TX-710 Auto-tunning problem

contadotempo

contadotempo

New Member
Hello, I've been a reader for a while but only recently decided to register.

I have acquired a Pioneer TX-710 that seems to have a faulty auto-tunning function on FM mode. Whenever the auto-tune function starts, by either pressing UP or DOWN, the tuner cycles through the whole FM spectrum and never seems to stop, even when the radio signal is detected as maximum (indicated by all the signal strength LEDs lighting up). On AM mode it seems to work just fine, stopping as soon as it finds a station with a good signal.

First thing I did was to acquire the circuit schematics: http://www.kallhovde.com/pioneer/tx-710l-om.pdf. Then I started fault-hunting. At first I suspected the LC7207 (Q13) "voltage synthesizer" (found a datasheet in Japanese http://pdf.datasheetcatalog.com/datasheets/700/501893_DS.pdf and also a service manual for a different tuner using the same IC, http://www.creapromedia.fr/lesdocs/sony/strs5l.pdf, page 7) since some voltages were a bit off according to the schematics. However, the previous owner then mentioned he replaced this IC with a new one, behavior was the same.

Then I noticed that the FM/IF System IC, the PA3001-A (Q7) was giving me 13V on PIN 7, which is described by the datasheet (http://nice.kaze.com/PA3001A.pdf) as "Tuning meter output", and should have 5.6V +/- 1.5V. I followed it through the circuit and found the PC4558C DUAL OP AMP (http://www.alldatasheet.net/datasheet-pdf/pdf/6768/NEC/UPC4558C.html) (Q12), which seems to be comparing the signal coming from the PA3001-A, to build the FM S-Curve (I might be wrong here) between PIN 2 and 3 of this OP AMP, outputting the result at PIN 1. The schematics say that PIN 1 should have a voltage of 4.7V. I get 25V (almost the positive supply voltage on PIN 8). I thought it could be this IC that broke down/short-circuited. I replaced it but got the same results. I cannot pinpoint why I'm getting such a higher voltage in these pins. What could it be?

This seemed relevant to the auto-tunning problem because PIN 1 of this OP AMP feeds PIN 13 (named S-Curve by the schematics) of the LC7207 "voltage synthesizer". I also asked the previous owner about the PA3001-A, he replaced it as well, but to no avail.

The previous owner of the tuner did tell me he accidentally short-circuited the darlington of the power supply (Q20), which in turn destroyed the 14V Zener diode attached to its base (D34). He replaced the darlington and the diode with a 15V Zener diode. Consequently, it raised the voltage a bit in some components. I did dismiss this since he also said the auto-tunning problem was already there before he got his hands on it. And also because the voltage on every terminal of the ribbon cable was approximately correct according to the schematics. I contacted with fellow user r_electronic, who made the thread http://audiokarma.org/forums/index.php?threads/pioneer-tx-710-problems.547087/, and he mentioned it could be a faulty power supply/insufficient voltage. I'm inclined to recheck it, should I?

I have uploaded a few photos as-well

20160118_011205.jpg 20160118_011055.jpg 20160118_011609.jpg

Thanks in advance for any tips.
 
Sadly not through normal functions, no. I can change the frequency by tweaking the small pink variable capacitor to the right shown in the picture bellow.
 

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Yes, all components I've mentioned in my first post have been replaced before.

Today I had some free time so I worked with the tuner a bit. It appears a new problem arose which I hadn't see before. Whenever a station is tuned manually (through the variable capacitor mentioned in post #3), the sound output is very low for some reason (I have to raise the volume on my amplifier almost to max). The better the signal (indicated by the signal LEDs lighting up), the lower is the volume. However if I tune to a frequency with no signal/no station (no LEDs lighting up), the background noise comes on full blast. What could cause this strange behavior? Power supply issue?
 
