Yamaha CT-7000 hifi tuner faults

[steveUK]

A Yamaha CT-7000 hifi tuner came to me not working. I have now got it into a state of basically working but there are still problems. So far I have replaced faulty transistors and blown fusible resistors on the power board, ALL electrolytics and tants on that board have been checked for value and ESR and just a couple of tants replaced. All of the fixed interboard connector pins and flying connectors have been thoroughly cleaned, and the tuning capacitor ground connections cleaned and electrolubed. All switches and pots have been cleaned. Oh, FWIW the red paint on both meter pointers had started to crumble and fall off, in the case of the signal strength meter, stopping movement. In ultra careful 'watchmaker mode' I carefully scraped the paint off the pointers using a fresh scalpel and ‘repainted’ them using a red marker pen, it worked brilliantly.


So now it will tune in to stations and sounds ‘fine’ but..


1 It lacks sensitivity

2 The ‘Station’ lamp does not illuminate when tuned in. But saying that, it has done so on occasions. The lamp itself is fine, checked.

3 Stereo reception is not happening

Any ideas please?

 

[clinic-audio]

Hello Steve

1) Disconnect antena

2) put your nedle on the " o " on maximum on the left



watch your tuning meter if you have this :



or this :



if your answer is the first one , then your tuner need to be re-aligned

 

[steveUK]

Thanks for that. All appears to be ok in that respect. However, I have changed my mind about it being a sensitivity issue. The set will receive, quite clearly, Radio Ashbourne (I'm in the UK). Now that won't mean much (obviously) to most people, but where I am sited, that is a very tough station to tune in to, most tuners simply cannot do it, let alone amidst a load of mush. This one does!

So the outstanding faults are:

* No stereo
* Station lamp works for first minute or so after cold switch on, then stops.

There are no ICs in the set. It's all discrete.

 

[Watthour]

So in essence, it is receiving and amplifying FM, generating and mixing IF, separating an audio signal, but not decoding the multiplex/stereo signal. That should be a hint.

 

[steveUK]

Thanks Watthour. So, stereo decoder section?

Out of interest, the service manual does not appear to have the procedure for re-alignment unless I'm missing something? It has a whole host of set up procedures but I cannot see any that are actual re-alignment ones as in "apply this frequency and adjust this core/cap" etc etc.

 

[Watthour]

It seems that those instructions are indeed elusive. The MPX board is completely discrete, but also completely confusing compared to other FM stereo decoders.

 

[bobschneider]

I had a problem with mine not decoding stereo. It turned out to be an issue on the control board (the large PCB on the bottom of the chassis, next to the power supply PCB).

 

[chazix]

No really cogent suggestions, just want to make sure it is known that there is a circuit description (among other things) at:

http://www.fmtunerinfo.com/yamaha.html

The description does cover the MPX decoder. I think a good thing to check first is whether the VCO is oscillating. Looks like it should be 76KHz free-running (before the two frequency dividers).

As for the "station" lamp being shy, I share (what I take to be) Patrice's idea that it suggests a discriminator issue. I know @steveUK said the tuning meter is OK, but at the risk of belaboring the point: Does the tuning meter deflect approximately the same amount left and right when tuning to either side of a strong station?

Cheers,

chazix

 

[steveUK]

Thanks so far guys. I'll report back.