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Old 12th Mar 2018, 12:24 pm   #1
David G4EBT
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Default Heathkit RF1U Signal Generator Restoration

I’ve had one of these little generators for some years now, which has served me well and works fine.

Last year I bought a non-worker from a rally, so was able to use the existing one for performance comparisons and to check for any wiring errors or whatever I found when I opened up the non-worker – not knowing whether or not someone had got there before me. I think the non-worker may be from the mid to late 1960s, older than the working one as it is darker in colour and has a separate orange and silver dial, whereas the working one I have is lighter in colour and the whole of the front of the case including the dial is screen printed. They’re nicely laid out and a compact size, measuring 24cms x 160cms x 140 cms. (9.5” x 6.5” x 5”).

I fondly remember the Heathkit era on the 1960s – 1980s when I often longingly browsed through the catalogues, but I'm sure I'm not alone in saying that the prices were far beyond my range. I’ve discovered a 1966 catalogue online which lists the RFIU - the kit being £13.18s 0d, and the assembled price being £20.8s 0d. To put this into perspective, when adjusted for inflation, this equates to £250 and £363 respectively in 2018.

http://heathkit.glosnet.com/heathkit.../1966_pg25.jpg

When browsing through the catalogues and seeing the wide range of kits, it’s a reminder of what a huge company Heathkit was in its heyday.
I can’t imagine how many designers, engineers and production staff it must have employed, not just in the USA but at Gloucester:

http://www.heathkit.org.uk/

The generator covers 100kHz to 100 MHz in six ranges, and calibrated harmonics from 100 MHz to 200 MHz as follows:

Range A: 100 KHz – 300 KHz
Range B: 300 KHz - 1 MHz
Range C: 1 MHz – 3 MHz
Range D: 3 MHz – 10 MHz
Range E: 10 MHz – 30 MHz
Range F: 30 MHz – 100 MHz
Calibrated Harmonics – 100 MHz – 200 MHz.
Accuracy, +/- 20% of dial calibration.

Output impedance 75 Ohms.
Voltage: Up to 100mV on all ranges.
Internal modulation approx 400 Hz, 30% depth nominal.
Audio output up to 9 Volts across 1 MegOhm
Valves:
V1: 12AT7/ECC81 - RF Oscillator
V2 ECF80 – Modulator and RF output.
Rectification is via a Sentercell selenium rectifier.

Some Heathkit equipment was factory constructed, but most was home-constructed, made much easier by the renowned Heathkit manuals which detailed every step of the way. Thus, the quality of construction can be variable. To aid the home constructor, the coil and bandswitch unit of the RF1U was pre-wired and factory aligned. Just one coil - the range F coil - consisting of four air wound turns of 22 SWG tinned copper wire - had to be soldered to the tuning capacitor. The turns are later either opened up or squeezed together to align that range. The coils and associated circuitry, along with the circuitry of V1 are contained in a screening box, bolted to the underside of the chassis.

The ‘U’ suffix on Heathkit equipment indicates that it was version modified for the UK market for 230 – 250V AC mains, whereas equipment without the 'U' suffix was for the American market, for 117 Volts mains and often had other diffrences. The first thing that’s evident on removing the case is that the thin two-core mains flex isn’t anchored inside the case – it’s simply tied in a knot as with similar Heathkit mains powered equipment of that era. The cable passes through a grommet on the rear apron of the chassis and poses a safety hazard as it can be twisted round in any direction, risking fracture of the soldered connection over time, which could then touch the metal cabinet. The mains cable doesn’t go directly to the two pole mains switch on the audio control potentiometer – it goes to a tag-strip, and from there, via thin twisted flex to the pot switch and back again to the tag-strip. As the mains cable isn’t earthed, that poses an added hazard. As I’d done with my original one, I fitted a 3-core flex, earthed the case, and fitted a cable restraint in place of the grommet to firmly anchor the flex.

On checking the output of the selenium rectifier, which - from a healthy rectifier would be 160V - it was down at 120V, albeit the AC input from the mains transformer to the rectifier was correct at 150V. Instead of the HT being 125 Volts after the 2k2 load resistor across the reservoir/smoothing capacitor, it was down at 95V, so it was evident that the rectifier was in need of replacement. More about that in the next post.

Pictures attached:

Pic 1: Front of the old (non-working) one that's the subject of this thread.
Pic 2: Front of newer one.
Pic 3: Newer one beneath chassis with the screening box in place.
Pic 4: Sentercel rectifier.
Pic 5 Above chassis.
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Old 12th Mar 2018, 12:45 pm   #2
David G4EBT
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Default Re: Heathkit RF1U Signal Generator Restoration

Replacement of Sentercel selenium rectifier:

Whatever other faults might exist, the first task was to replace the defective rectifier and get the Voltages up to spec.

