Pioneer SX-300

The Pioneer SX-300 was originally released in 1973 and stayed in production until 1976. It was a small, simple receiver that output 7W per channel. I found one unexpectedly at a nearby estate sale. It was sitting in a basement covered with dust, and the back panel was falling off.

Since the condition was unknown, and who knows how long it had been since it was powered up, a variac was used combined with a dim bulb tester to bring it back to life. This was done slowly to help reform any old electrolytic capacitors, and to ensure there were no potentially damaging short circuits. Luckily, it powered up successfully.

The problems

Any receiver this old will have several areas that need addressing. The volume pot, tone controls, and switches all needed cleaning with contact cleaner. The right channel was noisy as well.

The amplifier section primarily uses 2SC1344s and 2SC1345s, both of which are long out of production. Over time, they are susceptible to becoming noisy and failing. These in particular suffered from “black leg” syndrome, most likely oxidation, but it’s unclear whether this is indicative of an issue.

I used cold spray to see if I could isolate the noisy transistor(s) while the radio was playing. I was able to identify the culprit: a 2SC1344.

The repairs

Since these radios are worth a decent amount of money, and just from the historical value of them alone, I decided to re-vamp the entire radio to ensure it will play trouble-free for years. That meant replacing all the transistors and electrolytic capacitors, as well as a thorough cleaning of all the pots.

I substituted 2N5088 and 2N5210 transistors in place of the 2SC1344 and 2SC1345 respectively. These were compatible with the circuit topology and required no other changes. The footprint of these transistors was different, so I had to carefully bend them into the correct orientation before soldering.

All the electrolytic capacitors were replaced, mostly with Nichicon KL series. These are low leakage capacitors that are excellent for audio applications.

The tuner section worked beautifully and required no alignment.


RCA Dimensia MPA-120

RCA’s Dimensia line, aside from being pronounced like the neurodegenerative condition, was their top-of-the-line component system produced in the 80s. I purchased the MPA-120 amplifier that was part of this line from a Craigslist post. It was said to power on, but would eventually go into and would stay in protect mode.

It’s a cool chassis design with a separate VU meter for each channel on the front panel display. The amplifier topology utilizes STK modules as is relatively typical for many amplifiers from this era. It is rated at 120 W per channel into 8 Ω.

The problem

A visual inspection revealed a charred resistor that actually measured correctly with a DMM. Since the amplifier was portrayed as at least partially working, I powered it on and verified the behavior. The protection mode would kick in after being powered for a few minutes.

After checking the components surrounding the STK modules, and verifying no other visible damage, I began to suspect heat may be an issue. As mentioned above, the protection mode would engage after a few minutes. If the unit was powered off and on immediately, it would almost instantly go back into protection mode.

I used a heat gun to apply heat to various parts of the main amplifier board to see if I could get the protection mode to engage faster, rather than simply waiting for the entire unit to heat up. Eventually I found that heating the protection components themselves would trigger the protection mode.

The Unisonic UPC1237 was used as the IC for controlling the protection mode. On pin 7, a capacitor is used to control the muting period when Vcc is applied. In the MPA-120, a 100uF electrolytic is used.

The repair

Since no other issues could be found, either the protection IC or the timing capacitor could have been at fault. I removed the timing capacitor and checked the ESR, which returned with a normal value. I replaced the capacitor regardless to eliminate it as a possibility, and this turned out to be the culprit. The amplifier no longer went into protection mode even after being on for several hours. The capacitor must have had a high leakage current causing the IC to go back into protection mode.