Recapping Sanyo MR-909 tape recorder

While consulting a used item listing site, I recently came across an old Sanyo MR-909 tape recorder. The post was clear, the reel recorder was not working, but I still decided to buy it with the intention of fixing it, given the modest price.

In this article I explained how I reset the mechanics of the tape recorder, since, apart from the power light, which actually worked, the rest was completely stopped.

After spending several hours restoring the mechanics of the recorder, I immediately ran into other problems of an electrical nature: one of the two channels felt low and bad, while the other channel was even worse!

I first carried out a careful analysis, using the wiring diagram and some specific tools:

• a good quality digital multimeter;
• a signal generator (sine waves);
• a small single-trace oscilloscope.

The data were irrefutable and the problems were different:

• one of the four power amplifiers (the 2SB373 transistors) was short-circuited, since the voltage between the two transistors was quite different from half that of the power supply;
• the electrolytic capacitors were completely degraded, since the input signal, at the different frequencies, underwent not constant attenuation at the output and in any case different between the two right and left channels;
• the volume and tone adjustment potentiometers rattled when turned;
• one of the two amplifiers went silent when moved.

Fixing all of these flaws was quite time-consuming.

Sanyo MR-909, power final repair

Initially I thought that the problem was a blown transistor, so much so that I had bought some on the internet for a few euros. Disassembling the cooling fin of the transistor, I realized that this had to be isolated from the metal of the various supports and screws. In fact, one of the screws that screwed it touched the fin and put it in short circuit. Once the screw was isolated, the problem was solved and the voltage at the central point between the two transistors settled at about half that of the supply.

Replacing the capacitors of the tape recorder (recapping)

I already knew that electrolytic capacitors degrade over time. What amazed me was finding virtually all capacitors with particularly out-of-range capacities, with cracks on the casing and swollen.

I therefore had to replace all the electrolytic capacitors, doing a complete recapping.

Potentiometer lubrication

Regarding the potentiometers, I sprayed inside them some deoxidizing and lubricating spray, specific for electrical contacts. This method has given good results: now the potentiometers no longer make noise when operated.

Sanyo MR-909, re-soldering of components

When the behavior of an electronic circuit changes as a result of vibrations or deformations, it is always necessary to check whether the tin soldering of the components is done well. In my case, I found several leads of the oxidized components, whereby the tin did not make good contact. I then had to re-weld all the components, adding some tin where there was little and using a flux deoxidizer. The operation was quite laborious, but, fortunately, the components of each electronic board were not many! I also had to replace a couple of adjustment trimmers, as they didn't make good contact and opened the circuit when trying to adjust them.

Felt pads for tape recorders

An aspect not to be overlooked is the replacement of the felt pads that press the tape onto the playback head (playback). The original ones were very worn and hard, while for a better reproduction you need a bit softer felt. My choice fell on 2,5mm felt, the one used by children for chores. The felt must press the tape on the head where there is the sensitive part. As already mentioned in the previous article, each felt pad must press about 20-25 grams on its respective head.

Sanyo MR-909, various calibrations

The calibrations to be carried out on the recorder concern:

• the balancing of the two channels, by means of the trimmers VR1-3 and VR2-3
• the balancing of the two vu-meters, by means of the trimmers VR1-4 and VR2-4
• the bias adjustment, by means of the trimmers VR1-5 and VR2-5

To balance the channels and vu-meters I used the sine wave generator, first recording the frequencies on a tape and then evaluating their amplitude when played by the same Sanyo MR-909 tape recorder.

To adjust the bias, I recorded frequencies of 5KHz on tape, then evaluating in playback which bias voltage corresponded to an output of greater amplitude. I noticed that, with the tape at my disposal, I had to adjust the bias of both channels to 36Vpp.

Securing the tape recorder

This is a rather controversial issue: maintaining the originality of the tape recorder or securing it? The entire front of the recorder is made of metal, which could electrocute the user if the power supply phase had a leak, so I opted to secure the recorder.

As a suggestion, here are some tips for securing old appliances, radios or recorders:

• use, for the power supply, a double-insulated cable with also the earth conductor in addition to the two power supplies;
• connect the metal parts of the recorder to the ground;
• since it was missing, I added a second fuse in series to the secondary of the power transformer, before the rectifier bridge.

Sanyo MR-909 tape recorder, the final test

After all these interventions, both mechanical and electronic, here we are at the final test and adjustments. Fortunately, everything went well and the audio quality of the two channels is now very good.

The warm and enveloping sound of class AB equipment is far more pleasant than that produced by modern class D amplifiers. Of course this is only a personal opinion, but I urge readers to do a little experimentation before disproving what I affirm!

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