I removed the rectifier and in its place I fitted a small tag-board (actually a ‘turret board’), similar in size to the selenium rectifier, with a 1N4007 silicon rectifier and a series resistor chosen to drop the higher voltage produced by the 1N4007 down to the desired 160V. By select on test, I found that a 560 Ohm resistor met this requirement. I made a small aluminium stand-off bracket to mount the tag-board on the chassis, using the same mounting holes as the original rectifier.

The reservoir & smoothing capacitors consists of three 20uF electrolytic sections in one can. One is used as the reservoir, and the other two are wired in parallel giving 40uF for smoothing. The outer capacitor for the reservoir is identified is marked red. The three sections reformed well, with little leakage, but had they needed replacement, the old can could have been re-stuffed with modern Rubycon miniature electrolytics which have a high ripple rating. One 22uF for the reservoir and one 47uF in place of the two 20uF sections used for smoothing would have sufficed.

Of the 21 carbon composition resistors, nine were more than 20% high - some by as much as 40%, so while I had the generator dismantled I decided to replace those. Most of the capacitors were either disc ceramic or silver mica, and didn’t pose a problem. There were just two tubular paper ones, which I replaced as a precaution. The actual voltages attained will vary to some extent as compared to those shown on the circuitry depending on the mains voltage to the RF1U, which - at the time of testing - was 245 Volts.

After the remedial work, the voltages attained were close to those stipulated:

V1:
Pin 1: Spec 70V. Actual: 74V
Pin 6: Spec 65V. Actual: 63V
Pin 7: Spec -4.0V Actual: - 3.5V

V2:
Pin 1: Spec 55V. Actual: 57V
Pin 2: Spec 125V. Actual 128V
Pin 6: Spec 120V. Actual 124V
Pin 7: Spec 2V. Actual 2V.
Pin 8 Spec 1V. Actual 1V.

Accuracy:

On completion of the remedial work I checked the accuracy of the frequencies at the extremities of each band using my PW ‘Robin’ home-built frequency counter (designed by the late Mike Rowe, (forum name ‘Swordholder’), designer of the ‘Sussex’ Valve Tester. Back when the RF1U was originally marketed, few amateur constructors would have had a frequency counter at their disposal so the dial accuracy was important. Nowadays, we have the luxury of being able to set the frequency by reference to a frequency counter rather than the dial, so the dial accuracy is of much less significance. Nevertheless, the results showed that on all bands, apart from Band F, the RF1U was well within the stated +/- 2% range. Band E, as minus 3% at 10 Mhz and 4% at 30 MHz was perfectly acceptable:

Range A: 100 KHz - 300 KHz: Exactly to spec.
Range B: 300 KHz - 1 MHz: At 300kHz was 304 KHz (+1.3%). At 1 MHz was 980KHZ ( - 2%).
Range C: 1 MHz - 3 MHz: At 1 MHz was 990 KHz (-1%). At 3MHz was 2.98 MHz (- 0.7%)
Range D: 3 MHz - 10 MHz: At 3 MHz was exactly to spec. At 10 MHz was 9.75MHz (-2.5%)
Range E: 10 MHz - 30 MHz: At 10 MHz was 9.7 MHz (- 3%). At 30 MHz was 28.8MHz (- 4%)
Range F: 30 MHz - 100 MHz: Exactly to spec across the band.

More a tribute to the Heathkit design engineers than my handiwork!

Though replacing the rectifier was a simple task, I've gone into some detail with a couple of sketches which might help less experienced forum members should they have an RF1U which isn't performing as well as it should.

Pic 1: Old knotted 2 core mains flex replaced with 3-core and cable restraint.
Pic 2: Faulty Sentercel rectifier
Pic 3: Diagram of old and new rectifier circuit
Pic 4: Drawing of the new tag-board and mounting bracket.
Pic 5: The replacement rectifier board in place.

In the next post, I’ll include to pics of the waveforms and frequency readouts obtained.
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Old 12th Mar 2018, 12:57 pm   #3
David G4EBT
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Default Re: Heathkit RF1U Signal Generator Restoration

Waveform:

The RF waveform of the RF1U was checked on several ranges as far as my 20 MHZ Hameg 203-7 analogue scope could cope with, which proved to be a good deal higher than I’d imagined! The 'scope coped well up to 30 MHz, showed a good trace at 55 MHz and even a reduced trace at 88 MHz! Not just a good test for the scope, but for the new pair of e-bay-sourced 100 MHz scope probes at £7.60 the pair post free recently bought from a UK supplier.

The AF waveform was also good, and when used to modulate the RF waveform, the modulated waveform looked good too.

Pic 1: 465kHz unmodulated waveform
Pic 2: 465kHz modulated waveform.
Pic 3: 1 MHz modulated waveform
Pic 4: 30 MHz unmodulated waveform.
Pic 5: Scope trace at 55.5 MHz

The generator had a mixture of odd knobs rather than its original Heathkit knobs, so I bought a set of matching knobs to tidy it up.

