I spent some time playing around with one of my HP 3585A spectrum analyzers over the weekend to get a feel for its capabilities. While it’s only good from 20 Hz – 40 MHz, it’s a very capable tool within those limits!
Let’s start off taking a look at the AM broadcast band. I hooked a set of 75-ohm TV rabbit ears to the terminated input and set the analyzer for a 0.5-1.5 MHz sweep. Let’s see what came out:
Not bad! Those peaks are the local AM broadcast band stations. The marker is just left of center on 880 KIXI, our local oldie’s station playing period music. It’s great for vintage radios! Looks like the analyzer’s calibration is pretty good, too.
Then I hooked it up to a 3 MHz unmodulated output from my LogiMetrics signal generator:
Looking good! The levels agree within 0.5 dBm. How are those harmonics?
Decent! There’s the 3 MHz fundamental on the far left; the marker centered on the second harmonic at 6 MHz which is about -35 dBm down from the fundamental; the third harmonic at 9 MHz is about -40 dBm down from the fundamental. Respectable!
I’ll eventually use these for aligning pass-bands on radio IFs, among other things. More on that when I get there.
Cross-posted from the Rain City Audio Repair Blog:
This stunning piece of late mid-century modern German engineering, the iconic Grundig SO 191, recently came through the shop. It’s owner picked it up from an antique shop in nearly pristine cosmetic condition, but with a few electrical issues to sort out. It’d power on, but wouldn’t play! That’s definitely a problem, and so it was time for a full overhaul.
Other than a stain at the bottom of the grille cloth, this console was immaculate. It’s a massive, powerful, top-of-the-line console from the era: a total of 16 tubes, AM/FM/Shortwave and a powerful stereo amplifier with push-pull EL95s per channel driving three high-efficiency drivers, two forward-firing and one side-firing per channel for a total of six speakers.
It’s so powerful, it’s split up onto two chassis! The amplifier module itself is well regarded in audiophile circles and can command several hundred dollars on the secondary market. It’s paired with the tuner unit with a magic eye tuning indicator and the rest of the circuitry. The technology also really shows the era this was made in: while it has a very sensitive and high fidelity FM tuner, and the amplifier itself is stereo, there’s only true stereo output possible from a turntable or reel-to-reel: the FM tuner lacks an onboard de-multiplexer and has no provisions for an external one. When this console was manufactured, FM Stereo had only just barely been invented and wasn’t fully standardized yet. It’s unfortunate, but even still, it sounds great in dual-mono when fixed up!
Underneath, six coupling capacitors had been replaced with new film caps sometime in the ’90s, but the rest was all original. And the cause of the lack of output was very apparent: one lead of a power resistor in the B+ path was broken off, depriving most of the tubes of their power. That’s no good! A full re-cap of the amplifier, including Nichicon electrolytic capacitors in the signal path and KXG-series electrolytic capacitors in the power supply for extra long life.
I tested the forward and reverse resistance of the selenium rectifiers and they were within spec, so I left them in place. There’s a very large selenium bridge in a can on top of the chassis, and a single-plate selenium rectifier for the bias supply.
Up in the tuner section, it’s cramped as German radios often are – but with the amplifier and power supply on a separate chassis, it was surprisingly workable! I spot-checked resistors for tolerance as I passed them and every one I measured was within specification, as has been the case on nearly every German radio I’ve serviced. They sure built them to last! After component replacement, it was time for a first power-up. Not bad – but not great, either. There were a few issues. First was the power switch: the switch mechanism itself was damaged, somehow, and would never energize. Cleaning didn’t help and the switch was buried deep in the mechanism so the owner opted to bypass it and install a new switch on the power cord. Finally, the multiband tone control had an issue. Most of the bands worked, but the final treble adjustment which worked by pushing on a cable via mechanical linkage to change the bandwidth of the final IF transformer was nonfunctional. The cable was seized inside its housing, and cleaning and lubrication from both ends weren’t sufficient to fix it. The risk of permanent, functionality-killing damage was too great to overcome so we decided to leave that as-is. “Normal” through “Decreased” worked, but a treble increase wasn’t available after this fix. Then, an alignment, and ready to go home!
This radio is truly a marvel of high-end German engineering and with this service, it wouldn’t surprise me to have it last another 50 years. It’s a lot of work (and a fair bit of money!) to bring one of these back to life but it’s well worth it in the warm yet commanding sound they produce.
If you need your vintage radio serviced, Rain City Audio can help!
Cross-posted from the Rain City Audio Repair Blog:
A first for my shop, I recently worked on a Bose® 901 Series VI Active Equalizer! These are considerably newer than the Series I-IV that typically come through the shop, although they’re still getting to be 25+ years old at this point and that’s a long time for any piece of electronics gear, so I’d expect to start seeing more of these come through in the future.