I have a TX-720L which is similar to the 710. The fact it cant stop on a station is due to an adjustment on the board. I remembe ron mine I had to connect a DMM to a TP on the board and adjust it until I got 0mV on a tuned in station. Then it will stop. If you want, I can try to dig out my notes and see which TP it is on the 720. AFAIK they are pretty much the same tuner in a different style case.
 
contadotempo said:
Yes, all components I've mentioned in my first post have been replaced before.

Today I had some free time so I worked with the tuner a bit. It appears a new problem arose which I hadn't see before. Whenever a station is tuned manually (through the variable capacitor mentioned in post #3), the sound output is very low for some reason (I have to raise the volume on my amplifier almost to max). The better the signal (indicated by the signal LEDs lighting up), the lower is the volume. However if I tune to a frequency with no signal/no station (no LEDs lighting up), the background noise comes on full blast. What could cause this strange behavior? Power supply issue?
Click to expand...

What are the voltages at the supply? Its a 9 wire ribbon connector, pin 1 is ground.
 
I have found sometimes on these units that one of the voltage regulator transistors in the power supply can fail and pass too much voltage. It presented itself as an AC hum noise but only when the tuner was tuned in to a station.
 
Please excuse my late reply. I ended up rechecking the PSU and I found out the transistor/darlington (Q20) that I mentioned in my first post, was not replaced after all, I was wrong. While the schematics say it should have a 2SD836A, it in fact "originally" had a 2SD313. Now, I couldn't find enough information to prove they were equivalent, and also I felt like the "original" was always getting excessively hot. So I ended up replacing it with the correct one. To my surprise, after I did this the voltage on the dual op-amp I mentioned before "normalized" to about 8V. I was then able to put it at 4.7V (with no signal), like it says in the schematics, by tweaking the FM S-Curve potentiometer.

Sadly, it still has the same problems: It won't stop scanning and when I tune it manually to a station with full signal, sound/volume gets extremely low. The better the signal, the lower is the volume. Actually with full signal, there's almost no sound at all (measured less than 1mV at the speaker's output). As soon as I tune it into a a frequency with no signal, background noise come on full blast.

Measured directly on the PA3001-A (FM IF System IC) to try and understand what was going on with the output.
Measuring PIN 1 (labelled FM IF input on the datasheet), I think it works as expected. IF's amplitude raises when tuned a strong signal, and lowers when signal gets weaker. Measuring PIN 7 (labelled Detected Output (multipath HOR)), where I believe what you hear comes from, has completely the inverse behaviour. What could cause this? I thought it could be a bad IC so I got an extra PA3001-A quite cheaply on ebay and replaced it. Same behaviour. They can't both be bad, can they?

QSilver said:
I have a TX-720L which is similar to the 710. The fact it cant stop on a station is due to an adjustment on the board. I remembe ron mine I had to connect a DMM to a TP on the board and adjust it until I got 0mV on a tuned in station. Then it will stop. If you want, I can try to dig out my notes and see which TP it is on the 720. AFAIK they are pretty much the same tuner in a different style case.
Click to expand...
Yeah, I just checked the TX-720L's schematics it seems almost similar. If you have some time would mind confirming which TP is it? Thanks for bringing this to my attention.

glen65 said:
What are the voltages at the supply? Its a 9 wire ribbon connector, pin 1 is ground.
Click to expand...
I just checked the voltages after replacing the transistor I mentioned on the first paragraph. I got these values:
  • Pin 1: 0 V (GND)
  • Pin 2: 8.7 V (Schematics define as 8.4 V)
  • Pin 3: 13.75 V (Schematics define as 12.8 V)
  • Pin 4: 21.3 V (Schematics define as 20.0 V)
  • Pin 5: 8.2 V (Couldn't find defined in the schematics. Though it seems to supply the display tube, which is working correctly I believe)
  • Pin 6: 8.7 V (Schematics define as 8.4 V)
  • Pin 7: -8.2 V (Schematics define as -8.3 V)
  • Pin 8: 26.9 V (Schematics define as 26.4 V)
  • Pin 9: 8.9 V (Schematics define as 8.4 V)
Some pins have a considerably larger voltage because of the replacement of the 14 V Zener with a 15 V one. Could this cause such behavior? Thanks for your help so far.
 
