In my current project (sorry, it is a solid state Sansui Amp AU-6600), there is a problem with a secondary power supply. After 1 cap exploded last night, I intend to recap this Supply.

This power supply has fuses on both the positive input side and on the negative input side (between transformer and diodes) of about 36volts ac. Unfortunately, the fuses are different ..... I wish to make them the same as whichever is the original.

Fuses are 5mm x 20mm .... glass .... 250 volt, 1 amp

Positive supply input Capacitance is 220UF at 50v
Negative supply input capacitance is 100μF at 50v

Smoothing/output caps(10 and 4.7uF) are rated at 25v for the +25v and - 25v outputs.

1. One fuse has straight wire, whereas the other appears 'wound' or have a 'spiral' shape. I think the latter is a Slo-Blo fuse, but not sure. Whilst I have seen Slo-Blo fuses on power supplies previously, my guess is that the straight wire fuse is the original as all other fuses, whilst being different spec, are all straight wire ..... advice??

2. The input capacitors, that is, immediately after the diodes, are rated at 50volt. Shouldn't these caps have a higher rating??? Say 100v??

3. Shouldn't the smoothing caps have a higher rating also??

Cheers,
Ian

Spiral/spring fuses are slow blow.

Slow blow is used when the initial inrush current is high. I have to use slow blow on a valve based Hammond organ I own because the inrush current is about 4 times the operating current.

With power supply filter caps, I use rule of thumb of doubling the ac voltage, to accommodate peak voltage and some headroom margin for safety.

The rule of thumb I was always told was twice EMF. As I have said in another post, I will not use less than 500V caps on a 250VDC rail with a Silicon Diode, or Filament rectifier supplying heater tubes.

The same will apply if the Solid State rectifier is left with the rectifier & caps in circuit & the load is removed by a fuse. The caps then have to then be able to handle the surge voltage.

What is not clear is which set went bang. 25V is sailing too close to the wind on a 25V supply. Not less than double. If the supply is regulated to these caps check it.

The slow blow fuse has a time factor and I had these in a computer monitor. The fuse was 3A & the inrush 3A. These fuses can fatigue fail. Normally, if the current does not fall within 10secs they blow.
The spiral of wire provides the thermal inertia, to stop it blowing.

That was one of the traps with doubling fuse wire. Two bits of 5A do not create a 10A fuse:It's more.

Marc

Two bits of 5A do not create a 10A fuse:It's more.

... and a paper clip does not a fuse make.

Found one in a porcelain fuse carrier in a Federation house I bought in the 1980s. The carrier had been literally copper plated from previous fuse explosions, so the elderly occupant apparently decided that a paper clip will stop the problem for sure. That house got a rewire.

Probably "cloth rubber" wiring ? That stuff is great for starting house fires & I have seen a few houses with it, and know of one 130+ year old one, weatherboard, with it. I have told him to rewire starting at the power pole. I.e. rip it out & start again, new wire sockets, switches, fuse box etc....

Yes, 1920s vintage wiring in metal conduit. One power single point in each room, a number of which showing heat stress. I got the feeling that the elderly prior occupant had a radiator burning in every room.

Active and neutral reversed in some, too.

It's difficult to convince people that a rewire is required on safety grounds. It should be a requirement of insurance companies and electricity regulators that wiring in buildings over a certain age be inspected and certified. More cost I know, but compare that to the cost of a destroyed building and contents, not to mention human lives.

As a trade-qualified electrician I have seen a lot of things used as rewirable fuses and I have to say that I am glad they are illegal in new installations these days. Pieces of 2.5mm² wire, including the older, and also now illegal solid core type, wire coat hangers, fencing wire and multiple strands of 32amp fuse wire and the like do not make for good circuit protection regardless of when the house was built.

People also don't realise something about fuses - they are actually designed to run hot, which is why they are always installed in porcelain holders. When a 15 amp general power fuse is running a telly, electric blanket and a heater in the middle of winter, the fuse should be glowing a dull amber colour. If it isn't then it wouldn't be doing its job.

As for the ancient VIR cable in older homes. I'd agree that it should be replaced when sighted. Its gauge is too small for today's power requirements. VIR cable is actually quite stable inside their split-seam conduits. The problems are always at the points where mice can attack the insulation or the heat from light fittings can dry out the rubber and cause it to flake off.

