Broken Amplifier help! Technics SU600 (not a stompbox, sorry!)

[moid]

Hello everybody

Normally I just lurk during term time due to not having any time to actually make any effects pedals, but as of this morning I have a problem that needs quick fixing and I don't know anyone else to ask. My lovely Technics Stereo Amplifier (part of my hi-fi) died this morning. I've had it for 33 years and it's been used almost everyday with no problems until today when it no longer turns on. All the audio in my house plays through this amp, so the house has been horribly quiet today... None of the LEDs light up and no sound comes out of it. I did notice yesterday when I turned it on that it took about 30 seconds to power up (normally it takes about 3 seconds), but didn't think any more of it.

I've checked the fuse in the plug and that has continuity. I tried plugging it into other power sockets (no change). I've opened up the amplifier and found a fuse next to the power input. I checked that (it has a metal filament in it) and that also has continuity. My next plan is to clean the 33 years of dust there is inside it and look for obvious burned resistors or capacitors (there is so much dust that I can't tell if any are dead), but before I do that, I do recall reading somewhere about how amplifiers (even when unplugged and turned off) somehow store lots of electrical charge which makes touching the insides of them rather dangerous? Or is that just guitar amplifiers? Is there anything I should be careful to avoid touching or doing while cleaning the dust out? I've got a can of compressed air to clean it with or should I just wipe everything with a dry or damp cloth? Any tips to avoid me doing something stupid or painful would be gratefully received!

Thanks!

[stallik]

Been a few years and my memory is fading but I had a Technics amp which had failed. Took me ages to realise it had an overheat protection circuit in it and the cooling fan for the power transistors had failed. Caused silence but not sure about the display.
As I recall, the pcb was conveniently silk screened to indicate pre amp, power amp etc sections but nothing about this overheat circuit

[idy]

It's the big caps that are dangerous...although tube amps usually have higher voltages inside than transistors. Still a good idea to discharge them before you go poking around. People suggest 100k 1w resistor, an alligator clip to ground, an insulated probe.
The other rule is "one hand in your pocket."

[PRR]

This amp is +/-43V (or +/-30V) on the big caps. Unless you have a heart defect, brushing this will tingle like hell but will not kill you.

Check the switches you don't know about. This amp has a 4-8-16 ohm switch which gives the different power rail voltages. If it has been bumped into the in-between point, or grossly tarnished inside, no go. Work it back and forth and leave it all the way in the right setting. (If in doubt, lean toward 4 Ohms.)

You may also have a 100V-115V-210V-220V-240V switch. The in-between points are no-go.

It does seem to have a cut-out in the power transformer, also various fuses on the line side of the transformer.

You want a Volt-Meter. The big caps should show 43V (or 30V) each.

[Rob Strand]

None of the LEDs light up and no sound comes out of it. I did notice yesterday when I turned it on that it took about 30 seconds to power up (normally it takes about 3 seconds), but didn't think any more of it.

I've checked the fuse in the plug and that has continuity. I tried plugging it into other power sockets (no change). I've opened up the amplifier and found a fuse next to the power input. I checked that (it has a metal filament in it) and that also has continuity. My next plan is to clean the 33 years of dust there is inside it and look for obvious burned resistors or capacitors (there is so much dust that I can't tell if any are dead), but before I do that, I do recall reading somewhere about how amplifiers (even when unplugged and turned off) somehow store lots of electrical charge which makes touching the insides of them rather dangerous? Or is that just guitar amplifiers? Is there anything I should be careful to avoid touching or doing while cleaning the dust out? I've got a can of compressed air to clean it with or should I just wipe everything with a dry or damp cloth? Any tips to avoid me doing something stupid or painful would be gratefully received!

