G'day all,

I've been doing a lot of restorative work on my Beocord 1600, and I've noticed with a shock (literally!) that there appears to be a voltage potential between the black head shield assembly and the brushed aluminium backplate - I've measured at least 2.7VAC between the plate (and of course the brass locating pins and the head assembly mounting plate) and the backplate, but it feels like a LOT more (I've been belted by everything up to 415 3-phase, and I don't like it - this isn't as bad, but it hurts!).

This has been the case ever since I rescued the deck from a careless owner some years ago (it was literally in the dustbin, but not because of this "feature").

The front panel (black lower plate surround for the sliders) is at the same potential as the large backplate (which is, I hope, close to earth), but touching any of this chassis (when the headshell is fitted and touching the backplate) while the unit is powered gives a very nasty nip.

 Removing the head shield seems to stop this problem, as long as I don't create any circuit between the bare head assembly and the chassis, I'm fine, but that's not always possible when threading a tape!

I've looked at the circuit carefully, and I can see no reason why such a high aparent voltage should be on the head assembly (8004053), and I've tried my best to carefully insulate the head wiring (end of tape, read/write/erase head, etc, where it passes over the top edge of the assembly) from the backplate, but this doesn't seem to fix the problem.

It sounds very much to me as if the ground has been broken between the head assembly and the rest of the recorder, but there is little apparent distortion or noise (except when the head assembly shorts to the chassis), and the voltage is apparent even with the power switched off - so it seems there may be some leakage path somewhere that I can't find. The resistance between the head assembly earth and the chassis is about 180 ohms (when all power is completely disconnected), but this varies - but I can't find a capacitive path anywhere!

Can anyone offer any suggestions on where to check next?

Record bias can be a high (AC) voltage.
As you suggest yourself, make sure that all groundings are good, both on the heads, the metal plate holding the heads and on the boards.

I haven't done a lot of these recorders myself but I'm almost sure that a capacitive decoupling of the chassis is present somewhere. Maybe Tim can comment ?

If possible, try rotating the mains plug 180deg or - if this is not possible - reverse the live/neutral leads in the plug and measure again.

Martin

Is there any chance that this is part of the auto-stop mechanism?

Some tapes had foil leaders and when the end of the tape was reached it would form a circuit which would then trigger a release solenoid.

Obviously the parts involved had to be insulated from the rest of the chassis, by either mica shims or plastic washers, depending upon which machine it happened to be.

It possible that there has been tracking across the insulator, or maybe someone carrying out maintenance was not aware of the purpose of these insulators and they took it apart and put the wrong washer in the wrong place.

Martin and Dave,

 Thank you for the suggestions!

I've had to rewire the AC input to the recorder (and boy, I don't ever want to have to do that again - it's an absolute nightmare getting the 240V leads to the switch assembly, and the old insulation of the original wiring just crumbled ), and I'm 100% sure that the phase and orientation is correct.

The problem actually occurred when I first got the recorder, and I just assumed that some earthing point was floating, but your comments about the bias make sense. I'll check all the grounding and insulation item by item over the weekend.

Dave, I thought the same thing - maybe some leader tape got stuck - but the head assembly is definitely clear and clean.

I did have some problems with the autostop, the coil driver transistor kept blowing, but that was due to a short in the stop release coil, and that's out of circuit now until I rewind it by hand.

I did carefully monitor the +30V rail after the stop coil cooked, while running a tape test, and it does have a few hundred mV of ripple, but that's due to the main filter caps drying out (the ESR of the filter caps is getting a bit low for my liking too, which means problems with long-term regulation). Big fat caps are a bit hard to come by in Aus, and I'm trying to find an elegant way of replacing the paper electros - I'm thinking maybe gutting them and putting good quality equivalent modern caps inside the casings, then reassembling them.

But I'm going to first do a good long check of the mechanicals of the head assembly, then a careful check of the wiring and insulation, and then a good look at any capacitive coupling I can find.

The head assembly has been sparking since the day I rescued it, and I'm electrically qualified, so I'm very sure that the live/neutral orientation isn't the issue here - it's definitely a leakage or insulation breakdown problem local to the head assembly or the head drive electronics. I just haven't had enough time to sit down and trace through the whole supply path in that area...yet...

Thanks again for all the suggestions. I'll be sure to provide an update when (not if!) I've figured it out.

Cheers,

There must be a capacitive decoupling of the chassis somewhere. Mains filter, Y-caps or so.

- Where's Tim ?

Martin

I've found that replacing these big old caps in the original retainers by fitting more modern caps to be awkward, its surprising just how far materials science has moved on, and how much smaller the newer ones are.

One option is to use some 2mm rubber sheet to fill out the gap you just wrap it around to the desired width.

Well, I have some good news and some good news on the sparking (and capacitor) front(s).

I finally solved the sparking issue.

When I first retrieved the 1600, the aluminium cover panel (the brushed plate surrounding the tape turntables) had the two brass locating pins pulled out of their retaining holes. I repaired them by taking a little material from around the base of each pin, then setting each pin back into its socket using a long-life epoxy resin. What I didn't know was that the pins were now insulated from the plate by the epoxy.

Next, I'd tried to solve a problem with the end-of-tape kicker coil and assembly burning out. It turned out that the driver transistor (TR30) had gone short-circuit. So I replaced it with a modern unit (BD139), which seemed to fix the problem, so I fixed the transistor to the existing heatsink - and bent one corner of the heatsink accidentally, so it would now just come in contact with the underside of the aluminium cover. Since the coil overheated as soon as I'd reassembled it all, I assumed it was a coil problem and isolated the coil and the leads from the EOT board.

So now, the aluminum cover plate was at the collector potential of the driver transistor (-11V unreg), and as long as it didn't touch the lower front cover, it stayed at that potential. That was why I kept getting shocks when the head assembly cover was replaced - it is definitely grounded, and it does just touch the cover plate.

What that meant was that the power supply's negative -11V rail was effectively at -1V (due to wiring losses I guess), and the power supply went into overdrive. The great thing is, the unit kept operating, although with terrible hum and noise on the outputs - and when I cranked up the volume sliders (even with nothing plugged in to the speaker or headphone jacks) the amp boards and bridge rectifier were massively compensating for the sudden loss of 10V of negative bias. Hence the hum, overheating, and so on. I'd previously put it down to filter caps drying out (which they were), not thinking about other shorts.

That says it all for B&O, as far as I'm concerned. That power supply designer needs a gold medal.

With the caps, I elected to replace all board mounted electros with drop-in replacements, but all the external caps (2 filter caps on the power supply and 2 output DC-blocking caps) were gutted and had modern caps potted and resealed.

I've taken many photos of the whole procedure, so if anyone is interested, I'll put the shots and a description up somewhere we can all see it. The power supply/short fix isn't that interesting for other model owners, but the cap replacement might be useful.

I used siligum (two-part silicone rubber compound) to fill the cap bodies and support the new caps, and brought the new leads up through the existing cap lugs. It worked out better than I expected (but I had low expectations to start with).

Still, the offer's there. I'll post a separate topic to see if folks are interested (it's pretty simple really, but the first time it can be a bit daunting).

Thanks to everyone for their suggestions and comments. This is a great forum!

Cheers,

PC Pete