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Old 7th Dec 2018, 8:16 pm   #1
martin.m
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Default Hacker Sovereign RP18. Poor AM results

Can anyone help me solve a problem with a Sovereign RP18? VHF reception is fine and the sound is excellent, well up to the usual Hacker standard. However AM is very poor and seems to be lacking in sensitivity. The volume control needs to be turned up further than usual and the sound is slightly distorted and lacking in bass as though the stations are slightly off tune. I haven't tampered with the alignment but don't know the radio's history.

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Martin
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Old 7th Dec 2018, 8:20 pm   #2
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Default Re: Hacker Sovereign RP18. Poor AM results

It appears to have seperate RF/IF strips for AM and FM, it also has the AFXx series transistors in those strips.
Quick voltage checks on those transistors or check for shorts to the can.
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Old 9th Dec 2018, 12:26 am   #3
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Default Re: Hacker Sovereign RP18. Poor AM results

Thank you Frank. Thinking logically, the fault must be on the AM RF/IF panel. When I have more time I will replace the AFxs with BF450s (one at a time) and hope that cures the problem. I have a few Hacker radios and the AM performance is usually excellent.

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Old 9th Dec 2018, 2:30 am   #4
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Default Re: Hacker Sovereign RP18. Poor AM results

Just a warning that the use of BF450s as ideal AF11x replacements is a bit of an old wives tale. Yes, they will probably work, but are no better than many of the cheaper alternatives and worse than some. Search the forum for lots of opinions about AF11x replacements.
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Old 9th Dec 2018, 7:26 pm   #5
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Default Re: Hacker Sovereign RP18. Poor AM results

Thank you for the advice. I have never tried a BF450 but managed to buy some from an Ebay seller based in China for a bargain price (about £3 for 20 including postage). I have used a humble BC557 in a Perdio transistor radio and it worked fine. There has been a rise in price for the "vintage" transistors we use in our hobby and I would rather use a germanium transistor but feel that £6 for an AF127 etc is a bit steep.

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Old 9th Dec 2018, 7:33 pm   #6
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Default Re: Hacker Sovereign RP18. Poor AM results

Have a read of this web page, Mark contributes to this forum so he may be along to advise but it’s all in the web page.
https://www.markhennessy.co.uk/artic...ransistors.htm
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Old 9th Dec 2018, 8:21 pm   #7
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Default Re: Hacker Sovereign RP18. Poor AM results

These IF strips aren't very demanding and lots of PNP transistors should be fine. Designers didn't use AF117s because of some esoteric characteristic, it was simply that they were the standard RF transistor at the time.
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Old 9th Dec 2018, 9:00 pm   #8
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Default Re: Hacker Sovereign RP18. Poor AM results

With regard to Germanium transistors, I haven't checked lately, but Russian R.F. Transistors were available from various eBay sellers for £1 or less each in packs of maybe 10. These have leads about 25mm long, making it easy to fit them in place of AF11x types in most radios.
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Old 9th Dec 2018, 9:11 pm   #9
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Default Re: Hacker Sovereign RP18. Poor AM results

Quote:
Originally Posted by martin.m View Post
Thank you Frank. Thinking logically, the fault must be on the AM RF/IF panel. When I have more time I will replace the AFxs with BF450s (one at a time) and hope that cures the problem. I have a few Hacker radios and the AM performance is usually excellent.

Regards
Martin
Why assume that the transistors are faulty?

As the audio quality is good on FM, that indicates that the audio stage is working well enough, so yes, the poor audio when switched to AM does point to the AM RF/IF stages being suspect. But given that the AM RF/IF stages are working after a fashion, to my mind, it's much more likely to be electrolytic caps that have developed a high ESR/have lost capacitance, or carbon composition resistors that have gone high. Personally, I'd check the capacitance and ESR of C17, 19 & 20, which can be done in a jiffy with them in circuit with the set switched off.

AFxx transistors (in this case AF117s) generally short to the screen and stop working so you'd get no output to the audio stage at all if either TR1, 2, or 3 have failed. Snipping the screen often cures the fault, if perhaps only temporarily.

It's worth noting that the list of possible faults in the RP18 service data states:

Quote:

"Distortion on AM: Check the operation of AGC and the overload diode CR1'"

End quote.

(CR1 is an OA79 germanium diode).

To quote from Mark Hennessey's site where he goes into much detail about his experiences with AFxx transistors, he heads the page thus:

Quote:

"Of course, after 40 plus years of life, most electrolytic capacitors are suspect and the carbon-composition resistors have drifted high (and sometimes become very noisy)".

End quote.