Check the AC voltages at the output of the transformer.  Then the DC voltages after the rectifier and on the various supply rails.
https://www.audioservicemanuals.com/t/technics/technics-su/150519-technics-su-600-schematics

[moid]

First of all thanks ever so much everyone! I've cleaned the interior with canned air and wiped over it with a cloth so that I can see inside and nothing looks obviously burned or dead alas (at least to me anyway)

Been a few years and my memory is fading but I had a Technics amp which had failed. Took me ages to realise it had an overheat protection circuit in it and the cooling fan for the power transistors had failed. Caused silence but not sure about the display.
As I recall, the pcb was conveniently silk screened to indicate pre amp, power amp etc sections but nothing about this overheat circuit

Sadly this doesn't seem to have a fan anywhere in it... the PCB isn't obviously labelled into sections as you mentioned, but it does have labels on most parts (nothing about power though)

This amp is +/-43V (or +/-30V) on the big caps. Unless you have a heart defect, brushing this will tingle like hell but will not kill you.

Check the switches you don't know about. This amp has a 4-8-16 ohm switch which gives the different power rail voltages. If it has been bumped into the in-between point, or grossly tarnished inside, no go. Work it back and forth and leave it all the way in the right setting. (If in doubt, lean toward 4 Ohms.)

You may also have a 100V-115V-210V-220V-240V switch. The in-between points are no-go.

It does seem to have a cut-out in the power transformer, also various fuses on the line side of the transformer.

You want a Volt-Meter. The big caps should show 43V (or 30V) each.

Thanks, I will start measuring things and see what results I get. And I'll look at the switches you mentioned - there is a switch on the back for the ohms rating for speakers, so I'll look at that. I think to measure the capacitors I will need to take a lot of the amp to bits because the solder points for them are between the PCB and the chassis base.

It's the big caps that are dangerous...although tube amps usually have higher voltages inside than transistors. Still a good idea to discharge them before you go poking around. People suggest 100k 1w resistor, an alligator clip to ground, an insulated probe.
The other rule is "one hand in your pocket."

I can't discharge a capacitor without removing it from the PCB can I? Or can I? All the videos I watched about this seem to involve a capacitor that is already not attached to anything... I do have insulated probes and the DMM says it is rated to 1000V DC and 750V AC.

Check the AC voltages at the output of the transformer.  Then the DC voltages after the rectifier and on the various supply rails.
https://www.audioservicemanuals.com/t/technics/technics-su/150519-technics-su-600-schematics

OK will try that, I think the outputs are the 6 terminals at the top of the transformer (they all connect to ribbon cable that feeds into the PCB). What is the rectifier?

Thanks for the link to the schematic; I've photoshopped the images together in case it's of use to anyone (and increased the contrast to make it easier to read)

[moid]

Hello everyone

Some updates, although I suspect the answer is the transformer is dead



This is the power section, measured with the speaker switch set to 4ohms in case that matters. Everything seems fine until I measure the output of the transformer, which is 0.01V AC on all the terminals... so power is going into the transformer, but nothing is coming out...



This is the input of the transformer, I can't get a probe to the furthest input, but the first two are at 31V AC - does that sound like a normal value?

I've tried looking for any info about the transformer online (SLT5M479-W1), but the only mention of it is in a service manual for a similar technics amplifier to this one, so I assume that means it's not available anywhere

Do I now have to resign the poor amp to the dustbin? I guess I can't complain - 33 years of service is pretty impressive.

[Rob Strand]

This is the input of the transformer, I can't get a probe to the furthest input, but the first two are at 31V AC - does that sound like a normal value?

No.  You should see mains voltage getting to the transformer input terminals.


It might not be be the transformer (yet).  It might be something simple like a solder joint on the mains side or
perhaps even a faulty mains switch, or a bad connection around the power selector.

Plenty to check.

What you need to do is establish that *each* mains input line is connecting all the way through to the transformer.

*** Completely remove the power and disconnect the unit from the mains. ***

Using the ohms tester on 200 ohms trace:

1)  Transformer check
Referring to the schematic, the three wires correspond to the transformer primary winding
and the transformer primary winding through the thermal fuse (inside the transformer).
The direct transformer connection is available on transformer but there is no wire soldered to it.

