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

[moid]

Thanks anotherjim! you're totally right, I looked at the fuse in direct sunlight and what I thought was a 7 is actually a T! Just ordered some from ebay, hopefully they'll get here before the weekend. I'll be so happy if it's just a busted fuse

[moid]

There's some good news and some bad news! First - the amplifier is fixed Yay! Rejoice! Dance around madly because I can play records and listen to the audio on my computer correctly!  The bad news - I have totally wasted everyone's time   Sorry everyone, especially Rob. I took Slowpoke's advice and changed the fuse in the mains plug (I did previously test this and my DMM said it was fine), but I swapped in a new 13amp fuse and lo and behold the amplifier turned on! Which would've been totally awesome if I hadn't had been resting my thumb on the power section PCB at the time, with it right on the two prongs of the mains input... jump? yes, almost hit the ceiling! I tell you what, 240volts is quite a wake up; works faster than caffeine that's for sure. Also the noise I made scared the crap out of my wife... which is not the best thing to do when you're playing with electricity.

I guess with the duff fuse in the plug I was getting 97V out, but not the 240 the transformer needed for the amplifier to work? The mains plug shows 245 Volts now! And Anotherjim was right - this amplifier was supplied without a plug when I bought it, I remember my dad showing me how to wire the plug - the plug itself is so old that it says made in Britain on it!

So... all's well that end's well?

Thanks so much everyone, I really appreciate your input and help as usual. I'll get back to breaking stompboxes soon!

[Rob Strand]

There's some good news and some bad news! First - the amplifier is fixed Yay! Rejoice! Dance around madly because I can play records and listen to the audio on my computer correctly!  The bad news - I have totally wasted everyone's time

So... all's well that end's well?

Thanks so much everyone, I really appreciate your input and help as usual. I'll get back to breaking stompboxes soon!

Great news indeed.  No waste of time at all, that's life when you are debugging.   The good thing is you didn't need to find any hard to get/expensive replacement parts.

Which would've been totally awesome if I hadn't had been resting my thumb on the power section PCB at the time, with it right on the two prongs of the mains input... jump? yes, almost hit the ceiling! I tell you what, 240volts is quite a wake up;

Not good.  It's a risk when you work with this stuff.   I've developed a healthy sense paranoia over the years.

[anotherjim]

On safety, it amazes me that so many power transformers put the primary winding tags on the top side where it will be so easy to accidentally touch (our 240v) while the (usually) safer, low-voltage secondary tags hide out of the way down below. It wouldn't be so bad if they at least supply a safety cover (some do, but it's an optional extra that nobody stocks).
So, is there an electrical reason to place the primary of an EI transformer above the secondary?

[Rob Strand]

On safety, it amazes me that so many power transformers put the primary winding tags on the top side where it will be so easy to accidentally touch (our 240v) while the (usually) safer, low-voltage secondary tags hide out of the way down below. It wouldn't be so bad if they at least supply a safety cover (some do, but it's an optional extra that nobody stocks).
So, is there an electrical reason to place the primary of an EI transformer above the secondary?

I've seen the primary both on the top and on the bottom of transformers with either terminals or with wires.   As far as transformers inside of equipment go, the majority have the primary on the bottom  - the primary connections usually have wires.   There's also over wound types, and side connections (rotated transformers).

I've used transformers from this series in the past (originally from the UK).   I was never fond of those exposed terminals.   I used to cover the "spokes" out the top and the wire terminal with heatshrink.  Recently they have started to offer plastic shrouds for the connections which you buy separately.


Here's soldered primary at the bottom (very common in AU for 15VA's and 30VA's from 70's to 90's),
[Click to embiggen]

[duck_arse]

doesn't the australian standard say this needs a split bobbin? does it have a split bobbin obscured?


I have a question.

I took Slowpoke's advice and changed the fuse in the mains plug (I did previously test this and my DMM said it was fine), but I swapped in a new 13amp fuse and lo and behold the amplifier turned on!

