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Old 7th Jun 2008, 8:17 pm   #1
paolo67
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Default Racal RA17L BFO behaviour

Hi everybody,

here is Paolo from Italy, I just discovered the forum. I'm in the process of repairing and aligning my RA17L. I already went through IFs, 37.5Mhz filter, first and second VFOs, and the preselector. I found several leaking capacitors among the paper type and resistors that raised to more than 100% in value. The calibrator was completely non-working. I did not yet touch the quartz and 100kHz filters, and just left the 40MHz strip alone...

I' quite satisfied up to now, however one thing that makes me thinking is the BFO: when I switch it on, the meter jumps nearly at full scale. So I thought it must inject too much power into the detector. Then, I connected the spectrum analyser at pin 7 of valve V21, and I found 100kHz harmonics up to more than 5MHz, all almost equally powerful up to 1MHz and then progressively reducing. Everything goes if I switch the BFO off.

Do you think this behaviour is normal? It would explain the extra power.

The BFO control works and the trimmer does as well. All harmonics shift when adjusting the control or the trimmer.

Also, I found that dismounting the BFO unit it is not easy especially because of the three wires underside. I did not try yet... maybe someone found a more clever way of servicing it?

Thanks for reading, any help is appreciated.
Paolo
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Old 7th Jun 2008, 10:58 pm   #2
Phil G4SPZ
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Default Re: Racal RA17L BFO behaviour

Good evening Paolo and welcome to the Forum. Several regular contributors are familiar with the RA17 and its derivatives, and I'm sure you will receive help with your problem.

Without being familiar with this receiver, if the BFO is generating harmonics then its output must be badly distorted. The usual suspect components such as anode and screen resistors, feedback and cathode bias components should come under suspicion.

Good luck!
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Old 8th Jun 2008, 7:02 am   #3
paolo67
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Default Re: Racal RA17L BFO behaviour

Thank you Phil.

You are right, the output at the anode of the BFO valve is actually a badly distorted square wave. There are glitches when it gets flat at the top. The glitches are even more evident at the control grid.

The oscillator looks like a Hartley to me, so one would expect a certain level of harmonics. However in this case the circuit would work better as a harmonic generator

Paolo
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Old 8th Jun 2008, 2:46 pm   #4
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Arrow Re: Racal RA17L BFO behaviour

Hi Paolo & welcome to the Forum.

Yes, the BFO in the RA-17L is a Hartley.
The "badly distorted waveform" is not correct: hence the harmonics.

For the purposes of servicing it and fixing your problem, I suggest that you disconnect the three wires from their main chassis connections (making careful notes of where the wires go first, of course) and then withdraw the BFO assembly from the unit for a thorough overhaul.
Alternatively, you could make up simple extensions of these wires to test the BFO on the bench whilst it is operational.
The BFO as a unit is not a complicated affair. I suggest that you replace the two Rs at the anode of the valve (R132 1K; R133 4k7) and the two Rs in the grid feed (R126 100R; R125 47k). Similarly, the three HT decoupling Cs: C207 and C208 (0.05 uF each); C214 (0.1 uF) and C199 (220pF) in the grid cct.
[The O/P C (C215, 47pF) is unlikely to require changing - unless you want to increase the injection level - which can be worthwhile in giving improved resolution of ssb signals. I haven't tried this myself, but a change to something like 100 - 150 pF should help.]
If, having replaced those Rs and Cs, the problem remains - suspect the valve: CV138 = EF91.
By and large, this assembly is usually quite reliable - but considering the other defective Rs and Cs that you have found, a complete overhaul of the BFO assy. (as suggested above) would seem worthwhile.

If you have any more problems with your RA-17 - don't hesitate to come back. As G4SPZ has commented, there are several people here who have had considerable experience servicing these fine radios.

Good luck !

Al / Skywave.
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Old 8th Jun 2008, 8:36 pm   #5
paolo67
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Default Re: Racal RA17L BFO behaviour

Thank you Al.

I overhauled the entire unit as you suggested, but no improvement! I replaced every component including the valve with a NOS EF91. The unit uses a CV4014, this must be an equivalent of the CV138.

All the components were within spec. The only original elements are now the L and the two variable capacitors. The ohmic resistance of the L is 112+8, so no shorts apparently.

Actually the "worst" signal seems to be at the cathode: the 100kHz wave is superimposed by some very irregular, higher frequency oscillation. At the anode it is definitely a square wave, very flat at the top.

Now I will go through a deeper inspection, including bad wires/contacts...

Paolo
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Old 9th Jun 2008, 10:12 pm   #6
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Arrow Re: Racal RA17L BFO behaviour

Paolo -

Well, you seem to have a strange fault there. It sounds like the valve is being badly over-driven due to incorrect biassing.