It’s a wide, hefty black anodized aluminum case with mid-bass and mid-treble faders, a bass countour switch, LED power indicator, and tape monitor selector. Unfortunately interior shots aren’t available due to a camera mishap, but the design is fairly similar to the Series IV, except using slightly newer op-amp chips. Where the Series IV has four dual op-amp chips, the Series VI adds another active stage and uses a set of quad op-amps in addition to the duals, for a total of 6 op-amps in the circuit. It also features TO-92 package voltage regulators for the positive and negative rails, instead of just an unregulated power supply as earlier versions did.
As is standard practice, noisy old op-amps came out along with failing capacitors which were causing an intermittent signal in one channel. In went brand new Texas Instruments low-noise op-amps to replace, which will get this equalizer going as good as new.
After verification and burn-in testing, it’s ready to go home! With new chips and new Nichicon Fine Gold audio capacitors, this Active Equalizer should sound fantastic for a long time to come.
If your Active Equalizer isn’t working like it should, Rain City Audio can help!
Cross-posted from the Rain City Audio Repair Blog:
Another Bose® 901 Series I Active Equalizer came through the shop recently for a full rebuild. These are all nearly 50 years old and are definitely showing their age, and this one was no exception – it came in with no output on one channel and definitely needed a full rebuild to get it going again. The Active Equalizer is responsible for shaping the audio signal to match the Direct/Reflecting driver’s frequency response, and without it, you’ll never get the good sound the 901 series is known for.
This one was pretty dirty, but nothing some rubbing alcohol and elbow grease couldn’t majorly improve. Inside, most of the electrolytic capacitors had been replaced at one point in the late 1970s, but were still 30-40 years old and well beyond their design lifetime. The rest of the components were all original, including the unreliable small signal capacitors in the center.
These original 2N5088s have color bands for their gain group on the back – all but one were identical. It looks like someone at the Bose® factory grabbed a wrong one during the original construction! Such a minor difference wouldn’t be noticeable to your ears, but might be able to be seen with sensitive test equipment, so it’s not a big deal. The transistors on this one came out and were replaced, since after the initial component replacement it still wasn’t quite right. One channel sounded great, but one channel had lower volume and suffered recessed mids which gave it an odd sound. Time for some troubleshooting!
Using my oscilloscope, I compared channels to see where the signal got lost, using the built-in math functionality to show the difference between the two channels. Ideally this difference should equal zero, a flat line, with both channels being identical.
After further probing and testing at different frequencies, the difference and phase shift were frequency dependent. That pointed me towards the crossover and feedback circuitry, where it turned out that channel’s 22 mH inductor had gone open and was no longer giving the right curve shapes. Replacing both inductors fixed it right up!
Problem solved! With that fixed up, I installed a new neon power lamp and boxed it up to go home, good as new.
Fully refurbished, this one is going to serve well for a long time to come!
Another Bose® 901 Series I Active Equalizer came through the shop. These equalizers were manufactured between 1968 and 1973 and are really showing their age: most have degraded in one way or another, if not outright failed entirely, and this one was no exception. It came to the shop performing very poorly and no longer passing signal correctly – time for a full rebuild!
This particular one was purchased in Europe originally, and made its way over to this side of the ocean at some point in its history. Inside, it was all original, with the dark red paper-mylar capacitors showing the hallmark discoloration that means they’re degrading internally.
A full new set of components will have it good as new!
In this case, this equalizer received all new capacitors and resistors, and a new neon power lamp to replace the flickering original. Now, it’s good as new!
If your Bose® Active Equalizer isn’t performing like it should anymore, Rain City Audio can help!
Cross-Posted from the Rain City Audio Repair Blog:
This Bose® 901 Active Equalizer came in through the shop for the standard overhaul service. It was showing its age, as most of these equalizers from Series I and II will do these days – after all, components weren’t made to last nearly 50 years!
Cosmetically, this one is in fairly good shape. Bose® used a solid wood case on the first half of the production run of Series I, so they stay in better condition than the later ones which switched to particle board with a contact paper veneer on them.
While the current owner doesn’t live nearby, the equalizer does have an inscription indicating it belonged to “Ted Rodgers, Seattle” at one point in the past. Fun! Inside, it’s all original except the output capacitors which were replaced in the late ’70s or early ’80s.
First up, all new Nichicon Fine Gold electrolytic capacitors:
Then film capacitors:
And finally the resistors with new 1% tolerance, low noise, metal film units.
This particular one got some new grounding and shielding, and a full set of gold-plated RCA jacks for the inputs, too.
And we’re done!
This Active Equalizer is going to serve faithfully for a long time to come after this service, and it’s going to sound great.
If you need yours repaired, Rain City Audio can help!