Help with JVC JA-S44 Power meters adjustment

[gkwolfman]

In hindsight, I inadvertently dialed R507 & R508 adjust pots during my amp check up (the idle current & bias). So, I am needing to reset the power meters.
I have a question about the resistors? (507-508), I have them turned counterclockwise to do the reset, but as of now I don't have the dummy load and a signal source.

My question is, can I use my amp in this position without damaging the amp until I have the tools to do the power meter adjustments? Surely R507 & R508 have nothing to do with the wpc output of the amp?

I may try and download a signal/or/use a signal (sine wave 1kHz) from my desktop to my aux and does my 8-ohm dummy load need to be a measurable wpc (ie. 100 wpc)??

 

[mrk229]

Any sign wave will do. Phone app, PC app, CD with tone, etc. They have you calibrating it at 10 watts and then checking at 50 watts.

The loads need to be rated for at least 10 watts continuous. What do you have?

I'm pretty sure that the meter circuits have nothing to do with normal safe operation of the amp.

I can't think of one good reason why they asked you to turn them all the way down. before checking.

 

[gkwolfman]

I'm pretty sure that the meter circuits have nothing to do with normal safe operation of the amp.

You know, that is what I was thinking, but, this is the first time doing power meters and owning a JVC.

 

[gkwolfman]

The loads need to be rated for at least 10 watts continuous. What do you have?

I thought I read somewhere that using a set of speakers would work as a load? I have a set of Polk Monitor 7, and I have desktop speakers (Edifier R1280Ts) Power output: R/L: 21W+21W, Frequency response: 52Hz-20KHz.
I guess for what it is worth, I also read that power meters are just "eye candy" (as far as to accuracies), and I love "candy" is one of the reasons I got this unit, to pair with the big meters on my RT-1020L tape deck.

 

[mrk229]

I thought I read somewhere that using a set of speakers would work as a load? I have a set of Polk Monitor 7, and I have desktop speakers (Edifier R1280Ts) Power output: R/L: 21W+21W, Frequency response: 52Hz-20KHz.
I guess for what it is worth, I also read that power meters are just "eye candy" (as far as to accuracies), and I love "candy" is one of the reasons I got this unit, to pair with the big meters on my RT-1020L tape deck.

Yes speakers will work as a load but it is not a good idea. At the 10 watt level they want you to calibrate at it will be unbearably loud and you potentially risk damaging your speakers, especially if you go the 50 watt level.

If it is not something you think you will use often you can get something like this, all day at the big A site. They won't sink 100 watts for long without attaching them to a larger piece of metal. Even at ten watts they will get pretty hot pretty quickly. They would work for what you need to do for low dough.



A step up would be a pair of these. They are larger and don't require an extra chunk of metal for most short term uses.



What ever you decide to do be extra careful especially with the ones without the attached wires to not to short the speaker wires or have them both touch the metal body.

If it were me I would start by using the one watt level which is 2.83 Vrms across the 8 ohm load. Set the pots to read at the 1 watt tick mark.
Go to 5 watts which is 6.32 Vrms across the load and see if the meters track, then check at 10 watts which is 8.9 Vrms across the load and finally a quick trip to 50 watts which is the 20 Vrms level.

You may find that the meters don't exactly track across the range, fairly normal. As you said they are primarily eye candy. And like you I do like power meters as well.

 

[RVT1K]

I've always read that almost all "power meters" were in fact simply voltmeters calibrated for 8 ohms to read power. A true power meter would need to measure current and voltage at any given moment.

I would not be surprised if you could calibrate the meters with no load and just measure the voltage called out at the outputs.

 

[mrk229]

I've always read that almost all "power meters" were in fact simply voltmeters calibrated for 8 ohms to read power. A true power meter would need to measure current and voltage at any given moment.

I would not be surprised if you could calibrate the meters with no load and just measure the voltage called out at the outputs.

That is pretty much a true statement. Add to that a loudspeaker is a complex load not a simple 8 ohm resistor.

I think you are correct in that you could probably get close to the correct setting just by using voltage only with no load.

I just looked at that manual and nothing fancy in the meter circuit. I also noted that the amp is rated at 45 WPC and yet they are asking to check the meters at 50 watts. I find that a bit odd.

 

[RVT1K]

That is pretty much a true statement. Add to that a loudspeaker is a complex load not a simple 8 ohm resistor.

I think you are correct in that you could probably get close to the correct setting just by using voltage only with no load.

