Technics SU X840 – Initial Inspection

Another Ebay “does nothing, no power” casualty. (This is always a risky buy due to mains transformer loss – but mostly you get relatively simple faults in my limited experience – at best a surge blown fuse with not other damage, obviously.. )

Spotlessly clean, hardly used, no fuse blown, but I suspected the power transformer at first tests as one connection is open circuit but this could be a 110V connection point that this amp does not offer as it’s UK 240V model only, but need the manual to check (Yes, it is unused).

hfe_technics_su-x840_service

This leaves the secondaries that are questionable by initial odd readings.

Stereo Integrated Amplifier (1988)

Technics SU-X840

​_​_Specifications

Power output: 50 watts per channel into 8Ω (stereo)

Frequency response: 20Hz to 20kHz

Total harmonic distortion: 1%

Damping factor: 30

Input sensitivity: 1.2mV (mic), 3mV (MM), 200mV (line)

Signal to noise ratio: 73dB (MM), 84dB (line)

Channel separation: 60dB (line)

Output: 200mV (line)

Speaker load impedance: 8Ω to 16Ω

Dimensions: 360 x 103 x 300mm

Weight: 5kg

So checking these pins on the Peak ESR:

Pins 1-5; 13.7 ohm

Pins 2 -3; 4.6 ohm

Pins 2-4; >40 ohm

Pins 3- 4; 15.8 ohm

Pins 5-6; 4.4 ohm (This should not connect for the UK model)

Pins 6-7; 4.4 ohm

Pins 6-8; 10.2 ohm

Pins 7-8; 15.3 ohm

Looks like a secondary is gone…and here’s the proof:

The primary above shows 700Vpp (UK mains 240V RMS = 480V RMS pp = 480 x 1.414 = 679Vpp but showing more than 700Vpp here!) on x10 probes and 100V/div:

Only a couple of secondaries have a tiny 400mVpp signal at best, the rest are flat 0V. The white of the ribbon is pin 1 left below and marked on the main board. All others are the same.

Seems to have taken out the two FETS Q501-2 also as they are short from Drain to Gate – but I need to research FET checking in circuit.

Replacement transformer will be expensive…  these are a pain too as they have to be desoldered or cut from the board unlike the SU 600.

The power amp SVI 3102 A seems original by the old off colour heat paste.

Op amp not inserted fully – what happened to factory QA?

Guess the only fix for this is another broken unit with working transformer.

I can only dismantle this, remove suspect parts and check them out of circuit for something to do. I can cut the PT ribbon to the board as it’s soldered on anyway. Solder station should arrive this week.

The rectifier diodes failed on the Peak except 1, but because of black oxide on the legs – when scraped off it still caused a problem, but on connecting to the underside of the board, all passed as 2.57V @ 2.4mA diodes.

Beware bad connections to the component under test!! 

The legs of lots of components have black oxide type crap on them.

The 3300uF  50V filter caps test good at:

3005uF ESR = 0.04 ohm

3094uF ESR = 0.0 ohm!

I removed the 2 x D1265 NPN power transistors and the PNP B941 and they tested OK – no other obvious fails that could have shorted the PT secondaries or been damaged when the PT shorted..?