The danger these days is purchasing TPS cable that is not made in Australia. We still have a few wire mills here so it has me beat as to why we need to import garbage from China. One brand, sold through Bunnings and a few other outlets was recalled about a year ago because the insulation broke down after the cable was installed. The problem is, no-one keeps track of the cable they install and where it was installed - and why would they? It's all been top quality cable until now.

One brand, sold through Bunnings

Infinity and Olsent brands.

Actually Bunnings was about the only place that didn't sell it. You're probably thinking of Masters.

The number of sellers involved is a big worry:

https://www.recalls.gov.au/content/index.phtml/itemId/1061753

Getting back to the amp, has it been modified at some stage away from factory spec? If not I would trust the Sansui engineers to have designed the power supply properly as they are a well regarded company and are not known for taking shortcuts.

Do you have the circuit for the amp? It sounds like you are having problems with an aux supply for the low signal level sections. The main amp power rails will be fairly hefty with much bigger cap values.

I had a look at the circuit for it's big brother the AU-7900 and the power supply is fairly complex for a power supply in an amp. Most of the electrolytic caps are specified with a 50v rating with a couple at 63v so what you see in your amp sounds right. Given that you've had a cap explode I'd approach the power supply with caution to make sure the voltages are within the specs it was designed for.
Something silly like the mains voltage selector being set to the wrong voltage could cause caps to go bang!

Hi TV Collector,

Yes, that is exactly why I asked the question .... I would expect the Sansui people to have got it right. Yet, 36 volts ac into diodes into 50 volt working voltage electro caps.

I do have the circuit diagram. Caps are as per spec, however, when I eventually found the cap casing, it was different to all other caps in this part of the circuit .... originals were Elna ..... replacement cap was Nichion (?). Therefore, some-one else has tried to fix this amp before me ...... without getting to the actual problem.

The cap that exploded was one of the Input or Reservoir caps ..... C27
Aux supply ... yes.
Circuit and Service Manual can be found on www.hifiengine.com
Selector at 240volts.

I have replaced the cap today ..... but it is getting hot quickly ..... output voltages way too low. I have ALL circuits isolated ..... no load ..... pass transistor regulator, not SMPS .......

Started to work my way through the Aux power supply this evening ..... taking it slowly and carefully ..... maybe shorted diode putting ac on electro cap => bang????

Cheers,
Ian

originals were Elna

Not surprised that those didn't go the distance. Replacements would be Nichicon, whose caps had a bad reputation in the early 2000s.

50v caps on 36v in is right on the peak voltage. Not enough headroom for my liking. If the mains voltage rises (as it does), the cap is already in the danger zone.

For those interested in my friends Sansui amp ....

2 diodes in Aux power supply were shorted, therefore ac across electro cap.
Replaced diodes and gradually powered up each circuit ..... all good.
Amp is sitting on my bench with full mains power applied ..... no heat in components or magic smoke .... yet .... will leave for a few hours and run some safety tests on it before I go further.

These amps have a Relay across the audio output. ....possibly protection against dc on output or ensuring both channels are functioning or ...???

This Relay hasn't activated as yet ..... which suggests another problem. If anyone knows about this type of Relay ie what activates it.... I am open to advice.

Cheers,
Ian

These amps have a Relay across the audio output.

The relay (for each channel) is in a delay circuit to provide protection for the speakers against the 'thump' that occurs at power up. That is, the speakers aren't connected until a few seconds after power is applied.

Delay period is usually provided by a simple R-C circuit with a transistor driving the relay. Relay would have a reverse connected diode across its coil for back-EMF suppression.

The other thing to watch out for in some of the later 60's & 70's sets are oil filled caps. I have seen some of them exploded. These are often paper types in a PCB oil and should be treated with the same contempt as old electrolytics, waxed paper caps & anything with "Micamold" on it.

If you look on the bans & recalls site, or Energy Safe Vic. you should be able to get info on the cable sold by Masters & others. I would like to be a fly on the wall to see the liability bill?

There have been deaths in the last few years from VIR cable in metal conduit, due to corrosion & leakage to the metal.

Marc

If you look on the bans & recalls site

See post #8 above.