This Bose® 901 Series IV Active Equalizer came through the shop recently with a variety of intermittent issues impacting its performance. These later series equalizers are still getting to be quite old, so it was time for an overhaul.
The whole board comes out on the Series IV and later equalizers, making it very easy to service. This particular one was due to get a whole new set of electrolytic capacitors and op-amp chips, and a few upgrades: audiophile-grade film output capacitors for improved clarity and transient response, and precision low-noise operational amplifiers to replace the older style.
The original op-amp chips in this series have a tendency to go noisy over time, so they’re always replaced as a matter of course. Identical replacement chips are still available today, and bring the equalizer back to it’s original factory performance – but chip technology has advanced so much since then, there are a few improvements. In this case, precision Texas Instruments op-amps with 75% lower noise and 1/3 less distortion make a great upgrade and in combination with the film capacitors, really pack a punch.
Everywhere else, this equalizer received Nichicon Fine Gold electrolytic capacitors for the best possible performance.
All fixed up, and with new top of the line parts under the hood, this Active Equalizer is going to sound fantastic for many, many years to come.
If your Active Equalizer needs some help, how about Rain City Audio?
I’ve posted a few times about my HP 3585A spectrum analyzers over the past several months, and after chipping away at the problem I’ve finally solved the screenshots issue! It boiled down to a bad GPIB cable for my National Instruments PCMCIA-GPIB card. After replacing it with a new National Instruments USB-GPIB-B adapter, everything worked instantly the first time.
If you want to get yours set up to pull screenshots, follow along:
The HP 3585A is a bit older of a unit, in the first generation of HP digital spectrum anayzers (following up to the HP 141T analog spectrum analysis system) produced beginning in 1978 up through the early ’80s. Even today, despite it’s limited frequency range (20 Hz – 40.1 MHz), it’s a good performer with versatile connection options, a very flat tracking generator built in, and HPIB/GPIB connections for performing system measurements.
The service and operation manuals really show the age this was created – there was a companion control computer which could be used to extract screenshots and perform automated phase noise, harmonic distortion, and other measurements. Just load in the right cassette tape and key in a few instructions in Rocky Mountain BASIC and off you go!
To extract screenshots, you’ll need these things:
- HP 3585A Spectrum Analyzer (or another one which supports pulled plots)
- HPIB/GPIB Adapter and Cable
- Screenshot Software
1. HP 3585A Spectrum Analyzer
I got mine in trade for some radio repair labor, but they’re out there. Neither one of mine had been calibrated for over 15 years but when I attached an antenna and looked at the AM broadcast band, the counter was spot on for the stations, and it matches spot on with the calibrated output of my LogiMetrics signal generator. I might have gotten lucky since one of mine has the high-stability reference clock option but the other has the standard clock and is still spot-on. I get the impression these are quite reliable.
I’ve seen them on the local Craigslist for $500-1000, and they go for a bit more than that on eBay. They also turn up at Hamfests and so forth as they’re old enough and cheap enough they can make it onto most dedicated hobbyist’s benches, not just shops like mine.
2. HPIB/GPIB Adapter and Cable
Somewhat annoyingly, HP and most other test equipment manufacturers used the GPIB connector, an IEEE-488 interconnect. It’s basically a serial port, but supports daisy-chaining and multiple instruments on the same bus segment. It makes sense given how many of these were connected in systems and controlled centrally for automated measurements, but it’s not a connector that most people will have lying around, and since the only place it turns up is on lab equipment even used adapters command a pretty high price.
HPIB/GPIB adapters are widely available. If you have too much money lying around, you can pick up a brand new one from National Instruments for around $600…or, you can pick up a used one from eBay. They come in all shapes and sizes: old-school ISA cards, PCMCIA adapters, and newer USB ones. Unfortunately this is one area I’d recommend you set your search to “US Only” as I got burned on an adapter I purchased from China, but your mileage may vary of course. You’ll be looking at $200-300 for a good used adapter in most cases.
GPIB Adapters on eBay
3. Screenshot Software
You have a few choices of screenshot software for your HP 3585A spectrum analyzer, and most support several other types of analyzers as well.
