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Old 14th Apr 2021, 11:02 pm   #1
DavidAYorks
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Default Heathkit RF-IU

Hi,

I am trying to sort out my Heathkit Signal Generator. I admit to not knowing if this generator ever worked while I have had it.

However, it doesn't appear to have any output. I have checked it on two oscilloscopes and both agree.

So I am suspecting the oscillator. An 12AT7. Hartley oscillator.

It appears from the circuit diagram that only half of the valve is used for the first five ranges, with the other half being brought in for the highest range.
The first half uses pins 1,2,3.

Pin 1 is Anode, I have 70 Volt there. But I have nothing on Grid pin 2.

According to two versions of the circuit diagram I should have either -4 or -8 Volt.
The components around this half of the valve check out as ok.
So, before I get further into this repair (I have ordered a new valve) can anyone suggest why I would have no grid voltage.

Oh yes, I do have 23 mA of Anode current.

Thanks,

David.
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Old 15th Apr 2021, 9:48 am   #2
mickm3for
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Default Re: Heathkit RF-IU

Hi the stage is not oscillating check the switch if all other parts check OK. They go noisy and result in no feedback.

Mick.
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Old 15th Apr 2021, 12:39 pm   #3
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Default Re: Heathkit RF-IU

Thanks, I'll check that tonight.
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Old 16th Apr 2021, 2:04 am   #4
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Default Re: Heathkit RF-IU

The first question you have to ask of anything home-assembled is "has it ever worked?". If the previous owner is available ask him "what did you do to fix it?"
I have a Heathkit IO-102 scope here that I bought for a pittance - it was NZ$10 which may give you a clue. There were that many bodges in it that trying to extract the original circuitry from the actual was a nightmare. It still sits in the bottom of the "to-do" cupboard and every so often I open the cupboard and slowly close it again.....
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Old 16th Apr 2021, 9:59 am   #5
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Default Re: Heathkit RF-IU

I recently serviced my Heathkit amplifier and found a switch terminal which had never been soldered! The wires had been tightly twisted on in preparation and because of this it had worked fine! I only found it by chance looking into a different issue.
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Old 16th Apr 2021, 10:17 am   #6
David G4EBT
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Default Re: Heathkit RF-IU

Quote:
Originally Posted by DavidAYorks View Post
Hi,

I am trying to sort out my Heathkit Signal Generator. I admit to not knowing if this generator ever worked while I have had it.

However, it doesn't appear to have any output. I have checked it on two oscilloscopes and both agree.

So I am suspecting the oscillator. An 12AT7. Hartley oscillator.

It appears from the circuit diagram that only half of the valve is used for the first five ranges, with the other half being brought in for the highest range.
The first half uses pins 1,2,3.

Pin 1 is Anode, I have 70 Volt there. But I have nothing on Grid pin 2.

According to two versions of the circuit diagram I should have either -4 or -8 Volt.
The components around this half of the valve check out as ok.
So, before I get further into this repair (I have ordered a new valve) can anyone suggest why I would have no grid voltage.

Oh yes, I do have 23 mA of Anode current.

Thanks,

David.
The circuit does show that the stated voltages on pin 2 (-8V) (and pin 7, -4V) may vary widely, though of course, if the circuit is oscillating, there should be some negative voltage on pin 8. I've restored several RF1Us over the years, mostly from radio rallies or NVCF events, invariably not working, sometimes due to the rectifier failing, but as you have 70V on the anode that's not an issue in your case.

I've attached a table of voltages on the first two that I restored to show the results on the various bands.

As you no doubt know, the coil turret was factory built and aligned, and the cores of the coils are inaccessible to dabblers. However, the wiring of the switch, the valve holder and RF circuitry was left to the constructor using single coloured blue flex. The wiring around the valve-holder is a bit if a 'rat's nest', is cramped and for a novice constructor would have posed a challenge.

To access it you have to remove the screening can (unless it was a late model when the can and the mains RF filter was omitted). I've attached a pic of that part of the chassis, which - looking at the standard of construction - was, I think, home-built. But that said, I believe the Gloucester factory did use 'outworkers' who built kits at home unsupervised, so standards will have varied, though they will have been checked before sent out to customers.

The components in the part of the RF oscillator circuit in question are relatively few, and you say they've checked out OK. (IE: R3 56K, C8, 47pF, and the 1-8 pF trimmer which is used to tweak the final RF alignment slightly as needed). That trimmer affects all bands except the highest, the alignment of which is 'tweaked' by opening or closing the few turns on the coil attached to the self-wound tuning cap.

There is, I suppose, a slim chance that the 12AT7 might be duff and it's the easiest thing to check if a known good replacement is obtained. But that said, hobbyist signal generators spend far more time on the shelf gathering dust than in use so the valve will probably have had an easy life., unlike in a radio for example, which will have been used day out for hours on end in its early life.

I'd be inclined to print off the circuit and check out the wiring, the switch continuity and look for dry joints, open circuits and shorts, especially around the base of V1 to see that there are no pins touching, using a highlighter to mark the circuit paths as I check them.

