Panasonic RX-CS700 - faulty FM Stereo Reception

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MyOhMy

MyOhMy

Member (SA)
Hi All,

I received a Panasonic RX-CS700 today and the FM stereo reception is behaving rather oddly.

There is full stereo reception along the full length of the FM band and the 'FM Stereo' light indicates stereo reception throughout the full length of the band BUT, whenever any & all FM broadcasting stations are tuned into, the reception changes to 'mono' and the 'FM Stereo' light goes out. This seems to be doing the exact opposite of what it should be doing: stations in mono and stereo only in between stations!

At this stage I don't know if this is an easy fix or a Return To Sender job so any help would be appreciated. Reception on LW/MW/SW bands are working well and function normally.

If this issue is unlikely to be remedied by some basic servicing or simple fixing and could, more likely, be a more technically involved issue then I'm reluctant to open the box in the event the seller blames me for causing the problem by opening the box.

What I have done so far:
Worked the front controls thoroughly with no difference being made.
 
MyOhMy

MyOhMy

Member (SA)
toshik said:
FM RF stages adjustment needed.
Click to expand...
Thanks for the very rapid reply, toshik!

I don't even know what that means but it sounds very technical, like those words may be used alongside words such as 'calibration', 'circuit testing machines' and 'frequency responses' - and I'm pretty much out of my depth with these as well! :blink: :dunce: :huh: :w00t: :sad:

I'm pretty good with garden plants and propagation though! :yes:
 
Reli

Reli

Boomus Fidelis
So you are saying the stereo light activates only when there's poor reception? Sounds like it needs alignment, which requires certain diagnostic gear as well as the service manual with the alignment specs.
 
MyOhMy

MyOhMy

Member (SA)
Reli said:
So you are saying the stereo light activates only when there's poor reception? Sounds like it needs alignment, which requires certain diagnostic gear as well as the service manual with the alignment specs.
Click to expand...
Thank you, Reli - when I add this to the in reply from toshik it adds up to scary & technical stuff that wasn't disclosed to me so I'll be sending the box back at warp-speed! :yes:

Many thanks to y'all for the prompt replies.
 
Reli

Reli

Boomus Fidelis
It's probably the most common fault that sellers don't disclose. They just say the tuner "works". Not volunteering info is the same as lying IMO.
 
MyOhMy

MyOhMy

Member (SA)
Reli said:
It's probably the most common fault that sellers don't disclose. They just say the tuner "works". Not volunteering info is the same as lying IMO.
Click to expand...
I agree although I've been quite lucky so far. Being economical with the truth riles me.

Anyway, I made some vids of the problems, seller made a full refund to me and doesn't want the box back so........................I've disclosed to all & sundry the FM problem, there's also a broken speaker retaining clip (fixable, I'd guess) and a dodgy tape deck that may only need belts - so it's available, in whole or part, for anyone who's has an interest for free as long as any postage costs are covered. I'm always pretty broke but it you're worse off than me then I'll pay the postage. I can't be any fairer than that and if I could fix the FM problem it'd be a keeper.
 
BoomboxLover48

BoomboxLover48

Boomus Fidelis
In the radio circuit there are several components that need to be preset to specs for it to receive signals the right way. Manufacturers of these gadgets based on components when they integrate it to the circuit, locks down these values. In many cases a small change in variable resistor can affect these RF values. Coil adjustments are made by turning the core. A minor drift in specs can cause misalignment.

I have tuned up tube radios but never did any work on transistor types. My Great hobby buddy walked me through the process. Sadly he passed away a few years back. He was an engineer with Philips UK. This guy knew it all. I was in depression for months because we interacted for years and I learned a lot from him.

I was teaching Chemistry and Physics at Universities in the early times of my career. Now I know nothing! I have to refer a lot to understand it. I only have good hands and lots of patience. Getting old... and useless!


We have lots of knowledgeable members here like Superduper , James.... and more...

They spend time to explain problems and it's remedy in detail which is good for this site and other members.
 
MyOhMy

MyOhMy

Member (SA)
BoomboxLover48 said:
In the radio circuit there are several components that need to be preset to specs for it to receive signals the right way. Manufacturers of these gadgets based on components when they integrate it to the circuit, locks down these values. In many cases a small change in variable resistor can affect these RF values. Coil adjustments are made by turning the core. A minor drift in specs can cause misalignment.

I have tuned up tube radios but never did any work on transistor types. My Great hobby buddy walked me through the process. Sadly he passed away a few years back. He was an engineer with Philips UK. This guy knew it all. I was in depression for months because we interacted for years and I learned a lot from him.

I was teaching Chemistry and Physics at Universities in the early times of my career. Now I know nothing! I have to refer a lot to understand it. I only have good hands and lots of patience. Getting old... and useless!


We have lots of knowledgeable members here like Superduper , James.... and more...

They spend time to explain problems and it's remedy in detail which is good for this site and other members.
Click to expand...
First paragraph:
Gotcha, I think! Re the 'variable resistor': how could a small change (possibly) have happened? Has 'Mr Tampering' been at work or are such parts at risk of failure/shock damage? If it's all on a PWB I had a fleeting thought of comparing the PWB from my RX-C36 parts box for a compatible spare part or two if that's all that's needed but, if the 'pre-set spec' is different/unknown, then that's a no hoper from the start.

