Jump to content


Photo

Panasonic RX-CS700 - faulty FM Stereo Reception

Panasonic RX-CS RX-CS700 FM stereo no FM stereo

  • Please log in to reply
26 replies to this topic

#1 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 31 December 2015 - 09:33 AM

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.



#2 toshik

toshik
  • Members
  • 420 posts
  • LocationSt. Augustine, FL

Posted 31 December 2015 - 09:43 AM

FM RF stages adjustment needed.



#3 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 31 December 2015 - 10:03 AM

FM RF stages adjustment needed.

 

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:



#4 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 31 December 2015 - 10:19 AM

Just searched the 'net for "adjusting FM RF stages" - WOW, heavy stuff, man!  :-O   :



#5 Reli

Reli
  • Boomus Fidelis
  • 4,621 posts
  • LocationUSA

Posted 31 December 2015 - 10:30 AM

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.



#6 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 31 December 2015 - 10:43 AM

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.

 

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.



#7 Reli

Reli
  • Boomus Fidelis
  • 4,621 posts
  • LocationUSA

Posted 31 December 2015 - 11:31 AM

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.



#8 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 31 December 2015 - 12:36 PM

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.

 

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.



#9 BoomboxLover48

BoomboxLover48
  • Boomus Fidelis
  • 4,801 posts
  • LocationChicago, IL

Posted 31 December 2015 - 12:40 PM



#10 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 31 December 2015 - 12:46 PM

 

:   :bang:  Whatever did I do to upset you?!?!?!  :lol:



#11 BoomboxLover48

BoomboxLover48
  • Boomus Fidelis
  • 4,801 posts
  • LocationChicago, IL

Posted 31 December 2015 - 12:52 PM

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. 



#12 BoomboxLover48

BoomboxLover48
  • Boomus Fidelis
  • 4,801 posts
  • LocationChicago, IL

Posted 31 December 2015 - 01:20 PM

:   :bang:  Whatever did I do to upset you?!?!?!  :lol:

Oh no!

:w00t: 

I am a poor soul! I was also learning on the side...

:yes:  :-D  :-D  :lol:



#13 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 31 December 2015 - 01:47 PM

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. 

 

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.



#14 BoomboxLover48

BoomboxLover48
  • Boomus Fidelis
  • 4,801 posts
  • LocationChicago, IL

Posted 31 December 2015 - 02:14 PM

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:



#15 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 31 December 2015 - 02:20 PM

Ah, I see............."Oi, hands off the waxsh, blue eyesh!"



#16 ralrein1

ralrein1
  • Members
  • 956 posts
  • LocationShasta County in the real Northern Kalifornia

Posted 31 December 2015 - 06:29 PM

A picture of your new boomer would be appreciated.As you always take very good pics.😄😁😊

#17 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 31 December 2015 - 06:43 PM

A picture of your new boomer would be appreciated.As you always take very good pics.

 

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.



#18 Superduper

Superduper
  • Boomus Fidelis
  • 6,165 posts
  • LocationSomewhere over the rainbow, USA

Posted 31 December 2015 - 10:07 PM

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.



#19 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 31 December 2015 - 11:16 PM

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:



#20 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 09 January 2016 - 04:16 PM

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.

 

"........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:

 

*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.



#21 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 09 January 2016 - 06:45 PM

A further update:

 

This evening, I've left the radio playing in another room tuned to a strong signal (88.8FM) where the reception is mono and there is no stereo indicator light on.  I monitor the reception periodically and it will suddenly revert to stereo output with the stereo indicator light lit.  This will last no more than 2 minutes (so far) before reverting back to mono with the indicator light going out.  Most of the time (80%+) the signal remains mono.

 

It's very late here in UK so I plan to leave the radio on overnight and see how things are in the morning.



#22 BoomboxLover48

BoomboxLover48
  • Boomus Fidelis
  • 4,801 posts
  • LocationChicago, IL

Posted 09 January 2016 - 06:54 PM

A further update:

 

This evening, I've left the radio playing in another room tuned to a strong signal (88.8FM) where the reception is mono and there is no stereo indicator light on.  I monitor the reception periodically and it will suddenly revert to stereo output with the stereo indicator light lit.  This will last no more than 2 minutes (so far) before reverting back to mono with the indicator light going out.  Most of the time (80%+) the signal remains mono.