The fact that you have a scope to help in fault tracing is a big advantage.

I did re-draw the circuit as the one in the manual is a bit indistinct. I've attached both versions.

Though not relevant to you present issues, if you haven't already replaced the dodgy single insulated twin mains flex, restrained only by a knot, you might want to fit a new 3-core flex with a proper cable restraint and to earth the chassis.

The RF1U when working to spec is a capable little generator, more than adequate for hobbyist needs. It goes down to 100kHz, which not all generators do, so is useful for aligning pre-war radios which often had IFs well below 465 kHz. It also goes up to 100 MHz, (200 MHz on calibrated harmonics), when some hobbyist generators only go up to 30 MHz. (Trio SG402 for example). The dial accuracy when restored conforms to the claimed accuracy of +/- 2%, though nowadays, with access to low priced frequency counters that's not as important as it was back in the 70s.

There have been several forum threads on the RF1U which you might find helpful to check out.

Hope that might help a bit.

Good luck with it David.
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Last edited by David G4EBT; 16th Apr 2021 at 10:42 am.
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Old 16th Apr 2021, 9:46 pm   #7
DavidAYorks
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Default Re: Heathkit RF-IU

David, Thanks for that comprehensive response.

I got the new valve this morning.

First think I tried was a dose of switch cleaner on the wave-change contacts. This seems to have helped a bit, i do detect a signal on a portable SW receiver sat close to the generator. I have remover the screen from around the tuning coils to allow access to the valve pins etc.

With the genny apparently giving out some signal I changed the oscillator valve for the new one. No noticeable difference. At least I now have a spare.
Voltage checks around the valve base gives me the same Anode voltage. But I still only have 0.23 Volt on the grid (using aFluke DVM), and zero volts on the Cathode.

So I'll have to do a re-build of the oscillator wiring as something seems to be in the wrong place. I did find one wire that wasn't attached at one end. It was going o the step attenuator

But one thing puzzles me. (only one ?).

Looking at the circuit diagram (printed out on a largish scale to help) the wiper of the variable attenuator pot appears to go straight to earth and thus would not appear to be allowing the step resistors to do anything as they are connected across a short to earth. And the output (centre connector of the co-ax socket is also connected directly to earth.

Is this correct ? If so, how does it work ?

I'll go back to studying your post.

Many thanks.

David.
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Old 16th Apr 2021, 9:59 pm   #8
DavidAYorks
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Default Re: Heathkit RF-IU

David (G4EBT)

Just made an interesting discovery.

To answer my own question re the attenuator.

The circuit you provided is not the same as the one I downloaded.

Yours makes a lot more sense as there is not a direct short between the variable and ground. That part of the diagram is completely different to mine.

I don't know how this has happened unless a batch of bad diagrams was sent out and someone has uploaded one of these to the web.
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Old 16th Apr 2021, 10:12 pm   #9
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Default Re: Heathkit RF-IU

You can download the full manual of the RF1-U from the extensive Heathkit Archive thanks to forum owner Paul Stenning’s efforts at this link:

https://www.vintage-radio.info/heathkit

Hope that helps.

As to the attenuator network design, in all honesty I’ve never given it any thought!

Good luck with tracing the fault David.
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Old 16th Apr 2021, 10:34 pm   #10
DavidAYorks
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Default Re: Heathkit RF-IU

Thanks,

Here is a copy of the attenuator network on the version I download to this computer (I have full versions of the manual on another machine. The circuit on on them is ok.

Note the attenuator network.
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Old 17th Apr 2021, 9:28 am   #11
David G4EBT
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Default Re: Heathkit RF-IU

I see what you mean David - a direct short to ground on the RF output socket!

Imagine a novice building the kit, which would otherwise have worked fine, then the last bit of wire stops it in its tracks.

I mentioned earlier threads on the RF-1U which you might find of interest. Here are some:

https://www.vintage-radio.net/forum/...=Heathkit+RF1U

https://www.vintage-radio.net/forum/...=Heathkit+RF1U

https://www.vintage-radio.net/forum/...=Heathkit+RF1U


Hope you might find them of interest and that you'll soon have your RF-1U sorted.
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Old 17th Apr 2021, 10:35 pm   #12
DavidAYorks
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Default Re: Heathkit RF-IU

Well, good news and bad news.

The good news is the generator is operating on all ranges. The voltages are more or less where they should be.
The bad news is that there seems to be a problem in the step attenuator switch (also the fine attenuator variable is a bit rough, but that is easily fixed).

I disconnected the step attenuator from the circuit and connected the scope directly to the wiper of R13. Everything sprang into life. The attenuator appears to be wired correctly. but it isn't working right.

But it is usable as it is. I will add the frequency counter mod soon. And sort out the attenuator.

I suspect the step attenuator was pulling the output from V2 down to earth.

I will claim it as a success (80%).

Thanks for all the aid.

David.
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