13 Amp fuse? is that the standard plugtop-thing fuse the english comedians are always cracking funny about? and it was dead, you say? so what/where was the 800mA slo-blo one located? how does the 13A go before the 800mA one? or is something I miss?

[moid]

On safety, it amazes me that so many power transformers put the primary winding tags on the top side where it will be so easy to accidentally touch (our 240v) while the (usually) safer, low-voltage secondary tags hide out of the way down below. It wouldn't be so bad if they at least supply a safety cover (some do, but it's an optional extra that nobody stocks).

Kind of related to what you said - there was a plastic cover (loose) over the power PCB area that would prevent anyone who opened the amplifier up from geting shock if they had been stupid enough to touch that part while it was on... but I of course took the cover off so I could get to the power PCB to test it

[moid]

I have a question.

I took Slowpoke's advice and changed the fuse in the mains plug (I did previously test this and my DMM said it was fine), but I swapped in a new 13amp fuse and lo and behold the amplifier turned on!

13 Amp fuse? is that the standard plugtop-thing fuse the english comedians are always cracking funny about? and it was dead, you say? so what/where was the 800mA slo-blo one located? how does the 13A go before the 800mA one? or is something I miss?

The very same fuse mr duck! The fun thing was that with the DMM on that fuse I got continuity and it seemed fine - but switch it for a new fuse and the plug (and amplifier) suddenly worked! The 13 amp fuse was in the plug body itself - look below



So that plug was in the wall socket. The cable coming out of the plug went into the power input and power PCB of the amplifier. The 800mA fuse was part of the power PCB inside the amplifier.

[Rob Strand]

doesn't the australian standard say this needs a split bobbin? does it have a split bobbin obscured?

On the actual units you can see the design uses a split bobbin.  You can see the different wire thicknesses through the vent slots in the bobbin.

I don't have a copy of the latest Australian standard but I suspect it doesn't dictate a split bobbin; for example Toroid power transformers are always over-wound.  Typically the standards dictate creepage and clearance distance together with breakdown specs.

RS components have been selling this style of transformer since before 1990 (IIRC back then people referred to them as Radio Spares).   Over time the designs get revised to comply with changes to standards.   The common transformers in au (Ferguson/A&R Transformer/DickSmith/Jaycar/Altronics/Arista) have been revised over time for the same reason.   The incorporation of thermal fuses and recommended fuses in the transformer data is a result of changes to standards.  I rememeber back in the 70's when Ferguson were one of the few manufacturers to claim they conformed to standards!

[Rob Strand]

13 Amp fuse? is that the standard plugtop-thing fuse the english comedians are always cracking funny about? and it was dead, you say? so what/where was the 800mA slo-blo one located? how does the 13A go before the 800mA one? or is something I miss?

I was going to check out the fuse characteristic but I don't think it's that.
[For completeness:  I^2t rating of 13A BS1362 fuse is 870 A^2 sec whereas 800mA slow blow is 3.37 A^2 sec.
That indicates under pulsed loads the 800mA slow blow would be expected to fatigue first.]

Fuse are complicated because they can blow at currents less than the rating.   They fatigue over time when exposed to spikes etc.   Slow fuses are affected less than fast fuses but with the big rating difference of 13A vs 800mA even that might not account for the failure.

I think the simple answer is the 13A fuse probably suffered minor damage at some point which made it fatigue earlier.   Fuses in the mains plug can suffer mechanical shocks if the plug is dropped.   Fuses in the amp are unlikely to suffer mechanical shocks as often and the fact there are many springy mechanical mountings between the enclosure and the internal fuse holder will soften any blows.    Over 20+years the minor weakness shows its head.

It's all speculation based on a sample of one!

[bluebunny]

Eek! 

For my peace of mind (and yours, I hope!), please rewire that plug so that the cable grip holds onto the black outer insulation, and the live and neutral are trimmed to an appropriate length.

[anotherjim]

UK plug fuses don't have a good mechanical seal of the end caps for the fuse wire so fatigue/corrosion happens over time.
Anything made now will have a plug like this ready fitted & fused...