Measure the resistance between each of the three mains terminals of the transformer.

You expect to see a low DC resistance into the transformer: perhaps roughly 5 to 10 ohm for a 120V winding ; I'm assuming that's what you have.  The resistance across the thermal fuse should be less than 1 ohm.  When you are on the outer terminals the thermal fuse resistance you add to transformer resistance and your resistance measurement should reflect that.

One possible fault is the thermal fuse has opened.   That means you will get an open circuit across it but still see
the transformer DC resistance on the unused terminal.    In this case the transformer itself is OK.

Another fault is the transformer has opened and you will only see a short across the thermal fuse.

[If you have a voltage selector and a stack of transformer taps then identifying what is what won't
be straight forward.  It's important the thermal fuse is checked.  But if you measure across two
mains voltage taps it is normal to measure a near short circuit, this is where the small decimal
points can guide you about what is going on.]

If you see something wrong in this area stop there.

2) Wiring Check
- With the unit disconnected from the mains, enable the power switch.
- Using the ohms tester on 200 ohms trace *each* mains inlet wire through to the transformer input terminals.
- It's best to use the ohms scale and follow the wires by eye.  You want to see 0.5 ohms or less for the wires.
   Be aware if you put the meter on the wrong wire you will get false ohms reading (5 to 10 ohms) through
   the transformer, which is the wrong way through the circuit.  You want to follow the wires with < 0.5ohms
  all the way from the mains inlet to the transformer on *each* lead.

[moid]

Thanks Rob for all your suggestions. I unplugged the power and then unscrewed a lot of the amplifier and was able to get the transformer out, although it is still attached to the main PCB through solder (not the PCB that the power socket is attached to). I can't set my DMM to a 200ohm trace (I don't have a setting for this) but I was able to stick the black probe on one of the three input terminals and the red probe on one of the other input terminals and regardless of which I put where I got the same result: 16.7 ohms (see below). If I put the DMM probes to the six output terminals I get between 1.6 - 2.7 ohms resistance depending on which probe is on which terminal.



Regarding the thermal fuse - I can't figure out how to get inside the transformer - there are no screws and the metal plates it is made of appear to have been squashed / slotted together using some sort of hydraulic press... I have no idea if I can get into the housing - I assume the thermal fuse is inside this? Also the input terminals have thick transparent orange gunk all over them - it is solid and not sticky but I wonder if that should be there? It looks like something melted...

I will try your wiring check tomorrow; it's late here and I have to be up early for work

[mac]

Moid,

I have a Technics SA GX130. I had only one issue in more than 30 years -> the power switch which is non-mechanical. No power at all. I had to take it to Service, few $$$.

The Tape Deck TR515 needs repair too, stuck mechanism, new rubber belts and motors mantainance.
SL PG300 CD player tray motor sometimes gets stuck. A gently touch and problem solved! 

mac

[Rob Strand]

probes to the six output terminals I get between 1.6 - 2.7 ohms resistance depending on which probe is on which terminal.

Not unreasonable.  If you short your meter terminals together what ohm reading do you get 0.8 ohm or 1.0 ohm, 1.4 ohm?
The thing is to get the true ohm reading  you should subtract off the shorted ohm reading.   What that does is give you a more accurate reading and let you judge shorts or low ohms measurements better.

Anyway at this point the secondary looks OK.

Regarding the thermal fuse - I can't figure out how to get inside the transformer - there are no screws and the metal plates it is made of appear to have been squashed / slotted together using some sort of hydraulic press... I have no idea if I can get into the housing - I assume the thermal fuse is inside this? Also the input terminals have thick transparent orange gunk all over them - it is solid and not sticky but I wonder if that should be there? It looks like something melted..