I just looked at that manual and nothing fancy in the meter circuit. I also noted that the amp is rated at 45 WPC and yet they are asking to check the meters at 50 watts. I find that a bit odd.

The difference in deflection between 45 and 50 watts is probably tough to see.

The company I retired from made, among other things, electron-beam lithography machines and I tested/troubleshoot/prototyped/repaired all the electronics.
Since the electron beam is steered with coils -very much like a toroidal transformer with the beam shooting through the middle- very precise control of the amount of current in the coils was required. That required measuring the current very accurately and it was done with a sense resistor in series with the coil in question, usually on the order of 0.1 ohms. Measure the voltage drop across the resistor to get the current measurement and use that in the feedback loop for keeping the current correct.

Sometimes I toy with the idea of making something like that but I think a sense resistor that could handle enough current may be challenging.

 

[gkwolfman]

Well, let me throw this into the mix of things. After thinking about how low volume that I will be listening to my set up, the readings on the meters will be so low
that watching the meters will not be as satisfying. I want to see movement. So, how to make an adjustment for the meters to move more at a lower rate of volts??
I am thinking in the range up to <5 volts up to <10 volts. I will never play over 10 volts.

 

[ElectTech]

@gkwolfman, I have the JA-S44 and have restored it to a great degree, making some upgrades as I previously communicated with you in another thread.

JVC JA-S44 upgrade

I also like its VU meters very much. You may calibrate them as you please, but the tone to calibrate should be around 1 kHz because the meters' circuitry was designed around that frequency (i.e., the half-wave rectification, then smoothed by the 47 uF capacitor, and then going to the 680 Ohm + 22 k Ohm variable resistor, and then to the coil of the VU meter).

For simplicity, I suggest you:

1) Connect a 2 k Ohm (1/4 W or 1/2 W) resistor (*) to each speaker terminal, then inject a 1 kHz tone to the AUX input using your phone or other source.
2) Adjust the Volume until your multimeter reads 4.45 V at each speaker's output. (**)
3) Adjust the R507 and R508 variable resistors until you read 10 W on each of the VU meters. (***)

(*) If you already have the 8 Ohm, 100 W dummy loads as explained above, you should use them for sure. If you don't have any resistors available, then you could connect a corded headphone to the headphone jack in front of the amp and connect nothing to the speakers' terminals in the back of the amp. My point here is that there should be some current coming out of the STK-0050 Power Packs (see schematic) during the VU meter calibration. A headphone connected to the front headphone jack should be enough. If the tone coming out of the headphones at 4.45 V at the speaker's terminal is too loud that could damage the headphone, then a reduced voltage should be used.

(**) Some multimeters have a narrow limitation in measuring AC voltages. For instance, some multimeters may only be able to measure AC sine waves (the RMS value) for frequencies between 50 Hz and 400 Hz. If that is the case with your multimeter, I suggest you inject a 400 Hz sine wave from your phone (there are free apps for that).

(***) What we did here was change the decibel logarithmic scale of the VU meters by +6 dB. So the true power at the speakers is 1/4 of what is now showing on the VU meter. When you get your 8 Ohm (100W) dummy loads, then you should recalibrate as you desire.

In the end, when you play your favorite songs to the speakers at the average sound level you enjoy, then you can again recalibrate R507 and R508 accordingly.

The formula for the decibel scale is:

For Power ratios ----> dB = 10 log(P2/P1)

For Voltage ratios ----> dB = 20 log(V2/V1)

 

[gkwolfman]

(**) Some multimeters have a narrow limitation in measuring AC voltages. For instance, some multimeters may only be able to measure AC sine waves (the RMS value) for frequencies between 50 Hz and 400 Hz. If that is the case with your multimeter, I suggest you inject a 400 Hz sine wave from your phone (there are free apps for that).

I'm old school and have never updated to the internet on my phone?, I just use pay per minute and call and text only.
My multimeter has just two selections AC 600 & 200.

A update on where I am at, I have tried just moving the 507-8 resistors, slowly per instructions, plus just moving them just to move them, and it seems
like no change. Both power meters appear to be on spot and reads the low wattage 0 > .05 watts that I am listening to, like none of my turning 507-8 did anything to change.

So, just having bought the amp and wanting to play and set up the RT-1020L which I just got, I closed up the top of the amp.
With the meters with the readings it is showing, I can live with it until later.

