Bose® 901 Series II #101043 Repaired

August 4, 2015 1 comment

Cross-posted from the Rain City Audio repair blog:

Another Bose® 901 Series II Active Equalizer came into the shop recently for a full overhaul. These equalizers are getting pretty old and even if they power up and pass some sound, they’re almost certainly not performing as they should anymore and need some reconditioning to bring them back to peak performance.

You can see evidence of some outgassing from leaking capacitors on the shield foil inside the cabinet:

This one’s mostly original and frankly I’m surprised the owner reported it was making any sound at all.

The output capacitors had bulged out from the inside due to leakage and heating:

And the main filter capacitors weren’t doing so hot, either:

It looks like the main filters were replacements from some point in the past, too:

They’re all coming out to be replaced with new models:

Here’s one of the bad main filter caps. Nominally, this should be about 470 uF and have a very low ESR (<0.1 Ohms). Shown here measuring 6 uF and  140 Ohms ESR.

And an output capacitor measuring 19 Ohms ESR.

The silent channel had a totally dead output cap:

Time to replace parts:

Carbon composition resistors like these absorb moisture from the air over time and drift in value; this 22K 10% tolerance resistor has failed and no longer meets it’s spec of 19.9 – 24.2 kOhm:

The rest of the resistors are a simple swap-and-replace job:

The Series II and later generally have reliable film capacitors, but for the SI and SII they’re always replaced.

And finally to replace the electrolytics:

On a standard service, we’d be done here, but the owner requested upgrades to audiophile-grade film output capacitors, with a much lower dissipation factor to offer improved clarity in the output:

After testing, it was time to proceed with the other upgrade requested by the owner: gold-plated panel jacks. Panel jacks are a great upgrade if you’re planning to use new, thick interconnect cables (like Monster® or similar) but the modification is irreversible since the original jacks mounted on phenolic board are destroyed in the removal process.

And making the final connections:

With this, all resistors, electrolytic and film capacitors, and panel jacks have been replaced:

And a quick confirmation test shows everything is still perfect after the jack swap:

With that, it’s ready to go home! With all new top-quality parts and a thorough cleaning, it’s going to sound fantastic for a long time to come.

[Rain City Audio Bose® Active Equalizer Repairs]

T-Mobile’s WiFi Calling Not Quite Ready for Prime Time

August 3, 2015 Leave a comment

T-Mobile recently re-introduced Wifi Calling to their entire line-up of phones, and have made a pretty big deal about it. And on paper, I can see why – no more worrying about cellular dead zones, and you can take your U.S. number out of the country with you and still receive calls and texts as normal from anywhere in the world as long as there’s a WiFi network available.

Sounds great, right? This feature was available on the T-Mobile G2 Android phone several years ago, before disappearing for a generation of phones, and finally re-appearing on all current models. The only trouble? It’s just plain not very good. Much inferior to their previous version of the same technology, in fact.



I have a small, low-end LG phone which I use for business and seems to do “okay”, but on the Sony Xperia Z3, it’s very unreliable. And the extensive troubleshooting guides don’t cover an important situation that, really, should never exist in the first place: a successful connection that just silently fails to pass any calls or texts.

Arriving at home and auto-connecting to the wireless network? Expect, even though Wifi Calling shows its enabled successfully, to have to toggle it on and off at least once before you’ll get anything other than a beep-disconnect making a call or an error message sending a text. Then, it’s only a matter of time before it silently fails again, continuing to indicate it’s connected successfully but blocking all inbound and outbound calls and texts. In this state, even Google Voice calls won’t go through, as apparently those don’t legitimately use the data connection but still take voice channels.

There’s no error message, no notice, and apparently no heartbeat or auto-reconnect functionality built in. Combined with the fact that my home is basically a dead zone that even the Personal CellSpot can’t bring to life it’s very difficult to use my T-Mobile phone. The only reason I’m still with them is because of the price and plan (although, if I can’t use it effectively, maybe that’s not a good argument.)

Anyone else have a T-Mobile phone, and is frustrated by their inability to deliver on Wifi Calling’s promises?

Revitalize HP and Tek digital scopes with a NewScope LCD kit

August 1, 2015 Leave a comment

I’ve been trying to figure out if I want to bother repairing my old HP 1222A oscilloscope which I’ve been using as a waveform monitor attached to the scope ports of my Sencore PA81 Stereo Power Amplifier Analyzer. It’s been run hard over its lifetime from the looks of it, and the CRT is so dim now that with brightness turned all the way up to maximum, you can just barely see the trace if you shield it from external light. It’s an older model, from the late ’70s, but it makes a good audio scope.

While a replacement CRT seems hard to come by, I did stumble across an interesting solution for some of the later digital display oscilloscopes and analyzers: the Simmconn Labs NewScope modules! I’m not affiliated with them at all, but it looks like an interesting product that could preserve a lot of the highly reliable digital gear which just happens to be old and have worn out display sections.


