Heathkit RF1U Signal Generator

[DonaldStott]

Finally getting round to looking at my Heathkit RF1U Signal Generator and thankfully I have the detailed Thread from October 2017 by David G4EBT to guide me notwithstanding his thorough article in Vol 43 Autumn 2018 edition of the BVWS Bulletin - both excellent sources.

Voltage readings for the AC input and DC output from the selenium rectifier were close to those stipulated in the manual although there was a voltage drop of only 12V across the 2K2 (2.24kΩ) resistor - a bit low?

Valve pin voltages for both V1 and V2 were reasonable although a bit high! Thanks again to David G4EBT I now understand that on certain pins the voltages vary with the Range selected.

As expected most of the low value resistors are within tolerance while the high value resistors are all over tolerance, some by more than 20%, so these will be changed. I'll change the two paper, tubular caps as well just because I can when I've got the case open.

Not much else to do here apart from cleaning up the switchery, of which there is a lot, and sorting out the old mains lead.

But I have a bit of a mystery to solve - the schematic in the manual shows C12 across the modulation transformer (red & back wires) but I also have a 'phantom' 5.6kΩ resistor connected in parallel across C12 and which looks original?

Here is the schematic of the tag strip with C12, the 0.1μF tubular, paper capacitor: -



And here is a picture of what I have: -



I can't find that 5.6kΩ resistor anywhere in the manual - any ideas why it's there, what's it for and, more importantly, should it stay??

[orbanp1]

Hi Donald,

That resistor reduces the Q of the resonant circuit of the audio oscillator.
That also reduces the output of the audio oscillator.
Do measure/look at the audio output with a scope, with both the resistor attached and removed temporarily.
Measure and assess the output voltage as well as distortion of the audio signal.
Also verify the AM-modulation rate with and without the resistor.
The above will tell you if you need that resistor there or not.

Regards, Peter

[Sideband]

There may have been a change of the modulation transformer in later versions. Perhaps it was added to correct the waveform in some way.

If you are unable to check as suggested above, I would be inclined to leave it.

[DonaldStott]

Thanks the helpful responses - just need to remind myself what Q is in the context of the resonant circuit of an audio oscillator!

Once I get this Heathkit up and working then next on my To-do list is my Farnell 30-4D scope - so any measurements as described by Peter will need to wait until then.

Meantime I think I'll just leave the 5.6kΩ resistor in place as Sideband suggests.

[DonaldStott]

And while I'm here let's talk about Elephants in the Room - my Heathkit RF1U doesn't have an Under Chassis Screening Box!

[waran2005]

Neither does mine, nor does it have that resistor? I thought the box was missing, I guess they just did away with it on UK versions. My RF1U came from a scrap yard, has been bashed about a bit and so looks tatty but it works.

[David G4EBT]

I've owned and restored three over the years, and presently have another awaiting attention.

All have had the screening can, but I've never been sure what its purpose is, inside a metal cabinet. I'm wondering if it was to prevent the generator from casing RFI by radiating RF when outside of the cabinet for testing and setting up purpose? It raises the question why would Heathkit have fitted the screen if it wasn't necessary, but 'necessary' could be to comply with FCC RFI regulations at the time.

The box covers the V1 base, the bandswitch/coil turret and RF attenuator controls, leaving just the power supply, V2 base (audio section) and its associated components unscreened. I can't think why they'd want to screen all the RF components (except of course the tuning cap and RF output socket above the chassis), other than to prevent the generator from radiating RF when setting up and testing it outside of the case.

A few pics attached:

Pic 1 - underside of chassis with screening box removed.
Pic 2: Screening box in situ.
Pic 3: Above chassis view.
Pic 4: Circuit.

