Ok, I admit it, I need help.
Have a sony tafb930 for repair.
The fault started as sometimes not wanting to change input.
got to a point where he'd never turn it of because if he did, he'd have endless trouble getting the input selector to work.
It wont change inputs, and the input led's wont light up.
Fault finally became permanent, he then asked me to fix it.
Things I have done.
resoldered any obvious bad joints.
replaced i.c. 902.
replaced x900 ceramic resonator.
replaced c901 and c906.
tested r958 (68ohm, all ok)
tested d911 and d920, (in circuit) different reverse reading, but fine and identical in foward.
measured various voltages against schematic.
mostly they are fine,
except for the 4v (should be 2.2) at pins 11, 12, 13, 15, 16, 17 on ic 902.
Voltages on q908 are also out.
while probing junction of q908 and r913, an input relay clicked, and it started working perfectly.
on a power cycle, it went back to dead, and have not been able to get it functioning again.
while it was working properly, all voltages were as the schematic says they should be.
recap, fault is It wont change inputs, and the input leds wont light up.
All help greatly appreciated.
relevant section of schematic attached, measured voltages in red.
should add, even when its in faulty mode, unit turns on, comes out of protection mode, and amp modules run and can adjust their bias with no issues at all.
So to me it looks like the fault is restricted to the input selection area only.
Thanks.
Have a sony tafb930 for repair.
The fault started as sometimes not wanting to change input.
got to a point where he'd never turn it of because if he did, he'd have endless trouble getting the input selector to work.
It wont change inputs, and the input led's wont light up.
Fault finally became permanent, he then asked me to fix it.
Things I have done.
resoldered any obvious bad joints.
replaced i.c. 902.
replaced x900 ceramic resonator.
replaced c901 and c906.
tested r958 (68ohm, all ok)
tested d911 and d920, (in circuit) different reverse reading, but fine and identical in foward.
measured various voltages against schematic.
mostly they are fine,
except for the 4v (should be 2.2) at pins 11, 12, 13, 15, 16, 17 on ic 902.
Voltages on q908 are also out.
while probing junction of q908 and r913, an input relay clicked, and it started working perfectly.
on a power cycle, it went back to dead, and have not been able to get it functioning again.
while it was working properly, all voltages were as the schematic says they should be.
recap, fault is It wont change inputs, and the input leds wont light up.
All help greatly appreciated.
relevant section of schematic attached, measured voltages in red.
should add, even when its in faulty mode, unit turns on, comes out of protection mode, and amp modules run and can adjust their bias with no issues at all.
So to me it looks like the fault is restricted to the input selection area only.
Thanks.
Attachments
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Q908 is the reset generator and your measured voltages show a problem there. The collector should be pulled high via R924.
A scope check on Q908 would be useful as these kind of faults can be caused by excess ripple. You are showing 0.6 volts on the base. Where is that coming from ? It can only be via C906 or the transistor itself being faulty. DC voltage on the base has to be zero. As an ex service tech (I've seen it all
) the other real possibility is spillage of some sort.
Pull C906 out. If you still measure 0.6v on the base then either the transistor is leaky or there is something conductive on the PCB.
A scope check on Q908 would be useful as these kind of faults can be caused by excess ripple. You are showing 0.6 volts on the base. Where is that coming from ? It can only be via C906 or the transistor itself being faulty. DC voltage on the base has to be zero. As an ex service tech (I've seen it all
) the other real possibility is spillage of some sort.Pull C906 out. If you still measure 0.6v on the base then either the transistor is leaky or there is something conductive on the PCB.
Mooly.
Thank you.
I shall attack that tomorrow morning.
As its midnight here and way past my bed time
Sadly I no longer have a cro.
And can't afford one at present. Wahhhh, I want a cro again. Wahhh.
Boards appear clean, main amp board had some film (almost soot like. old age film ???) ? over it, but i've cleaned that (metho and toothbrush and cotton buds)
I was suss about leakage in c901, being all original and a 1,000uf 6.3v.
something in my head was steering me to that q908 area.
just didnt understand where the .6v was coming from.
anyway, thank you, i will try your suggestion in the morning and get back to you.
Thank you.
I shall attack that tomorrow morning.
As its midnight here and way past my bed time
Sadly I no longer have a cro.
And can't afford one at present. Wahhhh, I want a cro again. Wahhh.
Boards appear clean, main amp board had some film (almost soot like. old age film ???) ? over it, but i've cleaned that (metho and toothbrush and cotton buds)
I was suss about leakage in c901, being all original and a 1,000uf 6.3v.
something in my head was steering me to that q908 area.
just didnt understand where the .6v was coming from.
anyway, thank you, i will try your suggestion in the morning and get back to you.
Its probably also worth checking (maybe first before delving into small signal stuff) that the 5 volts B+ line that C906 couples to is clean. Any ripple (dried out reservoir cap) would give the problems you have. It would couple through C906 and hold the IC in reset. It would also show as the 0.6 volts B-E voltage you measure.
That might be worth a first look actually.
That might be worth a first look actually.
Weeeeeeeee.
Final update.
Fixed.
Pulled the front apart (again), replaced c909, 912, 900, 908. (prev did 901, 906)
Mainly due to them all being in the same physical location.
Plus I couldn't see any other electros on that board.
Do one, do em all.
Then replaced q908 (2sc2603) with a bc546 (swapping legs as needed)
Hooray, works like a fixed one.........
Thanks Mooly. Your my hero
Final update.
Fixed.
Pulled the front apart (again), replaced c909, 912, 900, 908. (prev did 901, 906)
Mainly due to them all being in the same physical location.
Plus I couldn't see any other electros on that board.
Do one, do em all.
Then replaced q908 (2sc2603) with a bc546 (swapping legs as needed)
Hooray, works like a fixed one.........
Thanks Mooly. Your my hero
Dare I say it helps when you have all the info, history, and some measurements to work from
Absolutely
My approach with anything like that would be to a scope both for DC and AC measurement. It reveals so much. Check the rails first, then go around every pin on the uprocessor looking for anomalies.
Its a classic genuine small signal fault with no contributing factors... the best
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