Adjusting signal with resistor

Alex SE

Alex SE

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To make long story short, if I want to adjust a signal level, is it properly to change value of R559 (and R560 for a second channel) ?

1647762086163.png
 
In the understanding of a circuit you need to know about Op Amp circuit gain designs. This is a non inverting op amp circuit and if adjustment is needed I would commonly put a Cermet trimmer in to work around the value that is in the circuit at the right place. R569 is not the right place- R555 could get a 68K resistor in series with a 20K trimmer and then you would have adjustment as you might need. The 68K provides a minimum amount of gain and then the wiper would take 68K to 88K allowing a lot of adjustment. I would also get rid of the 6.3V cap C553 and put in a 16V fresh cap minimum.
 
Skywavebe said:
In the understanding of a circuit you need to know about Op Amp circuit gain designs. This is a non inverting op amp circuit and if adjustment is needed I would commonly put a Cermet trimmer in to work around the value that is in the circuit at the right place. R569 is not the right place- R555 could get a 68K resistor in series with a 20K trimmer and then you would have adjustment as you might need. The 68K provides a minimum amount of gain and then the wiper would take 68K to 88K allowing a lot of adjustment.
Click to expand...

If that does matters, I'm trying to lower a signal, peak meters are showing 2db more than they should. That means that I need a lower resistance. How I was thinking is just to let more of signal go to the ground thru that R559. That was just a thinking. Anyway, doing what you have suggested means that I can without a problem put, say, 85K or 88K instead for 82K and it will do a job? I have a 100K trimmers. Will adjust them to 88K and put in place of that resistor. Then I can try with 90K or so. Thanks for advice :)
Sorry for asking, just trying to understand, that R559 is voltage divider or not? Isn't it the easiest solution to use such dividers for adjustments. I mean, send to a ground everything you don't need , and opposite.

Skywavebe said:
I would also get rid of the 6.3V cap C553 and put in a 16V fresh cap minimum.
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Last time I purchased caps for a whole deck I got some low-profile Nichicon caps but I never replaced those on a FL pcb because some of them are under a display, probably impossible to replace without removing a display itself and I'm kinda scared to desolder it (apply a heat on it). I'll check if the one you are talking about is somewhere beside or under a display. Anyway, I'm trying to understand, why should that cap be replaced by 16V?
 
Peak meter are suppose to be 3dB higher than Vu meters as this is the characteristic of a peak meter. Their reaction time and scale are made to operate that way. The only adjustment that you should do is set the 400 Hz o dB meter position when the specified output is had. Most good decks allow for plenty of adjustment so the modification of the deck is kind of strange and probably not needed. I have come across decks where the pots they used are near their end post- the wiper that is , and this is a unstable location. Trimmers should be in the 90-10% level of a trimmer otherwise circuits are way out of whack and you are trying to compensate for bad circuits with a extreme meter setting. In some cases I have increased the pot or trimmer value from like 2K to 5 K just because at the time the 2K were not in stock and that the 5 K will allow for better range adjustment- it does not ruin or upset anything as meter circuits are just display reference devices. I put in Bourns Cermet trimmers not the garbage carbon ones. The Ceramic substrate trimmers will not age and become intermittent like the carbon ones do.
 
Flouro tunes are easy to take off and put back. The only part that is delicate is the vacuum nozzle. This if broken means you need a new tube. I have taken tubes off to put in new drive IC's when needed not a lot but a few- it is no big deal to take them out.
 
Thanks for explanations, really. There are 3 points that can be adjusted:
1. Dolby (pb) level
2. Deck output level (which in my case is done by replacing IARC R41 and R42.
3. Peak meters

Now, all of those 3 are in chain, which means, touch first one and 2nd and 3rd are out of specs, touch 2nd one and 3rd is out of specs. In just my case, after adjusting dolby level, output is not 400mV as it should be, and It can be resolved by putting something like 11K or 12K instead of 15K. Next one is peak meters which should be at +2.8 with a test tape I have (let's say +3). That is what I have wanted to do for a long time ago but couldn't find out how to do. Watching SM, it seems that area at the picture is only area where mods should be possible.
I'll do as you said, 68K and trimmer, only to see what I need and will after that order resistors. Just great that there are plenty of resistor values out there.

Can you explain, if you have a time, that R559 I'm talking about is voltage divider or not? Isn't it the easiest solution to use such dividers for adjustments. I mean, send to a ground everything that exceeds, and opposite.
 
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First you offer a section of diagram and no telling what device this is from. You need to get the outputs from the circuit rather than worry about meter display first. Lack of getting proper output or test signals can be attributed to wrong measure technique or not correcting for circuit decay where it needs to be taken care of. Changing one thing in a chain does not affect the other parts but the voltage level might change which is what you apparently are after. Modifications that I do on decks have to have a good reason to do it not just because a adjustment can not be done. Like I was telling a trainee who is trying to calibrate a A4010S from Vietnam days, if you still have 60 old caps in the deck the calibration will not be possible but I am not knowing what decks you are working on.
 
Why get rid of 10V and 6.3V caps? Well first they are old most likely and then these voltage type caps have a high degree of failure to them and often times this is what the problems in a system are. My shop does not use anything less than 16 V caps. I do not allow any SMD parts in except for op amps which go into Walkmans.
 
