Yamaha R700 tuner won't stop searching for stations

[Benji01]

Hello guys,

I'm currently working on a yamaha r700 receiver. I repaired the power amplifier and recapped  the whole unit due to leaking capacitors.
But the tuner still has a problem. If you start a station search, the tuner will start searching for stations but the search will never stop. The signal meter shows singnal and the stereo light will come one during the search when the tuning passes the local stations but it won't stop and lock onto a station. If you press one of the preset buttons, the tuner will jump to one of the preset stations and the tuned frequency will slowly crawl upwards.
The tuner can't be manually tuned only if you use a little jig that is described in the service maunal, if I use the jig and tune the tuner manually, the tuner works fine.
I checked all the voltages to the ic's, everything looks normal.
Does anyone have an idea what could cause that problem or what else I could check ?

[wasedadoc]

Sounds like you first need to understand how the unit decides it has reached a station and should stop the search. Then investigate the circuitry which implements that.

[Kim Christensen]

the tuner will jump to one of the preset stations and the tuned frequency will slowly crawl upwards.

It's possible one of the tuning buttons is stuck, or there is contamination on the PCB making it look to the MCU like a button is stuck.

[Benji01]

thanks for your response.
I checked the corresponding pins on the ic and they only pulled high when the button is pressed so that is working correctly.

[perdrix]

My *guess* it that it stops a search when the AGC for the RF section detects a signal of "sufficient" strength.

David

[floobydust]

Do you have a schematic for the receiver? Elektrotanya's R-700 look like somebody was drunk when they scanned them and I could not follow it.

[Benji01]

Yes, I have the sm.
The scan that I have is also not that great because you can't read the voltages properly. I had to printed it out and piece it together with some tape.
I used the service manual from hifi engine sadly I can't uploaded it here because it's to big.

[floobydust]

I can't download from hifiengine, I get a server error message "Referer Error: unable to complete your request due to missing or incomplete referer data forwarded by your browser".
Can't contact any site admin etc. there, tried to post the problem:
Your posting was blocked by our spam filter
Your post has been flagged by our spam filters.
A moderator will review your post shortly.
Thank for you your patience."   

I'll see later if I can get this fixed. Otherwise you'll have to snip and black and white the schematic to get the filesize down or something.
Last I was looking for the signal telling the MCU that there is strong signal, while it is scanning. Might be "S-curve" but not sure.

edit: It does this on AM and FM?

[Benji01]

Hey thanks for your reply, the tuner does this both in am and fm.
I stitched the scans to a big plan together, not perfect but better than the original, I hope it helps.

[james_s]

If you had leaking capacitors I suspect you have a via that has been rotted out, or possibly a chip resistor that was damaged. If you can figure out how it senses that a station has been reached it shouldn't be too hard to trace that signal and see where it stops.

[Kim Christensen]

Last I was looking for the signal telling the MCU that there is strong signal, while it is scanning. Might be "S-curve" but not sure.

Looking at the block diagram in the datasheet for the LC7207, it sure looks like that's the way they detect a RF signal to stop the scanning. I can't read Japanese though.
The MCU's S-Curve signal seems to come from pin 1 of IC104 (Dual Op-Amp) on the RF/IF pcb so he might want to check in that area.

[floobydust]

It looks like the IC503 Sanyo LC7207 does the auto/man scan but which pin tells it "we're on a station, stop"... there's no English datasheet for it.
When the tuner has a good signal, that shows up on the signal strength meter and it turns off muting.
So it looks like the "S" or "S-Curve" signal is signal strength going into IC503. It's a pretty hairy circuit. Sometimes instead, the MUTE signal is used to tell it has a station.

Control 1 board, make sure VREF is 8.20V exactly or else that upsets the entire tuner. VR504 Ref. Voltage Adjust.
"SB" - VDD for the board 10.0VDC power.
"S" Can you measure and look at the voltage when you are on/off a station. I think this feeds IC503 and is not the FM discriminator S-curve but confusingly labelled that.

[Benji01]

I checked Vref that was spot on at 8,20V
"SB" was at 9,72V
I measured the voltage "S" and when a preset button is pressed, the voltage is 9,10V.
When the tuner is in search mode the voltage is 9,13V.
When the tuner is manually tuned with the potentiometer jig and the push button, if the button is pressed the voltage jumps to 9,23V and after about 3s falls back to 9,1V.
The voltage "S" does not change either while manually tuning or while searching, it stays at 9,1V.
I checked pin 1 of the op amp IC 104 and the attached spike occurs when the tuner moves past a strong station, the same spike appears when you manually tune it into a strong station.

