CRT SS7900 Audio Issue

Hi all, 

I've already had a good look on the web and on here for an answer but can't find anything so your input would be appreciated. 

I have a new SS7900 which makes a quite loud "hash" type noise even with the squelch closed. Also, when in scan mode you can hear an oscillator or clock chip running, a sort of ticking sound. 

It does this in all modes, on all frequencies, with antenna connected, disconnected and on dummy load. 
It's currently on a 10amp linear PSU but still does this when connected to a switch mode. 
I've tried it in garage, down the garden in the shed and it's currently in the home office. 
There's no other kit running when this is on, and my HF gear doesn't pick up any significant RF or mains interference.


It can be stopped simply by keying the mic, but it starts again as soon as the squelch is broken by a signal, or you press a button. 

The first unit was returned to the distributor who replaced it without a problem but the new one still does this. 
They have now offered a refund but it's a good rig other than this. 

I've been in radio for over 30 years but i'm stumped. 

Has anyone else got one that does this? 

Thanks, Steve

Have you tried running from a car battery.
That would be the final test and if it still does it then send it back.

Hi Steve,

Have you got all the controls whacked right up, (fully clockwise)?
i.e. is the Volume and RF Gain at full?

As you may be aware most modern electronic devices, (unless there is a muting system on the amplifier), produce 'hash' noise when the volume is full. You hear all the electronic noise from the circuitry. With most 'squelch' circuitry although the signal path is reduced it is not fully muted and the audio amplifier usually remains at full gain.

We also had a discussion recently about 'RF Gain' in that most will set it to the highest level and not always the best thing to do. The CRT radios have quite an open front end and pretty over-driven RF gain stage so will pick up a lot of noise even from it's own circuitry(!)


Try adjusting the volume down to a comfortable listening level when you have a received signal and keep it there, then adjust the RF gain down to about half way or even two thirds.

Let us know how you get on with that.


All the best,
Victor

Now been tested on battery and still does it. 

RF gain and volume to min and you can still hear the hiss when squelch is closed. 

Is this something these radios do but no else bothers about? 

Rig itself seems to work fine, albeit only running on a temporary indoor antenna (Genuine WA Modulator) until this is resolved.

I bet you find your problem is because the antenna is inside,  the original WA Modulator is very good mobile antenna proper old school and really still needs to be outside on a good ground plane

Steve,

I've had the same radio from new, I've tried this on multiple antennas as well as a Dummy load as you have mentioned previously and never experienced any of the issues you have described.

Have you tried as Victor mentions reducing the RF gain but at the same time increase the squelch control clockwise to deactivate the ASC?

Also, are you still hearing the noise if the RF meter is showing no signal received? 

73's

Graeme.

Hi All, 

It does it even with no antenna or dummy load, with RF Gain and Volume completely down. 

It just sits there hissing even with squelch on. It only goes silent if you briefly key the mic then it's ok until the squelch opens and closed, then it starts again. 

I would be happy to let someone listen if they wanted to call me, or I have an audio clip I can send. 

Radio seems to work fine, just doing my head in not being able to cure it.

May have to return it but it's the second one now. Must be an answer?

Steve,

Sounds very strange indeed, before you look at returning it, a picture / video they say is worth a thousand words.

Could you video the issue and let the guys on here see what it is doing before you pack it and ship it back?

Hopefully someone will have another option to try.

Regards

Graeme.

Hi Graeme, 

that would be great. 

I have an audio clip and can do a video but not sure how to upload to here.

Wow!  How annoying is that?! No wonder it's driving you mad

The only thing I can think of is the ASQ, auto squelch setting. Details are on page 18 of the manual.

If your manual isn't to hand there's a link to one as follows :-

SS7900 Manual

Maybe worth changing that setting and see how it goes?

Well Steve,

That certainly said a thousand words.

I have just tried my own 7900 and can confirm it is exactly the same, a quiet little hiss that I have never noticed before as my PSU fan was always that little bit louder.

Without seeing your video, I had the impression from your description it would be similar to the hiss from the squelch but no, it is a lot softer and in the background.

I'm guessing that it may be a trait of this particular model, sorry I can't be of any more help.

Regards

Graeme.

Righto Steve,

Had another play with my CRT 7900.

The noise seems to be evident when the Monitor Gain (NOG Function 2) is operated, as soon as this option is above 0 or OFF, the hiss remains and does not disappear after keying the mic. BTW, don't set it to 32 and key the mic or you are likely to scare the living daylights out of the dog (Guess How I know   )

What I did find is that if I insert an external speaker or use headphones, the Hiss is not evident at all?
Perhaps one of our expert guru's can help with an explanation on that one.

Hope that is of some help at least. 

73's

Graeme.

I got the idea from the video that the hiss was quite loud, but if your PSU fan can drown it out Graeme it's obviously far less obtrusive than first seems.

I like that you found out plugging in an extension speaker reduces or eliminates it. Possible cure?
(I must admit I run an extension speaker on most of my radios, the tiny speakers inside most leave a lot to be desired!)

