Yamaha CT-7000 hifi tuner faults - UK Vintage Radio Repair and Restoration Discussion Forum

stevehertz · 16th Mar 2019, 7:06 pm

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, and 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

It’s a complicated tuner and in its time a very high end one retailing at £600 in 1973.

stevehertz · 17th Mar 2019, 11:44 am

Update.

The station lamp does tend to illuminate when the set is first switched on, and after a few minutes it goes off. This to me suggests a sensitivity fault that gets worse as the set warms up. Similarly, stereo reception is not achieved, again pointing to a sensitivity issue. On an American forum it has been suggested that it could be due the need for a re-alignment. Of course, one is never sure if the preset twiddlers have been in action. Could it go out of alignment that badly naturally just by normal component ageing?

Radio Wrangler · 17th Mar 2019, 12:11 pm

Not really.

The front-end tuning is pretty broad, the IF selectivity is usually in ceramic filters and the thing that may need a tweak is the discriminator coil(s).

If signal level at the sig strength meter pin is that low, it probably disables stereo decoding.
You need to find which IF chip it uses. Monitor S meter voltage, AGC voltage and you'll need a decent sig gen. Injext 10.7MHz inti input of IF chip, wind up to get decent S meter and see if level injected is reasonable given the data sheet.

If OK here, work backwards towards antenna. change to RF for stages before mixer of course and see where the gain sinks.

After DC checks of course.... it might be that easy!

David

stevehertz · 17th Mar 2019, 12:45 pm

Thanks David. But before I absorb and action that, I have changed my mind about it being a sensitivity issue. The set will receive, quite clearly, Radio Ashbourne. 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.

Studio263 · 18th Mar 2019, 5:18 pm

I thought that these used IC gain blocks in the IF strip, coupled by ceramic filters.

It sounds like yet another case of a slightly misaligned discriminator, check what the DC output is when the set is apparently on tune. The stereo decoder won't be allowed to function until the signal is strong enough and the DC output of the discriminator is close to balance, so this is the first place to start.

This model has a touch sensor as part of the tuning knob which turns the AFC on and off, it makes fault-finding in this area tricky if you don't realise. All checks around the discriminator should be done with the AFC off of course.

Normally a slight adjustment of one of the coils is all that is needed.

16th Mar 2019, 7:06 pm	#1
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,831	Yamaha CT-7000 hifi tuner faults 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, and 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 It’s a complicated tuner and in its time a very high end one retailing at £600 in 1973. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

17th Mar 2019, 11:44 am	#2
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,831	Re: Yamaha CT-7000 hifi tuner faults Update. The station lamp does tend to illuminate when the set is first switched on, and after a few minutes it goes off. This to me suggests a sensitivity fault that gets worse as the set warms up. Similarly, stereo reception is not achieved, again pointing to a sensitivity issue. On an American forum it has been suggested that it could be due the need for a re-alignment. Of course, one is never sure if the preset twiddlers have been in action. Could it go out of alignment that badly naturally just by normal component ageing? __________________ A digital radio is the latest thing, but a vintage wireless is forever..

17th Mar 2019, 12:11 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: Yamaha CT-7000 hifi tuner faults Not really. The front-end tuning is pretty broad, the IF selectivity is usually in ceramic filters and the thing that may need a tweak is the discriminator coil(s). If signal level at the sig strength meter pin is that low, it probably disables stereo decoding. You need to find which IF chip it uses. Monitor S meter voltage, AGC voltage and you'll need a decent sig gen. Injext 10.7MHz inti input of IF chip, wind up to get decent S meter and see if level injected is reasonable given the data sheet. If OK here, work backwards towards antenna. change to RF for stages before mixer of course and see where the gain sinks. After DC checks of course.... it might be that easy! David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

17th Mar 2019, 12:45 pm	#4
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,831	Re: Yamaha CT-7000 hifi tuner faults Thanks David. But before I absorb and action that, I have changed my mind about it being a sensitivity issue. The set will receive, quite clearly, Radio Ashbourne. 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. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

18th Mar 2019, 5:18 pm	#5
Studio263 Octode Join Date: Apr 2007 Location: Hampshire, UK. Posts: 1,577	Re: Yamaha CT-7000 hifi tuner faults I thought that these used IC gain blocks in the IF strip, coupled by ceramic filters. It sounds like yet another case of a slightly misaligned discriminator, check what the DC output is when the set is apparently on tune. The stereo decoder won't be allowed to function until the signal is strong enough and the DC output of the discriminator is close to balance, so this is the first place to start. This model has a touch sensor as part of the tuning knob which turns the AFC on and off, it makes fault-finding in this area tricky if you don't realise. All checks around the discriminator should be done with the AFC off of course. Normally a slight adjustment of one of the coils is all that is needed.