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Dare I recap a tuner?

EPI-Center

EPI-Center

Super Member
i have a 39 year old sansui tu-9500 and was wondering if the sound would change dramatically if I recapped it? I know I'd possibly need to get it realigned, but I'm curious about the recap because in the solid state section people are happily recapping amps up the wazoo but I rarely see anything in here about tuner recaps. Why would 40 year old caps be a bad deal in speakers and amps but ok in a tuner?
 
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Well, Some would say 40 year old caps aren't good in anything. I think its recommended alot more with amps simply because if you recap a tuner, you should/may have to realign it, which is not a simple job, and has a set of necessary equipment (freq.gen., oscilloscope, etc.). Some older tuners are pretty well set on the current caps value as well, others have drifted, i've heard performance can be better with a aligned tuner, but no mention of new caps...

Keep in mind when recapping a receiver, you are usually also recapping a tuner. I recapped my 727, tuner and all, and it sound good, has good reception, and is well aligned.

Tuners are also a "source", if a cap goes bad, it's far less likely to take out a different part than say, a couple thousand uF filter cap shorting.
 
I don't believe you will find any electrolytic caps in a tuner that directly affect its alignment. Just don't mess with any of the tweaks in the RF, IF, and MPX sections, and you should be okay.
 
BinaryMike said:
I don't believe you will find any electrolytic caps in a tuner that directly affect its alignment. Just don't mess with any of the tweaks in the RF, IF, and MPX sections, and you should be okay.
Click to expand...

Ah, thanks for the correction:thmbsp:. I was just throwing that out there, as i'm not a radio expert of any sort. Good to know, as that's probably the reason my 727 was alright afterwards.
 
BinaryMike said:
I don't believe you will find any electrolytic caps in a tuner that directly affect its alignment. Just don't mess with any of the tweaks in the RF, IF, and MPX sections, and you should be okay.
Click to expand...

Not quite as simple as that. There are some specific section of the tuners where low-loss types are needed, and caps in the IF section could alter the tuning according to some, requiring the aforementioned alignment. Other cap changes could result in having to adjust the signal strength meter and tunign meter. However...I've recapped some of my tuners recently and it hasn't seemed to throw off the tuning at all. I am, however planning to get them aligned because I really want their best reception and sound, and having gotten them used (and possibly abused) I know they can't possibly be at their best right now.

Here's a couple I've done recently:

http://www.audiokarma.org/forums/showthread.php?p=3689437

http://www.audiokarma.org/forums/showthread.php?p=3624754

http://www.audiokarma.org/forums/showthread.php?p=3618286

John
 
I have a Technics SA-5460 receiver that I recently had recapped including the tuner side.

I have not used that receiver for a while weak channel IIRC got it back have been using its bigger brother a SA-5560. When i put the 54 back in service WOW :music: no adjustment were touched.
 
jheu02 said:
...caps in the IF section could alter the tuning according to some, requiring the aforementioned alignment. Other cap changes could result in having to adjust the signal strength meter and tunign meter.
Click to expand...

I would like to have a full understanding of this issue. Would you be kind enough to cite an example or two?
 
I wouldn't recommend messing with any of the components in the "radio" side (RF, IF, detector, MPX) of a tuner; replacing capacitors in the AF side is OK and might well add sonic value.
 
BinaryMike said:
I would like to have a full understanding of this issue. Would you be kind enough to cite an example or two?
Click to expand...

Sorry 'bout that. I was thinking of filter swapouts which could affect the perceived signal strength. Differences in insertion losses, etc.

John
 
Let's limit it to solid state tuners. In general, your OK doing caps in the power supply. Almost all SS tuners have regulated supplies, *unlike* tube tuners. The regulation keeps the output fixed to a reference, like a zener diode, so new caps won't change the rail voltages. Otherwise, changes to the RF, IF rail voltages may change the tuning, but we don't need to worry about it.

In the MPX area, some older solid state tuners had discrete component MPX sections. This applies to many tuners designed mostly before the early to mid 70's. After that time period, almost all tuners had chip based MPX sections. If you have a chip based MPX section in the tuner, replacing electrolytic caps caps won't change the alignment. In those designs, the key or critical tuned circuits use film or ceramic caps.

But if you replace the electrolytic caps in some sections of a discrete MPX section, including coupling caps, it may or will require some re-adjustment. Note, bad electrolytic caps are usually the reason these old MPX sections go bad and don't work in stereo. In general, other types of caps, such as film or ceramics, in general, never go bad and you don't need to replace them (we are only talking solid state here).

Bob
 
Bob@FM said:
But if you replace the electrolytic caps in some sections of a discrete MPX section, including coupling caps, it may or will require some re-adjustment.
Click to expand...

Can you explain how that is possible, given the broad tolerance of standard electrolytic caps?
 
Bob, I'm interested in this too based on these assumptions:

1. Tuner was originally aligned from the factory.
2. No physical monkeying with the tuner has been done (i.e. someone with a screwdriver "tightening" the screws on the tuning capacitor...)

I know it's a big assumption that the tuner came fully aligned from the factory, but if so, why wouldn't replacing defective or out-of-spec components "realign" the tuner back to factory spec? I'm certainly not saying to avoid alignments, because those assumptions would really only be possible if you knew the entire history of any given component. But knowing that the alignment is done by physically adjusting trimmers and cores and such, if they're never messed with, why would replacing capacitors, or resistors...require a realignment?

