National Panasonic RS-4150FD - VU Meter in Record Mode Only?

[Dancorp]

Hi everyone,

I am currently working on a RS-4150 FD. (i call it K.I.T.T )
Service manuals for this brand are very difficult to obtain.
For the moment I am proud to have managed to add bluetooth, but I would like to make the VU meters work all the time.
Indeed, these are Level/Tuning/Battery indicators.
The VU meters are only active in record mode.
I would like to modify it to force the VU Meter mode.

My idea was to find the link between the VU meters and the record switch.Is that a good idea?
Maybe if someone has a SM of the same generation, I could find some answers.

Thanks in advance.
Dan

 

[caution]

Check where the meters connect to the board and see what value limiting resistor it uses. Then, disconnect them and use the same value resistor when you attach it to the audio path. If you want them to bounce based on volume, tie them to the speakers with a higher value resistor.

 

[Dancorp]

Thanks a lot for your idea caution.
I'll try to check that.
I'll also try to follow wires between the meters and the record switch to understand how works the switch for them.

Dan

 

[Superduper]

Actually, I don't want to throw water on your project but I suspect that it's not going to be nearly as easy as it would seem. Firstly, although needle meters seems like simple thing but in reality, you need a proper driver. If your meters are calibrated to the low level record signals, just jumping it directly to a different part of the audio circuit might not have results you expect because you don't know the level of the signal at that part of the circuit. Most audio circuits have many layers of audio signal levels which get amplified or attenuated along the way. Secondly, most needle meters aren't just powered through a resistor. You'll first need to consider if the needle meter is a low level AC meter or a DC meter. Chances are, it's a DC meter. So in order to get the meter to respond, you'll first need to rectify the signal. If you look at any boombox service manual with needle meters, you will see that there are always diodes in that part of the circuit. It's for rectifying the audio signal, which is AC. Then there is the voltage aspect. A simple resistor in series won't work well as resistors only limit current, and doesn't reliably limit voltage. Instead, you are better off with a voltage divider network. Computing that with the max expected rectified voltage of the audio signal at the tap, and the meter top range voltage, you can design a proper voltage divider to ensure that the range is correct. Oh, and of course don't forget to factor in the voltage drop naturally present in the rectifiers.

Probably the easiest solution for you is to purchase a pre-made needle meter driver off eBay and connect the source to your speakers. Of course if you modify the circuit in this way, I hope your boombox has an ALC for recording, otherwise, you won't be able to set the proper recording level anymore.

 

[Dancorp]

That's a very complete explanation.
Thank you very much
I understand the complexity of setting up a circuit that can operate the meter within its working range.
The question remains whether "record levels" are also available during playback. If that's the case, I thought I could force the switch that swaps the function of the indicators to use them whatever its position.

 

[Superduper]

I think the RS Pannys were not released here in USA and I don't have a schematic or service manual for it. Every boombox is different so how one boombox does their meters could be entirely different on another so the circuit diagram is very important. Without it, it will be very hard.

As for record mode, are you saying that the VU meters don't even work during playback? That's unusual. If that's the case, then the switch is probably the record bar. But it's not so simple. Several things are probably switched at the same time including meter function from batt/tuning to VU and also function which could also cause the record bias to activate (which turns on the erase head) reverses the audio signal from the heads (reading to writing).

I don't know how badly you want to make this happen. You could always duplicate the onboard meter driver circuitry (including the rectification circuit, the voltage divider network and the driver transistors), put onto a small project board, and then source it from a preamp level audio signal source found by using a signal tracer on the record bar pins... then connect to the meters, it will probably work. Oh and don't forget to properly bias the driver transistors to class A, otherwise it may work "funny" such as nothing, then suddenly jump high. That's because the transistors aren't active until voltage at base exceeds about 2/3v, then suddenly they come alive. To resolve, you bias the base of the transistors to the active region. Oh, and feed the proper voltage to the transistor collectors (probably around 2-V).

 

[Dancorp]

During playback, meters are set to battery and tunning level.
I have to press the record button to turn them into vu meters mode.
I agree with you, the record bar switch many things but I don't want to push the entire bar. I want to follow meters wires to the bar and find the 3 pots (total6) used the switch the indicator source signal.
If I find them, I can try to force the record signal by soldering it to the output pin and cut signals used for tuning and battery levels

 

[Superduper]

Well, let us know how it turns out. Although it sounds simple enough, cut here, jump there, yet without a circuit diagram (and even with), I still think it may be more complicated than that. Remember that in play or tuner mode, the record circuit might not even be active.

 

[caution]

Good clarification Norm. Some boxes do have needles without an amp, like the C-100F, they just tap the preamps with a diode and resistor. That was sort of my line of thinking. Is it possible the needles on those units have a different type of sensitivity to allow that to work without an amp?

But Dancorp, your point about looking at the service manual for a close relative is a good idea. I found one for the RS-4300 here and although it only has a single needle and additional shortwave bands, everything else is essentially the same. The user manual here mentions the switchover to show audio during recording, like your RS-4150 does. "The recording will begin if the record button and playback button are pushed at the same time. The indication needle of the level meter will fluctuate during the recording depending on strength of the sound."

If you can determine that you have the same circuit it may be possible to figure out which switch sections on the record bar are responsible for providing audio to the needles during record mode.

