Is 70mv Bias correct for a Sony TA-F444ESII?

[northpaw]

I picked up a Sony TA-F444ESII integrated amp yesterday, and after checking for DC offset (good at 5mv and 0.2mv), listened to it through a pair of BA A60s. Sounded good except for some scratchiness/weakness on the left channel, which kept improving as I exercised the balance knob.

This morning I opened it up and cleaned the pots. I also wanted to check and adjust the bias. The bias pots were well labeled on the main board, as were the bias test points, as shown in the photo. The TP are well labeled as 70mv, which struck me as quite high. I measured across the TP, and presently there is only 3-4mv of bias. The amp does run fairly cool at this setting.

Not having a complete service manual for the TA-F444ESII (I have the circuit diagrams but no layout diagrams or the section on setting bias), I'd appreciate it if anyone can confirm and/or comment on the 70mv bias setting

I could find service manuals for TA-F444ES, and the ESX model, but both are quite different from the TA-F444ESII (e.g., there are only 2 large PS caps on the TA-F444ESII, not 4, and oddly, there appears to be no provision for DC offset adjustments on the TA-F444ESII). For supposedly similar models such as TA-F333ES and the higher-powered TA-F555ESII, the service manuals specify bias at much lower levels (7.5mv, 15mv, and 35mv). That is why the 70mv bias struck me as unusually high.

 

[restorer-john]

It's a missprint on the PCB or perhaps there's a tiny decimal point we can't see.

It's 7mV according to the service manual:

 

[northpaw]

Thanks so much, John, for the info and the scan. Greatly appreciated. 7mv makes a lot more sense.

You live in a beautiful area of the world. My son-in-law is from Brisbane, and a couple of years ago I had the opportunity to drive south from there along the coast, staying at his family's property south of Kingscliff.

 

[Goldie99]

As it happens, I'm also working on a TA-F444ES II at the moment, and can confirm the 7mV is correct - I've obviously got the same hardcopy manual.

Be sure to check all of the fusible resistors as well - they can all be measured in situ. Of the 22 fusibles in mine, 14 were way too high and were 'throttling' the amp to the point that it was unusable - simply changing the fusibles & adjusting bias has corrected >95% of the problem. Hopefully, a bit more switch cleaning will bring the rest back.

Alan

 

[northpaw]

That is really useful info about the fusible resistors. I will check them. When I tried the unit before buying, I thought it needed a bit more turn on the dial for a given sound level at first, but that was in an exceptionally large space with very small bookshelf speakers hooked up. Now in a living room driving old large Advents, and with the bias adjusted and the pot cleaning, it seems within the normal range.

I presume for safety, the fusibles should be replaced with the same. I've not come across them before, so I'd be delighted to know if there are particular brands/types (metal film or wirewound?) that you would recommend, or recommend to avoid? Thanks again.

 

[Goldie99]

Fusibles - you'll see at least 2 trains of thought on fusibles - depends which forum you use / who you listen to. They are notorious for drifting high with age, so some people opt for replacing with metal films, others insist on like-for-like replacement with new fusibles.... I'm still new at this, but as the Sony service manual lists them as "critical for safety", I just use new fusibles whenever possible.

They are not so widely available anymore (old technology). I got most of them in UK, but Mouser do carry a couple of ranges. You'll need to match fusible resistance & power rating.

http://www.mouser.com/ds/2/427/nfr25-239840.pdf
http://www.mouser.com/ds/2/447/Yageo LR_FRM_2013-774869.pdf

I'll be interested to hear how yours are doing after 30+ years.... not as bad as mine were by the sound of it, but I'll be surprised if there's not several well out of spec.

Alan

 

[northpaw]

Thanks, Alan, for providing that information, much appreciated. I will give the measurements a go this weekend, and report back on the state of the fusibles. I think this unit had a relatively easy life, very clean cosmetically and I found only a little dust inside.