First, there’s Keysight Screen Capture 2.0, a free download from Keysight (formerly Agilent, formerly HP). It’s a little more complex of a setup, but supports quite a few other HP products, and can also save and restore settings from the PC to the Analyzer which is useful if you’ve dialed in a custom measurement set-up for something specific It’s also a free download, although you might have to register a free account and give them your e-mail address to get access.
There’s also KE5FX’s 7470A software, part of the KE5FX GPIB Toolkit. It’s a full-featured set of open-source software to pull plots from a variety of instruments, the HP 3585A included. There are a few other applications in the package for use with a few different types of analyzers for making automated phase noise measurements, etc. but it’s main use is going to be the plotter emulator for pulling the screenshots.
Of the two, the KE5XP application wins. Both produce nice plots and work as described once set up. Unfortunately, though, I wasn’t able to get the Keysight application to actually save a PNG of the data it pulled. On two machines it just wouldn’t generate an actual file, but no errors were generated. So for that reason alone, I’m using exclusively KE5XP’s app.
With the connections plugged in and the software installed, it was a major upgrade. Plots went from looking like this:
This Bose® 901 Series I Active Equalizer came through the shop recently for an overhaul. It’s nearly 50 years old and the owner reported it had one channel beginning to drop out intermittently. A previous service a few years back replaced a few components, but age caught up with the rest and the only sure-fire fix at this point is a rebuild.
This equalizer was originally purchased in Europe, but was converted to 120V operation at some point, with a correct U.S. plug installed and the voltage conversion components already removed.
Most electrolytic capacitors had been replaced, but there were still a few which were original. The original resistors, and unreliable paper-mylar film capacitors, were all in place.
This particular one got some upgraded shielding around the power transformer and rear entry, and a new set of gold-plated RCA jacks on the main loop, along with the full overhaul of components and upgraded ERSE PulseX film capacitors for the output. These upgraded caps offer excellent transient response and virtually no dissipation factor, which further improves the already legendary Bose sound.
Is your Bose® Active Equalizer not sounding like it should? Rain City Audio can help!
When Kenwood’s engineers left to form Kensonic, they really brought their A-game, and this Accuphase C-200 pre-amp is no exception. It’s a mammoth device, built with exceptional attention to detail and top of the line components throughout. The owner reported it was sounding muddy and lacking detail, so it was in to the shop for an overhaul!
Like most equipment from the era, the capacitors go bad and cause poor sound, poor performance and left untreated can cause extensive damage in the event of a catastrophic failure. Fortunately, Accuphase built this one for service!
Underneath heavy double shielding, nearly all the critical circuits are on plug-in cards which are easily removed to re-work.
Modern caps are quite a bit smaller!
There’s just a few hiding on the back-side, but nothing too serious:
I measured distortion at <0.02% THD. And how about that frequency response?
Perfectly flat +/- 1 dB across the whole range! Quite a few parts were replaced during this overhaul:
With up-rated Nichicon Fine Gold and Nichicon MUSE Bi-Polar capacitors in the signal path, and an overhauled power supply, this Accuphase C-200 pre-amp should sound fantastic for a long time to come.
Need your gear fixed up? Rain City Audio can help!
I recently had another Harmony amplifier, the H400, through the shop for an overhaul. It’s owner, who previously sent me a Harmony H500 for service as well, took it to a local tech but wasn’t satisfied with the repair so he shipped it in to be re-done.
It’s a simple little amp, although it should offer good performance. There’s three tubes in a series-string arrangement: 12AX7, 50C5 and a 35W4. You might notice that’s not quite enough volts! Harmony used an interesting design here. The 50C5 and 35W4 are across the power line, but there’s 35V left to distribute and the 12AX7 can only take 12 in a series-string configuration.
Harmony got around this by using a 3:1 step-down transformer, mounted top left, taking that 35V and feeding 12V to the tube. It’s an interesting design choice! The other transformer on top is the output transformer to the speaker.
Underneath, it’s quite simple as well. There are a small handful of capacitors, a big multi-section electrolytic, and a few resistors. The previous tech reportedly “replaced some capacitors”. There’s definitely some evidence of work being done, but I couldn’t identify which ones were supposedly re-worked. Everything underneath was old, and time to come out.