Test sockets:

The three sockets (‘AF In, AF Out, RF Out’). are TV connectors known in Europe and Australia, (maybe elsewhere too), as ‘Belling-Lee connectors’, (IEC 61169-2 radio-frequency coaxial connector, type 9,52), whereas for many years, most test gear has featured ‘BNC’ connectors. Arguably the sockets on the RF1U could be replaced with BNCs, but they’re perfectly functional, neat, and original. Given that Belling Lee sockets and plugs are widely used for terrestrial VHF/UHF roof antennas, antenna signal amplifiers, CATV distribution equipment, TV sets and FM / DAB-radio receivers, they’re well up to the task, so I’ve left them in situ.

I hope the write-up might prove useful and of interest.
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Old 12th Mar 2018, 1:03 pm   #4
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Default Re: Heathkit RF1U Signal Generator Restoration

Interesting thread, David. A useful resource for others who may find an RF1-U has come into their possession.
I built an RF1-U generator when they first appeared on the market (late 1950s? Early 1960s? Not sure, now). Nice kit, looks like the grey model you show in pic 1. The building instructions were absolutely explicit and extremely clear.
The generator has served me well and still works fine after many years, a testament to the quality of the Heathkit product. It is a shame they are no longer around, but I guess that the old enemy, progress, has killed their market.

Tony
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Old 12th Mar 2018, 1:08 pm   #5
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Default Re: Heathkit RF1U Signal Generator Restoration

Thanks for the excellent write up. I actually have a list of things I want to get my hands on and an RF1U is one of them. This will be a great reference when that does finally happen.

On this subject, does anyone know what the output impedance of these is? I assume by the fact it uses TV antennna connectors, it's not going to be 50 ohms.

Also a good note with the Hameg oscilloscope. That's a very good result for a 20MHz scope!
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Old 12th Mar 2018, 1:14 pm   #6
David G4EBT
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Default Re: Heathkit RF1U Signal Generator Restoration

Quote:
Originally Posted by MrBungle View Post
Thanks for the excellent write up. I actually have a list of things I want to get my hands on and an RF1U is one of them. This will be a great reference when that does finally happen.

On this subject, does anyone know what the output impedance of these is? I assume by the fact it uses TV antennna connectors, it's not going to be 50 ohms.
Thanks for reading and for your kind words Mr B.

The output impedance is 75 Ohms, (as state in the spec in post #1).
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Old 12th Mar 2018, 1:31 pm   #7
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Default Re: Heathkit RF1U Signal Generator Restoration

Ooops. I actually read that and completely missed it. What a wally I am. Thank you
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Old 12th Mar 2018, 1:55 pm   #8
David G4EBT
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Default Re: Heathkit RF1U Signal Generator Restoration

Quote:
Originally Posted by Aerodyne View Post
Interesting thread, David. A useful resource for others who may find an RF1-U has come into their possession.
Tony
Thanks Tony.

Another popular signal generator for hobbyists is the Tech TE20D which pop up on e-bay from time to time, which also used two valves, but has a silicon rectifier.

Not quite as easy to work on as the RF1U as they have a rather compact PCB and it can be a challenge to relate the circuit to the actual layout. It's been covered in several threads on the forum. I think the dial on the TE20D has the edge on the RF1U dial. At £15.00 plus 7/6d post back in June 1970, it was a good deal cheaper than the ready made Heathkit RF1U (£20.8s.0d) was in 1966, but even so, it was by no means cheap. Respectively, at today's prices the TE20D equates to £225, (plus £5.00 post), in contrast to a ready made RF1U at £363 equivalent in 1966.

The TE20D features in the Henry's Radio advert on page 96 of June 1970 PW, alongside that other favourite - the Tech TE15 GDO (£11.10s 0d) at this link:

http://www.americanradiohistory.com/...PW-1970-06.pdf

Back then, I had to make do with a simple home-built F.G. Rayer P.E. design - even then I saved for weeks just to buy the Osmor coils!

For today's home-brewers on the forum, let's not overlook this one!:

https://www.vintage-radio.com/projec...generator.html
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Old 12th Mar 2018, 2:25 pm   #9
David G4EBT
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Default Re: Heathkit RF1U Signal Generator Restoration

One point worth mentioning is that the RF1U has an RF Mains filter consisting of two 1.1µF RF chokes and four .001µF capacitors, as shown on the diagram in pic 3 of the first post above. Arguably as the four caps are all from Line to Earth they should be 'Y' Class. However, as they're disc ceramic rather than tubular paper I've left them in place, though some may take a different view.