The fact you get 16.7 ohms on two terminal makes me think the transformer and the thermal fuse is OK.   For the three terminals, you got 16.7 ohm between the "common as drawn" and the other two.  Do you get a very low resistance between the other two?   I suspect the thermal fuse is OK and measuring across those two terminals is measuring across the thermal fuse.

That being the case I'd start tracing the continuity of the mains inlet through the fuse and switch and then to the transformer.   I'm really expecting some sort of bad solder joint along the way and that's the problem!

On one of the variants of the schematic the mains goes through two coils (possibly that's a single core).  The soldering could be bad in that area.  You can even get cases where the coil wires break.

[moid]

2) Wiring Check
- With the unit disconnected from the mains, enable the power switch.
- Using the ohms tester on 200 ohms trace *each* mains inlet wire through to the transformer input terminals.
- It's best to use the ohms scale and follow the wires by eye.  You want to see 0.5 ohms or less for the wires.
   Be aware if you put the meter on the wrong wire you will get false ohms reading (5 to 10 ohms) through
   the transformer, which is the wrong way through the circuit.  You want to follow the wires with < 0.5ohms
  all the way from the mains inlet to the transformer on *each* lead.

Hi Rob

First, sorry about the slow reply, my job is horrible at the moment. I finally got a few minutes this evening to look at the amplifier.

Thanks very much for your suggestion - I did that and the resistance stayed at 0.4- 0.6 ohms throughout the circuit with the exception of a few places where it drops to 0 as long as I measure the lines that run from the lower of the two metal prongs in the power socket input. This was notably the two outer prongs of the power connection from the power board PCB to the input of the transformer. The inner prong was 0.5 ohms, the other two were 0 ohms. I tested this by holding one of my DMM probes to the metal input tip of the power socket and the other to the part of the circuit I was testing. I tried both metal tips in the power socket because they both have their own path across the power PCB, and was careful to make sure I didn't accidentally hit the wrong part of the circuit! If I measure from the top metal prong in the power jack I get 0.3 ohms everywhere on that line.

So I guess that part of the circuit is working? I removed the power PCB and transformer from the chassis of the amp so I could get better access to them and discoverd the power PCB has a capacitor hidden underneath it that I couldn't see before. It's a 103 (10nF) and I measured it without any power in the circuit and got a reading of 11.5nF - so close enough to be OK I think? The schematic shows this capacitor as being 10nF.

You asked about shorting my DMM probes together? I get 0.6ohms when I do that.

The fact you get 16.7 ohms on two terminal makes me think the transformer and the thermal fuse is OK.   For the three terminals, you got 16.7 ohm between the "common as drawn" and the other two.  Do you get a very low resistance between the other two?   I suspect the thermal fuse is OK and measuring across those two terminals is measuring across the thermal fuse.

Sorry I'm not sure what you mean by 'common as drawn'? Do you mean put one probe on the centre prong and another probe on one of the two outer prongs? Presumably while it's plugged into the mains? Or something else?

That being the case I'd start tracing the continuity of the mains inlet through the fuse and switch and then to the transformer.   I'm really expecting some sort of bad solder joint along the way and that's the problem!

On one of the variants of the schematic the mains goes through two coils (possibly that's a single core).  The soldering could be bad in that area.  You can even get cases where the coil wires break.

There are two copper coils on the main PCB, but nowhere near the power PCB... I will try to take some photos in daylight.

Moid,

I have a Technics SA GX130. I had only one issue in more than 30 years -> the power switch which is non-mechanical. No power at all. I had to take it to Service, few $$$.

The Tape Deck TR515 needs repair too, stuck mechanism, new rubber belts and motors mantainance.
SL PG300 CD player tray motor sometimes gets stuck. A gently touch and problem solved! 

mac

Hi Mac, yes if I can't fix this myself I will have to look for a repair shop... I made the mistake of looking at what a new Technics amp costs these days and almost had a heart attack! £2000! I had no idea they were so boutique! I guess 15 year old me had good taste in hifi - that amp cost me £100 in the late 80s!

