Onkyo TX-4500 FM Quartz Lock Fluctuating

[Brivan]

Hi, everyone

I have an Onkyo TX4500 that has some FM tuning locking issues. The tuning (centering) meter barely moves and, when tuned to the strong point of the signal, won't hold a lock on it - instead, the "locked", "tuned" and "stereo" indicators blink about twice per second. The indicators are mostly illuminated, but very briefly turn off, then back on, again. I've had other TX-4500 and TX-4500 MKII receivers that have exhibited this same behavior before, but I've always been able to adjust out of it by going through the tuner alignment. I've gone through the alignment procedure several times on this receiver, however, without any joy.

I have a Meguro FM stereo signal generator that I rarely need to use, but I've connected it up and gone through the alignment with it and have ended up with the same problem.

Wondering if anyone has fought this particular demon before and what you may have done to exorcise it. I've refurbed many Onkyo TX-4500 (and MKII) receivers and they've been mostly trouble free but this one is giving me 'roids!

Thanks!

 

[chazix]

FM tuning locking issues

I don't have any bright ideas myself, but this is one of the exceedingly rare occasions where I think a thread would be best off in the Tuners subforum than in a general subforum. If that makes sense, I think you can click the "Report" link on your own thread, and ask the moderator to move it.

Reading between the lines, I assume that the sensitivity and quieting are normal, despite the lock shyness?

 

[Brivan]

I don't have any bright ideas myself, but this is one of the exceedingly rare occasions where I think a thread would be best off in the Tuners subforum than in a general subforum. If that makes sense, I think you can click the "Report" link on your own thread, and ask the moderator to move it.

Reading between the lines, I assume that the sensitivity and quieting are normal, despite the lock shyness?

Thanks for the reply! Yes, everything else seems pretty normal. I'll try reporting this thread and see if I can get it moved into the Tuners subforum. Thanks for the suggestion!

 

[hjames]

Moved to Tuners Forum as requested.
Good luck!

 

[chazix]

If only to give the thread a bump now that it's been moved, let me ask another dumb question:

If the FM MUTING/LOCK switch is set to OFF, do the TUNED (edit: LOCKED) and STEREO indicators stabilize?

(edit: Looking at the user manual, it seems like the TUNED and LOCKED indicators mean pretty much the opposite of what I would expect them to mean. Dang! I've been guilty of swapping those indicators in an Onkyo tuner, based on my mis-expectation!!)

 

[Brivan]

If only to give the thread a bump now that it's been moved, let me ask another dumb question:

If the FM MUTING/LOCK switch is set to OFF, do the TUNED (edit: LOCKED) and STEREO indicators stabilize?

(edit: Looking at the user manual, it seems like the TUNED and LOCKED indicators mean pretty much the opposite of what I would expect them to mean. Dang! I've been guilty of swapping those indicators in an Onkyo tuner, based on my mis-expectation!!)

Not a dumb question at all! I know a dumb question when I see one (usually when I re-read something I just posted!). With the muting function off, the "locked" and "stereo" indicators remain steadily lit and the "tuned" indicator goes dark - which, I believe is normal operation with the muting off.

 

[chazix]

Well, I'd hoped someone who's really well-versed in Onkyo lock mechanisms would be along, but it looks like you might be the foremost authority!

With the muting function off, the "locked" and "stereo" indicators remain steadily lit

This seems like it's at least a clue that it's just the locking circuit that is malfunctioning. That's probably not definitive, though.

I'm not sure if I will have the time, or even the ability, to wade through how locking works in enough detail to have specific ideas, but here are a couple of things you might look into:

There are two separate FM detectors at play here. The one that involves T101 looks like it's for the audio path, and the one that involves T104 looks like it's just for locking. Does the service manual cover how to adjust both?

In the hifiengine scan of the service manual, the schematic in the vicinity of dual transistor Q127 has some hand-written notes identifying adjustments for "center" and "width". I reckon these are involved in locking. Does the service manual cover how to adjust them (and has that been done)?

 

[Brivan]

Well, I'd hoped someone who's really well-versed in Onkyo lock mechanisms would be along, but it looks like you might be the foremost authority!

This seems like it's at least a clue that it's just the locking circuit that is malfunctioning. That's probably not definitive, though.

I'm not sure if I will have the time, or even the ability, to wade through how locking works in enough detail to have specific ideas, but here are a couple of things you might look into:

There are two separate FM detectors at play here. The one that involves T101 looks like it's for the audio path, and the one that involves T104 looks like it's just for locking. Does the service manual cover how to adjust both?

In the hifiengine scan of the service manual, the schematic in the vicinity of dual transistor Q127 has some hand-written notes identifying adjustments for "center" and "width". I reckon these are involved in locking. Does the service manual cover how to adjust them (and has that been done)?

Thanks for the reply and sorry for my late response - yesterday was kind of a cluster!

Yes, the service manual does describe the adjustments for T101 & T104. I'm pretty sure I've been through them, but it wouldn't hurt to recheck it. The service manual covers the adjustment for center and width as shown here:


R254 (width) has always been a fixed resistor soldered in place on every TX-4500 I've worked on. I've tried different resistance values here with little difference.

