Rotel Amplifier Repairs
Started having problems with one of my Rotel 610's, found it was the volume board transistors, decided to circumvent the problem by replacing the circuit with an opamp, worked out nicely...
This one sounded sick with poor sound from both channels, although through the power amplifier on its own it sounded much better. I decided to check the capacitors in the preamp. Most of them measured okay but around 50% higher than the rated value. I also checked the four transistors in the tone circuit and found that one had a gain of 4! This would be noticable as it should be around 200. After reassembly it sounded fine, another satisfied customer (myself).
This was cracking from the left channel, dry joints and a bad transistor on the tone PCB 2SC644 were the culprits.
This one was blowing the fuses. The o\p were blown in one channel, also replaced capacitors 6200µF 50V they had lost a large proportion of their value.
Changed PNP driver and input differential pair to 2SA970, they had been replaced with BC214 which had incorrect voltage rating (too low), the outputs had also blown, so these were replaced with BUX10's.
This one had blown a channel. The drivers, output and temperature sensor transistors and a couple of resistors were u\s and easily replaced as you would expect. Then came setting the quiescent, which in the manual says 140mV across a couple of test pins which are across a 0.47 Ω resistor! so the total quiescent would be 560 mA. I changed a resistor to allow it to be set to 80 mV.
Rotel RB-960\970BX Mk2
Both were dead as the owner had removed the fuses, the RH channels needed repairing. Farnell have most replacement transistors. The bridge rectifier needed replacing on one of them. Both these looked exactly the same internally.
This worked okay but the smoothing capacitors had signs of stress and bulging tops. They were replaced with 10,000 µF - twice the original value with same size case. These Rotel's and other models seem to have 'cheap brand' caps and benefit from being replaced when convenient.
There was distortion in the right channel, noticed there was no quiescent in that channel, turned out to be the 2SC2910\2SA1208 driver transistors. All the controls needed dousing with switch cleaner due to the amount of dust around the case internals.
Rotel RA-610 (1972)
This had excess offset volts at the speaker terminals about 200 mV. There is no adjustment on these, so an ad hoc method was derived. A 33k and diode (1N4000) was wired in series from the + decoupled rail to provide a small (+0.5 V) voltage to the + i\p differential transistor. A resistor of 82k to the base from the junction of the other r and diode provided the correct offset volts to reduce the offset to around 10-20 mV. I didn't go any lower to avoid altering the input resistance to the amp section.
Horrendous thump when switching one can be stopped by putting a delay relay in series with the speaker output.