Though in this case, the audio stage appears to be working well, the RP17 audio stage uses 12 electrolytic caps which - given the many decades they've been in service - if I owned one and it was one that I was going to keep, I'd be wanting to check those out, but I can well understand that if a set sounds good, most owners will want to leave well alone. Working on PCBs with a high component density can be quite challenging. The reason that Hackers sound so good is that they were built up to a standard - not down to a price, but the downside to that is they have a high component count.

Fortunately, Hackers come with copious service data - in the case of the RP 18, eighteen pages of it! Maybe that's how it got its name?

Hope that might help a bit.

Good luck with it.
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Old 10th Dec 2018, 12:48 am   #10
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Default Re: Hacker Sovereign RP18. Poor AM results

Frank, thank you for that link. It is a very informative read, especially the part about "fixing" AFxx transistors that have been affected by tin whiskers.

David, you have given me plenty to think about. The Hacker is one that I have been fixing for myself and I did change the "Daly" electrolytics on the audio panel as I came across one that was short circuit on another radio. I will have a closer look at the AM IF board when I have more time. The component count doesn't seem to be very high and I can make some cold checks. I didn't know that some multimeters can measure capacitors in circuit. The old school teaching used to be that "the only check for a capacitor is substitution". My own meter has seen better days and I may treat myself to a new one soon.

Thank you all for your comments and advice.

Regards
Martin
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Old 10th Dec 2018, 12:21 pm   #11
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Default Re: Hacker Sovereign RP18. Poor AM results

It's not worth "fixing" the AF117 etc, too much tin on inside of can on certain batches or production runs. They'll fail again. Some are fine. I've replaced with Russian "top hat" RF transistors in one Hacker RP18 AM IF. The FM strip was fine (AF114 I think).
I tried OC171, Silicon PNP 2N3906 (the emitter resistor on IF and L.O. meant Bias didn't need changed) and the same Russian Germanium RF on a Bush TR130 (version with 208 button), not any noticeable difference. It's DC coupled audio stages that are most critical, they need compatible parts. The LO & IF amps, especially with emitter resistors are not so critical. The FT is the most critical. In terms of gain, the AF11x parts had quite a wide spread of gain. Comparing the average may be misleading. Also transistor gain varies with bias (which is why AGC works), with a peak at a "medium" current. It drops both towards cut-off and towards saturation.
If it's working at all on AM then it's unlikely to be the AF117s. If there is no distortion on FM, then it's a fault on the AM IF strip, not the Audio amp.
The AM IF strip is far easier to work on than the Roberts R600 (dreadful wiring to switches and hard to remove Mullard Module). The R700 isn't as bad as R600 but still very much harder than the Hacker RP18. It's quite easy.
I tested my Hacker RP25A (dead audio) with the audio board from the RP18 as it is "compatible" and easy to unplug. The RP25A had a dead "Lockfit" transistor (Maybe BC148. Common when the PCB holes are not correct as a wire will fracture inside moulding with the stress, no doubt because the PCB was for a BC108. A 2N3904 worked fine (it's approximately the same as TO92 version of BC108 / BC148). The Lockfit was a short lived concept for machine insertion, as were the resistors with flat pins instead of leads. Autoinsertion soon managed almost any part on paper/card bandoleers. Fascinating to watch and very noisy (1984). Even in early 1970s the Surface mount was coming in more widely. The place I worked in 1980s started introduction in 1986. Much higher density and faster placement. PCBs "silk screened" with glue before placement. The smaller alumina substrates in late 1960s and early 1970s used screened on solder paste instead of glue, then ovens.
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Old 11th Dec 2018, 5:55 pm   #12
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Default Re: Hacker Sovereign RP18. Poor AM results

Thank you all for your comments and advice. I will put this one on the back burner for now and get a "Round Tuit" early next year. I don't sell radios or repair them for anyone else so there is no pressure but I am keen to discover the cause of the fault.

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Martin
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Old 8th May 2019, 12:01 am   #13
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Default Re: Hacker Sovereign RP18. Poor AM results

I finally got round to having a closer look at this radio today, removing the A.M. board completely for further inspection. I took out the three AF117 transistors and found that two of them had low resistance readings from the metal can to the collector pin. This model is "negative earth" so the shorts would have shunted the I.F. transformer primary windings. I fitted three new germanium AF178s and reassembled. This restored normal reception on MW and LW. The Sovereign RP18 is a nice sounding radio but I find it a little difficult to tune in on VHF as the AFC is on all the time. Once again thank you all for your advice and comments.
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Old 8th May 2019, 1:22 am   #14
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Default Re: Hacker Sovereign RP18. Poor AM results

The manual states the AFC can be switched off with the quiet control, The AFC setup is in the manual and also a modification used on later models, its just a wire link from what read.
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