Another Hacker RP33 with tin whiskers

[stevedee]

I'm a little embarrassed to admit that I can't remember how this radio came into my possession. It was probably just left in a house or flat that I bought in the mid to late 1980s. Its followed me from house to house, and was very lucky to have survived our last move 15 months ago when we finally down-sized, as so much of my stuff, that I could not give away, ended up in landfill.

Its not in great shape, but neither is it a wreck. Unfortunately the plastic face-plate has a 1" crack running from the tuning dial hole to the side of the plate. And the woodwork needs a bit of attention. The black cloth looks very dull and the set has small dots of white paint, probably ceiling emulsion!

As I don't have any of those big 9V batteries kicking around, I powered the radio from a black plug-in 16V power supply. Initially, all I could hear was a whining sound, rather like a small toy police car.

I was already aware that the AF117 transistors had a tendency to grow whiskers, tin whiskers. So the first challenge was to source replacements. I probably had a few of these AF117 transistors 15 months ago, But I donated all my 1950s/60s transistors to Mister Transistor: http://www.wylie.org.uk/technology/semics/history.htm

No matter. I'd read somewhere (maybe here) that silicon PNP general purpose transistors should work, so purchased a mixed box of 200, from which I selected four BC557B.

The first AF117 that I replaced within the IF screening can had signs of post assemble re-soldering, although I'd seen this kind of re-work before on new equipment, so may have been done during final test, way back in the 1960s.

As I removed each of the IF transistors one-by-one, none seem to have any case-to-terminal leakage. I finally took out the mixer/oscillator and found that was the hairy one!

With all four replaced (i.e. T1, T2, T3 & T4) the set fired up and I could tune in Smooth Radio & Talk Sport. But it was still noisy with the toy police car coming through, even with the volume turned right down. I don't have a circuit diagram, but it looks like this set always has a trickle of sound, so I disconnected the volume wiper and confirmed that the audio stage was 'clean'.

By now I was pretty sure that these noises were due to the black, plug-in power supply, so dug out an old 12V lead-acid battery (previously used to power our bird box) and gave it a quick charge. Sure enough, the radio is practically silent in between stations. Operation is marginal on 12V in that it seem to work at about 13V, but then stopped as the voltage dropped below 12.5V. (I need to retest this).

I am not sure how to power this radio. I think a pair of PP9 batteries cost around £15 which is not easy to justify for occasional use. The 16V power supply that I initially used will probably need quite a bit of work to silence! But for the sake of a couple of big electrolytics + resistor (or diode) maybe that's my best approach.

The cord drive has broken so I need to replace this, and I managed to find a drawing which shows the correct 'run' around pulleys.

So for the moment, I just have a few questions

Back in 2016/7, when I refurbished our Bush VHF55, I bought cord from some guy on eBay. Can anyone suggest a reliable source?

I don't have a circuit diagram for the RP33, let alone a service manual. So can anyone help me with any additional info & circuits?

I do have a circuit which I believe shows the audio circuit board used in this model. The identity on my audio board is A/204/4.

The rf/switch-bank board is A/33/14. The IF board is A/33/10.

Thanks once again for any advice.

P.S. I would have liked to have included some photos, but could not see how to do this.

[Mark Hennessy]

The service manual can be found for free on the Hacker Radio Group: https://groups.io/g/Hackerradiogroup

If you've not used groups.io before, there is a bit of a learning curve, but it's not too bad once you're used to it. The manuals are in the "Files" section. I scanned the RP33 manual for the group.

In general, many Hackers but especially the earlier ones are quite fussy about the power supply, and don't work well with switched-mode power supplies. Big caps won't work - you need a linear power supply and a voltage regulator like an LM317T or a 7818.

The easiest way to use batteries is to hollow out a couple of dead PP9s and install AA battery holders that take 6 cells. You can fit two of those in there if you want, so you can have a series/parallel arrangement to lengthen the time between replacements (or re-charges if you want to go down that route). For occasional use, AAs give reasonable life, especially on an AM-only set like this.

Watch out for the wire-wound pre-set on the audio board - I've found them to be quite unreliable.

As part of the various anti-spam precautions, new users are unable to add attachments while they are still under moderation. However, that restriction should have lifted for you now.

You can add photos using the "full" reply interface (not the "quick reply" box at the bottom of the thread that I'm using now). Depending on which theme you are using, the button you need to access the full interface is either "Reply to this topic" (green button) or "New Reply" (black button). Unless you've changed it, it'll be the former of those two. Once there, you'll see the box relating to attachments below the area where you type the reply. You can drag and drop files or browse for them.