In preparation, I have used the schematic I have to make a list of the 22 fusibles you mentioned and their ratings. All except two have a power rating indicated, so I presume those other two ((R516 and R566, 180 ohms) are the default 1/4 watt. All the other fusibles are labeled half watt.

 

[Goldie99]

R516 & 566 are actually specified 1/4W (service manual parts list), the others are indeed all 1/2W.

I had to use new old stock items for the 1/4W 180R's, from Cricklewood Electronics in London (an old, but reliable supplier), as well as 2X 180R 1/4W in parallel, for the 91R 1/2W replacements. If you get stuck for the 180R's, drop me a pm.

I suspect you'll find your 180R fusibles out of spec., as well as the 91R's - only the 10R base stoppers were OK in mine, everything else was out of spec., and all got changed anyway.

 

[restorer-john]

I too use 1/8, 1/4 or 1/2 watt watt resistors for fusible replacements, but usually carbon as they are more likely than metal film to 'let go' in situations where you need them to.

What I do is space them off the board about 5-10mm so if they attempt to fuse, they won't char the board.

Fusibles were a good idea, but they couldn't maintain their fixed resistance over time, I think the fusible link (some sort of conductive polymer I think), infects the carbon track and throws the resistance out.

Northpaw, glad you liked our neck of the woods 🙂 We live in a pole home on a few acres of heavy bushland, perched on the edge of a ridge, overlooking a valley and up into the border ranges and Springbrook National Park. (type Springbrook National Park into google images). We are about 10 minutes from the beaches.

This is what today looks like- it is supposed to be winter and it's 21.6degrees (71F), heading for a top of 30! (86F) today:

 

[ilimzn]

Fusibles are most commonly replaced by metal film because metal film resistors are usually of non-flammable construction. The idea behind fusibles was to sacrifice a series resistor rather than burning out other parts or PCB tracks. However, they were only half good at their job even without the reliability problems. In later constructions they were in fact replaced by small metal films, again with the idea of sacrificing them but without starting a fire (which carbon film resistors can, due to the combustibility of the pure carbon resistive film). It is generally NOT recommended to replace them with old stock fusibles (which is mostly what you can find) because they eventually develop the very same problems, just even faster since they have decayed somewhat just sitting on the shelf. They are especially not to be trusted if they were exposed to some sort of high current situation (perhaps by failure of other parts in the amp) but yet have not gone open, EVEN if they check out for resistance. Remember, there is a reason why they are not used any more - it has turned out not to be a reliable technology.

 

[niss_man]

I too use 1/8, 1/4 or 1/2 watt watt resistors for fusible replacements, but usually carbon as they are more likely than metal film to 'let go' in situations where you need them to.

What I do is space them off the board about 5-10mm so if they attempt to fuse, they won't char the board.

Fusibles were a good idea, but they couldn't maintain their fixed resistance over time, I think the fusible link (some sort of conductive polymer I think), infects the carbon track and throws the resistance out.

Northpaw, glad you liked our neck of the woods 🙂 We live in a pole home on a few acres of heavy bushland, perched on the edge of a ridge, overlooking a valley and up into the border ranges and Springbrook National Park. (type Springbrook National Park into google images). We are about 10 minutes from the beaches.

This is what today looks like- it is supposed to be winter and it's 21.6degrees (71F), heading for a top of 30! (86F) today:

My family and I love springbrook national park and are avid bushwalkers. I'd rank it as the best national park in south east Queensland.

 

[Goldie99]

I'm certainly no expert, but I would note a couple of points:

It is generally NOT recommended to replace them with old stock fusibles (which is mostly what you can find)

Agreed - but the 2 series I referred to at Mouser are current production products from Vishay & Yageo, specifically listed as fusible, flameproof, metal films, so I assume new, and not old stock. I also used other (new) fusibles from RS-Online in UK, but didn't reference them as I don't think RS-Online is available in USA, where the OP is. If anyone needs the RS-Online items - simply search "fusible resistor" on the RS-Online site, or use the following link

Through Hole Fixed Resistors | RS Components

New old stock items are certainly far more questionable, but were the best I could find specifically for the 180R 0.25W fusibles. Everything else I used was new, current production, as far as I'm aware.... so should hopefully last another 20 - 30 years.