 

It's very late here in UK so I plan to leave the radio on overnight and see how things are in the morning.

Pull the antenna all the way out and rotate it.  :w00t:  :lol:  :lol:

 

That Charles is not supporting FM radios. 



#23 Superduper

Superduper
  • Boomus Fidelis
  • 6,165 posts
  • LocationSomewhere over the rainbow, USA

Posted 14 January 2016 - 02:18 PM

 

*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.

 

IC's are monolithic semiconductors that typically looks like flat rectangular shaped centipedes.  The acronym is for Integrated Circuits and a simple description is that they are tiny circuit boards populated with electronic components and circuitry, then encased in an epoxy shell with leads protruding for wiring them up.  In addition to far lower cost than making up the same circuits individually, they are packaged this way to allow the repetitive use of the same circuits in different applications.  In any event, after your updated description of when and how the stereo light comes on, it's pretty clear that the IC's are "probably" OK and that some adjustment is what you need.  There are 2 possible adjustment scenarios.

 

#1, the stereo pilot signal adjustment is off and this is something that you can probably do yourself and is relatively safe in that misadjustment is not catastrophic and relatively easy to undo if you mess up.  In this case, an onboard potentiometer needs to be rotated to the centerpoint of the range where the stereo light illuminates.  If you have a service manual for this model, then you can leasily find which is the mpx stereo decoder and the associated stereo adjustment pot.  Lacking the service manual, there is another way to find which pot this is, it's generally easiest to start by looking up the datasheets on the various IC's in your tuner board.  Datasheets will usually tell you the purpose of that IC, such as AM/FM IF amplifier, MPX decoder, etc.  Once you've figured out which IC is the MPX decoder, then the datasheet will usually provide a sample circuit.  Luckily, you will find that most electronics manufacturers, with few exceptions, generally follow the sampe circuits very closely.  In this case, search for the variable resistor in the circuit that corresponds to stereo, 19khz, pilot, etc.  That is the pot to adjust.  Do not adjust the one that is indicated for separation.  It sounds hard, but it really isn't. 

 

#2, IF frequency or Local Oscillators are out of adjustment or alignment.  These aren't really DIY type of fixes due to that you will require a certain amount of rather expensive instruments to perform these, as well as a knowledgeable technician.  Some folks have tried experimenting by messing with these adjustment points (and there are many when you get to this point and some require bending of the coil loops to accomplish) but I don't recommend it.  IF we are speaking of AM, you might luck out since many AM adjustments simply require peaking and you can sorta of hear the difference but the FM adjustment is far more complicated and it's possible, likely even to throw the alignment out further, possilbly rendering the FM section almost useless.

 

#3, Band coverage.  This adjustment is performed to ensure that the range (i.e. 88-108) is properly adjusted.  When off, this is condition is typically evidenced by the stations not lining up to the proper spot on the dial.  This adjustment is possible for the DIYer to perform if you have accesss to a  FM signal generator.  The reason you need the FM signal generator is that you need the generator to create the FM signal for the adjustment.  Typically, you will have the generator output a signal at the specified frequency indicated in the service manual, usually a range extreme such as 88mhz, tune the tuner-dial to the same frequency then adjust at the specified locations until the tuner receives the tone outputted by the signal generator, then do the same for the other extreme such as 108mhz etc. and this procedure is repeated back and forth and repeated until no further improvements are evident.  The reason is that each adjustment not only affects the reception at the current point but also has affect at other end of tuner dial too.  By the way, an FM signal generator is not the same as an audio generator since it needs to output in the RF range.

 

This all probably sounds overwhelming and it's a science that is not easy to understand, and almost impossible to perform properly without access to proper instruments.  My only recommendation to you is try to see if you can find the MPX decoder chip, perform the adjustment and see if that helps.  Or take hi-res picture of the tuner circuit board and we will try to find that chip and the adjustment pot for you.  For some reason, I couldn't find a service manual for the RX-CS700, but do have one for the RX-CS710 amd 780. Perhaps it's the same or different, not sure.