The fuse wire is simply soldered or welded at the centre of the end caps. A partial/intermittent fault is possible that may pass a DMM continuity test, but any serious current flow causes a voltage drop.

The first thing I do with dead equipment is measure resistance between the AC plug live & neutral pins (cable plugged into the equipment but not AC supply of course) and switch it on. The load of whatever is inside (transformer primary usually) should show a relatively low resistance on the DMM.

[Rob Strand]

K plug fuses don't have a good mechanical seal of the end caps for the fuse wire so fatigue/corrosion happens over time.
Anything made now will have a plug like this ready fitted & fused...

After going over the thread, that has to be it.

I only remembered Moid checking the fuse in the amp back in reply #18,

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 he had actually tested the mains fuse way back in his opening post,

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.

[amptramp]

The worst fuses for corrosion are the bullet-nose fuses used in cars made in continental Europe.  Back when I had a Volkswagen (never again!), functions would cease to work then magically reappear.  I finally realized that you had to spin the fuses one turn while still in their holders about every six months or there would be more trouble.  Maybe if you put the original fuse back in, it would work properly since the act of removing or spinning a fuse is to grind off the corrosion.

Volkswagens need regular exorcisms to remain driveable.  Even Lucas electrics are better.

[moid]

Eek! 

For my peace of mind (and yours, I hope!), please rewire that plug so that the cable grip holds onto the black outer insulation, and the live and neutral are trimmed to an appropriate length.

Guess what everybody, I'm back! Like a bad penny, I always turn up. I have to resurrect this thread because the damn amplifier has died again

First of all I would like to reassure bluebunny (and any other anxious observers who were kindly concerned that I was about to electrocute myself again) that the power plug is now repaired to a hopefully higher standard (see below)



Onto the actual problem. Typically this has happened because I've finally got August off as annual leave so was looking through some layouts wondering what to build this Summer and went to turn the amp on to listen to some music at the same time and the amplifier no longer turns on (has worked perfectly since I changed the fuse in the plug). Yes, it's becoming obvious that the universe hates me, or at least there is some random spiritual entity that I have offended that is scared of me having the time to assemble a fuzz pedal and so throws these distractions at me to make sure I can't enjoy life! I assumed the fuse was knackered again, so put another one in the plug... no joy. I tried two other 13 amp fuses just being paranoid but the amp still doesn't turn on. I stuck my DMM probes into the two inputs of the plug (the end that connects to the amplifier - not actually connected to the amplifier at that moment) and got a reading of about 242V AC, which seems right for the UK, so I think I can assume the fuse is OK and power is fine.

Next I plugged the power cable into the amplifier and started taking readings of the power section. Here's where I'd like to check my method of doing this with you, because I got different results depending on where I put the black probe of the DMM, and I am worried I'm measuring things wrong. If I place black and the red probes on the two soldered prongs (one probe to each prong) of the PCB where the power comes into the amp I will get around 242V AC. However to measure the current elsewhere in the circuit should I be connecting the black probe to the ground point on the back of the amplifier (see below)



and then using the red probe to touch parts of the circuit?

Or should the black probe remain on the top prong of the power input?



And use the red probe to touch the rest of the circuit?
I suspect this is correct (black probe used as above) but before I start measuring the rest of the circuit and doing it wrong, I'd love someone else's opinion please

I'm being very careful not to touch any PCB areas with my fingers this time! I've removed the transformer so that I could take readings from where the power circuit connects with the transformer - if my power readings are correct and the way I'm measuring are fine I will progress onto measuring the transformer. At the moment the socket that connects to the transformer shows the following readings (first number refers to the socket closest to the camera / me) when the power button is on on the amp

01: 1.8V AC
02: 242V AC
03: 0.471V AC

Any help / opinions / commiserations would be gratefully received! I hope all your Summer's are going better than mine!

[idy]

Black probe close to power in, not that little ground post on the panel next to phono. Should not be different, but the post is for grounding a turntable. Maybe it is chassis ground.

