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Old 6th Jun 2020, 11:36 am   #1
Edward Huggins
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Default HACKER VHF "Herald" RP37A

Well this is my 2nd HELP posting in two weeks and my 3rd ever.
I have a near mint VHF Herald which has never failed from new. My version has the silver, large magnet, ELAC speaker fitted. It's untouched, the inter-station muting pot never altered.
The RF section is fine with super gain and mercifully tunes down far enough for Classic FM. The AF stage is loud and distortion free. There are 2 new PP9s in place. There are no AM components to worry about.
Now here comes the but.
BUT, there is a massive background mush that has just now developed.
Over the years, and learned from my Father, I've always described as "IF Noise". This is unaffected by the volume control. It badly intrudes over any received signal.
Under a glass, the board looks perfect, no loose components. These have been sensitively prodded (under power) with a wooden spill power and all seems secure....
Now there are real Hacker specialists out there that will know far more than me, but I'm wondering if the BC148 or either the 2 x BF194s in the IF section of the Board have failed? Or....?
Put simply, Edward (who has always wished he had learned more of his Father's RF and IF troubleshooting skills) needs some help!
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Old 6th Jun 2020, 11:43 am   #2
paulsherwin
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Default Re: HACKER VHF "Herald" RP37A

Mark is the real Hacker expert here, but this may well be the dreaded Lockfit problem. The easiest approach is simple substitution. The transistors are all general purpose RF/AF silicon types so you should have subs available. Watch out for the leadouts.

Don't replace Lockfits with NOS Lockfits! Use non-Lockfit equivalents.

I am now highly suspicious of Lockfits and tend to whip 'em out when I have a radio in pieces if it's an easy job, even in the absence of a fault.
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Old 6th Jun 2020, 11:47 am   #3
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Default Re: HACKER VHF "Herald" RP37A

If the noise is unaffected by the volume control, it surely must be from the audio amplifier and not the IF section.
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Old 6th Jun 2020, 11:49 am   #4
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Default Re: HACKER VHF "Herald" RP37A

Yes, it should be easy enough to establish if the fault is in the audio or IF section.
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Old 6th Jun 2020, 11:56 am   #5
dazzlevision
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Default Re: HACKER VHF "Herald" RP37A

Most likely a noisy transistor, but not necessarily a Lockfit - I've had noisy Germanium ones and (without looking at the circuit - not to hand) both types are probably used in the audio stages.
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Old 6th Jun 2020, 11:58 am   #6
paulsherwin
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Default Re: HACKER VHF "Herald" RP37A

From memory the output pair is Ge and everything else is Si Lockfit.
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Old 6th Jun 2020, 2:11 pm   #7
Edward Huggins
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Default Re: HACKER VHF "Herald" RP37A

Quote:
Originally Posted by Ambientnoise View Post
If the noise is unaffected by the volume control, it surely must be from the audio amplifier and not the IF section.
Well, that makes perfect sense - I just assumed it was pre-audio stage as it sounded so much like a possible IF stage fault. I'll go and poke around the seperate AF board whist it's under power. Many thanks to all so far.
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Old 6th Jun 2020, 2:40 pm   #8
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Default Re: HACKER VHF "Herald" RP37A

It would also be worth exercising the two presets on the audio amp panel between the batteries.

Nice sets. My parents bought theirs new in 1971 and still use it daily, though it had a decade off due to a dry joint in the tuner which the experts couldn’t locate.
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Old 6th Jun 2020, 3:10 pm   #9
Edward Huggins
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Default Re: HACKER VHF "Herald" RP37A

I'm kicking myself for not going straight to that naughty AF Board!
I've tracked down Mark Hennessy's excellent site "Transistors in Vintage Radios".
Under section 2.1, there's coverage of Lockfit problems and, serendipitously, uses the Herald's AF Board as an illustrative example the problem of inherent noise. Nickthedentist above mentions the 2 pre-sets on the board, which are picked up in the Hennessy piece, as well as the 1st and 2nd AF stage Lockfits being likely culprits....now where's me magnifying glass.....?
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Old 6th Jun 2020, 4:36 pm   #10
Edward Huggins
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Default Re: HACKER VHF "Herald" RP37A

Well an almost 100% result! Following the speedy input from Members here and my additional findings on-line, I adjusted the pre-set (VR3) which operates on the emitter of T1 (first stage BC108) on the AF board by a good 1/3 rd of a turn clockwise and it certainly has cleaned up the noise. Of course it will have now upset the operating voltage and as a result the gain is reduced by about 10/15% and with just a hint of distortion on music peaks. So I need to use more volume (and there's plenty left to crank) so it's an effective enough partlal solution. I'm sure this is a very common problem to our esteemed Hacker experts, but to those that have never needed to attend to their Herald's before, I hope it this will help them as it has me.
In the end, I guess the two BC108 (1st and 2 stage AFs) Lockfits will need to be replaced and then the correct voltage settings re-established. My thanks again to all who have contributed this afternoon!
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Old 6th Jun 2020, 5:37 pm   #11
steve.seven
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Default Re: HACKER VHF "Herald" RP37A

Hi Edward,

Just had a similar issue with my RP37A, very annoying rustling noise with the volume control at minimum setting.

