1950s Motorola Modded Standalone Tube Amplifier Measurements

July 6, 2015 3 comments

Cross-posted from the Rain City Audio Repair Blog:

Rain City Audio had an unusual project through the shop recently. It’s a Magnavox tube amp from the late 1950s. The owner brought it in reporting he’d purchased the amplifier, pulled from a damaged console stereo and modified to be a standalone stereo tube amp, from a Craigslist posting. After a scare involving un-inspected vintage equipment giving up the magic smoke unexpectedly, the amp came in for an inspection which turned up a few recommendations.

It’s a simple amplifier with a 12AX7 and pair of 6V6s per channel, using a 5U4GB rectifier tube. The chassis was thoroughly cleaned and polished, unnecessary parts stripped, and various jacks and controls were added to make it into a standalone product.

Underneath, the wiring was nice and orderly. The previous technician was a hobbyist who’d done good research, and the work which was completed was of surprisingly good quality. My only suggestion would have been to use shielded wire runs to the RCA jacks, but it proved not to be a problem in this implementation. The electrolytic filter, coupling and bypass capacitors (the 4-section can and two components below the chassis) were original, though, and that could cause a problem down the line.

The electrolytic capacitors in this amplifier hadn’t been replaced, but the paper ones had. In some later ’50s gear, it’s entirely possible the original filter capacitor can was still working, but it’s on borrowed time and should be replaced for sure. I soldered a terminal strip with a grounded lug to one of the can’s ground lugs to make a solid starting point for the can rebuild.

Attaching the filter capacitors, associated wiring, and dropping resistor to the terminal strip:

There were also two electrolytic coupling capacitors, 20 uF 25V capacitors, replaced with Nichicon Fine Gold 22 uF 63V electrolytics:

As well as a bypass capacitor, attached between the balance pot and ground. The old lugs for the replaced electrolytic can were cut off to ensure no connections could be made to them in the future.

While inside, I also touched up the solder joints for the new neon power lamp, which had broken free. Then, it was on to testing! This amplifier’s specifications are unknown, so it’s time to measure them.

Using the Keithley 2015 THD Multimeter, HP 3585A Spectrum Analyzer, and Sencore PA81 Stereo Power Amplifier Analyzer I made measurements of the amplifier’s characteristics. The channels are slightly different power. Measurable but not really audible. The amplifier measures about 10W per channel at 1.0V sensitivity. Output was highest into an 8 Ohm load, which means that’s the correct output impedance for the transformers.

At maximum output, volume control full clockwise, 1.0V input signal into 8 Ohms the amplifier produced 1.197% THD. While that sounds high, it’s not bad at all for the time and implementation. In addition, tube harmonic distortion is often considered pleasant to listen to, as opposed to the distortion generated by solid-state devices.

At normal listening volume of 5W, it was about 0.621% THD.

At 1W of output power, the amplifier produced 0.208% THD and at 0.5W it produced 0.129% THD. Driving very high efficiency speakers, this would be a low distortion hi-fi amplifier by standards even well into the ’70s or ’80s. The frequency response was “flat” +/- 3 dB from 70 Hz – 20 kHz.

Bass response in this case is almost certainly being limited by the amount of iron in the output transformer; with less iron, the core will saturate more easily and be able to transfer a smaller amount of lower frequencies. The coupling capacitors are sufficient value not to impact the frequency response in this case.

Overall, with a rebuilt power supply and new coupling and bypass electrolytic capacitors, this amplifier has been overhauled and should be reliable for a long time to come. It’s a little different looking, but it works well and sounds good – and that’s what matters!

Bose® 901 Series VI Active Equalizer #179043 Repair

June 29, 2015 Leave a comment

Cross-posted from the Rain City Audio Repair Blog:

Series VI Active Equalizers are the newest of the set and is on it’s second revision of production even today. Even still, the earliest ones are from the 1990s, and that’s getting up into the range where some gear might need service – especially if it’s seen either heavy usage or prolonged periods of storage over the years. This one came in for a full overhaul, explained in photos:

The owner reported it wasn’t equalizing properly as the reason for service. Every Active Equalizer gets full performance testing to confirm all the functionality, and it takes just a few minutes to confirm that a Bose® Active Equalizer is meeting its factory specifications. Most of the time this isn’t captured, but in this case I took some photos during the process to show just how performance verification works.

