Fostex E 16 blowing Mains Fuse...

[falderguitars]

Hi everyone,

I'm looking for some help fixing my fostex e 16 reel to reel. My band and i are all set to record our next EP - i persuaded them to do it 'properly' this time, and bought an e 16 on ebay.
Aside from a few issues with the tape counter running out, it worked fine - until yesterday, when it started blowing fuses.
I'd switched it off to swap some inputs/outputs, when i turned it back on, it died.
Checked the mains fuse (3amp) and it'd blown. Replaced the fuse, it blew again.

I've some experience with electrics - although mainly guitar/effects circuits, so working with mains voltage is a little outside my usual field of experience.

I'm fairly certain there must be a short circuit as when you switch it on (without power), there is connectivity between the live and neutral wires - this doesn't seem right to me.

There are 5 240V fuses inside the recorder (on the power supply board), and none of these have blown, so it seems to me the short must be before these, or surely at least one would have blown?

I suspected a broken mains switch, but i've isolated this, and it's fine. The next port of call is the voltage selector. The live and neutral wires go in, and 6 or 7 wires come out. Again, there is connectivity between ALL the wires going in and out, which seems unusual.
Incidentally, although it says voltage selector on it, i don't think it's actually doing anything more than feeding power to different parts of the recorder - it's not adjustable in any way.

Is there anyone out there who has experience with the e 16?

Does anyone have the service manual - particularly the schematics?

Any advice would be greatly appreciated.

Thanks,

Matt.

[falderguitars]

Forgot to mention, i've done a little research, and it seems the Fostex B16 shares some DNA with the E 16.
Is this correct?

If so, perhaps if anyone has service manual/schematics for this i might be able to glean some useful info from them - particularly regarding voltages.

Thanks again,

Matt

[The Scum]

Your measure of short on the power input is actually probably correct. The next component in that chain is probably the power transformer. Within the transformer, there are a couple hundred feet of copper wire between hot and neutral. Measured with a DC ohmmeter, it might be a few ohms...a continuity tester probably measures short.

The voltage selector is probably there to allow it to be configured for different line voltages - 110, 220, etc. Again, all in the vicinity of the transformer primary.

If the mains fuse blows, but the internal ones don't, I'd suspect something in the power supply itself. It's likely that the big caps in that part of the machine are failing, such that they're exceeding the inrush ability of the fuse.

For a manual, have you tried Fostex? Last time I contacted them, they were happy to sell me service manuals for some old synchronizers.

[falderguitars]

Thanks for the response.

I've emailed Fostex this afternoon, so hopefully i'll hear back from them soon.

I think you're probably correct on all counts. I built a guitar amp last year - a kit from ampmaker - that had similar issues with connections across the transformers reading as shorts, which the build instructions assured me were all perfectly normal. I'll measure the resistance tomorrow.

On closer inspection this afternoon, the volage selector IS adjustable. If you unscrew the fuse holder it comes apart and has the usual range of voltage options.

There's a transformer hidden in there somewhere to be sure - could be one of those toroidal thingys, as the wires disappear into a donut shaped enclosure after the voltage selector. Typically, this is buried behind a mass of wires and circuit boards, and i haven't been brave enough investigate yet

Wouldn't the fuses blow before power reached the caps though? There are 4 or 5 big old caps on the power supply board with the fuses, but the caps are rated either 63 of 35 volts, and the fuses are rated 250V - which would come first in the circuit?

That said, the area around the caps is discoloured. I've heard of 'leaky' caps - i always thought it was a figure of speech to describe a cap working at less than full power - is it a literal description? Do they physically leak? If so, you might've hit the nail on the head...

A schematic would be a godsend here, hopefully either Fostex or a kindhearted citizen will come up trumps with the service manual.

In the meantime, some new caps may be in order...

Thanks again,

Matt.

[Drone]

Leaky caps, refer to electrical leakage, but an electrolytic cap can go bad in two ways. One it can get old, the electrolyte inside dries up and it doesn't function as well. The other is it can get zapped by a voltage spike and it can puff up and the top cap will be pushed up, and occasionally you'll see white fluffy electrolyte surrounding the cap.



Here's a picture of the extreme case.

[The Scum]

Generally, cap "leakage" relates to electrical leakage as the dielectric in the cap ages. It's not something visible...

..With the exception of a certain era of caps that had faulty dielectric, that oozed out of the package. Interesting story behind that:
http://en.wikipedia.org/wiki/Capacitor_plague

But it's not what we're usually talking about with audio gear.

Usually, the cap degrades, the capacitance drops to less than the stated value, and the DC resistance goes up, so it heats up as it absorbs ripple, and the heat further degrades the electrolyte. That heat might also be the source of discoloration you're seeing.

The internal fuses are usually on the outputs of the power supply circuit, so that if an attached circuit fails short (say, a paperclip falls through a ventilation slot), the fuse pops before the associated supply rail burns itself up.

The big caps are usually in the power supply circuit, between the mains fuse and the internal fuses. If one of them were failing such that it drew a huge inrush current, the mains fuse would blow, but the output fuses wouldn't.

I would pursue Fostex for the manual...a lot of the scanned online manuals I've seen for other things are incomplete or illegible. If it's up to par with other manuals from that era, it'll also have exploded diagrams to help you disassemble & reassemble the machine.

[falderguitars]

Well Fostex have come up trumps with a copy of the service manual.

They even stock spares - although they come direct from Japan and take 3 months to get here, but that's life...

Looking at the pics of caps posted, i'm not so sure there's anything wrong with mine:





I'm thinking the discolouration might just be glue?

Puts me back to square one with the diagnostics. It must be something prior to the power board fuses though, which narrows it down to on/off switch, voltage selector, transformer or caps, if i'm reading the schematic right.

I think a trip to the repair shop is in order.

[Sean Sullivan]

Just because they look fine, doesn't mean they are.

To replace those power supply capacitors with quality parts that will last longer than your desire to record with it would cost less than $10 and take just a couple minutes to replace. Order from Panasonic FR or Nichicon HE capacitors of the same value at DigiKey and pop them in!

[Scodiddly]

That's probably glue in your photos.

What Sean said, though - equivalent caps are cheap and a good thing to try first. "Fix what you know is broken", and electrolytic caps are a pretty common failure in old gear.

Another thing to look out for is bad PSU diodes or bridge rectifiers. A short in one of those will almost always lead to blown fuses on powerup.