Hacker VHF Herald RP37A - Alignment Instructions

[Bristol603]

Dear All,

I have an RP37A upon which I am preparing to perform an FM alignment. I have never done an alignment before and I read with nervousness the warnings of how it is easy to make matters worse rather than better.

I have the service manual for the RP37 and this includes an addendum for the RP37A. The RP37A has a T309 tuner rather than a T305 as used on the RP37. The service manual addendum says the specification and test procedures are the same between the two models. However, the FM tuner alignment instruction step (i) refer to adjusting L4 to obtain maximum output when injecting a 90MHz signal with the pointer aligned to 90MHz. Problem - the T309 tuner does not appear to have an L4! Are there different alignment instructions for the T309?

In practice, the pointer alignment at 90MHz is spot on. With a 98MHz injected signal, the pointer is at 98.3MHz - close, but not correct. I can see TC1, TC2, L1, and L2, which are used for all the other steps (ii), (iii), and (iv). Before I touch anything I would like to know if the instructions in the RP37 service manual will work for the RP37A other than step (i). Looking at the circuit diagrams, it seems L3 in the T309 of the RP37A is the equivalent of L4 in the T305 of the RP37 and so I should simply adjust L3 in step (i) when aligning the T309 instead of adjusting L4 on the T305. Has anyone done this already?

Any other tips before I start turning the settings?

Thank you.

[mhennessy]

RF alignment is really easy. Often, you have to do it on a set with no instructions, but it's quite possible to figure out what does what.

It breaks down into 2 basic steps:

1. Oscillator adjustments (to get the pointer accurate)
2. Tuning adjustments (to optimise the sensitivity)

But before doing either of these, get the pointer datum right! This is the mechanical adjustment of the pointer on the string, and will be wrong unless it's been recently done. On just about all Hackers, with the tuning capacitor set to maximum capacitance, the pointer should align accurately with the right hand edge of the apertures on the tuning scale (note the RP37 manual is vague on this point). A lot of people dive in and do the adjustments without attending to this first.

Taking the oscillator first, this is a coil and a capacitor. The coil is adjusted near the bottom of the scale (perhaps 90MHz), and the capacitor is adjusted towards the top (perhaps 98MHz). The numbers aren't critical, and I often use 88 and 100MHz.

These adjustments will interact, so you have to repeat them, hopefully making smaller adjustments each time. With practice, you learn to deliberately mis-adjust each one, knowing the other one will pull both. Stick at it though.

In the case of the RP37A, the oscillator coil is, as you say, L3, and the capacitor is TC3. I just look for the set closest to the oscillator/mixer transistor TR2, if that helps... If wrong, it's obvious because the oscillator adjustments are very "touchy", unlike the others.

Having done that, there's no need to touch them again. Don't accidentally do it (we've all been there!).

The remaining adjustments are for sensitivity. As before, coils for the LF end, capacitors for HF. Like the oscillator, they interact, so you have to repeat each end a number of times. At each end, peak both coils or caps - note that each of those will interact. All good fun!

Needless to say, don't touch the IFT on the tuner module. In theory, you should do the whole IF strip - including IFT1 on the tuner - before doing the RF stages, but in practice, the IF seems to be a lot more stable than the FM adjustments, and I only run through it if there's good reason to do so. To do IF properly, you need a wobulator or equivalent.

Good luck!

Mark

[Bristol603]

Thank you. I will be brave and give it a try. As you say, the pointer datum was 0.5MHz out before I reset that. I wonder why that wanders?

[mhennessy]

I suspect the cord stretches. Sliding the adjustable pulley by the right amount to restore the jockey wheel bracket to its correct vertical alignment usually fixes the pointer datum at the same time.

[Bristol603]

I have just had a try, but the sharpened pieces of plastic I was trying to use to turn the inductors aren't up to the job. I have ordered a proper set of trimmer tools and will try again when these arrive rather than risk damaging anything by not using the proper tool.

I am glad you say the IF adjustments can be left alone, as that does remove some extra possibilities to make matters worse rather than better.

[Bristol603]

I have now done the RF alignment and learnt a few lessons.

The CK alignment tools I purchased were not up to the job. The slot in the inductors is narrow and they were reasonably stiff to turn. The result was that the tiny plastic end of the adjuster soon sheared off. I ended up using a jeweller's screwdriver - I could not adjust whilst watching the response as the metal screwdriver influenced the inductance. Instead I did a sequence of measure, power off, adjust, power on, measure, power off, ... until I found the maximum point. Slow, but it did work out.

I found the first adjustment of TC3 and L3 to get the tuning scale properly aligned took a while, the adjustments were sensitive and do interact with one another. Thank you for the tip of adjusting one beyond the desired point so the when you adjust the other the first is pushed back to the correct spot. This made converging on the correct adjustment for both ends together much easier/quicker.

Having done this first adjustment, I adjusted TC1, TC2, L1, and L2. I found the measured output was much, much less sensitive to these adjustments and the output was at maximum with these adjustments as they were already set. Is that lack of sensitivity to these adjustments typical?

The final result is that the scale is now properly aligned across the full range. The sensitivity has improved as I can now capture an additional 6 clean signals compared to the original 6 I could capture in my test just before I did the alignment. In practice all but 2 of these are BBC national signals from non-local transmitters for stations I already had, but I do now have the option of 2 additional local stations.

Overall - not as tricky a job as I had feared. Thank you for your help.

PS - can anyone suggest a source of more robust plastic alignment tools?

[Radiocruncher]

Well done on aligning one of these. I did one recently that was way out of alignment and it took ages to get it right. As for alignment tools, I use ceramic screwdrivers that were made by Draper. I did a search but as usual they are no longer available. Try this link 8PCS Adjust Frequency Screwdriver Anti-static Plastic Ceramic Set Slotted and Phillips 90MM Color Blue https://www.amazon.co.uk/dp/B01MRGT5..._drEhzbFJRC2FX. A Google search for ceramic screwdrivers will bring up lots of hits.

Regards

Graham

[Bristol603]

Dear Graham,

Thank you for that link. I have ordered as set of ceramic screwdrivers.

Nigel.

[mhennessy]

Sorry for the delay in responding; I've been away with just a phone for access...

Sounds like you got there in the end. I agree that good plastic tools are difficult to find. Mostly I use small metal ones - very carefully, as the core is obviously brittle - but it's not ideal. There's no need to turn the power on and off - just ignore what happens while the screwdriver is near the coil.

Yes, the sensitivity adjustments are much less critical. You have to use very low levels from the RF generator to optimise them, but often they are fine where they are. Makes sense, given what changes over time is the oscillator.

[Goldieoldie]

If the cores are stiff, I little IPA will help to penetrate and act as a lubricant it will then of course evaporate
If still not turning heat the core a little with a tip of a soldering iron.This will help especially if there's any wax around
Cheers Pete

[Bristol603]

Thank you for those tips they are very useful.