In the near future, can I PM you for tips on any updates I may have to do with my JA-S44?
By the way, I am liking this JVC matched with the polks monitor 7. I am listening to KSHE Klassics 2 from the pc for my radio (Tuner) hooked to the aux input of the JVC.

 

[ElectTech]

Ok no problem. Yes, you may PM with questions. (I am an electrical engineer, and I won't advise you to do anything harmful to you or your equipment.)

What I suggest you do, based on the information you gave, is to align the VU needles as follows:

1) If you have Audacity on your PC, you can generate a 1000 Hz sine wave tone as follows. You can download Audacity for free via the link below if you don't have it already installed on your PC.

Audacity download



1.1) If you don't want to deal with the Audacity program, you may play the 1000 Hz tone from your PC to your amp via this YouTube link:

1000 Hz Tone

2) Connect a headphone to the headphone jack on the front panel.

3) Set the stereo/mono switch on the front panel to MONO.

3.1) Loudness can be ON or OFF. The subsonic filter can be ON or OFF.

4) Make sure that the BALANCE potentiometer is at the center.

5) Play the 1000 Hz tone from your PC to the AUX input of the amplifier. Ensure the speakers are not connected, or the speaker selector is in the OFF position. Hearing this 1000 Hz tone is very annoying, and if you have other people and pets around, it is even worse.

6) Without wearing the headphones on your head, but holding them near you, raise the VOLUME until you sense that the sound is loud enough on the headphones, but not too loud. Then set R507 and R508 so that each VU meter reads 1 Watt.

7) Connect your favorite speakers and listen to your favorite songs. Recalibrate per item #6 above as needed so that the VU needles stay aligned and in a range that you like.

PS:
On your comment "Both power meters appear to be on spot and reads the low wattage 0 > .05 watts that I am listening to, like none of my turning 507-8 did anything to change."
Answer: Since it is a passive set of VU meters, this is because the signal of the music is so low in voltage that it is not able to sufficiently charge the 47 uF capacitor (C503 and C504) that feeds energy to the coil of the VU meter, so the R507 and R508 variable resistors won't have much effect. If this problem persists, you may try replacing R505 and R506 (680 Ohm each) with a 100 to 220 Ohm resistor (1/4 W) to see if there is improvement.

 

[ElectTech]

@gkwolfman If you will use your JA-S44 constantly, and want a better and permanent solution for the VU meters, here is what I suggest:

If you have a technician who can build an active circuit for the VU meters, in place of the passive one, he or she can leverage what JVC did for the KD-A5 cassette deck of the same era (here is a picture of the equipment I have). Like the JVC JA-S44 amp, the KD-A5 uses few capacitors, resistors, a diode, and a variable resistor, plus it adds a dual opamp to be used for both channels. Inside the JA-S44 there is a 25 VDC voltage available that can be used for the new opamp (like an NJM4580D). The cost of all components should be less than 15 dollars, and a small PCB will fit easily inside the amplifier. The technician should build the circuit on a breadboard to find the best component values for the VU meter. It should be a fun project, I would do that with my amp, but I prefer to leave the VU meters' scale original.

 

[gkwolfman]

The technician should build the circuit on a breadboard to find the best component values for the VU meter. It should be a fun project, I would do that with my amp, but I prefer to leave the VU meters' scale original.

Thanks, I will keep this for future reference. But I think I will do as you and keep the vu meters as is, original, and just watch from the RT-1020L thru its' meters.
The Polk Monitor 7c speakers pairs with the JVC JA-S44 real nice. I have them set at shoulder height or so and I don't have the amp on very high using the volume from the tape deck thru source and tape one on the amp. This way I use the vu meters on the tape deck to watch. Problem solved. Plus, I really don't mind the tweeters SL2000 on the Polks, especially with my old failing ears.

 

[ElectTech]

@gkwolfman That is a very nice Pioneer reel-to-reel. I don't have any, only cassette decks. My father had an AKAI 4000DS that was built like a tank; it never broke. I remember I recorded hours of music off FM stations overnight using a timer, then selected the best songs and recorded on cassette tapes. I still have some of the cassette tapes.

 

[gkwolfman]

I remember I recorded hours of music off FM stations overnight using a timer, then selected the best songs and recorded on cassette tapes. I still have some of the cassette tapes.

Ahh, I remember those days past, before Napster. I think I had about 9 of those laserline 100 count tape holders fastened on the wall full of cassettes.