They are especially not to be trusted if they were exposed to some sort of high current situation (perhaps by failure of other parts in the amp)

Agreed - the manual for the Sony TA-F444ES II indeed states the same, albeit specific to the fusibles in the power amp section only. Having measured all of mine, I'd recommend to extend that to all 22 of them.

Alan

 

[northpaw]

Fusible resistor measurements

Well, Goldie was right at the beginning, there are divergent opinions on the fusibles. Lots of good discussion here, but before I chime in, let me provide the results of my measurements on the 22 fusibles on this ~30 yr old integrated.

R 505, R 510, R 555, R 560____ 227 228 229 226 Nominal Value 220/0.5w

R 512, R 562____ 122.8 128.9 Nominal Value 91/0.5w

R 515, R 565____ 81.6 81.9 Nominal Value 82/0.5w

R 516, R 566____ 188.7 188.4 Nominal Value 180/0.25w

R 530, R 531, R 527, R 528____ 9.9 9.9 10.0 9.9 Nominal Value 10/0.5w
R 580, R 581, R 577, R 578____ 10.1 9.9 10.0 9.8 Nominal Value 10/0.5w

R 526, R 576____ 468 465 Nominal Value 470/0.5w

R 529, R 579____ 100.5 99.2 Nominal Value 100/0.5w


So it looks like this Sony is in surprisingly good shape, all considering. The two 91 ohm resistors are out of spec and high, and the two 180 ohm resistors are near the top end of spec, but all the rest look very good.

 

[northpaw]

Northpaw, glad you liked our neck of the woods 🙂 We live in a pole home on a few acres of heavy bushland, perched on the edge of a ridge, overlooking a valley and up into the border ranges and Springbrook National Park. (type Springbrook National Park into google images). We are about 10 minutes from the beaches.

This is what today looks like- it is supposed to be winter and it's 21.6degrees (71F), heading for a top of 30! (86F) today

You do know that you are living on the flanks of an extinct volcano, right? The main vent was roughly at the present location of Mt. Warning, and you are located on the northern apron. Probably good soils for growing things. I'm a geologist by trade, and don't stop working even while on vacation. I was there in your summer, but you could tell from the vegetation the region was solidly tropical. I mean mango trees, bougainvillea, etc... It did seem like a paradise.

Edit: I should mention that my son-in-law chose Cabarita Beach at sunset as the place to propose to my daughter.

 

[northpaw]

Thanks ilizn, John, and of course Goldie for the discussion and insights on fusibles, and Goldie for those convenient links to Mouser for current production. They indeed have stock in the half watt versions, and small quantities can be ordered.

In the near future, I plan to replace the two that have drifted high, and will probably order others so I have them handy in case some of the others drift higher. This was a single-owner unit, and I believe it was not used much recently (last several years), perhaps even longer, and therefore I will check the fusibles from time to time in case they might start drifting more rapidly with more frequent use.

I did do a search looking for cases where fusibles in Sonys have failed and did not come up with much (not that everyone with this problem feels compelled to post about it), but nonetheless it is clear that these resistors have failed enough in the universe of audio gear to generate strong feelings from those that have dealt with troubleshooting them and not wanting to have them come back, such as from disappointed customers.

I'm inclined to follow Goldie's advice and stick with fusible replacements (at least the half watt ones), as they are cheap insurance, I don't mind checking them from time to time, and I'd rather replace them than deal with troubleshooting and replacing trashed transistors, even if that is infrequent. But if I was repairing for someone else, for money, I might feel differently.

Thanks again to all for the discussion. I've learned a lot.

