with a Roku or other Internet radio hooked up permanently! This was a great project. I love working on these top-of-the-line sets, seeing how they’ve been treated in the past, and how they’re being used in their homes – very few of these exist anymore, and I’m lucky to have had the opportunity to work on this one.
My workbench has been absolutely swamped lately, so I’ve been rather slow on writing up the projects I finish. I’ve finally had a little bit of time to put some of this down on paper and the first thing cleared off the bench is a Bose 901 Series I equalizer, serial number 31131 brought back to life!
The set’s owner reported it was giving him noise and distortion and definitely needed to be serviced. That’s pretty standard with these Series 1 equalizers, and many Series 2 – the parts are just so old they degrade and fail and can’t deliver the kind of performance the Bose 901 Series 1 speakers deserve. He also requested I update the RCA jacks, as the old style jacks were spaced too closely together to support the high quality Monster cables.
This one came to me with some evidence of having been repaired at least twice over its lifetime. One of the 100 uF electrolytic capacitors was replaced, as were the two 500 uF primary filters. The two replacement main filters were actually a bit of an upgrade and were much newer, doubled capacity at 1000 uF each which will slightly lower the noise floor even further and provide better quality power.
It’s very easy to see with the original set of capacitors where the problem lies. The red capacitors are very clearly discolored, and spot-checking the resistors revealed they were badly drifted as well. Definitely time for a complete overhual.
Attention to detail and precision are key in refurbishing a 901 Equalizer. There are over 80 components which need to be replaced to restore the unit to complete functionality.
Finally, it was time for burn-in and listening test! I hooked this up to my Series 2 speakers. The Series 1 and 2 equalizers and speakers are compatible, so this provides a perfect live test. My listening lab is in transition right now, so I’m using them on a temporary setup but they still sound absolutely amazing.
After several hours of burn-in testing to ensure solid operation. I was satisfied this one was back to perfect shape. It’s actually in one of the best condition equalizers I’ve had the pleasure of working on – an exceptionally low noise floor, and perfectly operational switches. This one will really make the 901 speakers it’s attached to really sing.
If you need your Bose 901 Series 1, 2, 3 or 4 equalizer repaired, I can take care of that back to meeting or exceeding its factory specifications with precision manufactured components. Click through for more details!
I recently got to fix up another Bose 901 Series 1 equalizer which I received for repair. These are some of my favorite electronics to work on – they’re easy to work on and each one has its own history. Every one of these I’ve seen has been slightly different and this one was no exception.
This one in particular has 4 separate repairs. One is especially interesting.
The last one is somewhat clever. A 10K resistor, probably 5W, across those terminals is the modification to run this equalizer on 240V in Europe or similar. It’s been jumped with a solid piece of copper bus wire taking it out of the circuit but still leaving it in the equalizer if conversion ever needs to happen again.
Top-off testing was next. The neon indicator lamp in the power switch was flickering badly – it had likely been losing neon through the metal-glass interface very slowly over the past 40 years. It’s a neon lamp attached directly across the AC mains with a voltage dropper/current limiting resistor in series. The total power consumption is a few mA at line voltage.
Here it is removed from the circuit. The lamp/resistor combination is actually a single component – they’re welded together. I replaced it with an NE-2A/150K resistor combination, I believe the resistor is 1/8W the draw is so small. The envelope size of the new bulb is about half that of the old one, but it fits in well from the bottom to let wire tension keep it in place better.
After burn-in testing, the equalizer checked out perfectly! It has incredibly clean switches. The others I’ve serviced are much improved after cycling but can hang up the first time they’re used and these didn’t even need cleaning.
This one is going to be a great performer for a long time, and these are a lot of fun to work on.
I recently had a chance to repair another Bose 901 Series 1 equalizer. This makes quite a few of these that I’ve written up on here. One of my favorite things about seeing copies of the same model is getting to pick out the individual variations that happened in the production run and any repairs that have happened over the years, and this one is no different.
This one is new to my bench, it’s a 240V model! I haven’t had this one across my bench before – but the circuitry is identical except for the addition of a single extra resistor. Fortunately, I’m equipped for that!
