Heathkit RF-1

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Mike H
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#1 Heathkit RF-1

Post by Mike H »

Is this a project? Well so happens it's turned into a bit of a refurb job, so guess it is.


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Was ist das? It am do be a Heathkit RF-1 valve RF signal generator. And it's pretty cool, in an olde fashioned steam radio valvey sort of way. :D

It's not without the odd problem tho. I had the case off for a quick butchers inside, all seemed OK, however the case screws (only two, through the back) are a bit odd, not lining up with the threaded holes. For some reason the internal chassis plate they screw into is being pushed sideways so the holes are offset. This became apparent upon trying to replace the case, and it has to be pushed and held down to the front panel (face down) on one side, to close properly, even then the screw holes don't like lining up. (Because the chassis panel bends.)

I eventually figured out what was going on - the top pip of the ECC81 valve's glass was pressing against the inside of the case, because its socket had been changed at some time, for a skirted one, which has to be mounted on top, and makes it all too high.


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I found a normal plastic socket which can be mounted from underneath, so then I somehow need to rewire this:


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Mike H
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#2 Re: Heathkit RF-1

Post by Mike H »

Managed it eventually. Only took six hours! (not including meal breaks.) Kept wondering whether brain surgery is easier.


Rewired with other socket:

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Also why do the two HT supply wires have to be bare heavy gauge, very awkward to bend, one refused to fit anymore, so right, put in two red insulated wires instead.


Below, ECC81 in situ, and case and screws now fit properly. Huzzah! Image

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Overall view - the thing that looks like an untidy spring is the F band oscillator coil:

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'Topside' of chassis panel:

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Bottom left is a tapped audio choke, for the AM modulator oscillator. Like the RF oscillators, it is a common anode Hartley, where the cathode goes into a tap in the coil, hence to ground. The top of the coil (tuned circuit) drives the grid. Series DC blocking capacitors are charged via grid to cathode forward diode action so that the grid DC point drifts negative, i.e. the triode makes its own negative grid bias. The net result is the triode only conducts on the peak positive cycles (the rest of the time it is in cut-off), so just adds a pulse on each positive peak to keep the oscillation going.

Both halves of the ECC81 are used as a RF oscillator. #1 does all bands except F. #2 does F band only cos it goes into VHF, so needs a slightly different design that is 'hard wired', i.e. doesn't use the band selector switch. Each oscilator is actived by switching the anode supply between them, so only one is working at a time.

The audio oscillator uses the triode section of a 6AN8 triode / sharp cut-off pentode. The pentode section is the amplitude modulator. It is given a mix of the RF riding on the AF, so AM RF comes out. The kit can also take audio in, which could be interesting. On the glass of the 6AN8 here is printed "Made for Heathkit by RCA". The ECC81 has "made in Britain", so evidently a replacment at some time.

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Mike H
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#3 Re: Heathkit RF-1

Post by Mike H »

Here's the current issue:

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These are some kind of strange (archaic?) UHF connector. Previous owner has (he says) made up a couple of adaptors to convert to BNC sockets. Unfortunately they're a bit ropey, the centres are loose so won't press against the centre contacts, or might if you attach a BNC cable first.

After some time fiddling about with them I've decided I'm going to replace them with two BNC chassis sockets, it turns out the panel holes are practically the right size.

Onwards...


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IslandPink
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#4 Re: Heathkit RF-1

Post by IslandPink »

popcorn.gif
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Mike H
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#5 Re: Heathkit RF-1

Post by Mike H »

Thank you for your interrest Image
 
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Dave the bass
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#6 Re: Heathkit RF-1

Post by Dave the bass »

Bah, he's being all 'sarky' Mike, knock his popcorn over his lap. That'll learn him! :)
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#7 Re: Heathkit RF-1

Post by IslandPink »

Oy :!: Shut up at the back there TheBass.
I was enjoying looking at that fine air-vaned capacitor amongst other things. It reminds me of all the bits of Kit my dad used to bring home from work or buy surplus for bits, when he was making things in the late 70's.
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Mike H
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#8 Re: Heathkit RF-1

Post by Mike H »

Today it has acquired new BNC sockets:
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RF1_2310.jpg
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The original are some kind of obscure (read as, obsolete) microphone connectors apparently.
Last edited by Mike H on Mon Dec 17, 2018 7:06 pm, edited 1 time in total.
 
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Mike H
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#9 Re: Heathkit RF-1

Post by Mike H »

Previously, I had changed the electrolytics (a dual) for two more modern ones. Not sure what state the original is in, as consensus says the RF-1 dates from around early 1960's. But anyway means can up the values a bit.
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RF1_2303.jpg
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Also treated the "PSU" to a bridge rectifier.
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RF1_2305.jpg
 
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#10 Re: Heathkit RF-1

Post by Mike H »

The mains input is still the original American 115 - 120V with 2-pin plug, so I am using a small autotransformer, having the 2-pin socket, seems to work well. There are two 20 nF ceramic caps connected between each mains line and the chassis / case. I was going to leave this alone but just by chance I found a topic on another forum where the guy was talking about one he'd just got, tried it out, the ground lead for his 'scope popped off and so then the 'cope was looking as a large AC at mains frequency. These two caps were producing a roughly half mains AC relative to his 'scope's earth (bear in mind the 'scope would have a large input impedance, 1 Megohms is standard for a modern one). So what I did was remove the caps and instead have one 10 nF across the transformer primary, which is otherwise isolated. I can only imagine the reason for having these caps in the first place was to prevent RF leaking out on the mains lead?
 
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Mike H
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#11 Re: Heathkit RF-1

Post by Mike H »

Anyway, the peas Delia resist aunts, here it is working, so I don't seem to have c0cked anything up with all the surgery (much)...

I am using a battery AM radio to "listen" to the output modulator valve...

The audible tone is it's own internal 400 Hz oscillator ...
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Cressy Snr
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#12 Re: Heathkit RF-1

Post by Cressy Snr »

Shoddy! :D
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#13 Re: Heathkit RF-1

Post by Dave the bass »

Cressy Snr wrote: Mon Dec 17, 2018 9:03 pm Shoddy! :D
Agreed... to the power of 400 times a second.
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Mike H
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#14 Re: Heathkit RF-1

Post by Mike H »

Ahh thank-yew.....

:D
 
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#15 Re: Heathkit RF-1

Post by Mike H »

More photos - after a bit of hiatus with some cheap BNC leads from eBay
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#1. 400 Hz audio output:
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RF1_2311.jpg
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...and it modulating RF (100 kHz):
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RF1_2314.jpg
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
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