Looking at the circuitry in this and similar radios there are a lot of paths tied into the audio chain. All the following microprocessor functions tie in to it :-

ECHO/MONITORING
SQ/ASQ
NB/HI-CUT
etc.

So probably worth playing with these function settings first.

I did start explaining some of the radio design, Nch MOSFETs Squelch, THD% in the audio chain etc. Then thought better of it
It can all become a bit argumentative(!)


If your radio and Steve's radio both do it, then perhaps it's inherent in the design? As you discovered and perhaps others too, no one ever noticed it before or doesn't bother them?

Got to admit my own HF radio still has a background hiss even when the squelch kicks in if the volume is loud enough. I can also hear the microprocessor noise on scan under the same conditions....but then I never run it that way usually and no one else ever complained about it on this particular model

I only use squelch on VHF/UHF FM, never on HF SSB and don't usually frequent UK FM CB so I've never noticed it before. But I'm not going to let it bother me



Hopefully either you find a cure, (Graeme's extension speaker cure sounds good), or live with it.

Steve can obviously return it as he mentioned but I think you'll find fault/noises/quirks in pretty much every radio if you look/listen hard enough.
(Some Hams put radios in Faraday cages and listen for 'birdies'!!! Yes, as mad as it sounds!)


All the best,
Victor

Firstly I really appreciate the feedback on this guys. 

I've tried the settings as per latest suggestions but no different, the noise is still present and this includes on external speakers. However that's given me a cunning plan, extension speaker with separate volume control, so I can reduce the background hiss which is actually noticeable in my home-office environment. As the rig's volume control has no effect on the background noise, all I need to do is whack it up a bit to compensate for the extension speaker.

Other than that it's going back, any thought on what to replace it with, thinking President Lincoln 2 V3 perhaps? 

Thanks as well for inspiring my first You-Tube video lol.

Hi Steve,

Pity we were unable to eliminate the issue entirely for you.

If you were looking at an alternative to the 7900, I believe your idea of the Lincoln 2 V3 is a decent choice.

If this is the latest model also known as the Lincoln 2 plus the radio is all mode (AM/FM/SSB/CW) and covers the full 10m band as standard (28.000 - 29.700MHz) plus 12m (24.880 - 24.990MHz). It can be expanded for continuous coverage of 24.715 - 30.105MHz (text borrowed from a known dealer site) another radio worth considering is the President McKinley, very good reviews with similar properties to the Lincoln


73's


Graeme.

Just took a quick shifty on the Lincoln II v3 schematics....

Similar circuitry to the 7900 funnily enough......

HOWEVER,

There is none of the N-MOS/Echo circuitry and a clever addition to the Audio output chips feedback/gain circuitry.
If you hear low hiss or microprocessor noise on that bleeder then you've got superhuman hearing or a very fussy sod

I wish you the best with whichever the way you go.

Oh, and well done with your first YouTube vid. Here's hoping you haven't got a cat


All the best,
Victor

If you really want to change your radio because of that hiss, which IMHO didn't really or wouldn't bug me but that's me , you wont like the Lincoln 2 ,I remember when they first came out first off it was a insult to the original icon President Lincoln of the 70s/ 80s plus the Lincoln 2 when they first came out basically where crap there are videos on YouTube to prove my point that's why there has been so many different versions of it trying to get it right which as far as know they still haven't, me I would go for the Mc Kinley it's a far superior radio plus its proper Uniden made who made all the old original President radios back in the day .

I can vouch for the president McKinley,I have had mine for about two months, and now i,v got to know all the functions,it's a great radio. I have EXPANDED  mine which couldn't be easier just a matter of cutting a wire loop,and moving a link.

I get 26watts on FM and about the same on USB.The RX is brilliant with some nice filters etc. All in all a VERY good radio.

Redsprite.

and in the video when tx the mic the chans was going up there is a prob my self take it back and get your money back its a
new radio and it should be better , we have one its my young bros and his sits there and very quiet hope you get sorted and were did you buy the radio from,

Steve,

What was your final decision? 
Has the 7900 gone back again or have you decided to put up with the quirks?

Regards

Graeme.

Hi Graeme, 

at present CRT China are looking into it thanks to Simon The Wizard. 

In the mean time I've now bought a President Mckinley which is excellent. 

I'm keeping the SS7900 for now as I like the flexibility of programming and being able to change frequencies more easily than the Mckinley's SPAN mode, but if they offer a solution then it will go back for exchange or repair as the hiss is still a pain in the a***. 

All that said, there is naff all on around here anyway. 

Steve

Glad to hear it is being looked at Steve.

Fingers crossed they are able to resolve the issue for you.

Please keep us updated, not just for my own 7900 but for others that may come across this post in the future.

73's

Graeme.

Hi..i am new to the group and decided to join after finding this link through Google.
I am having the same problem with the hissing noise.
Only had my SS7900 for a few hours, but i will be investigating the problem over the next week or so.
I shall be looking at the firmware and the circuit design for any possible glitches.
Will let you chaps know what i find.

OK I will admit I've never owned one of these radios , but 1. Did you buy them from the same seller?, if so they may have brought a bad batch and dont know, and 2. It maybe the nature of the radio they all do it .