This one looks like it’s in great shape except for a tiny corner that’s cracked but not yet separated.
It’s an incredibly simple switch. A 3W resistor dissipating about half that amount in series with the AC and the transformer, dropping the line voltage to 110V and feeding the standard circuit. It doesn’t look like this model has been repaired.
Capacitor replacements went according to plan, although one set of capacitors ended up being defective from the factory so I ordered a different set. Shown below are the good replacement filters.
All replaced! Precision 2% tolerance resistors, German-manufactured film capacitors, and modern replacement electrolytic capacitors. This equalizer powered right up and sounded great on every setting immediately with no further troubleshooting required.
Quite a few parts were replaced during this process.
I picked up this Grunow 750 “World Cruiser” radio from eBay a little while ago for an incredible deal and now it’s time for it’s turn on the bench. These radios are fairly uncommon and frequently sell for several hundred dollars, so I was excited to be able to pick one up for under $100 with shipping.
It’s in remarkably good shape, despite the eBay seller packaging it in form-fitting cardboard with no padding whatsoever and the chassis unsecured in the cabinet. The fact it arrived as anything other than a pile of broken wood, bent metal and shattered glass astounds me – it was by far the worst packing job I’ve ever seen an Internet seller provide.
The radio looks like it sat somewhere very dirty, and possibly was briefly inhabited by a rodent. There are a few chewed-on spots, and some fiberglass insulation was dragged into the cabinet. It doesn’t look like whatever lived there was in it very long, however, as there’s no rust, the damage is very minor and there wasn’t a lot of “fill” material brought in.
I set to cleaning and examining. One IF transformer is missing it’s grid cap, that’ll be a bit annoying to replace.
You can see around the edges where it looks like a rodent did some chewing.
It looks like it also chewed through the output transformer leads.
This is a big radio with a big chassis to match, accepting 7 tubes 6D6 6A7 6F7 75 76 42 80. It can receive 2 shortwave bands and the AM Broadcast Band, features a tuned RF amplifier, and double-tuning on the broadcast band for extra selectivity. For the double-tuning, it uses a 4th segment on the tuning capacitor. It’s very rare to see a 4-segment tuning gang on a superhet and it’s a definite indicator of quality.
The underside is built a bit like a tank, with multiple sets of shielded coils. Fortunately, the sides of the chassis are bolted on allowing easier access to the components. It would be impossible to work on otherwise.
With the sides off and the coil covers removed, it’s a lot easier.
I’ll be working on the radio this week, testing all the coils and transformers and then replacing the out of tolerance resistors, new capacitors, and repairing the IF transformer grid cap and output transformer leads. The radio will also need a new cord as the old model was badly frayed and chewed and so it was discarded.
A friend I’ve made through the antique radio community is also a repairman himself, but ran into some scheduling difficulty with a piece of work on his bench and referred the chassis over to me for service while he dealt with his scheduling conflicts: an RCA 9X561, from 1950.
This RCA is a simple and straightforward radio which came to me with 75% of the work completed; my task was to finish off a few odds and ends to bring it up to 100% condition. This turned out to be a bit more interesting of a task than I’d suspected it would be.
I apologize in advance for the poor quality of all photos that will appear on my blog now and into the foreseeable future. Both of my digital point and shoot cameras have since given up the ghost after 5 and 7 years of service respectively – I’ve been taking these photos for quite a while using my cell phone’s built in camera. It delivers acceptable quality in most situations, but unfortunately, its lens has been scuffed beyond the point of buffing out and all future images are going to be somewhat poor definition. (If you find my writing here useful and care to donate securely via PayPal, it would be much appreciated!)
The radio, speaker and chassis arrived alone without the case.
Two capacitor leads were broken, as was a lead from the second IF transformer.
I replaced it with a new segment of wire to replace the now too short broken one.
One of the capacitors which had come disconnected was C3, a 0.05uF bypass capacitor located physically near the oscillator section shown in the center of the photo below.
I reconnected and set out the process of final tests.