The file size limits are fairly generous here, but you might need to down-size images if they're straight from your camera. Limits are discussed here: https://golbornevintageradio.co.uk/forum...p?tid=4835

[ppppenguin]

I can't add much to what Mark has said - he's one of the UK's Hacker gurus - but even if you don't have some old PP9s to provide the cases any old assembly of 12x AA cells will work. May not be as elegant but it will do fine. If you make your own mains PSU, any reasonable transformer and rectifier will do the job, giving at least 20V unregulated. A quick and dirty trick to make 18V regulated is a few diodes or a couple of LEDs in the 0V leg of a 7815 (which many of us will have in the junk box) will work. It won't be very well regulated and the short circuit protection will be compromised but that doesn't matter here.

Also pleased to hear that BC557B worked well instead of AF11x. This raises the question: Is it worth trying to replace AF11x with germanium devices at all? There are plenty of very ordinary silicon devices such as the BC55x series and 2N3906 that should do a good job for pennies. In theory, some re-biasing might be needed. Is it required in practice?

[Refugee]

I use necklace cord for tuning.
You can get it from most places that sell sewing and knitting supplies.

[stevedee]

Thanks for your feedback guys.

I've joined the Hacker Radio Group and now have a copy of the service manual.

Just ordered a couple of AA x6 battery holders, so when I have the radio running on 18V I'll be able to measure emitter voltages in the oscillator & IF stages and compare with the quoted values for AF117 transistors.

DC current gain for BC557B seems to be 5 or 6 times higher than AF117.

Sensitivity and S/N ratio is amazing on this set compared to my feeble Bush VHF54.

And here are some photos...

   

   

   

   

[ppppenguin]

The DC current gain of a transistor is not a very important parameter. It just needs to be sufficient so it doesn't draw too much current from the base bias arrangements. If a designer is foolish enough to make his (or her) design strongly dependent on DC current gain they should rethink their design.

Contrary to a common preconception, a transistor in common emitter mode is voltage controlled. Not current controlled. The base current is simply a nuisance that you have to design around.

[stevedee]

I've just installed new AA battery holders, fitted 12 x AA and powered it up.

Wow! What a great little radio the RP33 is?

I can see now that my Bush VHF54 really needs a lot more work to bring it anywhere near the performance of this Hacker.

I have soldered the power leads together and used terminal blocks for insulation & strain relief.

   
   

Here are a few voltage readings which may be of help to others:-

• Full supply: 19.3Volts @14.4mA (no signal), 16-18mA (with music), 20mA (with very loud hum from led desk light)
  

• RF HT: 7.88Volts
  

• T1 emitter: service manual 900mV, with BC557B 561mV
  
• T2 emitter: service manual 900mV, with BC557B 608mV
  
• T3 emitter: service manual 900mV, with BC557B 584mV
  
• T4 emitter: service manual 1.55V, with BC557B 1.384V
  

• T1 base: service manual 1.05V, with BC557B 1.11V
  
• T2 base: service manual 1.00V, with BC557B 1.21V
  
• T3 base: service manual 1.00V, with BC557B 1.21V
  
• T4 base: service manual 1.85V, with BC557B 2.01V
  

   

[PerdioPal]

One reason for two batteries is that is is very unlikely to connect both backwards and blow up all that expensive germanium. Nice idea.

[stevedee]

So just to close the loop on this one, I've now reassembled my Hacker RP33 Autocrat and its working well on both LW & MW.

   

What grabs you straight away is that the control knobs are wrong. Clearly a smaller diameter than the originals and without a marker line for the volume control, they also sit at slightly different heights. This indicates that they were probably fitted at the same time as the volume/switch assembly was replaced (i.e. this nylon shaft has been cut down to suit the replacement knob).

Also, the body of this replacement volume/switch must also be longer than the original because one of the switch solder tags was virtually touching the IF can, with a small strip of black insulation tape in between. So I've now filed it down a little.

   

The new battery holders fit into the case reasonably well, with a few strips of draft excluder to stop them wandering too far.

   

I replaced the cord drive with a length of 0.6mm thickness cord I found on eBay. Its made from nylon ...yes, I know! Didn't notice that when ordered, so lets see how long it lasts.

I didn't spend too long cleaning this set, so it has a lot of what Flog It! call patina, including white paint ingrained in the fake leather covering. And the other aspects of the various mods & alterations (including a wonky aerial socket) are now part of this sets history ...I tell myself!

   

If I carry the set upstairs, I can just pick out Radio Caroline on 463m (648kHz) within the background noise!