The CV4014 is correct. Equivs. are CV138 and EF91.

I don't know if you have the manual to hand, but the d.c. voltages at the valve electrodes should be:
Anode 180 v dc;
Screen 210 v dc;
Cathode 0 v dc.

You seem to have elimininated the major components in the cct. - including the valve.
I assume that you know the history of this set and can verify that the BFO cct. hasn't been modified in some way - easy to check, since the cct. is quite basic.
Apart from measuring the dc voltages (as above) to see if that throws any light on the problem, then the next logical thing must be to suspect the BFO coil itself.

But let's see what others have to say on this before we seriously entertain going down that route.

Incidentally, I see that the BFO O/P cap. in the '17L is 47pF. In the forerunner (RA-17) the value is 10pF. I suppose this cap could be faulty - see what the waveform looks like when this cap. is disconnected from the rest of the unit. Also make sure that the valve screening can is in situ. (clutching at straws now )

Al / Skywave.
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Old 10th Jun 2008, 9:49 am   #7
paolo67
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Default Re: Racal RA17L BFO behaviour

Thank you Al.

Here how I progressed: I replaced the L! I dug in the junk box (there's probably more value there than anywhere else...) and I found a coil with approximately the same inductance as the original (about 18mH), same size and a tap! How lucky I am I thought.

The tap of the original is, based on the resistance, at 90% of the coil while the one of the replacement is at 50%. Also, the original coil is completely impregnated of some white insulation material (while the replacement is not). Another difference is the core: the original has a fixed core, the replacement is on air, although I could insert a core but it would significantly increase its inductance (8x).

The harmonics are gone (40 dB below the 100kHz signal) and waveforms look quite good at any point, especially at G1 it is nearly a perfect sinusoid.

Here the voltages I measured (the anodic voltage is 227V, the valve is the original CV4014):

Anode 222 Vdc
Screen grid 222 Vdc
Cathode 0 Vdc 500 Vac (pp)
Control grid ? Vdc (unmeasurable with my equipment) 600 Vac (pp)

So the A-K current is then (227-222)/5.7k = 29uA. According to your figures, it should be (210-180)/4.7k = 6.4mA.

The output after C215 is approx. 2.7 Vpp when the BFO is in circuit. I tried several values for C215 with no significant change. This is a direct effect of the small A-K current I think.

Most important, the BFO works quite poorly: although I can tweak the trimmer to have 100kHz, it is impossible to decode any SSB signal, just no way.

I've been surprised to find such a high voltage in the tank. It would explain why the original coil is so accurately insulated - to prevent arcing.

Now, my guess is that the position of the tap is making the difference here. However, it is an oscillator, probably one of the most critical kind of circuits, and it is far beyond my knowledge to re-calculate proper values for the components.

A poor man idea would be to try a lower value for R125 (or R216?) to get less negative bias at G1. However, I'm worried of the already very high voltages there...

Paolo
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Old 10th Jun 2008, 6:31 pm   #8
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Arrow Re: Racal RA17L BFO behaviour

Hi -
There's a lot of data in that last Post of yours; for the time being I'll limit this reply to three issues.

The thought has crossed my mind that if you have recently acquired this radio and perhaps don't know too much about its history, the coil may have been taken out at some time and re-fitted - and re-fitted incorrectly - so that the cathode tap is closest to the 'top' of the coil, instead of the 'earthy' end. Under these conditions, I would expect the cct. to behave exactly as you have described; too much positive feedback and the valve being over-driven, etc., etc.

The white insulation material is simply Racal's method of keeping damp / moisture out of the coil.

As regards the inability to resolve ssb transmissions, (and assuming that the BFO is on freq. at 100 KHz and that the IF amp. does not have a poor response curve), the usual cause of this with BFO injection to an AM / envelope detector technique (as used here) is simply insufficient BFO injection (via C215). At present, I can't find my records to tell me what level to expect - but off-hand, 2.7 v peak-peak sounds far too small. I would have expected something like 20 v p-p. The small level of injection that you have could be due to the substitute coil you are using. Which takes us back to my first paragraph - as above.

Al / Skywave.
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Old 10th Jun 2008, 8:04 pm   #9
paolo67
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Default Re: Racal RA17L BFO behaviour

Hello Al,

got you. No, it was correctly fit, that meaning the tap close to the "earty" end.

I am *very* new to the receiver, I got it from eBay for just 100 Euro. The auction had just 80 visits, I suspect the seller placed it in the wrong category or something. Looked like it was lying somewhere for years.

