I have a Technics SU-X501 amp which I got to learn electronic fault finding. This is my first project fault finding on amps. I have done electronics with Arduinos and I have built small amps and valve amps so have some knowledge of electronics but never gone into service repair which is where I would like to go.

I would like some help with the current amp I have as the fault on it seems to be more difficult to find than I intended to start with. Initially the amp was reported to not start up. I found four PCB tracks cut on the front panel which supplied the LCD and the 6.1V / GND to the Microcomputer. I have bridged these out and now it powers up with display.

There was a power issue the relay would click on/off and the negative -15 voltages across the board would fluctuate. I have re-soldered the regulator transistors in as they looked like dry joints and now the supplies seem all ok.

The issue I have now is with the selector chip TC9164N. initially when putting a signal through the amp there was no control over the selection of the input. A signal on CD would play no matter what was selected on the control panel. So, I changed the selector input chip but I still have the same issue.

The supply voltage to the chip is correct 14.8v on pin 28 and -14.8v on pin 1. If I put a 1000 Hz signal through the CD input I get this signal on the inputs to the chip at pins 8 and 21. I then get the signal on pin 9 output for right channel no matter what is selected on the front panel but nothing out on pin 20 for left channel no matter what is selected. I have checked the DATA signal and seem to get some data through when selecting an input on the front panel but the CK signal seems to not do anything, not sure what I should be getting on this pin or what I should be getting on ST pin.

I have removed the coupling capacitors to the pre and power amp C301 & 302 but the fault is still the same. Maybe the power amp has blown this chip a second time or the chip was faulty from supply. I have ordered two more selector chips to try.

I am now a little stuck not knowing what next to try. Has anyone worked on this amp and had similar fault. Any advice help would be great. I would love to get this fault fixed so I can move onto the next learning curve. Thank you in advance for any help

Service Manual
https://cyqvlqsfs.files.wordpress.com/2015/08/technics-su-x501-user-manual.pdf

 

That 3 wires go to the MCU IC602 (M50754) at pins 16,17 & 19 (DATA, CLK, STB2).

You can check with a scope and measure the voltages there. Datasheet of TC9164AF attached.

Allen

 

The wires start from TC9164 as (a,b,c) ----- J201 becomes (d,e,f) ----big cable---J601 and becomes (a,b,d) ----(a.b.c) -----IC602.

Wire (a) goes through 100 ohm resistor and collector of Q609---Q610.

The diagram is messy and hard to trace. You have to print them all out and connect the sheets with transparent tapes.

Allen

 

That 3 wires go to the MCU IC602 (M50754) at pins 16,17 & 19 (DATA, CLK, STB2).

You can check with a scope and measure the voltages there. Datasheet of TC9164AF attached.

Allen

Hi Allen, thanks for the reply. I was thinking this today, I would expect some CLK signal to come from the processor. But I do wonder if the chip is damaged as the left channel has no signal output. I will have ago tonight. If i can work it out I will post the oscilloscope traces. I have looked over the data sheet and the DC voltages seem ok.

 

The wires start from TC9164 as (a,b,c) ----- J201 becomes (d,e,f) ----big cable---J601 and becomes (a,b,d) ----(a.b.c) -----IC602.

Wire (a) goes through 100 ohm resistor and collector of Q609---Q610.

The diagram is messy and hard to trace. You have to print them all out and connect the sheets with transparent tapes.

Allen

I have printed out the schematics on A3 and followed it all through. i see the Data signal going through the 100 Ohms resistor and the transistors. Not sure what these transistors are for they come off the muting circuit so I guess this is a way of muting the signal either on power up or by user. I have checked the DATA signal at the chip and it does look like it is getting through. The trace is showing a short pulse signal different for each input selection so I believe this part is working

 

This is another link for the manual

https://elektrotanya.com/technics_su-x501_sm.pdf/download.html

 

The SM has a very good block diagram and I think you can follow this diagram and inject audio tone into the inputs, set the modes switches, and hear if the tone appears at the correct output.

Since you already has the signal/tone generator, you can easily construct a small audio Amp using LM386 where the output is connected to a pair of ear-phones. Like what I have done...



1. My mini LM386 amp. 2. Two of them made at diff times. 3. signal generator using two germanium transistors.






Allen

 

Sorry for the late reply, i had some issues with using the phone as a signal generator. My new generator has now turned up so i can carry on with the testing.
I have removed the IC201 (TC9164N input selector) and wired in the generator at 10000Hz stright into the output pins 9 and 23. I found a dry joint at R302 right channel which I have sorted. The amp will now pass the signal through to the output speakers both channels with volume control. So it looks like the power amp and tone controls all work ok.
I checked the DATA, CK, ST signals to the chip and I was getting the following.
DATA - 1 pulse different width for each selection
CK - different number of pulses for each selection
ST - NOTHING
So checking the front CPU the ST was pulsing one pulse each selection. I found another broken track next the the CK track which was also broken.

• This is where i am at the moment. I have repaired all tracks and have the DATA, CK, ST signal at IC201 with the chip removed.
• With the chip back in i then get a lot ac noise on the data signals and with no signal inputs connected.
• playing music through the amp it plays very well no hum and I have all tone/volume controls
• When selecting other inputs the amp still does not change channels but has a click on the speakers each time a channel is select

This is the second selector chip fitted and still I have no selection of the inputs. The noise on the data lines may be an issue. I have an ESR tester on the way so I can test all the caps to see if they are failing.
I am also thinking setting up an arduino to test these chips see if they are failing from supplier or failing when fitted.

Below is another link to the manual through my google drive let me know if this does not work.

https://drive.google.com/file/d/1o9IBB3qA9oTIFPELvhgNkEvBqX7wqp0A/view?usp=sharing

 

Finally got the amp changing over with selector switch intput. Found another dry joint on the Clk pin to the board and I believe the DATA track on the PCB was shorting the the CLK track so I have isolated this track seeming as I have now done a direct link from the input pin to the CPU bypassing this damage.

I powered the amp up on the bench and it played very well both channels and I now have input selection. Everything was worked great.

Boxed the amp up and cleaned it, powered it up for another test showing off to my son who is also interested in learning and on power up the magic smoke appeared. No son this is not the smoke machine effect it has blown out the voltage power transistors now.

I did do some soldering on the regulators as one pin looked bad and I did not clean off the excess flux. Could flux conduct enbough to blow out the regulators or is this amp just refusing to work.

Will have to now order some new transistors and try again

 

Got some time testing on the amp last night and have found the problem was with R702 and zenor D702 which supply the base to the power transistor Q704. I tested Q704 and that was all ok. Replaced the burnt-out components and powered the amp up. All supply rails are now correct, and no more smoke . But after about 5 min playing music I get a click and no more sound. Powering the amp off and on I get sound for a while the again relay drops out and no sound. I believe this is now going into protection mode but not straight away on start up. I hope this is not the power chip and this becomes expensive.