#24 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 14 January 2016 - 03:11 PM

Soperduper, what can I say???  I can't thank you enough for your time and effort in posting this comprehensive reply and also for imparting such expertise in a manner I can understand with a little more concentration than usual.  Many, many thanks indeed.  :bow:

 

Yes, all you've written is new and unfamiliar to me but I'm darn sure I can put my best efforts into the work needed.

 

#1: All clear on this (or will be once reread!)

 

#2:  Not having the instruments for this I feel this step will not be an option.

 

#3:  An FM signal generator looks to be quite an expensive bit of kit so, if it's financially viable, I'd see if this could be carried out by an expert with his/her own generator.  As the box cost me nothing, there's now a value to balance against the cost of expert work.

 

 

I do not have the service manual for this RX-CS700 but I do have one for my RX-CS800 so this may well be of help.

 

One thing I will do is print off and read over & over the info you've provided to use as a step-by-step guide as my memory won't allow me to remember all of the info in 'one big lump'.

 

I have to say again:  I'm incredibly grateful to you for taking the time to write such a comprehensive reply, it's most generous of you.

 

Many, many thanks.

 

:yes:  :thumbsup:  :rock:  :hooray:



#25 Superduper

Superduper
  • Boomus Fidelis
  • 6,165 posts
  • LocationSomewhere over the rainbow, USA

Posted 14 January 2016 - 05:22 PM

Expensive is relative, but for any boombox collector with large number of radios and with sufficient hunger to learn, it might be worth it to purchase one and some other appurtenant instruments to be able to play doctor on their collection.

 

There are a ton of instruments that one might need to be able to perform all the adjustment procedures typically prescribed in service manuals.  Some aren't even common anymore since new radio designs don't need much adjustments like older gear, typically they either fail entirely or work perfectly.

 

As for buying old instruments, it could get overwhelming and some are junk or have such limited ability that you are going to collect 3 or 4 pieces just to get the combined ability of a single unit so all that needs to be considered.  Also since the adjustment is only as good as the quality of the instrument used to perform the adjustment, units that are crude or out of calibration won't be much good.  You should be able to trust your instruments, otherwise, it's a crapshoot.  Is the erroneous readings due to bad tuner or bad tester?  Can you trust the reading you get, or is the lack of a reading due to malfunctioned tuner or an insensitive instrument?   Diagnosing the diagnoser won't be fun, trust me.

 

In short, instruments typically required for tuner work includes:

AM, FM RF Signal generator.

Stereo FM RF signal generator to diagnose MPX.

Marker generator (some sweep generators have this feature build in, most don't)

Sweep generator

Audio generator for general audio troubleshooting or to inject tone into the RF generators, if they don't have internally built in tone generators (most do).

Millivolt meter, although most good modern DMM's will suffice.

Oscilloscope (the more capable and the more modern, the easier the work).

Frequency counter (for MPX pilot signal adjustment).



#26 MyOhMy

MyOhMy
  • Members
  • 1,883 posts
  • LocationSleaford, Lincolnshire, UK.

Posted 14 January 2016 - 05:47 PM

I knew it, I just knew it :bang:

 

Only joking! :yes:  :lol:

 

To be honest, it all looks very in depth from what you've just added although I really do appreciate your additional info as I feel much better informed.  I'll do as much as I can from the info you previously provided then take it from there.

 

I think it's probably a bit late in the day for me to begin acquiring all manner of technical equipment along with the required knowledge although at least some simple test equipment, I now see, as being essential for the more routine aspects of BB maintenance.

 

When I mentioned value, the value isn't solely of a monetary nature, there is also value in learning new skills and, importantly, learning to respect the technology for what it is - no matter what it is.  It's not 'just a radio/cassette of $$'s value.

 

Thank you once again for taking the time to be so very helpful.

 

:yes:  :rock:



#27 T-STER

T-STER
  • Members
  • 1,425 posts
  • LocationUK

Posted 14 January 2016 - 06:05 PM

Very interesting read, we are so lucky to have access to such expert knowledge. Thanks Super, i know this post was to help Mom but i have just read this page 3 times and have bookmarked it to re-read, a lot to take in there!





Also tagged with one or more of these keywords: Panasonic, RX-CS, RX-CS700, FM, stereo, no FM stereo