 

[ilimzn]

There is an old saying that goes something like this: transistors act as fuses for fast acting fuses, to protect them from burning out. There is quite a bit of truth in this, and especially in small signal stages where a strategically placed problem can make the apparently small current circuit draw such currents that it goes up in flames.
Fusible resistors are not there to protect the transistors in most cases but to prevent a problem from propagating catastrophically and by fire (literally). Fusible resistors fall into the 'slow blow' fuse category - keep this in mind. They can sometimes save a component, like when a base resistor goes open on an output stage transistor, but you never know what the actual state of this part is then, aside of checking it with a curve tracer (and knowing how and what to look for). When things like that happen, a good tech will preventively change all power transistors anyway - and the blown fusible. So as i said - keep in mind this technology and idea was phased out later on. There is a reason for it.

 

[northpaw]

Thanks for pointing out these issues, ilimzn. If I understand you properly (my electronics knowledge is pretty shallow), the problem is that a fusible resistor may sacrifice itself, but still fail to save other components from damage, and further that it may be difficult to diagnose that damage, requiring any prudent repair effort following the fail out of a fusible to include replacing those other components anyway.

So if I stick with fusible replacements, I may or may not help myself with protecting the other parts of the circuit. I get that from what you said (not happy about learning that, but appreciative). I can also see the benefit of a design based on more standard resistors and therefore avoiding the inevitable degrading of fusible resistors, which probably is more common than the failing of other circuit components (thinking here of the case of reasonable users that don't abuse their equipment).

But these thoughts are also tempered by two considerations with regard to the Sony I now own:

(1) as an owner/user, I have the option to go incrementally with the fix of a fusible resistor failure, and see if the replacements also promptly fail or other damage manifests itself in terms performance I can hear, or otherwise detect. I understand that this would probably make less sense for a repair shop situation.

(2) As Goldie alluded to, Sony seems to believe there are safety considerations that dictate the use of fusibles in those particular circuits. I take this to mean that if a standard resistor is used, the damage created elsewhere could create a dangerous situation such as a fire, as you alluded to.

So I'm not sure if you are advocating the use a standard resistor instead of a fusible (but what other alternative would I have if a fusible is not to be used?), or if you simply wished to point out the pitfalls of designs based on fusibles, which you have done very well.

If I were to use a standard resistor in place of a fusible, do you have an opinion about whether I should (or need) to do anything else to mitigate the safety risk, or is that risk no worse than the risks present in the use of all the other amps out there that do not use fusibles at similar places in their circuits? That is, has Sony used fusibles in place of other means of insuring safety, such as internal fuses? I know my Adcom and NAD amps have quite a few internal fuses, but the only fuse I've noticed so far on this Sony is on the power switch. It does have some other protection circuitry, but I am not knowledgeable enough to understand/evaluate its role vis-a-vis the circuits with the fusibles.

I note that restorer-john seems to take a middle ground in the replacement of fusibles, by using a low-watt carbon resistor that is capable of failing in dire circumstances, but otherwise does not drift on its own.

Thanks again for your comments. They are very useful.

 

[restorer-john]

...You do know that you are living on the flanks of an extinct volcano, right?..

Yes, we've climbed and trekked most of the peaks around here and I first climbed Mt Warning when I was 11, (39 years ago).

Geologist huh, I could do with your help right now as we are about to get land/soil tests done for our house extension- our land is so steep only the wallabies can traverse it! Lot of piles, footings and poles I'd say. 🙂

Back to the discussion, your 444 appears fine. I'd leave the fusibles alone.

I use this Xuron 573 to give any heat producing resistors an opportunity to 'breathe' by spacing them consistently and securely above the PCB:

 

[ilimzn]

Thanks for pointing out these issues, ilimzn. If I understand you properly (my electronics knowledge is pretty shallow), the problem is that a fusible resistor may sacrifice itself, but still fail to save other components from damage, and further that it may be difficult to diagnose that damage, requiring any prudent repair effort following the fail out of a fusible to include replacing those other components anyway.