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Glad you found the problem in your PSU - not sure if I said further up, but I recently had a 720L which had a nasty hum on the output but only when tuned in. Turned out it was the voltage regulator passing full rail voltage to the circuits. Replaced it with a higher rated transistor and that settled it.

Looking at my schematic and adjustment procedures for the TX-720L, it says... "Obtain a reading of DC 0V between terminals no. 38 and 39. The adjustment point referenced here is "T5 (A)" - its to the left of PA3001-A on my schematic. Pins 38 and 39 are next to each other and labelled "T.METER"

With your inverse tuning issue with volume... I can't really help there but it sounds like the muting circuit is working in reverse...?
 
Thanks for confirming the TPs! I believe the equivalent TPs for the TX-710 are 30 and 31. As I was going to make the adjustments and found out the ferrite core screws were shattered (one of them only a bit on the top).
Darn this tuner is full of bad surprises. I'll see if I can scavenge any from old radios, otherwise I'm screwed. Thank you for your help and time, I'll report if I make any progress.
 
Thats really unlucky - someone has been rough to that tuner... hopefully you can find a replacement - do you know the ratings for the broken one?
 
Tried to measure its dimensions, it seems to be 5mm height and 5mm diameter. Sadly, I'm not sure about other ratings. It seems to be one of these (10.7MHz HF Transistor Radio IF Ferrite Core), but with a different size: http://www.ebay.co.uk/itm/111769342332
They don't seem to be specified in the schematics either. It's going to be a tough one to find.
 
Looking through my schematics.... if I'm correct, its T5 that your talking about. My part list calls it a "FM DET transformer". Maybe you could try and find a replacement?
 
I think it's an FM Quadrature Detector (it seems to be connected to PIN 8 and 9 of the PA3001-A, labeled "QUAD OUT", "QUAD IN" respectively on the datasheet)...? I think...? Do I stand any chance of finding a replacement for something so specific? Or in the worst case, is there a chance of rebuilding it?
 
Next time your poking around check the voltage on pin 3 of Q402 IC LC7207 "Voltage Synthesizer". I'm curious and looking at the De-tuning circuit, transistors Q503 and 504. They may of been toasted when the PS was shorted.
The IC's can't take high voltages the condition you found the set in with the voltages doubled might of damaged them.
 
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zebulon1 said:
Next time your poking around check the voltage on pin 3 of Q402 IC LC7207 "Voltage Synthesizer". I'm curious and looking at the De-tuning circuit, transistors Q503 and 504. They may of been toasted when the PS was shorted.
The IC's can't take high voltages the condition you found the set in with the voltages doubled might of damaged them.
Click to expand...
Pin 3 seems to be giving me 6.9V, quite a bit more than the 6.4V specified on the schematics. What does it mean? Also, what's the purpose of this pin? It's labelled "BUC" which sadly doesn't say much.

I also checked the transistors Q503 and Q504 (which seem to be equivalent to Q17 and Q27 on the TX-710, right?), desolderer them, tested their conductivity for any shorts and luckily they seemed to be fine. After re-soldering I measured their voltages and they seem to be almost satisfactory:

Q17 (Q503 on the TX-720):
- Emitter: 0.637 V (Schematics say it should be 0.6 V)
- Collector: 0.617 V (Schematics say it should be 0.5 V)
- Base: 0.09 V (Schematics say it should be 0 V)

Q27 (Q504 on the TX-720):
- Emitter: 1.3 mV (Schematics say it's GND)
- Collector: 0.09 V (Schematics say it should be 0 V)
- Base: 0.630 V (Schematics say it should be 0.6 V)

Q17's collector seems to have a little bit of extra voltage and I don't like the little voltage going through Q27's emitter. Could this mean anything?

Thank you for your time and help.
 
When it comes to tuners my skills are to the point of looking for the obvious. Quite a big step from last year.
A few mV's from the scat is ok but the .6v between the emitter and base is the difference between the transistor being on or off.
 
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