They’re really interesting upgrade modules, which add color and clarity to the old display screens, and have some other interesting features like saving traces, etc. which may not have been available on earlier models. These come in several models, designed to work with:

  • 3577A, 3577B Network Analyzer
  • 3562A Dynamic Signal Analyzer
  • 3563A Dynamic Signal Analyzer
  • 8756A Scalar Network Analyzer
  • 4145A, 4145B Semiconductor Parameter Analyzer
  • 8566A, 8566B Spectrum Analyzer
  • 8568A, 8568B Spectrum Analyzer
  • 8567A Spectrum Analyzer
  • 8753A, 8753B Network Analyzer
  • 8757A Scalar Network Analyzer
  • 8702A Lightwave Component Analyzer
  • 8720A Network Analyzer

And some Tektronix scopes, too:

  • TDS 520A 540A 620A 640A
  • TDS 520B 540B 620B 680B
  • TDS 520C 540C 580C 680C
  • TDS 520D 540D 580D
  • TDS 644A 644B 654C
  • TDS 684A 684B 684C 694C
  • TDS 724A 724C 724D 714L
  • TDS 744A 754C 754D
  • TDS 784A 784C 784D 794D

Worth checking out if you’re in the market!

[NewScope LCD Modules]

Sony ST-J75 Recap and Alignment

July 31, 2015 Leave a comment

Cross-posted from the Rain City Audio Repair Blog:

The Sony ST-J75 is a pretty well regarded, great sounding stereo FM tuner from the early ’80s. It’s one of the first forrays into digital tuning, with a frequency read-out, programmable memory, and seek/scan functionality and variable muting driven by a microprocessor.

This model features FM only, accepts a 75 or 300 Ohm antenna, and outputs left and right channels at fixed 0 dBm (0.77V) level. Inside, the board is quite well laid out and accessible:

The owner reported it didn’t sound great and generally needed to be reconditioned. Taking the cover off, you could see where the capacitors had been outgassing due to failures!

The bottom cover comes off, too, allowing service without removing the board from the enclosure:

Component replacement was uneventful:

First power-up, it started pulling in some stations, but was fairly far out of adjustment.

Time for alignment! The service manual has a very detailed alignment procedure for this receiver, including adjusting various voltage references, clocks, levels, and offsets. This process used a large assortment of test equipment including an HP 3585A spectrum analyzer, Sencore PA81 Stereo Power Amplifier Analyzer, Sencore SG80 AM/FM Stereo Analyzer, and Keithley 2015 THD multimeter.

First up was setting the levels:

Next, to align the discriminator:

Adjusting stereo separation was next.

Using the SG80 and the PA81, I checked the existing stereo separation and found it to be approximately -40 dB; this figure is within spec and did not require further adjustment.

FM Stereo contains signal information up to about 17 kHz, then drops off to allow for separation of the 19 kHz stereo sub-carrier which contains the stereo coding on the sub-carrier. Because this 19 kHz is within range of the audio frequency, MPX decoders have a 19 kHz rejection trap which removes the remaining 19 kHz carrier from the audio, but leaves the decoded stereo information. This adjustment should be set for a minimum. The service manual procedure for this adjustment was somewhat complex and in fact I was not able to complete it the way the manual recommended.

Fortunately, though, Rain City Audio has considerably better test gear than what Sony imagined a stereo shop might have at the time, and using the HP 3585A spectrum analyzer I was able to see and adjust the 19 kHz peak correctly.

The 19 kHz peak is shown at this marker, below the prevailing signal level:

With that, all sealed back up! It sounds great in both mono and stereo now.

Classic looks, and sounds great!

The Hewlett-Packard 5451A Fourier Analyzer

July 27, 2015 Leave a comment

1972: the year launching the Space Shuttle program, the completion of the monument at Stone Mountain, Watergate, the first female FBI agents to join the force….and the new HP 5451A Fourier Analyzer, bringing unparalleled performance to the worlds of acoustic and vibrational measurements.


Learn more about this historical instrument in the June 1972 issue of the HP Journal.


Stereo Aficionados

July 25, 2015 1 comment


Want an Acer Chromebook? They’re really cheap right now on eBay.

July 24, 2015 Leave a comment

Back in June, I’d picked up an Acer Chromebook CB5-311 from Amazon. At the time, it was $377 for a pretty high-end machine with 12+ hours of battery life, a quad-core nVidia Tegra CPU and Kepler GPU with 4GB of RAM and a 1080p screen. Even at that price, I was really happy, but they’ve gotten MUCH cheaper through Acer’s official eBay store.

If you’re comfortable with a manufacturer refurb unit, you can pick up the same model for a shockingly low $259.99 with free shipping. At that price, there’s almost no reason not to if you’re in the market!

[Acer Chromebook CB5-311 on eBay]


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