Bose® 901 Series IV Active Equalizer #244305 Repaired

May 30, 2015 Leave a comment

Cross-posted from the Rain City Audio Repair Blog:

Even the later series Bose® Active Equalizers are getting older, and the Series IV is coming up more and more. I just had another one through the shop, this one sent to me here in Seattle from north of the border from Calgary in Alberta, Canada. The owner reported it was no longer equalizing the signal and the controls had no effect on the sound, nothing a few new parts couldn’t fix.

This particular unit came in for the standard overhaul service, which for the Series IV includes all new electrolytic capacitors and brand new, low-noise op-amp chips.

Inside, it’s hard to tell if this unit was serviced previously or if it was just built that way; three of the op-amp chips are identical but a fourth was of a different brand. All came out, though, and were replaced with Nichicon Fine Gold electrolytic capacitors and ST precision op-amps.

Finally, back in the case and ready to go home!

If your Bose® Active Equalizer is having trouble, Rain City Audio can help!


Marantz 2270 Stereophonic Receiver Overhaul and Adjustment

May 26, 2015 2 comments

Cross-posted from the Rain City Audio Repair Blog:

I recently got to work on another iconic piece of hi-fi history, the Marantz 2270 stereo receiver. I’ve had this model through my shop before, as well as a couple of other 22xx receivers and they’re well built and generally pretty straightforward to service. This particular model offers 70W/channel at less than 0.3% THD into an 8 ohm load, with frequency response from 7 Hz – 50 kHz.

This unit’s owner pulled it out of storage and started listening, but reported the sound steadily got worse and worse. Good thing he sent it in before something worse happened! The capacitors were in pretty bad shape, including a visibly leaking main filter capacitor:

There are quite a few circuit boards in this receiver which all need serviced for a full recap. With only one or two exceptions, all parts were replaced with Nichicon Fine Gold electrolytic capacitors. There’s the FM MPX board:

The regulated power supply board:

Tone amplifier board:

Phono Pre-Amp board:

FM IF strip:

New main filter capacitors, which happen to be much smaller and offer better airflow:

New capacitors on the final modules:

The AM board:

And not pictured, the demodulator and quieting circuits, which are in a tough spot to photograph but had 3 or 4 capacitors on each board. Then, it was time for power-up testing. First was to adjust the regulated power supply’s reference voltage, which had drifted high up to 38.35V over the years.

With that squared away, it was time to check and adjust the DC offset and idle (bias) current before hooking up the speakers. One channel’s DC offset weighed in at 69.4 mV, the other at 11.5 mV; both were reset to 0.

Functional testing revealed that the FM circuitry was suffering from poor reception and distortion, but the owner declined to have that function repaired, so it was on to the AM board for an alignment:

Several fuse lamps were burnt out and when they arrived, it was time to install. Seen here an old fuse lamp and a new LED fuse lamp. The LEDs offer similar brightness and evenness, but draw much less power and generate less heat, which further relieves some stress from the receiver’s power supply and will help it to last longer.

Looks great! With the cover still open, it was a good time to do some frequency response tests:

Flat +/- 1 dB, not bad at all! It sounds fantastic, too, and registered < 0.1% THD across the power band, significantly beating it’s factory specifications. Doesn’t she look great?

Quite a few parts came out:

All in all, this Marantz should offer many more years of faithful and powerful service back at it’s home in sunny California.

Rain City Audio Stereo Repair

ANOTHER fine achievement in radio set design

May 23, 2015 1 comment

Radio Retailing, Apr. 1929

Bose® 901 Active Equalizer Series I #37582 Overhaul

May 22, 2015 Leave a comment

Cross-posted from the Rain City Audio Repair Blog:

Another Bose® 901 Series I Active Equalizer came through the shop just recently for a full overhaul. These Series I equalizers were made from 1968-1972 and most are beginning to experience component failures which rob them of their performance. In addition to an overhaul, this one came in with requests to upgrade the output capacitors and install new gold-plated jacks with all silver-bearing solder.

Inside, it looks like it’s all original.

All new parts installed:

Testing before replacing the jacks:

New panel jacks in the rear, for the main input and output connections:

Quite a few replaced parts!

If your Active Equalizer isn’t working well, Rain City Audio can help.

Need 45V batteries for your old radio?

May 21, 2015 Leave a comment

Rayovac has been making batteries for over 100 years, and their batteries were commonly found in use with many types of battery-operated tube radios. Most commonly, radios would operate on 45V batteries with a tap half-way (for 0-22.5-45V). Two in series would give you 67.5V and 90V for bigger radios. This style has been out of production for many years, but Rayovac has a good modern replacement for the older Type 783 batteries, the 45V-HD.


It has connections for a modern connector as well as the old-style connections:

tab2 tab1

If you have a ’20s or ’30s battery radio, and want something more original than using a stack of 9V cells, this looks like a good bet!

Yours for $19.95 with free shipping from Amazon.

“We’ll See You at the Trade Show!”

May 20, 2015 Leave a comment

Radio Retailing, Apr. 1929

Factories Making CeCo Tubes

May 19, 2015 Leave a comment

CeCo Plant No. 2, now partially in operation and to be finally completed in April - located at Providence, R.I.

CeCo Plant No. 2, now partially in operation and to be finally completed in April – located at Providence, R.I.

Radio Retailing, Apr. 1929


Get every new post delivered to your Inbox.

Join 104 other followers

%d bloggers like this: