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Repairs

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.