Found the problem to be a poor contact of the wiper on the pre set mid point variable pot RV3.

After moving the wiper back and forth and cleaning the track with Servisol switch cleaner problem solved.

If you need to reset the mid point setting you need to adjust RV3 to obtain a reading of 8.9 Volts measured at the junction of R14 & R15 on the A205 AF amplifier board, this figure need to be set with a supply input voltage of 18.00 Volts.

Hope this may be of help to you.
Regards,
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Old 6th Jun 2020, 7:15 pm   #12
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Default Re: HACKER VHF "Herald" RP37A

I would certainly change the BC148s. They aren't at all critical - anything NPN from the BC108 family will do, as will the 2N3904 and many other jellybean types. You can use pretty much any small signal Si type you have to hand. You will need to reset the quiescent current afterwards of course, but that's a good idea anyway after doing any work on radios of this type.
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Old 6th Jun 2020, 9:39 pm   #13
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Default Re: HACKER VHF "Herald" RP37A

I've just replaced a BC148 with a 2N3904 in my Herald this evening.

It had started motor boating so I just replaced the electrolytics on the audio board. Job done and sounding good. Put board back in and no signs of life. Would you believe TWO problems. One new capacitor had gone dead short robbing first transistor of HT, and I'd caught the same transistor in tightening the little plastic nut nearby. All it takes to send a lockfit to the underworld. They are not destined for heaven.

David
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Old 6th Jun 2020, 10:32 pm   #14
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Default Re: HACKER VHF "Herald" RP37A

Quote:
Originally Posted by Edward Huggins View Post
In the end, I guess the two BC108 (1st and 2 stage AFs) Lockfits will need to be replaced
BC108s are not lockfits. BC148's are lockfits. I doubt the BC108's will need replacing. I had trouble with one of the AC128's in mine....think it was driver or pre-driver. That was giving intermittent crackling. Just replaced it with a new AC128.

If you need to replace BC148 you can use BC548 although as stated, they are not critical.
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Old 6th Jun 2020, 11:21 pm   #15
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Default Re: HACKER VHF "Herald" RP37A

Bit late to this (been busy all day), but glad my webpage helped

Yes, on occasion I've had T4 (AC128) go intermittent. Many would describe it as the driver, probably because many older text books did, but it's more accurate to call it the VAS (voltage amplifier stage). This circuit doesn't have drivers, as the VAS drives the output pair directly, but if there were additional transistors between those two points, they would be called "drivers". I'll get me coat!

In addition to the transistors and presets, some of the front end resistors can go noisy. I think I mention that in the article...

Sounds like several people have had a more productive day than me!
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Old 7th Jun 2020, 12:14 am   #16
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Default Re: HACKER VHF "Herald" RP37A

Quote:
Originally Posted by Radio Wrangler View Post
I've just replaced a BC148 with a 2N3904 in my Herald this evening.
At least you broke it rather than letting it die of old age

Seriously though, they are bad news. If you have the boards out and can get at things easily you should replace them. It costs next to nothing.
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Old 7th Jun 2020, 12:37 am   #17
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Default Re: HACKER VHF "Herald" RP37A

You've got me worrying, Paul. I had the thing on the bench and the scope on it and I wondered about replacing the second stage, but didn't. Those snap-in pins were awkward to get out. Aye, I think I'll swap it tomorrow.

David
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Old 7th Jun 2020, 1:03 am   #18
paulsherwin
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Default Re: HACKER VHF "Herald" RP37A

If everything is working normally then you might as well leave things as they are, given that the Lockfits may last for decades. My point is simply that once you have the radio in bits in front of you, you might as well change the dodgy transistors.

If you melt the solder on the pins and rock the transistor it will come out easily enough, even without a solder sucker.
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Old 7th Jun 2020, 9:06 am   #19
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Default Re: HACKER VHF "Herald" RP37A

Quote:
Originally Posted by Radio Wrangler View Post
I've just replaced a BC148 with a 2N3904 in my Herald this evening.

It had started motor boating so I just replaced the electrolytics on the audio board. Job done and sounding good. Put board back in and no signs of life. Would you believe TWO problems. One new capacitor had gone dead short robbing first transistor of HT, and I'd caught the same transistor in tightening the little plastic nut nearby. All it takes to send a lockfit to the underworld. They are not destined for heaven.

David
Motorboating can be caused by poor connections at the multiway plug/socket.
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Old 7th Jun 2020, 9:57 am   #20
Philips210
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Default Re: HACKER VHF "Herald" RP37A

I am looking forward to seeing Mark Hennessy's instrument for assessing the noise generated by the Lockfit transistors.

Regards,
Symon
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