The IF chain was fine, but the radio seemed not to want to oscillate. I went through a fair bit of troubleshooting – inject the IF signal into the cathode and grid of the 12SA7 mixer to replace the local oscillator, swapping known-good tubes, using another radio to find an oscillator beat note – nothing at all. The odd part was that it wouldn’t work even with an injected oscillator signal – something which should definitely get the radio going again, and generally identifying the problem with the oscillator.
This one is very simple. A set of coils, a resistor and a capacitor. The coil was intact on both ends and the resistor and capacitor were within tolerances, so I took another peek under the circuit. It turns out that I’d made a mistake reconnecting the broken lead. C3, the bypass capacitor, was connected at the junction of C2 and R2 (indicated above in red) which had the result of completely bypassing the local oscillator to ground – including when I would inject a signal from the generator on pins 5 or 6. The correct location for the connection was the lower left circle indicated in green.
I moved the capacitor from the converter tube over to a tie point on the secondary of the first IF transformer, and the radio pulled in stations instantly.
That missed connection was an easy way to kill a few hours troubleshooting, but it all turned out in the end. The radio is back to my friend to return to his client later this week.
My friend recently purchased a 1991 Mazda Miata from its original owner, in immaculate condition and with incredibly low miles. Except for one issue: the Air Bag light on the instrument cluster was indicating a trouble code. The number of blinks of the light indicates the fault and the light was flashing 10 times, meaning the System Down Fuse had opened. This fault keeps the air bag system from working resulting in reduced safety in the event of a crash, so it’s important to take care of. Used modules can run around $100, with new computer modules starting over $200.
As these cars are getting to be over 20 years old, these module issues have been known for a while and the cause identified: faulty electrolytic capacitors cause the thermal fuse to blow, disabling the system. Capacitors are at the root of pretty much every electrical problem, it seems. We decided to try repairing the module after some research that showed it’s a common problem with a fairly straightforward fix.
This rest of this article demonstrates modifications to your car’s occupant restraint system that are not approved by the manufacturer and if executed improperly could very likely result in your serious injury or death from the air bag failing to deploy, or deploying unexpectedly.
The repair involves sensitive components which can be damaged by even slightly improper handling, furthering the risk of an unexpected failure.
This information is provided only for experienced automotive and electronics technicians as an academic exercise, and KF7LZE is not liable for any consequences arising from following or failing to follow these instruction.
The module is a little blue package that lives up near the steering wheel on the 1990-1993 Miatas. This part wasn’t used in the entire range of the first generation’s production due to revisions that happened along the way. It was also used in similar years of the Ford Taurus, the Mazda RX-7, and there may also be other Ford and Mazda cars using the same air bag module which have similar faults.
We unmounted the board from its housing, then got down to business by removing the bad capacitors:
If one capacitor of a set is bad, it’s very likely the rest of them will be soon. This board uses 5 x 100uF 35V electrolytic capacitors and 3 x 10uF 35V capacitors, all rated at 105°C, along with an assortment of other components that aren’t subject to failure the same way. The System Down Fuse is the long, red-tipped object parallel with the connector on the right side of the photo. A quick continuity check revealed yes, it was in fact open.
From the black spots around the bottom of C7 (center-left in the photo, lower right of R42) you can clearly see the electrolytic fluid had leaked from the bottom of the cans and etched the board a little, but the damage wasn’t that bad. Rubbing alcohol took off some of the residue, but it’s more cosmetic than operational damage and since this board lives inside of a plastic housing itself located inside the steering column nobody is ever going to see it. Replacing the capacitors was very straightforward: de-solder pads, pull old caps, insert leads, and re-solder pads. My Hakko de-soldering tool makes this job very easy, but with any tool it’s important not to overheat the joint or the traces could de-laminate from the board and that usually means the part is destroyed.
Here’s the thermal fuse on its housing. The fuse is the center component, and wrapped around it is a flexible trace completing a circuit between the two center pins. I’m not sure what purpose it serves, but it makes a complete circuit so it’s important to save it. It might be a current sensing winding around the fuse to send a signal when the inflators are triggered, but that’s just speculation based on its placement. Be very careful – on this one, the foil contact pads on the flexible trace split from the mounting points and it was very frustrating trying to get them back together during reassembly.