[floobydust]

"S" looks stuck high, the schematic shows 5.6V TR109-E or 4.2V IC503 pin 13, I think it should be the same point though. I would next measure more around IC104 and compare voltages to the schematic.

[Benji01]

So I checked the voltages of IC 104 and 105.

IC 104   V schematic   V measured
PIN1    3,6V             9,14V
PIN2    4,4V             3,92V
PIN3    4,4V             4,20V
PIN4    GND             GND
PIN5    4,4V             4,2V
PIN6    4,4V             4,21V
PIN7    4,4V             4,21V
PIN8    10V             9,86V



IC 105  V schematic   V measured
PIN1          10V           0V
PIN2          0V         9,8V
PIN3   -                 2,18V
PIN4   -                 9,83V
PIN5           6V         9,76
PIN6         10V         0,015V
PIN7     GND         GND
PIN8           0,2V         0,16V
PIN9           6,4V         6,28V
PIN10       9,9V              9,81V   
PIN11        0V          0V
PIN12        6V          0V
PIN13      -0,5V              8,47V
PIN14        VCC              9,85V


TR107                     TR109
B 0V                        B 8,47V
C 1,2V                     C 9,11V
E 4,12V                    E 9,11V

TR 108                  TR108 when manually tuned
B 0,16V                 B 4,8V
C 0V                      C 4,2V
E 4,2V                   E 4,2 V

The screenshot shows a peak that occurs when the button for manual tuning is pressed.
It occurs IC 105 On Pin 6,5,9,10,11.

[floobydust]

These measurements are in FM, on a station or not?
TR107-B is fed from power via R144, so I would not expect 0V there. TR120 switches power on to the AM tuner section I believe, so that is what leads me to think it's in FM mode.

What I am thinking is the output of IC104 pin 1 is way high, which appears to indicate no signal strength at all (for scanning). It is inverted, you would think signal strength = more voltage, but no. Working backwards...
TR107-C at 1.2V is too low which leads us to IC108 pin 20. That voltage should go up with FM signal strength. IC108 input is fed by the FM detector at pin 18. The FM signal strength meter is working OK though? I guess look at the pin 20 on and off a station.
This tuner has a delicate balance, so if the muting or signal strength or LO frequency is out, it will do some strange things.

[Benji01]

Thanks alot, I found the problem with the Tuner.

TR107-C at 1.2V is too low which leads us to IC108 pin 20. That voltage should go up with FM signal strength. IC108 input is fed by the FM detector at pin 18. The FM signal strength meter is working OK though? I guess look at the pin 20 on and off a station.
This tuner has a delicate balance, so if the muting or signal strength or LO frequency is out, it will do some strange things.

This was the important quote that helped me.
I tuned the tuner manually into a really stron station and then measured the voltage on pin 20 of IC 108.
Now I turned T105 till I had 4,179V on pin 20 of IC 108.
Now the tuner works perfect, it locks onto staions and everything works.
Thanks alot guys.

[floobydust]

That's great  it's got complicated tuner control and T105 would be the FM discriminator alignment.
That would cause weak signal level inside the tuner chain so it works but scanning and muting are confused, see zero signal strength or reception. Just a theory.
As long as it stays out of landfill, these vintage receivers are worth repairing.

[james_s]

I wonder how it got out of alignment in the first place? One of those situations where it stopped working so somebody fiddled with every adjustment they could find?

[Benji01]

I don't really know.
The guy that owns the receiver said that the tuner hasn't been working correctly for a long time so he only used it as an amplifier.
When the power amplifier died he gave the receiver to me. I don't think that he tried to get the tuner to work by just fiddling with the adjustments but I don't know.
I know that the guy moved a lot maby it just moved during a car ride.

[james_s]

I suppose vibration could have done it over time, or maybe the core in the adjustable transformer cracked at some point or some other issue. Either way nice job fixing it, it's great when a something like this comes together and works.

[Benji01]

Thank you, but without help I would not have been able to repair the tuner.
A massive thanks to all that helped me find the fault.