Using the HP 3324A Synthesized Function/Sweep Generator as my signal source and the Sencore PA81 Stereo Power Amplifier Analyzer as my load, I supplied a 500 mV RMS input signal at 125 Hz test signal and recorded the level as the 0 dB reference.

Following tests at 35 Hz, 225 Hz, 650 Hz, 1.85 Hz, 3 kHz, and 13 kHz this confirmed the equalizer’s curves were fully functioning per the service manual.

Finally, I measured distortion at various frequencies with a 1 or 2 V RMS input signal. For this test, I used the Keithley 2015 THD Multimeter’s internal low-distortion source. At 1 kHz, distortion should be below 0.09%:

At o.o18% THD, it’s coming in 5x below the factory permitted distortion floor after service. With that, it’s ready to go home!

Rain City Audio Bose® Active Equalizer Repairs

Bose® 901 Series V Active Equalizer #130806 Repaired

June 24, 2015 Leave a comment

Cross-posted from the Rain City Audio Repair Blog:

More and more, even the newer generation Bose® Active Equalizers are showing signs of age. Series V and VI have been making appearances here and there, but seem to have picked up quite a bit lately as components age up past their working life. This typically manifests as any of a number of problems: low hum in the audio, loss of variable controls, loss of bass, loss of volume, and distortion. Number 130806 in the shop was no exception, but nothing an overhaul couldn’t take care of.

Series V equalizers were produced in the mid to late ’80s, generally, and are an incremental improvement on the iconic 901 Direct/Reflecting speaker design.

Series III and later active equalizers switched designs and used resistors and film capacitors which are highly reliable and don’t need to be wholesale replaced like they do on Series I and II models, but there are still plenty of electrolytic capacitors to replace. In addition, the op-amp chips in these have been known to suffer from some reliability problems several decades later, so on all models with op-amps they’re replaced as a matter of course.

All the electrolytic capacitors are replaced with Nichicon Fine Gold replacements, of course.

All fixed up!

Performance testing showed this equalizer was back in top of the line condition, and with the upgraded capacitors, it’s very likely to sound even better than new!

If your active equalizer needs service, Rain City Audio can help.

Eric Engineering Model 357 High-Fidelity FM Tuner

June 23, 2015 3 comments

I recently got to work on something quite unusaul, the Eric Engineering model 357 hi-fidelity tuner. Built by obscure outfit Eric Engineering in Santa Monica, CA in 1961, there’s relatively little known about these units. It’s a fairly simple ratio-detector mono FM tuner with a dual mono audio output but no MPX output. The no-frills control panel offers only an on-off switch and tuning knob connected to the dial pointer.

No schematics are available for these units, but fortunately they’re pretty straightforward, and this one came to the shop in good shape. It wasn’t receiving anything, though – time to pull it apart.

It uses five tubes, 12AV7, ECC85, ECC85, 6AU6, 6AL5, and a selenium rectifier.


I replaced the selenium rectifier with a terminal strip and silicon diode, and relocated the three capacitors and their associated dropping resistors to the new strip. The power supply used a 3-stage RC Pi filter with B+ being finally taken off the third filter cap.

I also replaced the Bumblebee line-to-ground capacitor with an X1Y2-rated safety cap, the ratio detector electrolytic (which in this case was most likely responsible for the lack of audio output), the output capacitor, and both dial lamps:

Finally, it was time for an alignment. The Sencore SG80 AM/FM Stereo Analyzer comes in handy for that. This design used both a ratio detector, and I believe lacked a limiter on the FM, so it’s a bit more sensitive to variations in the received signal strength than some other designs, and a ratio detector offers lower fidelity than a full discriminator, so an alignment was a must to get the most out of the tuner.

The best way to align an FM receiver like this is through distortion analysis which shows precisely how well the radio is working at converting the RF into audio. By bringing the front end into alignment by adjusting the oscillator for best tracking (which can also be measured through distortion), then work through the IF chain starting at the input to the discriminator or ratio detector. Adjust each element for lowest distortion, then move and repeat for the next stage back.

The alignment had drifted with age, while on station the dial was mis-aligned and it was receiving with excess distortion.

The oscillator adjust is a variable core near the front of the radio:

Then the antenna pre-selector:

But it only significantly cleaned up with adjustments to the IF chain:

There we go! Finally settled at 0.029% THD, which is quite respectable.

