Pete,

I'd love to see a circuit or article on this.?


Donald KX8K

On Mon, Jun 19, 2023 at 04:08 AM, Jinxie wrote:

I guess you'll also wear thick rubber gloves and welding goggles to compliment your ultra-safe approach. ;-)
If you're going to be using an isolation transformer, that GFI will be somewhat redundant, don't you think? How's your earth fault current going to arise?

I agree. With an isolation xfmr, a GFI can only detect a *double* fault: The xfmr is not actually isolating, AND you also have an unplanned path to Ground. It's a little hard to imagine how that would happen.

Sort of related: A household GFI breaker can be used to make an adjustable resettable fuse, with one fixed R and one pot. The idea is that the GFI is sensitive to a certain fixed current - it opens when that current is exceeded. The fixed R causes a V-drop proportional to the load, and the pot determines how much of that V-drop is "seen" by the GFI.

Pete

I guess you'll also wear thick rubber gloves and welding goggles to compliment your ultra-safe approach. ;-)
If you're going to be using an isolation transformer, that GFI will be somewhat redundant, don't you think? How's your earth fault current going to arise?

On 6/18/2023 4:20 PM, Laurence Taylor wrote:

A variac is nice, but a dim bulb can be more useful; if you're uncertain
of the condition of a set

Not if you start a voltage and current metered Variac at 0 volts and slowly bring it up. I've been powering up sets for the first time that way for more than 40 years without damage.
--
Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA

Dale H. Cook <bridgewaterma@...> wrote:

One places an incandescent light bulb in series with the hot from the AC Mains.

This has been used for decades by those who restore antique radios, and we call it a "Dim Bulb Tester."

I don't use one because I own an isolated, fused and metered Variac.

A variac is nice, but a dim bulb can be more useful; if you're uncertain
of the condition of a set, a dim bulb will tell you at once if there's
anything approaching a short, without running the risk of blowing the
fuse or anything else, as the current will be limited by the lamp. You
can also see at a glance if anything nasty is happening, like
transformer flashover.

A variac is good for providing a known reduced voltage (also a good
idea) but I'd use it as well as the series lamp, not instead of.

--
rgds
LAurence
<><

Instead try to realise, that there is no sig.
~~~ Random (signature) 1.6.1

On 6/16/2023 5:56 PM, Jeff Green wrote:

I'm almost embarrassed to offer such a piece of simple test equipment.
...
One places an incandescent light bulb in series with the hot from the AC Mains.

This has been used for decades by those who restore antique radios, and we call it a "Dim Bulb Tester."

I don't use one because I own an isolated, fused and metered Variac.
--
Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA

I would probably use two logins to create two accounts and put half on each drive. Would be within 15G limit on each that way. Each drive could have a link to the other. No cost.

Regards

开云体育

Looks like a low cost version of 8901A …?

On Fri, Jun 2, 2023 at 12:16 AM, <swallowp@...> wrote:

as described by Jacques Audet in the original thread and sketched by Mikek in this thread.

?Just so I don't take credit that's not mine, I only re-labeled the sketch in English from the original Dutch(?).
????????????????????????????????????? Mikek

On Fri, Jun 2, 2023 at 06:05 AM, john davis wrote:

does not state frequency range ?

?

Well, the oscillator (TM1247) is 20-300 MHz. Presumably it covers this range, although as the note says .... up to 4V depending upon frequency. (one suspects that it rolls off at the extremes)

>?in my book there is no TM5727 ?

Get another book 8-)

Thanks Mikek,

Yes that scheme would require even more power, a total of about 1.5 watts just to get 20 mW into the Q measuring circuit. Another method might be to cascade a couple of transformers with lower turns ratios. I wonder if anyone has a pic of the transformer HP used in their Q meter that transformed power from a 75 ohm source to some very low value.

Morris

Back in the Q meter thread someone gave just enough info to make it sound interesting, but not enough for me to understand.
Their design was for 50Ω, They used a 40Ω and 10Ω in series and put the transformer across the10Ω to reduce the turns count.
In that case, I think the goal was 0.02Ω. Yes, that does require you to input more? power.
57Ω in series with 18Ω gets your 75Ω impedance for the amp. 18Ω to 0.5Ω is a impedance ratio of 36 or a turns ratio of 6.
That is a much easier transformer build with less strays to worry about.
But 2/3 of you power is dissipated in the 57Ω.
? Just throwing out the idea.
?????????????????????????? Mikek
???????????????
?

My biggest hurdle in producing an exciter for the TF1245 has been? the design of the matching transformer. The TF1245 requires 0.5 watt at an impedance of 0.5 ohm which means a current of 1 ampere into a resistive load over the frequency range of 50 KHz to 50 MHz. The power requirement is more than most other Q meters and is mostly dissipated in the resistive divider inside the TF1245. The power used for the actual Q measurement is only 20 mW. That is why modifications to the Q meter to allow direct injection without the resistive divider are so attractive. However I don't want to modify the TF1245 as I also have a TF1247 that I might want to use and live in hope of finding a TM5728A as well (or masking a suitable exciter ;-).

Generating half a watt of RF in the required frequency range is not difficult. I have tried a number of toroidal core arrangements for the transformer to no avail. The HP power amp I have copied likes a load of 75 ohms as far as I can gather from the schematic so the transformer needs an impedance ratio of 150:1 or a voltage/turns ratio of about 12:1. I have some large binocular cores on order an will plunge into the black art of transmitter style transmission line transformers and report back.While waiting for them I will try glueing some smaller toroids together to make one larger one that might be able to cope with the required power without saturating.

Watch this space!

73,

Morris VK3DOC

I've got the TF1245A with both the TF1246 & TF1247. The TF1247 also comes with an 'adapter' (TM5727) to 50 ohms. Presumably this could be used in reverse to match 50 ohms to the low impedance of the Q meter (although I haven't tried it)

Michael
NV3Z / VK2BEA / G4NYV