Bose® 901 Series II #65097 Overhaul

This Bose® 901 Series II Active Equalizer, #65097, just came through the shop for an overhaul. It worked briefly, but failed quickly on its original components. Nothing that an overhaul can’t take care of!



This particular unit had its three main power supply capacitors replaced, but the rest was all original.


Time for new parts:






After all new components, upgraded output capacitors, and a thorough cleaning and testing session, this Active Equalizer works and sounds great!

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1934 RCA Victor T4-8A Bluetooth Upgrade

From Rain City Audio:

This was an interesting little project which came through the shop, a 1934 RCA Victor T4-8A cathedral radio. This radio had been fully restored, down to the bare metal of the chassis and completely re-wired by a boutique restorer, then it was on to Rain City Audio for some specialized finish work for its new owner.



The restorer did an immaculate job, removing and polishing every component and re-painting the transformers, running all new wire and all new components.


First up was the addition of a passive mixing network and some coupling capacitors. This radio uses an interesting, reflexed arrangement for its IF/Detector/1st Audio stage, but tapping into it is straightforward.



On the backside, the radio got a switched outlet for powering the Bluetooth receiver, and some small adhesive pads to hold it all in place. I shortened the length of the power cable between the USB power supply and the receiver for good measure.


Configured this way, the radio receives AM and has its volume controlled through the front panel; the Bluetooth audio’s volume is controlled by the source. And with this, the 1934 radio gets a new life playing anything its owner chooses.

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1951 EMC 211 Service Tube Tester Refurbish

From Rain City Audio:

I don’t often work on test equipment, but sometimes I take a tube tester or similar for service. This one in particular is a 1951 EMC Model 211 tube tester, a quick and simple emissions tester offering a handful of socket types for commonly needed tubes of the era.

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This one had been stored in its original box for many years and was in very good cosmetic condition.

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By the time this tester was released, 7- and 9-pin miniature tubes were common (and Nuvistor tubes turned up occasionally, although never entered wide popularity as was expected.) Octal tubes were largely relegated to just being rectifiers and output tubes, and Loctal tubes had disappeared almost entirely. Owing to a lack of compatible sockets, this tester isn’t able to test anything prior to the mid ’30s when Octals were released.


It’s an exceedingly simple design, with one adjustment, an electrolytic and a paper capacitor in the circuit. One resistor appears to have been replaced at some point in the past, too.

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Not much to it!


Shown here working! This tester isn’t especially accurate or sensitive, it’s the sort of thing a technician might bring out to a home on a service call rather than a benchtop model with more features, but it broadly agreed with my Precision tester. Good as new!

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Bose® 901 Series II #114658 Repair

From Rain City Audio:

This Bose® 901 Series II Active Equalizer, #114658, just came through the shop for repair. It had stopped performing the proper equalizer functions, and so was time for an overhaul.




Inside, it looks like the smaller electrolytic capacitors were replaced, but the large ones were all original. The first filter had blown out its positive end, no wonder this wasn’t working well anymore!




Time for all new components:






After an overhaul, it’s back to working and sounding just like it should!

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Sony TA-2000F Preamp Overhaul

From Rain City Audio:

This venerable Sony preamp was seen at Rain City Audio last September 2015 when it came in for an evaluation. That check-up revealed a clean bill of health at the time, but over the past several months of active service, the original components worn down and started creating problems.



This preamp is very overbuilt, with the different circuits on modular plug-in cards for easy removal.









Component replacement was uneventful for the plug-in cards, although there were some strange value capacitors that I’ve never run into before, like some 150 uF capacitors, and a set of capacitors rated at 3.15V.


There were 4 more capacitors hidden on the phono amplifier board behind the main panel:



Time to replace the main electrolytic capacitors as well. The originals were replaced with new long-lifetime, high-ripple Kemet models made in the EU, and upgraded to screw terminals.






Now for checks and adjustments:




Everything cleaned up and came into spec nicely! Time to measure with the Audio Precision System One.

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Overall frequency response looks great, and zoomed in, the channels are ~0.3 dB separated from each other. Very close!

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And look at that almost perfectly matched RIAA curve for each channel after service.


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Quite a few parts came out of this one! It’s going to sound fantastic for a long time to come.

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Bad Caps in Everything

I just had a bit of a scare with my Audio Precision analyzer. The 2004-era Dell OptiPlex GX270 which drives the AP hardware decided it would refuse to power up – just flashing the “ON” light for a fraction of a second, then back off again. Since that analyzer is pretty central to what I do in the shop, this was a major problem.

It’s not the easiest piece of gear to get running, either. If you have an APIB interface card, the legacy APWin software will work in Windows XP – but if you’re using the parallel port redirect VXD, you’ll need Windows 98 because VXDs aren’t allowed on any later OS.

A quick inspection inside the old PC showed the problem immediately…bad caps on the motherboard.

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Fortunately, Dell made a large number of these OptiPlex GX270s. I picked up another one on eBay, swapped the hard drives, and fired it up. DSC_0040

Fortunately, it worked the first try! Everything is as it should be. I’ll probably make an attempt at replacing the capacitors on the old motherboard, just to ensure I have a backup PC around if there’s any trouble, but for now the problem is solved.

Posted in Computers, DIY, Electronics, Test Equipment | Tagged , , , , | 3 Comments

Bose® 901 Series II #71033 Overhaul

From Rain City Audio:

This Bose® 901 Series II Active Equalizer, #71033, came into the shop for an overhaul recently. It was showing some signs of age, in this case, a bad left channel. Nothing Rain City Audio can’t fix!




This unit was mostly original, although some of the electrolytic capacitors may have been serviced previously. No matter, though: all old parts were replaced with new.


The owner also requested some upgrades, including output capacitors, new transistors, and the addition of a switch to the power cord since his receiver did not come with switched outlets.


An inline power switch, since the owner’s receiver doesn’t have switched outlets and the Series II is always-on.


Testing showed all was good!


After a full overhaul and control cleaning, it works and sounds great on all functions and settings!

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