A few comments on the pics:

Firstly, I don't find the 'rat's nest' layout devised by Heathkit very easy on the eye, but appreciate the need - particularly with RF - to keep component leads and inter-wiring as short as possible. Using only blue PVC insulated flimsy wire all over the place with no colour coding in unhelpful, (as is the illogical way they wired the mains switching circuit), but I guess it simplified construction to not have several colours of wires for say AC, HT, heaters, AF, RF etc.

The circuit isn't a scan of the Heathkit manual as it wasn't good enough to publish in an article I submitted to the BVWS for the Autumn 2018 Bulletin, so I re-drew it to tidy it up, but it's identical to the original. The components inside the screening can are shown within the dotted lines.

I take my hat off to the draughtsmen who drew all he intricate step-by-step drawings in the conventional manner on pre-CAD days. Even more of a challenge for the more complex kits such as communications receivers and transmitters.

[DonaldStott]

Thanks again David for yet another informative and detailed response.

I agree with you regarding the ubiquitous " ... use of blue PVC insulated flimsy wire" and was a bit confused initially by " ... the illogical way they wired the mains switching circuit"!

If one were of a mind to reinstate the 'missing' Under Chassis Screening Box what are the dimensions of the enclosure and what material is recommended.

[Jimbo]

The RF-1U was obviously a popular model for Heathkit, and it clearly went through several updates of its external appearance [David, G4EBT has discussed this in his previous postings], so it is quite likely that the internals are also ‘variations on a theme’. There are scanned copies of the assembly instruction manual on the Internet, but unless one has the actual manual that was shipped with the particular version of the kit that is to hand, it is impossible to know if there were supplementary addenda or revision notes attached to the manual for a particular version.

In my own example of the RF-1U, the below chassis screening box is present, but there is no sign of the 5.6k resistor across C12, [neither does it appear in the circuit diagram or wiring layouts in the particular scanned version of the assembly manual that I have]. However, in my version, the RFC filter inductors and some of the capacitors on the primary side of the transformer [as shown on the G4EBT circuit diagram and in the diagrams and pictorials in the version of the manual that I have], are completely absent. Tag strip 'MB' shown in the pictorials does not exist and the primary connections to the transformer share tags on strip 'MA'. The metal chassis is punched to receive a machine screw with a countersunk head for strip MB but that's it! All I have is two ceramic disc capacitors [one each from the live and neutral sides of the mains supply] taken to the central chassis earth tag on strip MA. I suppose I could re-introduce the missing tag-strip MB where it was originally intended to go and hard wire the missing components. [G4EBT had some correspondence with 'Mr Bungle' in his own original post of the Web-site in connection with substituting new X/Y rated capacitors for the original Heathkit ceramics, but the missing 1.1mH RFC inductors should not be too hard to find].

My own version of the RF-1U is, I think, one of the later ones with a beige painted front to the case, and light brown knobs with a darker brown insert. The text below the knobs simply states MODEL RF-1U, with no mention of ‘Daystrom Gloucester’ seen on other models. I can only think that the omission of the filter components was an economy measure that Heathkit might have made in the latter years of production. As David G4EBT notes in his post above, certain elements of the design might have been to comply with contemporaneous FCC RFI requirements so it may have been considered as overkill on UK variants of the kit.

I am certain that there will be a ‘Heathkit’ specialist somewhere on this forum who will know all the answers!

[David G4EBT]

Originally Posted by DonaldStott

Thanks again David for yet another informative and detailed response.

I agree with you regarding the ubiquitous " ... use of blue PVC insulated flimsy wire" and was a bit confused initially by " ... the illogical way they wired the mains switching circuit"!

If one were of a mind to reinstate the 'missing' Under Chassis Screening Box what are the dimensions of the enclosure and what material is recommended.

Some pics of the screening box most recent one that I've acquired to restore when I get round to it. The screening box on others I've had have all be aluminium, but for some reason, this one is copper-plated 1mm thick steel. (Thought it was copper till I tried a magnet on it!). I've drawn a quick (not to scale) sketch of the dimensions. (The box is held in place with five 6BA RH screws and nuts).