In addition to the issue with the capacitors, the 35W4 was dead and needed to be replaced. Not to worry, though – they’re cheap and readily available.
All new capacitors, with the former multi-section capacitor replaced with a terminal strip. One resistor ended up needing to be replaced, too.
Hooking it up to my spectrum analyzer with tracking generator, it looks pretty great! It’s mostly flat from about 40 Hz to 15 kHz, rolling off on either side due to the transformer. It has very low distortion, too – this would be a good mono music amp for the time, in addition to being a decent little practice amp.
Fully serviced, it’s going to serve reliably for a long time to come. Another classic, preserved for the future!
Bose 901 Series III Active Equalizers are uncommon compared with the other series, but I did have one through the shop just recently. The Series III design – while a great performer when in good condition – was susceptible to reliability problems with the somewhat more complex circuit design. The Series III was the last generation Active Equalizer to use discrete transistors; while functionally interchangeable, the later Series IV used op-amp chips.
This one originally came from Europe and was fitted with a 220V plug. Voltage conversions aren’t generally possible with later series, but the Series I, II and III were easily convertible by just removing a couple of parts and changing the plug end.
Starting with the Series III, Bose used high quality resistors which don’t drift, and good film capacitors – but like most other electronics, it’s the electrolytics which cause the problems. In this case, the owner also requested an upgrade to audiophile-quality film output capacitors.
This equalizer got a full set of new Nichicon capacitors, including Nichicon Fine Gold capacitors in the signal path, and top of the line ERSE PulseX film capacitors with excellent transient response and a vanishingly low dissipation factor for the best clarity possible in the output signal path.
That’s a few parts pulled out! The new replacements are all up-rated and will provide many years of reliable service to come.
Do you need your active equalizer repaired? Rain City Audio can help!
Cross-posted from the Rain City Audio Repair Blog:
I don’t typically service portable transistor radios, but I make an occasional exception for something especially unique or interesting. The Telefunken Bajazzo Sport 201 is no exception – it’s a high end piece of German engineering, a great performer with unique styling.
These radios make great portables, with surprisingly loud volume from such a small speaker and very good audio quality. They also had provisions to be mounted to a car’s dashboard. I’m told certain 1969-1972 Porsches might have had those docking stations installed. I hope the owner find one to complete the rest of the set!
It’s got a beautiful dial face with clear markings, and tellingly, this one is a U.S. Export version! You can tell because the FM band goes the full U.S. range, 88-108 MHz. Back then, the German FM band stopped before ours.
It’s built very accessibly, too. Remove 4 fasteners and the speaker pops out on short leads which allow it to be moved. There’s only a handful of electrolytic capacitors, the rest are film domino caps or “polystyrex” polystyrene film capacitors which don’t go bad.
This one has been serviced once before. For some reason, this 0.22 uF capacitor has been shorted across. Everything appeared to operate normally, though.
The previous service replaced one electrolytic capacitor, as well. There were 5 electrolytics in the unit in total, four filter capacitors and the tiny one was a 0.5 uF electrolytic capacitor which was replaced with a film cap. It came in for mostly preventive maintenance and an alignment.
Electrolytic caps were all replaced with Nichicon Fine Gold, and the tiniest was replaced with a polystyrene film 0.47 uF 250v capacitor.
There appear to be two radios which share this model number, with slightly different band coverage. Shown here, the German schematic. The trimmers for the FM section are all identical but otherwise the adjustments are all different. I found schematics and partial instructions for both, but only the German version had full alignment pointers. Unfortunately, either this version or this board revision had differently arranged trimmers for most functions to the point where I couldn’t complete the alignment. I did, however, use the distortion analyzer to align the FM for best performance and adjusted the dial tracking.
Ultimately, while this one didn’t receive an alignment for Shortwave or Longwave, that’s not really the end of the world. There’s nothing broadcast on Longwave except for some aircraft navigation beacons these days, and with the decline of shortwave, it’s unlikely the 3′ telescoping antenna would be able to pick up much anyway. With all that, back in the case!
Looks and works great! Another great piece of radio history set up for a long future.