Sort of... let me illustrate what I mean with some statistics. This is what I find in the amps I have restored or fixed over the years (and there have been MANY):
- 10% dead fusible (*) and all other components intact in units that use fusibles
- 89% dead components consistent with the failure mode and history (in case of cascade failures) AND dead fusibles (*) in units that use them
- 1% fusible resistor managed to save one component out of a handful involved in a failure - MAYBE (*), it was as likely it would have been saved by a regular resistor in the same place.

And here is where the (*) comes in - before there were fusibles, regular resistors were used. After fusibles have fallen out of grace (sometime in the late 80s - early 90s) regular resistors of a non-flammable construction were (and still are) used. All in the exact same 'critical' places. Fusibles were not magically inserted into parts of circuits, their positions are the same as regular resistors used for the same purpose - usually series connected to something that could fail, with the idea of limiting current and eventually expiring because of overheating. In some cases fusibles replaced real fuses and today there are other technologies used for such purposes. YET - I have yet to discover a regular resistor used in such a position that has quietly drifted or even gone open while at the same time still looking perfect - which is something that does not happen to regular resistors, they will look burned, and believe me, this helps with fault finding because you can see it.
The thing with fusibles is that they do NOT protect components, but rather in the event of component failure to short, protect the rest of the circuit by current limiting, without emitting smoke or (perish the thought) flames. A regular resistor will do the protection part (if well dimensioned) but it WILL smoke. I would not be surprised if it turns out fusible resistor use had more to do with the appearance of a failure to be a fire hazard (due to smoke) i.e. a legal issue, more than anything else.

(1) as an owner/user, I have the option to go incrementally with the fix of a fusible resistor failure, and see if the replacements also promptly fail or other damage manifests itself in terms performance I can hear, or otherwise detect. I understand that this would probably make less sense for a repair shop situation.

What you decide here has more to do with your won peace of mind. If you know in advance that there is an inherent failure mode, a possibility of failure happening in the future is not such a big problem as you already know the most probable culprit. What I would do is, replace the drifted ones (92 and 180 ohms), bring the amp back to full spec, and decide if I'll keep it or not.If yes, perhaps given some time, invest it into a more complete overhaul.
It should be noted that not all fusibles are created equal. They do not ALL inherently fail, some can work for ages. But in a situation where you are making thousands of amplifiers, this statistic is important. In the case of one amp, yours, it might not be, once the resistors have been verified. One thing which you still might want to do is find out which ones heat up the most - if you can't keep a finger on them, these are likely to fail first if they do.

(2) As Goldie alluded to, Sony seems to believe there are safety considerations that dictate the use of fusibles in those particular circuits. I take this to mean that if a standard resistor is used, the damage created elsewhere could create a dangerous situation such as a fire, as you alluded to.

For Sony - and all other manufacturers really -a 'critical' resistor is one whose specs must stay the same as in the schematic. Mostly this has to do with size. This is because in 99.99% of other cases, you can replace a resistor with one of 'the next size up' without a problem, or a carbon one with metal film (or the other way around), etc. Not so for positions where fusibles were used unless you want to risk other damage.
That being said, if you looked at other service manuals, before or after the time fusibles were widely used, you will find all the same resistor positions (amp circuits are not that different, in some cases carried over 99& or even cut down in newer devices!) just with regular resistors. They will still be specced as critical, and it will usually be the size that matters. In the olden days, they would sometimes use a regular carbon resistor just enclose it in a small glass weave tube so it could not flame out. Today they use small metal film ones. Many ways to skin a cat, and the cat has been around a long long time.

So I'm not sure if you are advocating the use a standard resistor instead of a fusible (but what other alternative would I have if a fusible is not to be used?), or if you simply wished to point out the pitfalls of designs based on fusibles, which you have done very well.