On those that I've worked on, they've all performed perfectly well on all ranges six fundamental ranges from 100kHz to 100 MHz when testing without the screening box in place before putting the generator back into its cabinet, which tends to confirm my suspicions that as with the mains filter circuitry, Heathkit only incorporated it to meet FCC Regs, but maybe the UK regulatory authorities of the day (PMG, Home Office) took a more relaxed view so at some point, Daystrom at Gloucester dropped these 'refinements'?

As Toyah Wilcox once said 'It's a mystery'.

I have first hand knowledge of that 'cos it's what she whispered it in my ear when I asked her how come she looks so good at sixty?

I get around!

[DonaldStott]

Following a slight delay while I replaced the three sockets on the Heathkit Signal Generator - they are now BNC sockets - I came across another puzzle associated with the wiring and connections to the large electrolytic triple can!

Here is an extract from the schematic for that section:-



And here is a photo of what I had (apologies for the poor quality):-



You will discern from the schematic that the direct feed from the Rectifier (before the 2k2 resistor) should go the single capacitor in the triple can marked YEL while the feed from after the 2K2 resistor should go to the two capacitors in parallel in the triple can marked RED and PLAIN. Hopefully you should be able to see that in my photo what I had was transposed!

David G4EBT clarified this for me by illustrating the correct and incorrect way in which the power supply capacitors can be wired up:-



What I did was to disconnect all components and wiring in this locality and to start again but this time following the schematic and circuit diagram exactly.

The triple can was removed from the chassis and rotated so that it's orientation matched the schematic and while it was out of circuit I checked the values of the individual capacitors. They all fell within the range 22μF to 24μF, which isn't too bad as they should be 20μF? Similarly the 2k2 resistor measured 2.24kΩ out of circuit.

So with the wiring and connections to the large electrolytic triple can sorted out what are we now measuring?

AC Mains : 244V AC
AC IN to Rectifier from Transformer : 167V AC

DC Output from Rectifier : 168V
DC after 2K2 Resistor : 134V

So that's a drop of 34V across the 2K2 Resistor - using Ohms Law we arrive at a current draw of 34/2,240 = 0.01518A, which is near enough 16mA?.

And Valve Pin Voltages - figure in brackets are the Spec voltages from the Trader Sheet while the other figures are as measured:-

V1
Pin 1 (70V) 81V
Pin 6 (65V) 73V
Pin 7 (-4V) -0.3V

V2
Pin 1 (55V) 52V
Pin 3 (125V) 134V
Pin 6 (120V) 131V
Pin 7 (2V) 1.8V
Pin 8 (1V) 0.7V

So much better now that we have that triple can sorted?

In addition, all resistors that were over-tolerance now replaced, two paper caps replaced, mains cable properly restrained and a general tidy up of wires and connections.

With my new BNC sockets I can now move on to checking the AF and RF output from this Heathkit Signal Generator but at the back of my mind is that we know that the person who wired it up made a mistake here but how many more are there?

[Radio Wrangler]

You could perhaps drop the hum level a bit by going to a bridge rectifier arrangement. Takes the DC component out of the transformer core, too.

Really, David, selfies at your age!

David

[DonaldStott]

Originally Posted by Radio Wrangler

You could perhaps drop the hum level a bit by going to a bridge rectifier arrangement. Takes the DC component out of the transformer core, too.

Thanks David but unusually for something I'm working on there is no appreciable hum!

A bridge rectifier would be any easy upgrade with this Heathkit but I'm of the opinion that if it ain't broke etc.

Let me focus on testing the AF and RF output over the weekend and I'll see how far that gets me?

[Herald1360]

Originally Posted by DonaldStott

Following a slight delay while I replaced the three sockets on the Heathkit Signal Generator - they are now BNC sockets

Sneaking back to this issue and the holes left in the panel, I think it would look better if the holes were filled, but with some suitable countersunk head screws and nuts.