Careful about words; "current" you measure (in amperes) by putting the meter in series and allowing power to flow through it, dangerous at high voltage. Voltages are what you are measuring, and what you should be measuring.

[anotherjim]

Because you don't have a grounding wire on the mains plug, the AC voltage ought to be measured between L & N at the transformer's primary input. However, Neutral is grounded somewhere back in the supply feed but it doesn't prove the plug neutral is connecting if you measure the live from the chassis ground.
*Brits should probably say "earth" for the plug/socket safety pin and not "ground" but the rest of the world seems to understand ground better.

[PRR]

...Brits should probably say "earth" for the plug/socket safety pin and not "ground" ....

"PE" will fool all the Americans.

If you have some sort of AC voltage on the PT secondary, stop fooling with the 240VAC side! It works, obviously.

On the secondary I normally stick Black in any handy screw-hole, call it common. In any domestic Hi-Fi, chassis IS common. The phono screw WILL also work, but don't grip a probe too good, unless you smoosh it out of shape.

Take the Red probe first to the BIG power caps. Expect zero on a common wire, plus 43V one side, negative 43V the other side.

And beFORE you connect any speakers, be sure you see near-ZERO on the Speaker terminals!

[moid]

Thanks chaps for the fast replies! I can't do anything to test things until Wednesday (off to see my mother who lives a very long way from us). I was hoping to do some testing today, but one of our kitchen drawers decided it would jam instead (it was the one with all the knives in it of course) so lost most of the day to dismantling / drilling holes until I could free the stuck object - a knife had got stuck inside a drawer compartment and then also jammed behind part of the carcass of the cupboard, what a day!

Black probe close to power in, not that little ground post on the panel next to phono. Should not be different, but the post is for grounding a turntable. Maybe it is chassis ground.

Careful about words; "current" you measure (in amperes) by putting the meter in series and allowing power to flow through it, dangerous at high voltage. Voltages are what you are measuring, and what you should be measuring.

Brilliant, thanks very much Idy, I'm glad I was right (I was assuming it would be the same as an effects pedal, but when I noticed that post it did make me question myself). Sorry about the language confusion, my electrical knowledge is not very good.

Because you don't have a grounding wire on the mains plug, the AC voltage ought to be measured between L & N at the transformer's primary input. However, Neutral is grounded somewhere back in the supply feed but it doesn't prove the plug neutral is connecting if you measure the live from the chassis ground.
*Brits should probably say "earth" for the plug/socket safety pin and not "ground" but the rest of the world seems to understand ground better.

Aaahhhh so 'earth' is a British thing? I have always wondered if the two terms meant the same or were different. I'm not sure which of (two of) the primary inputs are the Neutral one (I've worked out the central one is live!). So shall I try a reading with red probe on the centre input and then hold black to each of the other inputs to see what AC rating they show? Thanks anotherjim

If you have some sort of AC voltage on the PT secondary, stop fooling with the 240VAC side! It works, obviously.

On the secondary I normally stick Black in any handy screw-hole, call it common. In any domestic Hi-Fi, chassis IS common. The phono screw WILL also work, but don't grip a probe too good, unless you smoosh it out of shape.

Thanks PRR - when you say PT secondary you do mean the seven prongs that voltage comes out of the transformer? I assume I should see a lower voltage reading on those prongs? I will test them all and get back to you on Wednesday.

The other reason I queried where to place the black probe was because I assumed that if it touched any bare metal on the chassis, that would be fine to establish ground; but when I tried this I got really low voltage readings on the power input prong (it went from 25 - 32 V AC!) which seemed really odd to me when I was sure it would be about 240V AC at that point. Maybe the metal has some sort of coating on it that would give a strange reading?

[PRR]

> I queried where to place the black probe was because I assumed that if it touched any bare metal on the chassis, that would be fine to establish ground; but when I tried this I got really low voltage readings on the power input prong (it went from 25 - 32 V AC!) which seemed really odd to me

You do not have the Earth/Ground pin connected. There is no connection from chassis back to 240V power source. (That's fairly common in small hi-fi.)