[mac]

I made the mistake of looking at what a new Technics amp costs these days and almost had a heart attack! £2000! I

For only £1,999.99 I sell you mine, free shipping! 

mac

[Rob Strand]

Thanks very much for your suggestion - I did that and the resistance stayed at 0.4- 0.6 ohms throughout the circuit with the exception of a few places where it drops to 0 as long as I measure the lines that run from the lower of the two metal prongs in the power socket input. This was notably the two outer prongs of the power connection from the power board PCB to the input of the transformer. The inner prong was 0.5 ohms, the other two were 0 ohms. I tested this by holding one of my DMM probes to the metal input tip of the power socket and the other to the part of the circuit I was testing. I tried both metal tips in the power socket because they both have their own path across the power PCB, and was careful to make sure I didn't accidentally hit the wrong part of the circuit! If I measure from the top metal prong in the power jack I get 0.3 ohms everywhere on that line.

So I guess that part of the circuit is working?

Well, from that test you would think so.   However, if the both mains wires connect all the way through from the mains plug through to the transformer then you should be seeing mains voltage at the transformer in normal operation.    Your mains voltage measurements in Reply #6 don't show this.   So I'm not sure what's not matching up here.

I'm also not exactly sure which version of the wiring you have.  Three mains terminals on the transformer implies the lower version on the schematic.   (That's assuming three transformer wires in Reply #6 are in fact the main input.  Your 16.7 ohm measurements in Reply #6 look in the right ball-park.)

Your mains voltage is 110V but the simpler mains wiring in the lower part of the schematic shows only 220V and 240V.  For this version there is no mains *outlet* on the back of the unit and no mains voltage selector switch (see upper mains wiring).

It would help if you can confirm exactly what version you have.

I removed the power PCB and transformer from the chassis of the amp so I could get better access to them and discoverd the power PCB has a capacitor hidden underneath it that I couldn't see before. It's a 103 (10nF) and I measured it without any power in the circuit and got a reading of 11.5nF - so close enough to be OK I think? The schematic shows this capacitor as being 10nF.

You asked about shorting my DMM probes together? I get 0.6ohms when I do that.

Yes.  What that means is the 1.6 ohm and 2.7 ohm secondary measurements in Reply #6 are actually 1.0 ohm and 2.1 ohm.   As a rough judgment that looks OK for secondaries.

The fact you get 16.7 ohms on two terminal makes me think the transformer and the thermal fuse is OK.   For the three terminals, you got 16.7 ohm between the "common as drawn" and the other two.  Do you get a very low resistance between the other two?   I suspect the thermal fuse is OK and measuring across those two terminals is measuring across the thermal fuse.

Sorry I'm not sure what you mean by 'common as drawn'? Do you mean put one probe on the centre prong and another probe on one of the two outer prongs? Presumably while it's plugged into the mains? Or something else?

So in reply #8 (upper/lower as per your pic) you have two resistance measurements on the transformer:
- Upper to center terminal 16.7 ohm
- Lower to center terminal 16.7 ohm.

If your mains wiring follows the lower version of the wiring in the schematic then the transformer has three terminals.  two are across the windings and two are across the thermal fuse.  Based on that I would guess measuring from the Upper to the Lower you should see close to 0 ohm.   As a sanity check you should confirm this is true.   If you follow the mains wiring you should see PCB tracks going to only two of the three transformer terminals.   The unconnected terminal should be where the thermal fuse connects to the winding - as per lower version of main wiring in the schematic.

There are two copper coils on the main PCB, but nowhere near the power PCB... I will try to take some photos in daylight.

It's possible your board doesn't match the mains wiring on the schematic.  If you see two coils far from the mains wiring then I suspect those are around the power amplifier IC (L401 and L402).   Those aren't the coils shown on the lower version of the mains wiring.

