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Old 30th May 2020, 2:44 pm   #1
Backtoreality
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Default Hacker Mini Herald RP17

I've had a Mini Herald RP17 for a few years now and when I first got it I stripped it down and replaced all of the dodgy capacitors, Hunts and electrolytics. I also replaced the AF117s with 2N3906 silicon transistors. The radio worked but the volume had to be full on to hear anything so after testing the AF117s I replaced Tr 1 with the only good one. The radio worked but was still very quiet. I have since bought a load of germanium transistors from a seller in Germany, AF127s, AF137s, AF186s, AF200U, and a couple of large packs of Soviet transistors from the Ukraine so I thought I'd remove the AF117 and silicon transistors.
Having checked a load of AFxxx transistors, with my Chinese tester, I seem to have gains ranging from 50 to over 200 and I would like to know if I should try to use specific high, medium or low gain transistors in specific points of the front end of the circuit ie. Tr1, Tr2 and TR3.

Also a year ago I bought a scrap front end just to get a cream button for the switch bank of my radio, and I wonder if there was a mini herald which did not use the middle button as an on/off switch as my scrap chassis has all 3 switches interlinked and the middle button pops up when either of the other buttons are pressed?
Regards,
David
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Old 30th May 2020, 6:52 pm   #2
Paul_RK
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Default Re: Hacker Mini Herald RP17

Quote:
Originally Posted by Backtoreality View Post
...and I wonder if there was a mini herald which did not use the middle button as an on/off switch as my scrap chassis has all 3 switches interlinked and the middle button pops up when either of the other buttons are pressed?
That's what happens with the original RP17. The RP17A has the central button as an isolated on/off switch, and is a slightly larger set, using two PP9s rather than the RP17's PP7s. The amplifier of the later version is completely different, and physically markedly shorter, which helps accommodate the larger batteries.

Paul
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Old 31st May 2020, 11:01 am   #3
Backtoreality
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Default Re: Hacker Mini Herald RP17

Well I'm pretty sure mine is the RP17. It uses two PP7 batteries and has a transformer driven output stage with a metal rectifier between the output transistors. Maybe just a later version before the 17A?
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Old 31st May 2020, 11:25 am   #4
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Default Re: Hacker Mini Herald RP17

Now working well on Medium Wave with three AF137s in place, no selection of gain done I just banged them in. Two Erie resistors replaced which had gone high. Nothing on LW at all so I think I'll check the waveband selection switch next. Two photos attached.

I'm still wondering about gain selection of the transistors for my scrap chassis which I want to get working so I can compare the two. I'm going to use 3 old stock Soviet transistors but I need to replace the capacitors first.
David
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Old 1st Jun 2020, 12:08 pm   #5
mhennessy
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Default Re: Hacker Mini Herald RP17

Honestly, I wouldn't worry about the current gain of the transistors - it really doesn't have the effect on signal gain that many people seem to believe.

The signal gain is about 40 times the collector current, multiplied by whatever resistance is present at the collector. So, nothing to do with current gain, or hfe. Ok, hfe has an effect on input impedance, but how much difference that makes depends on the overall circuit - usually not a big deal in practice.

For a "quiet" radio, the first thing to check is the gain of the audio stages - details are given in the manual, but basically a 1kHz tone at 20mV (assume RMS; manual doesn't specify) should result in 600mW at the speaker. As I measure volts, not watts, I reckon that's 3.9V RMS, which implies a pretty health gain of 195, or 46dB. I can dig out one of mine to verify that if necessary.

Next, I would "treat" any AF11Xs that are present in circuit. I know that not everyone approves, but I haven't had any whiskers return in at least a decade of doing this. For a set like this, where things are easy to access, I wouldn't mind having to "re-treat" the transistors now and again, but naturally if these are hidden inside a Mullard module, then I'd be more likely to swap them out.

It's worth saying that I have had sets that were absolutely fine on MW but dead on LW, and zapping the oscillator transistor fixed the problem. Weird...

Then I would look at the AGC arrangements, and check the associated capacitors haven't gone leaky or open. There's two in the RP17, of course.

With other Hacker sets, I've had problems (especially on LW) with the rail decoupler. Definitely worth a look.

Then I'd take a look at the general DC conditions. As transistor signal gain depends on collector current (which is how AGC works), this is important. The manual gives some nominal voltages.

Finally, I would go though the alignment. On an AM set it's fairly easy, but it's obviously best if you have access to test gear. If doing this gets a large improvement, you know "the Phantom" has been there before you - normally the improvements are minimal in my experience.

If this still hasn't fixed it, then there's something more tricky going on. Resistors going high might have been picked up by the DC conditions check, but perhaps not all. I've had problems with capacitors inside the IFTs - usually the glue on the yellow tape corroding the leads. If there's evidence of someone else playing with the IFTs, check they haven't cracked the slugs. Check the ferrite aerial hasn't been cracked and repaired. Dry joints and dirty switch contacts hopefully go without saying.

But I've yet to come across a Hacker that didn't come good in the end. And they are well worth the effort. I admit that I've a soft spot for the RP17
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Old 5th Jun 2020, 4:59 pm   #6
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Default Re: Hacker Mini Herald RP17

Thanks for that info Mark!
I've spent the last few days working on the scrap front end I mentioned in my first post. I replaced all of the electrolytics, a Hunts capacitor, one Erie resistor that had gone very high and all 3 AF117s which had been removed by the previous owner. I used 3 Soviet germanium transistors (n416G ?) from a batch of 50 I bought a couple of years ago.
I connected the "scrap" front end to my RP17 amplifier and turned it on. With the amp on half volume I got some very loud crackles while tuning so I looked at the variable capacitor and noticed that the fixed vanes on the underside were moving and shorting out against the moving vanes. I carefully pressed them into place and by trial and error got them in a position where they didn't short. I then superglued them into place.
Tuning the radio on MW with the volume on full brought in a very quiet station so I looked at the coils and noticed they all looked as though they had been played with. Adjusting the oscillator coil brought the station volume up to the point where I had to turn it down so I adjusted the first IFT and the signals came in loud and clear across the band. Success!
That made me go back to my original front end so I plugged it in to the amplifier and adjusted L4 (oscillator) with the radio tuned to Radio 5 Live. Adrian Chiles loud and clear! Looking at the core of the first IFT L5 showed that someone had been at it, the core was broken at the top! Little bits of core were lying around in the top of the coil. I carefully removed the bits and the remains of the core and replaced the core with one I had in store. I peaked R5 live again and tried the LW. There was nothing across the band until I got above 1700m when I heard a very quiet Irish accent. RTE 1 LW! It was at the wrong end of the band so I went back to the MW and discovered that R5 Live was miles away from where it should be on the dial so I alternately adjusted the variable capacitor and L4 until it was at the correct wavelength.
I then went back to the LW and found that RTE 1 was nearly at the opposite end of the scale where it should be so I tweaked L4 again (very slightly). Radio 4 came in loud and clear next to 200m and all of the stations on the MW were in the right place with good volume. The radio sounds better than ever in the time I've had it.
Next step is back to the 'scrap' front end to check the positions of the stations on each of the wavebands. I don't have a 'dial' plate for it so my next step is to make one from a piece of clear acrylic and mark it with frequencies/wavelengths.
David
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