It would still be possible to return to original and you'd already have the fixings to hand.

[turretslug]

I prefer the 4-hole flange BNCs to the single D-shaped hole types for replacing Belling-Lee coax sockets (actually, I greatly prefer them generally...).- two of the holes on a diagonal can be filed to slots to suit the original securing holes in the panel, no wounds to the original panel and a BNC socket that won't ever rock back-and-forth in its hole with a tight plug.

Also, the idea of using a bridge as a one-day addition- the HT will rise a little as a result of greater circuit efficiency, so a resistor can be used between bridge and reservoir to equalise. Ripple lowered further and a better charging waveform form-factor- transformer runs still cooler, a virtuous circle.

[DonaldStott]

Hoping that I now have the wiring and connections fixed, including that issue associated with the triple capacitor can, it's time to move on and test this Heathkit RF1-U.

What I did first was to check that the audio oscillator was working by plugging my test lead into the AF OUT socket and applying it to the slider of a volume control on a working radio, in this case the currently dismantled Roberts R303. Thankfully I heard a loud tone from the radio speaker, the volume of which I could vary by adjusting the AF Output control.

To double check this I plugged the test lead into my Toward Function Generator which has a Frequency Counter function and this displayed a fairly steady 615 Hz - tried to find this value in the documentation but don't know if this is correct?? In any case I'm satisfied that the audio oscillator is now working!

To check the RF I plugged the test lead into the RF OUT socket and draped the lead over a working radio - again the Roberts R303. The Range switch of the Heathkit RF1-U was set to Range B, the Modulation switch to EXT. MOD. and the Coarse and Fine Attenuators to their maximum clockwise rotation.

I then swept the tuning of the RF1-U across the AM IF frequency - around 450 - 470 kHz. There was a pronounced squeal from the radio speaker which was adjusted to the lowest pitched squeal until a 'zero beat' was more or less achieved. This was at a frequency of almost exactly 470 kHz on the RF1-U dial.

Pages 18-20 of the Heathkit RF1-U contains further much more detailed instructions on calibration and alignment but some of this is currently beyond my current skills and expertise!

For me this more or less wraps up restoration of this Heathkit RF1-U and I am happy that I now have a working signal generator!

Anything else I need to check before putting it back in its case??

[DonaldStott]

Apologies, typo in my last Post - it should, of course, read:-

"To double check this I plugged the test lead into my Topward Function Generator which has a Frequency Counter function ..."

[DonaldStott]

Originally Posted by Herald1360

Sneaking back to this issue and the holes left in the panel, I think it would look better if the holes were filled, but with some suitable countersunk head screws and nuts.

It would still be possible to return to original and you'd already have the fixings to hand.

Point taken Chris so I've reintroduced the original nuts, washers and bolts so at least I'll know where they are.

Certainly improves the look of the fascia:-

Originally Posted by turretslug

I prefer the 4-hole flange BNCs to the single D-shaped hole types for replacing Belling-Lee coax sockets (actually, I greatly prefer them generally...).- two of the holes on a diagonal can be filed to slots to suit the original securing holes in the panel, no wounds to the original panel and a BNC socket that won't ever rock back-and-forth in its hole with a tight plug.

Also, the idea of using a bridge as a one-day addition- the HT will rise a little as a result of greater circuit efficiency, so a resistor can be used between bridge and reservoir to equalise. Ripple lowered further and a better charging waveform form-factor- transformer runs still cooler, a virtuous circle.

I did see those 4-hole flange BNCs and note your point about them being more secure - the ones I have seem adequate for this Heathkit which will only be used infrequently. Good to know that I have a Plan B if I ever encounter any problems with them rocking back-and-forth.

I also note your helpful points about the benefits of a bridge rectifier but I need to move on from this Project as my To-Do pile is getting bigger by the day! As I said earlier this would be an easy upgrade with this Heathkit.