It peaked up very nicely! Ratio detector FM was more of a budget design but it still manages to sound quite good.

Another one of these obscure FM tuners saved! If your tube gear needs fixed, Rain City Audio can help.

Categories: Uncategorized

1974 Sony STR-6200F Receiver Overhaul and Alignment

June 19, 2015 2 comments

From the Rain City Audio Repair Blog:

I recently got to work on a pretty big, fairly rare Sony receiver from 1973-74: the STR-6200F. It’s a powerful receiver featuring 60W per channel of power available, with quite a few inputs and outputs, and a strong FM tuner built in.

The owner brought it in complaining of poor sound quality. Not particularly surprising given it’s age!

First up was component replacement. All the on-board capacitors were replaced with new Nichicon Fine Gold electrolytics. This receiver had seen some minor service in the past: there was evidence of soldering, and one transistor on the FM discriminator board was replaced at some point.

This receiver was laid out pretty well. Most of the boards were easy enough to slide out, flip up, and service from beneath.

Time for the lower boards:

The power supply was a little tricky to re-work, but not too bad. Sony soldered many of the leads directly to pads on the bottom, so you had to be careful to make sure it didn’t come apart.

The power amplifier board was similar.

Time for the main caps. Two smaller ones, and two huge ones.

The new capacitors used screw terminals, so I soldered spade connectors to the ends of the old wires for a strong, durable connection.

All set!

Time for a first power-up. Not smoke, and it sounded okay, but not at all like it should. Time to check the adjustments. First, the power supply reference voltage. It was pretty close, but I adjusted it anyway.

Rather than provide test point jacks, the service manual called out junctions of resistors for probing the voltage. Not my favorite method but it works. Then onto DC offset, which was WAY out of spec. One channel reading 73.1 mV, the other 100.2 mV. Both reset to zero, though.

Bias was also way off, starving the transistors of power and introducing some distortion. Both channels call for 25 mV across their emitter resistors, but it was more like 5 mV per channel. I adjusted both up to 20 mV, which provides plenty of power but will run a tiny bit cooler.

Checking the temperature of various components on the driver board with a laser thermometer while playing into a dummy load.

The sound was much improved – and the measurements showed it, too!

With all this, the amplifier itself sounded fantastic, but the FM left something to be desired. This is a pretty well-regarded FM tuner, so something wasn’t right. I hooked up the FM test signal generator, distortion analyzer, and dummy load and measured the REC OUT port and found that it was receiving with fairly high distortion, indicating the alignment needed to be adjusted. Then proceeded to align the FM through distortion analysis, adjusting the various cores in the order described in the service manual. Instead of aligning curves on an oscilloscope, though, I made each adjustment for minimum measured distortion. This is faster, more accurate, and more reliable than an oscilloscope alignment, thanks to modern technologies.

Adjusting the front end:

Centering the discriminator:

And finally the discriminator’s DC balance:

There we go! Much better, exactly where I’d expect it to be. The sound really cleaned up, too, now it’s bringing in clear highs and powerful bass over the air just like it should.

All told, 76 components were replaced in this overhaul, followed by amplifier adjustment and FM alignment.

Another classic preserved! This Sony is going to sound fantastic for a long time to come, and it looks great!

Bose® 901 Series II Active Equalizer #117185 Overhaul

June 15, 2015 Leave a comment

From the Rain City Audio Repair Blog:

Another Bose® 901 Series II Active Equalizer came through for an overhaul and some upgrades. These equalizers are necessary to provide the curve shaping needed to get the great sound the 901 Direct/Reflecting speakers are known for. The Series II was made in the mid to late 1970s, and the original components are at the end of their working life.

This one looked like it was all original inside.

Partially re-capped, and with some upgrades. In this case, in addition to the standard Nichicon Fine Gold electrolytic capacitors, the output capacitor for each channel is replaced with a Nichicon film capacitor instead of the electrolytic. The coupling capacitors, too, are Nichicon.

This equalizer is going to sound great for a long time to come, really bringing the 901 system to life.

Rain City Audio – Bose® Active Equalizer Repair

Can you say high ESR? [dead cap]

June 11, 2015 Leave a comment


That’s right, 35 Ohms ESR. No surprise, every MIEC cap I’ve ever seen has been garbage.

Categories: Commentary, Electronics, Photos Tags: , ,

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