Skywavebe said:
First you offer a section of diagram and no telling what device this is from. You need to get the outputs from the circuit rather than worry about meter display first. Lack of getting proper output or test signals can be attributed to wrong measure technique or not correcting for circuit decay where it needs to be taken care of. Changing one thing in a chain does not affect the other parts but the voltage level might change which is what you apparently are after. Modifications that I do on decks have to have a good reason to do it not just because a adjustment can not be done. Like I was telling a trainee who is trying to calibrate a A4010S from Vietnam days, if you still have 60 old caps in the deck the calibration will not be possible but I am not knowing what decks you are working on.
Click to expand...

It is RS-B965. All caps are replaced with Nichicon UPW/UES and Panasonic FM/FR (except those on FL display pcb). PB level is OK, output can be, as I wrote earlier, adjusted by replacing R41 and R42 (10K) with 11 or 12K. It is tested and it works. So, last point is peak meter and there are only 2 components between those R41/R42 and display. Practically, what you see on that picture in my post is everything that have to do with signal to display to do, from letter f to a pin 25 (SIGL) on a IC.
Here is a picture where that signal is coming from. Green line is showing signal coming from Dolby chip. Next, signal goes to output and the other side goes to a display pcb to that f letter on that picture I have posted earlier.

1647796520012.png
 
Skywavebe said:
Why get rid of 10V and 6.3V caps? Well first they are old most likely and then these voltage type caps have a high degree of failure to them and often times this is what the problems in a system are. My shop does not use anything less than 16 V caps. I do not allow any SMD parts in except for op amps which go into Walkmans.
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Thanks, I have learned myself something new. Have checked, have only 10V 100uF. Will order 16V. So, it means that all caps can be replaced with 2 step higher voltage, of course if size allows. Some of caps I have replaced in that deck are 1 step higher, say 35V instead of 25V, but not all and I didn't know that even 2 steps is safe. Just love places like this :)
 
I ALWAYS go up on voltage if I replace caps. Always.

I try to tighten tolerance is possible too, to get back to the design intent. I don’t have fancy test gear so I spend a few cents more per cap.

I always go up on temperature too. Generally, new caps “up-rated” end up the same size as their elderly counterparts.
 
J!m said:
I ALWAYS go up on voltage if I replace caps. Always.

I try to tighten tolerance is possible too, to get back to the design intent. I don’t have fancy test gear so I spend a few cents more per cap.

I always go up on temperature too. Generally, new caps “up-rated” end up the same size as their elderly counterparts.
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Smaller tolerance is always best. Was wondering how manufacturers could get equipment up to specs when all parts are 5% tolerance. For example, I have never seen 10K resistor in Technics decks which was higher in value than 9.85K. Nothing special, but when you have 10 or more in the path, that can make little higher difference at the end of path.
After researching it seems that Nichicon UPW and Panasonic FM and FR are most used as polarized and Nichicon UES as bipolar caps. Mouser and DigiKey is good source for all components, no risk for counter. Only problem is that I have to order for at least 50$ to get free shipping to EU so couple resistors or caps is not alternative :)
 
The big question is why can you not get 400mV at output. The manual I have downloaded from Hi Fi Engine is terrible as it never gives a playback calibration procedure or tell you what the specified output is- you are all left to guessing. The 400mV at the specs is just the nominal level but does not mean the specified output level. Maybe it is? 400mV would be -8dBV and -10 dBu. That manual never specified any of the standards used. It seems to be more concerned about how to take the unit apart but skips over a large requirement of the calibration method. This is a real bogus manual as it has more pages of what you do not need and no pages of what you do need. Page 12 of the 555 manual at least says this. This company seems to be more about selling cheap stuff than doing a good job- now I know why the Japanese never buy National.
The R559 is simply as load resistor.
 
The tolerance of these parts have very little significance except for resistors in gain structures. 5% or 10% is good enough. The caps +20% would not cause any problems at all as all they do is pass a signal. The amount that all you guys worry about makes no difference at all. The caps that are more critical are the Mylar for EQ effects and any in the bias oscillator so that the frequency remains in spec to some degree- there are coils to adjust that usually. This is not Aerospace or NASA equipment and as you can tell from the service manual they did not put much effort in even providing the documentation that is needed for play and record cal. This is Another Technics product I am not impressed with.
 
I agree that SM sucks, lot of information is missing. Don't have all of Technics decks but only some from RS-B and RS-BX series. Interesting is that SM for all of them states that only output should be adjusted to 400mV and nothing else. As we all know, there are many cases where tapes recorded with dolby on one deck are not compactible to other decks. The reason is that dolby level is not up to specs.
When I first time adjusted one deck to output level 400mV, first thing I have noticed is that dolby level was about 20 or 30mV under the specs. Then I have adjusted pb level so that voltage on a dolby IC is correct. What I got is output level higher than it should be and not just that but peak meters are off. Same thing on the other Technics model. Now comes the question: If all of their decks are adjusted by checking output level and not dolby level, how the hell they expected decks to be compatible with other decks?!
Anyway, RS-B965 is a really good deck when all mods by Alex ANT are applied. From factory it is just a middle class deck. I have two of them and really like them.
Anyway, weekend is over and I'm not gonna have a time to test a solution you said, but will do it next weekend and will be back with results. I know most of people think that peak meters are just an eye-candy, but I believe that there are some like me who cares about all details. Information like this should be shared, for those who are perfectionists and care.
 
Makymak said:
Sorry, Alex. At Dolby level, the output of these decks is about 545mV. The 400mV are for the 160nWb test tape and is something 2.8 dB below the Dolby level.
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Yes, in my case it should be 545mV and +2.8db as a test tape I'm using is 200nWb and not 160. With 160nWb tape peaks should be at 0db. Problem was that it was more than 570 on output if I remember correct and peaks are at +5 and +6db, which I have to reduce.
 
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