Hi,

I've recently bought a rig, and it seems to work pretty well except for a really annoying hum, I can't seem to find what the issue is! I've tried it into a dipole at 1:5:1 and a dummy load, I've tried with audio connected and without, and in different locations but it's always there.

When I first power on the hum is loud for a second or so, it then dies off completely for another second before gradually re-appearing over another second or so and then staying continuous.

I've attached a couple of pictures, I've heard that the gear is good so I'm sure it's nothing (in fact probably something I've done wrong, but in fairness I haven't done anything except connect an antenna!)

Another issue is it's slightly off frequency, so it's 0.05 MHz off frequency, it comes in tuned and stereo at distance at .5 instead of .0, this kind of board doesn't have dip switches so am unsure how I would tune back up to the required frequency.

Any help is much appreciated!

Sounds like mains hum (50hz).

I'm not particularly good at Electronics so may well be off the mark, but the first thing I'd do is put screening in between the Toroidal Transformer and the Exciter and amp stages.

Another possibility is that the supply isn't up to providing sufficient current for the rig, or the supply is a little dirty with poor smoothing, etc'.

I'm having difficulty seeing the power supply part of the rig from my phone so can't really see the circuitry.

Had a couple of these come to me with similar probs, it's a regulation problem to exciter. Give it it's own regulator and hum should go away, also replace left right cables with screened cables. If you move antenna away does hum quiet down? And do you get hum when lid is removed from box?

Also remember the position the black volume pot is in on the exciter pcb, turn it to zero and see if it tunes to .0 on frequency? Could be stereo knocking it out slightly

All your tests should be done in to a dummy load. If you connect an aerial indoors you'll definitely get a bad hum. It's quite common for people to try stuff in their bedroom and think it's not working properly but the cause is too much RF radiated by an antenna close by.

Are you 100% the frequency is 50KHz off? Have you checked the frequency while it's on the load?
I would not play with the volume too much because the stereo level is quite crucial, but like Mr Berlin says can appear to do funny stuff with the frequency if incorrectly set.

I'm fairly sure I know who built that and I'd also be surprised if the driver does not have it's own voltage regulator or that it would be sold humming but you never know!

Plus shouldn't that driver be screened in it's own little box?

teckniqs wrote:Plus shouldn't that driver be screened in it's own little box?

Not really, those type are pretty immune. Lots of people use them in rigs without screening with no issues. The doubler is the key here.

Of course for best practice you're correct it should be screened but in reality they work fine without.
You'll also see pictures of driver boards right next to the PA without screening and people will jump up and down saying how bad it is but in reality it can work just fine if the right circuit is used.

Thanks for your responses so far. I would love to be able to fix this up myself but I think it's gonna be too difficult. Anyway since I bought new I'm gonna email the builder and ask what he can do for me.

Analyser wrote:Not really, those type are pretty immune. Lots of people use them in rigs without screening with no issues. The doubler is the key here.

What is that exciter? Haven't seen that exact one before.

It looks very similar to London builder 'N's' drivers from back in the late 90s but possibly slightly larger with a few additional components???

Yes perhaps... the doubler looks familiar!

Although the PA says 'Enigma' it also looks like the way N builds things, I recognise some of those particular components - especially that Sony Digital cheap Chinese copy of the BLF244 which were in his rigs driving 1407's about 15 years ago!

I hadn't looked at the PA, but yes it does look like a little brother of the famous BLF244/SD1407 design, with those striplines.

At first glance of that rig I must have assumed from the size/shape of the board, that the exciter was one of the Dutch ones and then didn't look at it in any detail. It's clearly not. This one's a half frequency design (probably - I can't read it) based on an MC145170 programmed on a single frequency by what looks like a pic12C508 or whatever it was back then... It's all classic N.

teckniqs wrote:Although the PA says 'Enigma' it also looks like the way N builds things, I recognise some of those particular components - especially that Sony Digital cheap Chinese copy of the BLF244 which were in his rigs driving 1407's about 15 years ago!

Gotta correct you techniqs

S ony were not chinese

They were made for S @ cricklewood and N, they had to order something like a thousand pieces to get good price so teamed up, then when you order that many you get to print what you like on them. So S at cricklewood and the lads did S ONY... notice it has a space.. The rest of the letters and and numbers were randomly picked by each person.. They were actually made by semilab and a very good transistor! This is going back years and years

Enigma looks like the way N builds...... hat off to you hahaha.. You nearly solved the puzzle there!

RF-Head wrote:

Did you sort out his hum?

Wonder if the hum's anything to do with parking the unshielded inputs to the stereo coder, its unshielded active filter stages and its power supply about an inch from the PA output?

About hum... If transformer screw is not properly tightened, it can produce phisical vibration which can actualy modulate carrier by vibrating oscillator coil. Somebody explained this on the old forum in thread about printed coils in NRG oscillator...


Sent from my iPhone using Tapatalk

True but:

Uksoundz wrote:When I first power on the hum is loud for a second or so, it then dies off completely for another second before gradually re-appearing over another second or so and then staying continuous.

So sounds like the power output (and therefore input) are changing over the VCO lock-up period, which is leading to either an RF induced effect, or an issue with the PSU regulation.

Not sure really, but I sent it back beginning of last week, he seems fair so hopefully he'll get the hum fixed and it on frequency. He said it'll take around a week and then get it sent back.

I'll keep you posted!!