EDIT: My pea-brain may have answered my own question...all the initial components have their respective tollerances, yet each individual component affected the initial factory alignment (or any subsequent ones) with wherever they fell in their tollerance group. (i.e. a 470uF cap could go from 376uF to 564uF and still fall in the 20%). So...one could theoretically replace components w/o needing a new alignment if one had measured all the specs of each component when new (or last aligned) and could later match them with replacements. Yes? No? Obviously, the likelihood of having any of that information, or taking the time to record it, is slim to none, so a fresh alignment will compensate for these differences.

John
 
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The old electrolytic caps have many characteristics like ESR, and actual capacitance value, that are taken into account when the alignment of circuits is done.
When you put in new modern low ESR caps, of a slightly different value, you may disturb the MPX alignment. In the old discrete MPX sections, electrolytic caps are used to couple tuned sections in the 19 and 38kHz sections. Changing the ESR drastically will change the tuning, i.e. it is like changing the value of a resistor in a tuned circuit.
I saw this in old units like the KLH 18 MPX section. Replaced all the electrolytic caps, and had to re-align the MPX section.

But please don't take this out of context, as I put specific bounds on this effect above.

It it *only* applies when they are used for coupling between tuned circuits, etc, *not* in power supplies or bypass caps, audio stage coupling, etc.
Bob
 
The other place I forgot to mention where you'll find an electrolytic in a tuned section is on the secondary side of an FM ratio detector. The ratio detector is the one where the 2 diodes go in opposite directions, unlike the Foster Seely where they both go the same way.
Most Scott tube tuners used ratio detectors, and many top Yamaha and Sansui SS tuners had them as well, including the popular TU-717.

In both tube and solid state FM tuners with ratio detectors, there is usually a 10uF electrolytic cap across the secondary side of the ratio detector. I don't recall the effect of changing it, and whether it requires re-alignment, as I always re-align every tuner after rebuilding it, and have for over 10 years. Maybe someone else can comment.
Bob
 
Bob@FM said:
In the old discrete MPX sections, electrolytic caps are used to couple tuned sections in the 19 and 38kHz sections. Changing the ESR drastically will change the tuning, i.e. it is like changing the value of a resistor in a tuned circuit.
Click to expand...

Thanks for all of your comments. My understanding of RF technology is weak, but it seems that resistance in tuned circuits only affects Q, not resonant frequency, so that retuning couldn't compensate for resistance change. Are you saying that ESR in the coupling cap governs the degree of interaction between two tuned circuits that tend to pull each other away from their (unequal) assigned frequencies?
 
BinaryMike said:
Thanks for all of your comments. My understanding of RF technology is weak, but it seems that resistance in tuned circuits only affects Q, not resonant frequency, so that retuning couldn't compensate for resistance change. Are you saying that ESR in the coupling cap governs the degree of interaction between two tuned circuits that tend to pull each other away from their (unequal) assigned frequencies?
Click to expand...

The frequencies in a multiplex decoder circuit are not really RF. Also,
changing a capacitor does mean a different capacitance value, so retuning
is in order as stated.

Another comment: in a low-Q circuit, the resonance frequency is indeed
affected by the resistance value. Just draw the circuit and do the
math exactly, rather than approximately as is OK with a high-Q circuit.
 
BinaryMike said:
Thanks for all of your comments. My understanding of RF technology is weak, but it seems that resistance in tuned circuits only affects Q, not resonant frequency, so that retuning couldn't compensate for resistance change. Are you saying that ESR in the coupling cap governs the degree of interaction between two tuned circuits that tend to pull each other away from their (unequal) assigned frequencies?
Click to expand...

Dunno.

Get the schematic for the KLH 18 MPX, and Sherwood S2300 and analyze them. Maybe you can then tell us why I observed that re-capping required re-alignment. I have not done the breakdown, and as to why, it's only conjecture, because the only components replaced were the electrolytic caps.

Bob
 
electroking said:
Another comment: in a low-Q circuit, the resonance frequency is indeed affected by the resistance value. Just draw the circuit and do the math exactly, rather than approximately as is OK with a high-Q circuit.
Click to expand...

Cannot agree. R does not appear in the equation for resonant frequency, even when R is present (and it's always present).
 
BinaryMike said:
Cannot agree. R does not appear in the equation for resonant frequency, even when R is present (and it's always present).
Click to expand...

Take a parallel circuit consisting of

1 - ideal lossless capacitor C;

2 - ideal lossy inductor made of series connected R and L.

The resonance occurs when the impedance becomes real and large.
The impedance would become infinite if it were not for the resistor.

It can be shown through some simple complex (!) algebra that the
resonance condition is :

L*(1-OMEGA^2*L*C) = R^2*C.

In the lossless case, this would reduce to L*(1-OMEGA^2*L*C) = 0,
with solution OMEGA_res = SQRT(1/(L*C)).

In the present lossy case, this becomes:

OMEGA_res = SQRT(1/(L*C) - (R/L)^2).

The presence of the loss mechanism in the coil lowers the resonance
frequency. A similar calculation could be done with a lossy capacitor,
or with both elements lossy. Please do trust circuit analysis over
simplified statements in textbooks. Best regards.
 
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I stand corrected. When Q is less than about 10, which occurs frequently in audio circuits, parallel resonance has no simple definition. Electrolytic capacitor ESR could therefore influence alignment to some degree in discrete MPX decoders, if the capacitor appears in a parallel resonant circuit (perhaps serving as a DC blocker). One might argue that such a circuit was poorly designed, possibly by a 'vintage engineer' such as myself. :D
 
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