Sansui Amplifier Repairs
I got this to play with! It worked okay but sounded rough, the valves had little or no emission 6L6GC in the output, when they were replaced the bias cathode voltage was as it should be but noticed the grid voltage was changing on a couple of the valves. This was due to the coupling capacitors, the 0.333µF Elna ones were leaking +ve volts from the preceding stage. Needed a good dousing in switch cleaner in all the pots and switches. Poking around I found the HT was at 530V! Way too high for the 6L6GC max anode rating couldn't see why this was so high as it should be 488V or so, the secondary volts on the transformer was also higher than it should have been! Voltage selector was correct, so ended up with a low value resistor in the a\c to the rectifiers, sounded okay afterwards.
This worked of sorts, the front headphones worked, but not the speakers. The back power amps were holding the protection relay off, the VU meters and headphones are fed from before the relay. There were many fusible resistors open on both power amp boards, mainly 150 and 180ohm. When these were replaced the relay operated the front speakers okay but not the rear ones, problems in the 4 channel decoder which I don't repair (does anyone?). Dry joints through solder points on the 4 channel board were also found causing pops and crackles, most of the switches needed contact cleaner soaking, worked okay in 2 channel and sounded fine.
This was working on the RHS channel only, the left made popping noises. One of the 5A fuses had blown and the 2SA640 transistors needed replacing in the power amp. The phono input was also noisy and used the same type of transistors.
This one was not working but the power light was! The 1A fuses were blown on the regulator board and a diode was short circuit. I replaced all four with the usual ones 1n400x again, nothing when powered up. The speaker relay not energizing, there is a 33µF capacitor that expires feeding the protection circuit that needed replacing, worked okay afterwards.
This was not working, not surprisingly, found the left channel had blown loads of resistors and transistors. I replaced them and after powering it up, luckily, on a mains limiter as it was in overload. Turned out the preset for the quiescent was open so the outputs were being turned fully on. Replaced both presets 100 ohm worked fine, replaced several capacitors on the driver and PSU boards while I had it in bits!
This one had blown the 2.5 Amp fuses. The output transistors had also blown. I used TIP41\42 as replacements and set the bias preset to 470 ohm. I also replaced the driver transistors and one of the temp sense transistors by using a BC182L. It also needed the panel lights replacing 8V 300mA.
The speaker relay was switching on-off, I eventually found that four 220 ohm fusible resistors were open circuit and 6.8 ohm resistors in the o\p section were high. All the 470 µF & 100 µF 50 V capacitors on the power amp and PSU were also replaced. Lots of glue around them was also cleaned off, sounded fine afterwards.
This was heating up too quickly, the quiescent was too high and couldn't be adjusted down to what it should be. Needed several resistors replacing on the power amp board and many dry joints going over.
The phono pre amp had burnt up the 820\150\220 ohm resistors and the o\p transistors. These were replaced but there was a problem with the regulated PSU which blew the series transistors when it was powered up. It proved difficult to repair the phono section with -25V on the o\p, I suspect the board had carbonised and was conducting.
After using the amplifier for a while through the line inputs I noticed something was not right. The sound was blurred, checking the left channel quiescent found it at 0.6 Amps. Further checks showed the transistors were okay, however, turned out to be the amp was oscillating at some MHz. I found the capacitors (the square black ones that gave problems in the AU-919) had lost their capacitance, mainly the 10pF ones.
Vintage integrated amplifier from Sansui dating from around 1965. There was no HT (High Tension) to the driver valves, there was a short under the large resistors that drop the voltage for the different HT sections.
Sansui AU-317 Mk2
Several faults on this one, there was 3 V on the o\p of the right channel which was caused by the 18V Zener in the power amp s\c. The quiescent couldn't be set as it wasn't present! The 82 ohm resistors were virtually o\c so after setting the quiescent to the recommended value the RHS channel wasn't audible, the 68ohm resistors in the tone circuit were o\c. Line inputs were okay, the phono input wasn't though, one channel had buzzing, someone had replaced transistors with BC560C and not noticed the difference in pin outs. I replaced all the transistors in the phono and tone sections.
Sansui CA-3000 Preamplifier
Lost LH phono channel, +\- 47 V supply to the phono boards disappeared intermittently then the RH channel phono started playing up, the plug in board connectors had been butchered. I had to replace the connectors with wire links on all 5 boards and wire link all the through links on the main circuit board, over 100 of them!. There were several dry joints on the phono eq boards as well, sounded okay after all that.
Sansui AU-317 Mk2
Noisy transistors; a991, a992, c1845 in the RHS turntable input and also a noisy transistor found in the tone circuit a992. After these were replaced the amplifier sounded fine.
This had nothing working. The 25 V was missing, there was a pre-set adjustment on the board which did nothing, a modification had bypassed it! All three of the transistors in the regulator required changing then I noticed the power amps bias was wrong, all 4 and the o\p DC levels were around 400 mV. Several of the transistors in the vario-matrix circuit needed replacing, 2SA726\C1312 due to one channel not working and crackles\pops through the speakers, after all this it worked fine!
Monster Power Amplifier from 1975. Reset bias, left channel was taking 2 Amp's quiescent, should be 180 mA total per channel, 15 mV across emitter resistors.
Protection kept kicking in intermittently, the NPN transistor in the over current section had a gain of six!(TR605 2SC945), also the solder had separated from the copper giving bad connections on several pads on the power amp PCB.
The amplifier wouldn't come out of protection, because the +\-32 V wasn't present. The capacitor across the 13 V Zener was o\c and the resistors from the bridge rectifier were o\c (68 ohm), at power on from cold the thyristor would trigger in the protection circuit, a small value capacitor across the gate-cathode stopped it, more damage was done to the PCB around these components as the heat from the resistors had carbonated the resin. The 2SC1313 transistor was s\c b-e, be careful the transistor connections are reversed compared to normal Japanese TO92's, the ever present glue on the power amp capacitors was removed. Produced excellent sound when reassembled and set up.
There are fusible resistors used in the power amp section and the preamp section that are going faulty, ie high resistance for no reason, they are 150 ohm in the power amp and 82ohm in the pre amp, the sound becomes distorted and weak when they are the wrong value. There are 2 sets of the 82ohm resistors, one is on the RH vertical board and the others are under the front cover behind the switches. I would recommend replacing the capacitors on the PSU board also 100 µF 63V and 220 µF 63V, and remove the glue holding them in place as over time it begins to corrode the components.
From 1975, the filter board had dry joints and the transistors were noisy. I used BC184LC's (from Farnell). Both channels of the preamp board needed replacing (2SC871r), they must have had a bad batch of them? The fault was crackles and pops through the speakers. A general re-soldering on the power amp board fixed it.
Horrid little amp. Somewhere there was mixing between the channels out of phase, volume pot seemed the fault but turned out not to be.