I've highlighted the four caps in the pic below.
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Old 12th Mar 2018, 3:05 pm   #10
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Default Re: Heathkit RF1U Signal Generator Restoration

I rather like the ceramic ones. They are less exciting than the alternatives if anything goes wrong!
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Old 12th Mar 2018, 3:19 pm   #11
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Default Re: Heathkit RF1U Signal Generator Restoration

David, it must be 20 years or more when I had my first RF1u. When it came, the Selenium rectifier was U.S. I did the same as you but my work is not in the same league as yours. The repair work you've done is so neat, I will have to get mine out and try to bring it up to your standard.
Keep up the good work!
Kind regards
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Old 12th Mar 2018, 5:44 pm   #12
David G4EBT
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Default Re: Heathkit RF1U Signal Generator Restoration

Thanks for reading the thread Kevin, and for your words of encouragement.

Incidentally, the Hameg scope I used is one that I bought from 'Marc in Manc' a couple of years ago for spares.
I've yet to get round to fixing the minor issues with it, (volts/div cal pots), but it bimbles along quite nicely.
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Old 12th Mar 2018, 10:49 pm   #13
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Default Re: Heathkit RF1U Signal Generator Restoration

Not surprisingly, given my love of Heathkit, I have an RF-1u, however I've never been 100% happy with its modulated waveform. When I once used it to line a set up the tuning points were different from a generator with a more textbook modulation shape.

However looking at the output spectrum doesn't show any lop sidedness to the side bands (see attached)
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Old 13th Mar 2018, 8:08 am   #14
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Default Re: Heathkit RF1U Signal Generator Restoration

Great write up I have one to do that I got from a fellow forum member.
but yet another case style
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Old 13th Mar 2018, 9:50 am   #15
David G4EBT
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Default Re: Heathkit RF1U Signal Generator Restoration

That's an interesting contrast. The dial is screen printed like the lighter coloured one of mine, but in is painted gray like the darker one of mine, which has a metallic dial. Both that one and yours have larger dials than the lighter coloured one as the dial goes right to the top of the case. On the lighter coloured one, the dial is smaller and the term 'HEATHKIT SIGNAL GENERATOR' is printed above the dial.

My Darker one on the top LH corner states 'Heathkit Daystrom' which I think means the Gloucester factory, and yours has 'Heathkit' in the same place, but in a 'cursive' font. For some reason, Heathkit knobs don't age well over time, and just as the knobs on my darker one had to be changed to the ones that I fitted, on yours, only the central tuning knob is original - the 'chicken head' knobs are replacements, but don't look out of place.

I guess that the different front panel designs reflect what Heathkit considered to be in style at the time. I imagine that the RF1U had quite a long run spanning many years - anyone's guess how long. Despite the high price originally, they seem to be fairly common. As long as the coils haven't been messed with, by reference to the full manual, the RF1U can be checked through step by step to find any errors.

Does yours work at the moment - have you checked the output of the selenium rectifier or is on your 'to do list'??
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Old 13th Mar 2018, 11:11 am   #16
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Default Re: Heathkit RF1U Signal Generator Restoration

Hello David,

Just to say I really enjoyed reading your account of this overhaul as well as the background to Heathkit. Your write ups are always clear, detailed and informative .

Those historic prices are interesting, too.

It sounds like you have a great piece of test equipment on your hands now; pretty amazing accuracy!
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Old 13th Mar 2018, 11:27 am   #17
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Default Re: Heathkit RF1U Signal Generator Restoration

Quote:
Originally Posted by David G4EBT View Post
Does yours work at the moment - have you checked the output of the selenium rectifier or is on your 'to do list'??
Its on my to do list when work quietens down as doing 6 day weeks at present
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Old 13th Mar 2018, 9:23 pm   #18
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Default Re: Heathkit RF1U Signal Generator Restoration

UK Heathkits had four distinct phases. First came the grey finish with grey skirted knobs, then the two tone silver grey with black skirted knobs. IMHO these were the best looking. Then came the light brown case with brown skirted knobs and finally brown case, but the knobs are a different style and have no skirts.
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Old 2nd Jun 2018, 1:34 pm   #19
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Default Re: Heathkit RF1U Signal Generator Restoration

Thanks David! I hauled my RF1-U out to use last week for the first time in years and found it had stopped oscillating due to deterioration of the rectifier. I was going to replace it with a silicon job and then found your timely post so I didn't have to mess about finding the best value for the series resistor! Brilliant timing! It's now working well again.
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Old 2nd Jun 2018, 8:06 pm   #20
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Default Re: Heathkit RF1U Signal Generator Restoration

Many thanks for this thread and your repair details David - like you I used to drool over the heathkit catalogues years ago and now they often turn up cheap on internet auction sites, have been happily collecting all the ones I'd have any use for - have at least 3 of this sig gen and they are all of the post #14 coloration.
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