Actually it came with several problems:

- the three wires below 2nd VFO were badly fitted - miscontact - producing random problems
- with 2nd VFO dismounted, I checked every component. ALL the paper capacitors were bad: brown/black colour, evident dielectric leak. Several resistors largely out of tolerance. Fortunately, no problems with the frequency determining components. The 2nd VFO is PERFECTLY aligned in frequency
- deafness at any band above 500kHz - fixed after alignment of 1st IF and replacement of the two paper capacitors that showed dielectric leak. This filter looked just crazy at the beginning
- 100kHz IF checked and realigned. I own an old HP141T 110Mhz with its also old tracking generator (nixie!) that make things slightly easier
- 37.5MHz strip checked as per manual. The drive at TP3 is OK
- harmonic generator and mixer checked work fine not yet dug deeply there
- preselector roughly aligned with tracking generator, peaks everywhere on all bands. Amazing sensitivity! Do you know why Racal did not make it tracking? Other receivers of the same age and class have it. BTW the preselector is an essential element: I live very near to a hugely powerful 900kHz transmitter, without preselector I can hear it at 5.400 for example
- calibrator dead. Also here, I overhauled it completely, now it works fine (easy job here...)

Apart from BFO, the receiver is extremely accurate and enjoyable to me. The 6.5 and 13kHz filters make it perfect for HF and MW BCL. I did the procedure for overall receiver gain measurement and it exceeds specs (A3).

Of course, I am at the very beginning with it. Everything looks new and there is a lot to learn... but this is why I bought it!

Paolo
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Old 10th Jun 2008, 9:06 pm   #10
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Arrow Re: Racal RA17L BFO behaviour

OK Paolo: all points in your above Post noted.

I cannot think of any further helpful comments on your BFO problem.

Re: the other / general comments on servicing this receiver: if you haven't already done so, there are a lot of entries on this Forum detailing repairs & measurements that I and others have done and the results so obtained. Put them all together - and you can get a lot of useful info.

Persevere with it - IMHO the Racal RA-17 (& its derivatives) is one of the finest valved comms. receivers ever built.

Al / Skywave.
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Old 11th Jun 2008, 5:55 am   #11
paolo67
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Default Re: Racal RA17L BFO behaviour

Thank you Al for everything. Your suggestions were vital for me.

Obviously we discovered that the BFO coil is defective on my unit, I'll try to find a better replacement for it, possibly the original component.

Paolo
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Old 11th Jun 2008, 4:12 pm   #12
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Default Re: Racal RA17L BFO behaviour

Keep an eye on E-Bay. I have seen RA-17 BFO assemblies appear there from time to time.

Al / Skywave.
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Old 12th Jun 2008, 6:02 pm   #13
paolo67
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Default Re: Racal RA17L BFO behaviour

Here how I decided to temporarily fix the problem while waiting to find a replacement:

I removed the white insulation material and discovered that the tap of the coil - which is made of litz and machine-wounded so impossible to "rework" - was partially disconnected.

I fixed that and then the opposite problem arose: insufficient drive. The coil is definitely defective

Here what I did: I inverted the two ends of the coil. Of course now the valve is over-driven. To compensate that, I replaced R126 with 1.2Megs Now I have approx. 15Vpp after C215 and the correct dc voltages at anode and screen. Waveforms look quite sinusoidal. I found the value experimentally: smaller values produce distortion, with higher values the feedback is insufficient.

One point I noticed is that the alignment to 100kHz must be done when the main variable capacitor is half way. This is actually quite obvious because the oscillator must span +/-8kHz. Anyway, there is another stable 100kHz position when the capacitor is fully open.

Point 5.4 of the manual says "...the meter reads approximately half f.s.d" at zero beat and the system switch in "Check BFO" position, and this condition is met. To me, this means that, when receiving with BFO on, the meter will never read less than half scale.

The listening test is quite satisfactory - although not exceptional: very large SSB signals cannot be decoded, somewhat acceptable because they can be handled using the MAN position.

A special thank to Al for his support. I am now looking for a BFO unit, if eventually someone in the group would have one beyond his needs please let me know.

Paolo
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Old 13th Jun 2008, 7:31 am   #14
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Thumbs up Re: Racal RA17L BFO behaviour

Hi - yes, it's me again

I'm very pleased to see that you have found a temporary fix - and also that you were not afraid to do the mechanical detective work on the suspect component. Quite a few people would have said "right, it's got to be the coil. I'm not going any further with this until I get a new one!". And the receiver just sits there - and waits - and waits. To delve into a component such as this requires a certain amount of bravery, since there is always the risk that you might make matters worse in so doing (speaks from bitter experience)

As regards the problem of resolving strong ssb signals on the Racal, it is my experience that this is about normal: hence, the use of the IF gain control in the MANual op. mode - as you have found. It must be remembered that the RA-17 was designed in the years when ssb transmissions were far less common than they are today. Plus, Racal did make a range of external 'bolt-on' ssb adapters for this receiver - which, I feel, proves the point.

Anyway, well done & good luck with it.

Al.
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