As you can see, the foil came off the terminal when it was removed. Not good, but not the end of the world either – it’s fixable.
Here’s another view of the board, showing the replaced capacitors mounted up.
Repairing the foil trace was a delicate process. The original connection was a very small contact area, and when it snapped off it removed a bit of foil. There was a thin layer of resin-like insulation over the remaining portions of the foil wrap that needed to be scraped off to expose the bare metal underneath. We first tinned the metal contacts on the fuse mount body, then heated the foil contact pad from the back while applying pressure to force the foil pad into the metal terminal. Once the solder starts to flow, remove the heat but continue to apply pressure – rolling the iron back so it wasn’t applying heat, but could apply pressure as the joint cooled. This took quite a few tries – looking at it wrong the first few times caused it to break off, taking a little more foil off each time. It finally made a good, solid connection and we wrapped it back around the new thermal fuse.
There was a lot of controversy about the thermal fuse replacement on various Miata forums while we did pre-op research. Most commenters who have attempted this repair in the past have been held up on a lack of information about the part number for the thermal component. I can only assume the flexible trace wrapped around it completely destroyed the part numbers on the fuse for most other modules – this particular car has been garaged its entire life, so maybe it was luck. For a safety-critical part such as this one, it’s important it have the right ratings or it could fail to allow the air bag to ignite in a crash or cause an electrical fire after a crash. Several people suggest to replace it with a 1/4W 10 Ohm fusable resistor, a standard metal-film resistor, or a standard fuse instead. I don’t recommend this shortcut.
…replacing it with a standard fuse, metal film resistor, or whatever would circumvent its primary function. Thus, the bottom line is that replacing it with anything but an identical item would risk air bag deployment at the improper time…
After unrolling the film from the old thermal fuse, there were still some very faint, but readable, part numbers listed – very surprising. It’s a Motorola part, which goes well with the Motorola controller chip onboard. Part number 4283A.
The bag the old fuse is resting on gives away the next step a bit. When looked up in the NTE Cross Reference Search, 4283A brings up a modern part number replacement that happened to be stocked at my favorite local electronics shop, Vetco. It’s an NTE8139: 141°C, 15A thermal cut-off. That’s a definite part number! And it’s available in modern production. No need to worry about replacing it with a different part and changing the operation of a safety-critical circuit when an identical component will do!
This bears repeating: the air bag system down fuse in a first-generation Mazda Miata is an NTE8139.
The new fuse gets installed and soldered into place, wrapped again in the original flexible trace that covered it before. It’s very important to remember this is a heat-sensitive device, and you’re soldering to it. We only applied heat for about 10 seconds max at a time, and it was just barely enough to get it to take the solder. The first 10 seconds were a cold joint with the solder holding it in place, the second 10 seconds reflowed the cold joint to be a proper joint to ensure the fuse wouldn’t open up while it was being installed. If you overheat the thermal fuse, you’ll destroy it.
Finally, clip the extra-long leads down to size, reinstall the board in the housing, the housing in the car, and fire it up: The light came on briefly at the start like it should, then blinked off. NO CODES! The air bag system passed all self-diagnostics. We think it’s ready to protect him in the unlikely event of a collision, but there’s of course no way of testing that short of crashing the car. The controller thinks the system is fine, and the parts were replaced with identical new replacements, but full system functionality testing is impossible.
The total cost of the project was about $8 worth of parts, and a couple hours standing over a soldering iron.
This is a fairly straightforward rework job, but you must take special care to not overheat the thermal fuse when installing the replacement or it will fail. I’d recommend using an a solder clip or other heat-sink between the terminal and the fuse body, and it really helped to have two pairs of hands working on this to hold the fuse in place while the other person soldered. If you don’t have a friend who can hold it in place, definitely use a soldering assist device.
And remember, use this information at your own risk. You should not attempt this repair yourself, and KF7LZE is not responsible for the consequences of failing to follow these warnings.