Live chassis TV and antenna input questions

[staticmind]

Hello,

Nearly having finished my B&O Capri 606 build, I decided to post this question separately, as it is very general.
When I had everything assembled today, I connected the TV to the mains. It is a live chassis set.
I have an RF modulator (VHF) that I had connected as well and powered on. This is AC mains powered as well.

Turning the TV on, I was met with a short buzz from the speaker and a "pop". The result is that one 1n capacitor on the 75 ohm to 300 ohm matching transformer has been fried (please refer to attachment).

My guess of what happened is that the TV chassis was on live and "ground" on the RF modulator was neutral.

My questions are:
- Having not powered the TV up again (trying to get the courage), is it likely it has caused further problems, or should I just be concerned for the RF modulator?

- Is there anyway to make this foolproof without using a (big) isolation transformer for the AC? What about the inline isolators for the antenna input?

- Out of curiosity - having live mains on the antenna input would mean that one could shock himself by touching the TV aerial and a earth/ground connection?

Best regards, Troels

[Welsh Anorak]

I doubt the TV is damaged, though do check the coils on the islator for continuity or burning and, if the mains fuse is blown, for any power supply damage. I think your rectifier is a valve so should be OK. However the modulator might well have suffered - check with another TV. The capacitors in question offer isolation for the chassis so replacing them both with suitable higher voltage components should get it back working.
You could always add another isolated aerial socket in series with the set's own socket to be absolutely sure - modulators give a high output so attenualtion won't be a problem.
I don't think you need to go the trouble and expense of an isolating transformer, though I would check the chassis is at mains neutral.

[SiriusHardware]

In the very distant past I came across a couple of examples where a UHF transformer- isolated aerial input socket with its output connected internally to a live chassis had obviously been broken and replaced with a standard panel mount aerial socket. Not too clever.

[TonyDuell]

I would fit class Y2 capacitors here (replace both of them), or is there a good reason for not using them in this circuit?

[Maarten]

The RF input may have been replaced over time with a non-isolated type during a previous repair. Normally, B&O uses Philips input connectors that contain a few 390p or 470p Y-class capacitors in series with both the core and the mantle of the input.

[Restoration73]

1n disc ceramics rated at 5kV should be ok. However in UK practice a 1M resistor would
be in parallel to provide static discharge path.

As mentioned if a non reversible power plug was used (or perhaps with neon if an earth contact is provided on the type of reversible socket used) the problem would not arise.

[staticmind]

RF modulator turns out to have survived - tried it on another TV. Rectifier is 1N4007, since the PSU section has been modified. The old selenium rectifier was severely lossy.

The antenna input is original and hasn't been tampered with.

I don't understand how the capacitors would offer chassis isolation - they block DC, but wouldn't they just pass the 50 Hz 230Vac with minor attenuation?

[Nuvistor]

The capacitors are 1nf, capacitive reactance at 50hz is over 3M ohm. The capacitor values in post #5 would be even higher reactance.
This type of aerial input was standard for live chassis TV’s, very important safety part of the circuit.

The one that failed was obviously faulty, has per previous advice replace both.

[Cobaltblue]

These are small value capacitors their Reactance at 50Hz will be over 3 MegOhms

Nuvistor already posted the same

Cheers

Mike T

[kan_turk]

Originally Posted by staticmind

I don't understand how the capacitors would offer chassis isolation - they block DC, but wouldn't they just pass the 50 Hz 230Vac with minor attenuation?

1nF offers a very high impedance at 50Hz & low impedance at VHF and so will provide isolation from mains but will pass RF without attenuation

Rgds
John

[staticmind]

Thanks guys. That of course makes sense.
I only have 1 nF in Y5F grade from AVX, 3 kVdc - but no AC rating. Maybe it is a bit risky using them here, so will order Y2 graded ceramic discs.

[Radio Wrangler]

Those would be the right parts for the job.

Failure of either of these parts can cause the TV antenna and the feeder cable to it to all go live at dangerous voltage. Anyone on the roof, or in the attic touching the antenna would be at risk. There are no warnings on TV aerials to tell anyone that touching them has risks, and their life may depend on two small components and which way round the mains is connected to the TV.

David

[Maarten]

Originally Posted by Restoration73

1n disc ceramics rated at 5kV should be ok.

Agreed, that's roughly the peak rating for Y2 capacitors as well. The chance that they'll fail short is small anyway.

To the TS: Y2 (voltage rating and safety approval) is not the same type of rating as Y5 (material grade). 3kV would indeed be on the low side, only use in case of emergency, though 2 in series for each position would be okay.

[staticmind]

Thanks David & Maarten
Glad I wasn't a TV aerial service technician back in the days, not without a voltmeter or neon screwdriver at least.
I have ordered a bunch of 1nF Vishay / Cera-mite 440L series Class X1/Y1 - just to be safe.

On a similar note: In a LOPT section which sees 475V pulses (not sinusoidal AC) would such AC rated caps (any X or Y) be preferable to use compared to normal ceramic disc caps rated for 3 kV DC? I'm asking since the same type of 1n capacitor was used here, but I replaced it with a 3kV rated replacement. After all the AC rating of the old 1n capacitors were also unknown...

[Maarten]

While the X and Y safety classifications specify an AC rating of roughly the mains voltage as a part of their definition, that's not the point of using those. Y2 capacitors have a peak rating of 5kV for incidental peaks to conform to the norm for 'double insulated equipment' and are supposed to fail open to prevent shock risk. Their construction is often the minimum the manufacturer can get away with while still conforming to the classification.

Normal MKP / FKP and ceramic disc caps as used in the LOPT section have a DC rating printed on them but the AC rating is in the datasheet and by rule of thumb often around 1/3 of the DC rating. Their strong point is that they can withstand continuous steep pulses as seen in such power stages, defined as a high dV/dt rating, without going open. They will sometimes fail short after a heavy pulse load for a long time. In that case, I always overrate them when replacing (2kV -> 3kV) which is usually good enough to keep the set going for years.

I'm curious though, what capacitors B&O used originally. Ceramic caps in that position would likely only fail short if damaged by a lightning strike, but they may have used paper capacitors which will fail short whenever they feel like it? In that case they didn't use the trusty Philips RF entry that they used in other models but might have cobbled something together themselves.

[staticmind]

That makes sense. The AC rating I didn't find, but it seems quite normal for general purpose ceramic discs. The 3 kV ones I have, are the series 5NT ( https://www.digikey.com/htmldatashee...-class-ii.html ).

I'm unsure of the origin of the capacitors B&O used, but they are ceramic. They are encapsulated in a phenolic like resin but it is a bit soft. B&O and I believe LL (Linnet & Laursen) as well, used flat/spade connectors up until the mid-sixties for the antenna. I attached an image of the conversion I made between the flat connectors and coaxial. I have no idea how the antenna cable itself looked like, other than it must have been some sort of twin-lead construction, but 75 ohm characteristic impedance.