Hacker RP38A Hunter - weak/sibilant FM - UK Vintage Radio Repair and Restoration Discussion Forum

richieyork · 26th Jul 2019, 1:42 pm

Hi all

Quick question: Acquired an RP38A - tatty but complete - car boot find. Has been converted to mains (two transformer PCB's, fullwave rect on each and cap smoothers - each producing 9v).

Powers up fine and DC voltage is correct at 18V or thereabouts.

MW/LW performance is strong and clear. FM is very insensitive and sibilant and lower volume than AM. It also 'hums' when tuned into the centre point of a station. No lively hiss between stations as i'd expect..

I've thoroughly cleaned the wavechange switches with contact cleaner and they are working correctly.

The set is riddled with Lockfits - my suspicion falls on them (i've ordered up some MPSH10's to try out) and/or C27 discriminator cap.

Most of the electrolytics are the blue sleeved variety (like Philips branded??) - i'd always thought these are stable with age or am I wrong? Happy to replace all...

Before I start to dismantle and shotgun replace the lockfits/electrolytics do my suspicions sound correct

I don't have suitable equipment other than multimeter to sort this issue out, so any known stock faults that could cause this problem id appreciate knowing.

Thanks all,
Rich

Mike. Watterson · 26th Jul 2019, 4:16 pm

14V to 16V is more ideal as 18V is peak fresh batteries.

If the FM issue was a lockfit then it likely wouldn't work. Also they are not like AF117s, it's poor assembly stressing the leads and causing an internal wire fracture is the majority of lockfit faults.

1) Make sure DC is 14V to just under 18V, not over it.
2) You need more than a DVM/DMM otherwise you'll be pointlessly changing parts.
3) Mostly the blue sleeved Electrolytics are OK, but they can dry out. Since it's "drying out" you can just use jumper wires etc to "hold" a good capacitor across one. Try the discriminator electrolytic first.
4) Don't tweak any coils / presets without suitable test gear.
5) I'd not at all "shotgun" except with paper dielectric caps in battery or mains valve sets.

ronbryan · 26th Jul 2019, 5:16 pm

If you check the polarity of the voltage across the discriminator electrolytic with your multimeter, when tuned into a local FM station, you will find that the positive voltage is applied to the negative terminal on the capacitor - i.e. the silk screen polarity marking is incorrect and the electrolytic is reverse biased. If you fit a new component, it makes sense to connect it such that the capacitor is connected the correct way round, disregarding the silk screen marking.

It is worth checking that one of the two germanium FM discriminator diodes is not faulty. You should be able to easily check this if your multimeter has a diode test facility.

Ron

G6Tanuki · 26th Jul 2019, 5:28 pm

Yes, I'd be looking into the discriminator - as mentioned, the electrolytic capacitor *should* be wired in the non-instinctive sense (+ve to ground) and if either or both of the diodes have 'aged' they can make tuning/recovered-audio distinctly odd and the AFC act like it's home to a poltergeist.

paulsherwin · 26th Jul 2019, 6:22 pm

It's not the case that lockfit problems result in a dead set. They produce all sorts of strange symptoms when they fail. They may not be responsible for this fault, but I would change them while you have the set in pieces as it's a cheap and easy job. Do them one at a time in case there are compatibility problems, and take care over the leadouts.

26th Jul 2019, 1:42 pm	#1
richieyork Pentode Join Date: Dec 2010 Location: York, North Yorkshire, UK. Posts: 212	Hacker RP38A Hunter - weak/sibilant FM Hi all Quick question: Acquired an RP38A - tatty but complete - car boot find. Has been converted to mains (two transformer PCB's, fullwave rect on each and cap smoothers - each producing 9v). Powers up fine and DC voltage is correct at 18V or thereabouts. MW/LW performance is strong and clear. FM is very insensitive and sibilant and lower volume than AM. It also 'hums' when tuned into the centre point of a station. No lively hiss between stations as i'd expect.. I've thoroughly cleaned the wavechange switches with contact cleaner and they are working correctly. The set is riddled with Lockfits - my suspicion falls on them (i've ordered up some MPSH10's to try out) and/or C27 discriminator cap. Most of the electrolytics are the blue sleeved variety (like Philips branded??) - i'd always thought these are stable with age or am I wrong? Happy to replace all... Before I start to dismantle and shotgun replace the lockfits/electrolytics do my suspicions sound correct I don't have suitable equipment other than multimeter to sort this issue out, so any known stock faults that could cause this problem id appreciate knowing. Thanks all, Rich

26th Jul 2019, 4:16 pm	#2
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Hacker RP38A Hunter - weak/sibilant FM 14V to 16V is more ideal as 18V is peak fresh batteries. If the FM issue was a lockfit then it likely wouldn't work. Also they are not like AF117s, it's poor assembly stressing the leads and causing an internal wire fracture is the majority of lockfit faults. 1) Make sure DC is 14V to just under 18V, not over it. 2) You need more than a DVM/DMM otherwise you'll be pointlessly changing parts. 3) Mostly the blue sleeved Electrolytics are OK, but they can dry out. Since it's "drying out" you can just use jumper wires etc to "hold" a good capacitor across one. Try the discriminator electrolytic first. 4) Don't tweak any coils / presets without suitable test gear. 5) I'd not at all "shotgun" except with paper dielectric caps in battery or mains valve sets.

26th Jul 2019, 5:16 pm	#3
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Hacker RP38A Hunter - weak/sibilant FM If you check the polarity of the voltage across the discriminator electrolytic with your multimeter, when tuned into a local FM station, you will find that the positive voltage is applied to the negative terminal on the capacitor - i.e. the silk screen polarity marking is incorrect and the electrolytic is reverse biased. If you fit a new component, it makes sense to connect it such that the capacitor is connected the correct way round, disregarding the silk screen marking. It is worth checking that one of the two germanium FM discriminator diodes is not faulty. You should be able to easily check this if your multimeter has a diode test facility. Ron Last edited by ronbryan; 26th Jul 2019 at 5:24 pm.

26th Jul 2019, 5:28 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Hacker RP38A Hunter - weak/sibilant FM Yes, I'd be looking into the discriminator - as mentioned, the electrolytic capacitor should be wired in the non-instinctive sense (+ve to ground) and if either or both of the diodes have 'aged' they can make tuning/recovered-audio distinctly odd and the AFC act like it's home to a poltergeist.

26th Jul 2019, 6:22 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,966	Re: Hacker RP38A Hunter - weak/sibilant FM It's not the case that lockfit problems result in a dead set. They produce all sorts of strange symptoms when they fail. They may not be responsible for this fault, but I would change them while you have the set in pieces as it's a cheap and easy job. Do them one at a time in case there are compatibility problems, and take care over the leadouts.