Interesting note about the tuner schematic. The one I've been working from is a little further down in the service manual and I didn't pay much attention to this particular tuner schematic until now. The notation points out R282 that runs in parallel to R254 (shown as a fixed value - contrary to what the alignment procedure says). I don't recall if more or less resistance at that point in the circuit created more tuning meter swing, but I'm betting it's less resistance since my resistance swap in place of R254 didn't make much difference. If I have a moment today, I'll try to pull both R254 and R282 and substitute in some varying values (via decade box) and see what happens.

 

[Brivan]

Here's the schematic I was working from. No signs of R282, but it's definitely on the board - right next to R254. Sheesh, I feel a little dumb, right now.

 

[Brivan]

Today I removed R282, as well as the 1K variable resistor I had in place of R254. I soldered in some short leads in place of R282 and connected my decade box to them. It didn't matter how much resistance I had selected, it made no difference on the tuning meter width or the oscillating "tuned" "locked" and "stereo" indicators. I also should mention that, previously, I replaced Q127 with two KSC1845 transistors, with no difference in tuning meter response or oscillating tuning lights.

 

[Brivan]

Last week, I pulled T104 & T105 and had a look at the internal capacitors. They looked dried up up and bad. Just putting a slight amount of pressure on them caused them to crumble. I have no idea what the value of those caps are supposed to be, so I guessed and subbed in some 82pF caps that I had used in some tuning section transformers in some Realistic receivers. Surprisingly, I was able to adjust both T104 & T105 and stabilize the quartz lock circuit so that the "tuned", "locked" and "stereo" indicators remain solidly lit when a strong FM station is tuned in. That's one hurdle, I guess.

I still cannot get the tuning center meter to move more than just a fraction. I subbed in resistances for R254/R282, as well as R252. I might try altering resistance values of R248 and R250 to see if that makes any difference.

 

[two.dogs]

The tuning circuit description in the 4500 service manual is not-so-much. "Touch of the tuning knob causes the ham to be initiated..." It helps to read the 6500 manual and even the T-9 tuner manual because they are better written and have similar design.

 

[Brivan]

The tuning circuit description in the 4500 service manual is not-so-much. "Touch of the tuning knob causes the ham to be initiated..." It helps to read the 6500 manual and even the T-9 tuner manual because they are better written and have similar design.

Thanks so much for that information! I'll download both manuals and see what I can find out.

 

[dly66]

Bookmarking this. Have a TX-4500 Mk II that was working fine after restoration suddenly had similar behavior. When tuning a station center meter goes to one side and all three lights blink about twice a second. Pushing in the button to disable quart lock kills the locked light but tuned and stereo would stay stable for extended periods. But would notice if on for a long time (maybe an hour) it would start losing the stereo.

Searched for ideas this thread giving me some things to look at. Been on the shelf until I have a better idea what to attack.

 

[Brivan]

Bookmarking this. Have a TX-4500 Mk II that was working fine after restoration suddenly had similar behavior. When tuning a station center meter goes to one side and all three lights blink about twice a second. Pushing in the button to disable quart lock kills the locked light but tuned and stereo would stay stable for extended periods. But would notice if on for a long time (maybe an hour) it would start losing the stereo.

Searched for ideas this thread giving me some things to look at. Been on the shelf until I have a better idea what to attack.

I've had a few Onkyo TX-4500 MKIIs that have exhibited roughly the same symptoms as you describe (and like what I'm having with the TX-4500). For the MKII, going through quartz locked and tuning meter center adjustments have always taken care of the problem. I have a signal generator but don't use it too often. For these adjustments, I found that tuning to a strong signal as close to the center of the dial (98Mhz) as possible works fine.

 

[Brivan]

The tuning circuit description in the 4500 service manual is not-so-much. "Touch of the tuning knob causes the ham to be initiated..." It helps to read the 6500 manual and even the T-9 tuner manual because they are better written and have similar design.

I found a copy of the Onkyo T-9 service manual, but could only find a service manual for the TX-6500 MKII - nothing for the TX-6500. Would you happen to know a source for the TX-6500 service manual? Thanks.

 

[two.dogs]

I just have the MKII manual. The TX-8500 uses the same tuner board as the 4500 but not much description in the service manual. You've mentioned that the lights blink, but are there performance issues as well? Does you signal generator produce a good sine wave at the outputs? Good stereo signal?

 

[wojcimierz]

I found a copy of the Onkyo T-9 service manual, but could only find a service manual for the TX-6500 MKII - nothing for the TX-6500. Would you happen to know a source for the TX-6500 service manual? Thanks.

Hi Brivan,
It's been quite a long time since your last post but you haven't left a feedback if you've resolved your problem. Have you figured it out?

 

[Brivan]

Hi Brivan,
It's been quite a long time since your last post but you haven't left a feedback if you've resolved your problem. Have you figured it out?

My apologies but I have no notes on what I did to resolve the issue. The nearest I can recall is that I likely went through the alignment procedure another time or two, then finally got it. Some tweaking of T251 outside of what's specified in the manual may have done the trick.

 

[wojcimierz]

My apologies but I have no notes on what I did to resolve the issue. The nearest I can recall is that I likely went through the alignment procedure another time or two, then finally got it. Some tweaking of T251 outside of what's specified in the manual may have done the trick.

Thanks Brivan,
I will then look into alignment procedure and try to play with T151 (I think that's what you ment, not T251 )