I have no plans to do anything with this box other than keep it for spares. You never know, parts from the RX-CS700 may even get my cheapo RX-C36 parts/doner box on the road! :lol: It's a bit back to front but maybe worth a look see on a rainy day.
 
BoomboxLover48

BoomboxLover48

Boomus Fidelis
Trimmers can go bad from dirt, debris, or oxidation on contact points. If it is the tape section we can spray little deoxit , turn them a little, bring it back to it's original position for cleanup. Trim caps I just leave them alone!

As you can see some very vulnerable components are locked in place with molten wax. If Mr Tampering touched them they can cause lots of chaos. If he tried to squeaky clean anything in the RF and IF section he might have caused trouble. :-D :yes:
 
MyOhMy

MyOhMy

Member (SA)
ralrein1 said:
A picture of your new boomer would be appreciated.As you always take very good pics.
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It's not looking it's best at the moment - I think it spent a dirty night out! Seriously, it only needs alight clean as there's no stubborn grime, no battle scars just one or two very light scuffs which are small and difficult to see. The speaker grills need a little elbow grease or a paint touch up but that's about all.

I've had another play, I think the tape deck will be an easy fix and Line In works fine in stereo so all may not be lost. If I ignore (!) the lost stereo on FM it'll be a very good box for Line In, headphone use (but not stereo FM) and other waveband use. It'll be all it should be except for FM being mono. These small Pannies certainly have a sound quality that's not often found for the size of the box.
 
Superduper

Superduper

Moderator
Staff member
It is possible that your problem is an improperly functioning FM decoder chip and not an RF adjustment issue. Simply swapping out the decoder with a new one is cheap enough to try and within the abilities of many of the folks here, but I suspect this won't be any better news for you since it does involve desoldering an IC with fairly narrow pin pitch, ergo requires a bit of technical skill. On yours, virtually all of the tuner functions is dependent upon approximately 3 IC's. If your RF was out of alignment, you would observe very poor reception, or stations received not centered on the dial markings, (example, receive 98mhz when needle is pointing at 96mhz) or a situation where stereo is captured when the tuner is slightly askew from the "strong" part of the signal but when the tuner is adjusted to the strongest part of signal, then light extingquishes. That you are getting a functioning light throughout the entire band except when stations come in is highly unusual and not what I generally see on tuners slighly out of alignment. On a tuner that requires slight RF adjustment, what I would expect to see is a stereo light that only lights slightly off center from an MPX station, not on dead spots where there is clearly no stations broadcasting.
 
MyOhMy

MyOhMy

Member (SA)
Thanks, Superduper - I had to read that twice! :blink: I got the drift despite being thrown off a little by high-tech talk, I'm also not familiar with the parts/components yet as this is a whole new area for me and my remembery isn't good. I understand what you're saying for the most part as you've not only explained quite well (for ME) but also detailed some variances allowing for a more detailed definition of the problem. Cuh, and here's me thinking I described it all so well the first time - I've so much to learn!

What I need to do next is return to the physical FM tuning to check how the finer details of the tuning operation fit into one of your descriptions as closely as possible. I'll also set it next to my RX-CT800 to directly compare how close the stations are to the markings on the display in each tuning window to get a better idea of how far the indicator travels to effect a mono or stereo reception.

However I do this I'll be reading your post as I do it and posting here LIVE! :yes: :lol:
 
MyOhMy

MyOhMy

Member (SA)
Superduper said:
It is possible that your problem is an improperly functioning FM decoder chip and not an RF adjustment issue. Simply swapping out the decoder with a new one is cheap enough to try and within the abilities of many of the folks here, but I suspect this won't be any better news for you since it does involve desoldering an IC with fairly narrow pin pitch, ergo requires a bit of technical skill. On yours, virtually all of the tuner functions is dependent upon approximately 3 IC's. If your RF was out of alignment, you would observe very poor reception, or stations received not centered on the dial markings, (example, receive 98mhz when needle is pointing at 96mhz) or a situation where stereo is captured when the tuner is slightly askew from the "strong" part of the signal but when the tuner is adjusted to the strongest part of signal, then light extingquishes. That you are getting a functioning light throughout the entire band except when stations come in is highly unusual and not what I generally see on tuners slighly out of alignment. On a tuner that requires slight RF adjustment, what I would expect to see is a stereo light that only lights slightly off center from an MPX station, not on dead spots where there is clearly no stations broadcasting.
Click to expand...
"........a situation where stereo is captured when the tuner is slightly askew from the "strong" part of the signal but when the tuner is adjusted to the strongest part of signal, then light extingquishes.......".
Your description accurately reflects the situation, Superduper.

".....That you are getting a functioning light throughout the entire band except when stations come in is highly unusual and not what I generally see....."
This appears to be no longer the case for some reason! I've tuned/retuned/worked the radio over the last few days and the FM light is no longer on at all times, now only when next to a broadcast station (as in the first quote, above).

I've now taken a short video of the problem on one station only but this is indicative of all FM stations.
The station I'm tuning to is broadcasting on 88.8 FM, stereo indicator light is towards the bottom left of the screen:
https://vimeo.com/151254172

*Could you briefly explain 'IC's' to me, please? It sounds like a very precise soldering job you've mentioned? I do have a couple of old donor boards I'm happy to practice on to see if my skills are up to the job.
 
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