Both, really. If you can find fusible replacements for a reasonable price, modern ones will surely be better than old ones. However, they will always be more expensive than regular resistors. And - the latter is what the industry now uses in those places 99.9% of the time. Again, if it was me, I would not even bother with fusibles unless they were really, really, really needed (have not yet found such a situation, though)

If I were to use a standard resistor in place of a fusible, do you have an opinion about whether I should (or need) to do anything else to mitigate the safety risk, or is that risk no worse than the risks present in the use of all the other amps out there that do not use fusibles at similar places in their circuits? That is, has Sony used fusibles in place of other means of insuring safety, such as internal fuses? I know my Adcom and NAD amps have quite a few internal fuses, but the only fuse I've noticed so far on this Sony is on the power switch. It does have some other protection circuitry, but I am not knowledgeable enough to understand/evaluate its role vis-a-vis the circuits with the fusibles.
I note that restorer-john seems to take a middle ground in the replacement of fusibles, by using a low-watt carbon resistor that is capable of failing in dire circumstances, but otherwise does not drift on its own.
Thanks again for your comments. They are very useful.

Nothing more should be needed except for rising the resistors off board, such as restorer-john does. I would not use carbon films in places where the possibility of flaming could be an issue (as I said, in the old days they would put a small piece of glass weave tubing to prevent full exposure to air so pxygen supply would be limited to prevent flaming of the carbon), and I would (and do) use metal film - however it is sometimes difficult to find them in anything less than 0.5W size. This size is actually the same a 0.25W or sometimes 0.33W carbon film, simply because the former can work at higher temperatures. Still, in the case of failure, one is looking at much more than the usual extreme of the spec so in most cases 0.5W metal film will do (sometimes 0.33W can be found as well). Check the current rating and calculate with the specified resistance to get approx. size (current rating will be much higher than actual current, by 2-3 times), i.e. W equivalent of the resistor that replaces the fusible. Usually the size is a good clue - it should be approximately the same or one seze smaller (metal film...) as the fusible it replaces.
You have also given an example of internal fixed fusing, which is the way to handle protection of fairly high current circuits - but this was never the realm of fusible resistors in the first place, so you should be OK there.

 

[Goldie99]

Many thanks ilimzn - I appreciate the extra info / details - as a relative newcomer, I've read numerous posts citing the unreliability of fusibles etc., but few that have bothered to go into much detail or explanation. It's good to find someone prepared to spend the time to explain things.

I'd realised from the datasheets (specifically, the times to fuse) that the fusibles were not being used to protect components, but were to limit current in the event of other failures. I assumed the safety reference from Sony referred to the fusible characteristics of always failing 'open' and / or being 'flameproof' - so, in looking for suitable replacements for the fusibles in my amp (where 14 out of 22 were already so far out of the spec., the amp was unusable), those were the features I concentrated on. With limited experience - I tend to go with what's in the manual... usually.

As I could readily obtain 18 / 22 of the necessary values as new, replacement fusibles, which I assumed would be at least as reliable as the originals (but probably better), that seemed like a no-brainer, and still does.

Where I had more difficulty was specifically for the 180R 0.25W and 91R 0.5W items - for which I've not seen new replacement fusibles anywhere, only the (not so) old stock 180R / 0.25W items from Cricklewood Electronics. I did consider metal films at the time, but didn't find those values in a 'flameproof' grade - so opted for the new old stock items instead, especially as I could just use them directly for the 180R positions, and put a couple in parallel for the 91R 0.5W positions - 2 birds with one stone as it were....

Again, with limited experience, I don't have a good frame of reference for how most standard metal films tend to fail with over-current - e.g., how likely they are to flame, or not. Since there are specific flameproof metal film grades available, I assumed a worst case scenario, and restricted my potential choices specifically to the flameproof grades, but couldn't find the relevant values needed..... esp. at 0.25W. Are you saying that 'standard' metal film grades are also just as suitable to use, e.g., not just those metal films that are specified as 'flameproof ' ?

It should be noted that not all fusibles are created equal. They do not ALL inherently fail, some can work for ages.

If any more proof were needed - my daily amp, used up to 12 hours a day for the past ca. 30 years since I bought it new, is a Sony TA-F500ES, still running strong.... with 16X original fusibles... another to add to the to do list (I'm ashamed to say I've never checked them) !

Anyway - hope that explains my 'logic' a bit, and thanks again for the added info.

Alan