[Slowpoke101]

This is an interesting fault. I do have a few observations to note.

1. Moid is in the UK which generally uses 220 - 240VAC so the amp would most likely use that voltage.
2. The 220V transformer's primary only uses 3 connections whereas the multiple input voltage transformer uses 7. The pictures that moid has uploaded shows only 3 pins.
3. The measured voltages are odd. Where were the test probes placed?

I suggest that a continuity / resistance test be done on the power lead itself. I've had those fail in some very odd ways before. Also, resolder the power switch as there appears to be a fractured solder joint there.

Edit: Also test the resistance of the power switch when it is on. They do fail and not always in an obvious manner. Don't forget to resolder the switch.

Both Rob and Jim have mentioned in their comments below some interesting things about mains appliance plugs used in the UK that I had forgotten about. But it did remind me of another little fun issue with older equipment with cylindrical glass fuses. Replace the fuses even if they test OK. That includes any in the amplifier itself. After many years of use the fuse element develops minor fractures which don't cause any problems until you pass current through it. A multimeter doesn't pass enough current so the fuse seems fine but the fuse won't work when you try to power up something like a transformer. The fuse goes open circuit but when you take the fuse out and test it with a multimeter it now tests fine. This is a rare fault but it does happen. Now some people will say BULL#@IT!!. Feel free to do so, but it happened to me twice last year. The fuses were between 12 to 17 years old and not in a heavy current situation.

[Rob Strand]

1. Moid is in the UK which generally uses 220 - 240VAC so the amp would most likely use that voltage.

So he is!  That puts a spanner in the works for the 97V measurement he made earlier on!
That reading could be entirely spurious.

Maybe it's the mains plug?  Don't UK mains plugs have fuses?  Worth checking.

[anotherjim]

If it dates from the times when UK electricals were supplied without any mains plug and before they started dressing the wire ends with bootlace crimp ferrules - you had pre-stripped solder tinned wires to screw into your plugs terminals. This was always a reliability problem since the solder was lead-based and lead flows under pressure so contact gets looser and dirtier over time.

[moid]

I made the mistake of looking at what a new Technics amp costs these days and almost had a heart attack! £2000! I

For only £1,999.99 I sell you mine, free shipping! 

mac

I'm glad to see that mate's rates are still observed on DIYStompboxes I'll give your kind and generous offer some thought! Also sorry to everyone for disappearining, work has been awful. I now have a whole week of holiday (yay!) so will start replying to everyone's helpful suggestions.

[moid]

I'm reading through all the suggestions (thanks very much everyone!). I apologise for not explaining that I live in the UK so my local electricity runs at 220-240V. I'm so used to thinking in 9VDC for stomp boxes that I completely forgot that everyone else in the world runs their electricity at very different amounts!

I will try changing the fuse in the plug itself as Slowpoke suggested, and I did find a fuse near the power input to the amplifier which is a glass one with a wire inside - my multimeter says it is fine, but I'm heeding Slowpoke's advice and wil try to get a replacement... which might be rather hard. The use has 7800mA 250V written on it, and I can't find those for sale anymore. Is it OK to use 8 Amp fuses instead? I can get some on Amazon quickly which are the fast blow glass fuse type:

https://www.amazon.co.uk/Pack-F8AL-Fast-Blow-Glass-Fuses/dp/B01M5CHTUE/ref=sr_1_7?crid=17486DM7CVV1X&keywords=8%2Bamp%2Bfuse&qid=1649685138&sprefix=8%2Bamp%2Bfuse%2Caps%2C99&sr=8-7&th=1

I was unsure about swapping for a higher amp fuse because I'm sure someone once told me if I did that and the components in the circuit were rated for less than the fuse, those components could fry before the fuse would trip? I might not have remembered that correctly, but it sounds logical.

[anotherjim]

It's T800mA. T for time delay or slow blow.