The first page of the RS-4300 schematic is the right half of the sheet, and page 2 is the left half. If you look at the meter on page 1, you can see all of the lines for the various inputs (mic, tape, radio, aux) all passing right above it from the left half the sheet (on page 2) and converging on an area to the right of the needle, going through some resistors.


The 4300 has a single three-position function switch, S2, for (T)ape or (R)adio or (S)leep, Sleep is radio only, you just play or record a tape until it stops for your "timer." It looks like the 4150 has a separate tape/radio switch to allow you to fall asleep to tape or radio.

Section 12 of the record bar, S1, toggles the needle between battery/tuning in playback and audio signal for recording, but i'm not sure exactly how it does this, because it looks like it just runs 9V through R204 and TR21. Battery level just comes straight through R206 normally. If you can determine that you have the same basic circuit and two sections of the record bar indeed tie directly and only to the needles, you could try cutting and jumpering them if you're feeling adventurous.

 

[Dancorp]

With the help of two people like you, anything is possible!
I have to go to work, but I'm watching all this tonight!
You guys are awesome.

 

[Superduper]

Eric, I took a look at the schematic and the diagram is missing a trace. AT first glance it appears that the base of TR21 is connected to the collector, but if you study the voltage specs, you know this isn't possible as the numbers don't compute. As well, it makes absolutely no sense for the base of the transistor to be tied to the collector because the transistor will merely function as a dead short as it would always be ON. Therefore, the logical conclusion is that there is no union between B/C of TR21. Then if you look more closely, the trace is missing, probably from a poor quality original that was omitted by the 1-bit scanning device due to a high contrast threshold. So looking further to the left, if you can imagine that the connection at the base continues on left, you will see that the trace continues on onto the 2nd page and then it will be connected to some audio source.

Indeed, if you study carefully, you will see that the audio signal is rectified and the meter on VU is driven by a driver.

As for the C100, meters indeed can have great variations in level of sensitivity. Some are more precise and some are just coarse for fun looking. You can find meters that will read millivolts and some can go 12v easily. In this case, are you sure that it's driven directly through a resistor and diode only? Perhaps the preamp you are speaking of is actually the meter driver itself. A driver for the meters do not have to be a discrete transistor, a dual channel IC can function perfectly like that because it is, for all practical purposes, a dual amp and perfectly capable of driving 2 meters independantly. More and greater study will need to be looked at to be sure but in general, you wouldn't want a meter to be driven directly like that because it simply won't perform as stable without a driver or buffer amp, and the load could drag down an unbuffered audio signal. Especially in a preamp, you want it to be as noise free as possible because any amount of noise will be amplified into horrible output. This is especially true why preamps have ripple filters but the output amps don't. It's a matter of garbage-in, garbage-out.

 

[Dancorp]

Well, that's a lot for my brain to understand because I'm not an audio electronics specialist.
I have taken the liberty of merging pages f (and I added a few wires that seemed to have disappeared)

https://drive.google.com/open?id=1Q4gdwD1tqqHP5qME3AeUh8kJMzyciTqF

if i'm lucky, meters will be directly connected to the record bar on my RS4150 too.

To answer some of your questions, the RS 4150 is also equipped with a 3 position switch (Tape/Radio/Sleep).
Bluetooth has been integrated via the mixing function of the tape. (I am lucky to have a double switch on the tape mechanism allowing to activate the amplification without activating the motor)

 

[caution]

are you sure that it's driven directly through a resistor and diode only?

Preamp direct. Take a look for yourself here. The needles are just above the center of the schematic, and tie to the output area of IC401 through some 18K resistors.

 

[Superduper]

OK, I see. In this case, I believe it's possible that the BA343 rohm chips primary drive function is the meter driver circuit because every other tap to that location has to be re-amplified for the mixing and record circuits (or buffered). The additional buffered stages means that the load imparted to the output of the IC by the input of the downstream amplification stages is very low, which is why this leads me to believe that it's primary function is to drive the meters. I do not believe that this IC is a preamplifier for the output signal however (that you would hear from the speakers) because the source for the HA1392's seem to originate at a point before this IC and seems unrelated at all to the BA343 chip. However, to be honest, it's very difficult for me to follow large schematics on a computer screen anymore since I need to explode it quite a bit in order to just see the components but then it's like looking at a large quilt through a microscope -- constantly having to move the subject around to see where one thing leads to another, but by then, I've lost track and give up. I think I have a large paper copy of this schematic somewhere -- I'll have to try and dig it up. Doesn't improve my eyes one bit but at least I can paste the diagram right up to my face.... geez it's no fun getting old, lol.

 

[Dancorp]

Sorry to disturb these discussions too complex for me, but I managed to force the needles in VU meter mode.
As expected I found a direct link between the needles and the recording bar. So I soldered the two pins and isolated the 3rd from the rest of the circuit.
It works like a charm !

Thank you again for your help and interest in my request.

Sone pics and a video : https://photos.app.goo.gl/6ziaSysxB3awYvBn1


It's time to put some light in there!

 

[Superduper]

Good news.

But what happens when you record now?

 

[caution]

Congrats Dancorp!


Wiser, Norm. You're getting wiser But no, you're reading it correctly. That IC is on a small board next to the upper tape deck, and ties to the edit/dubbing buttons, monitor jack, and amp/deck1 PCBs,


If you remember what the inside of one of these looks like, there are two main PCBs stacked on top of each other, the top one has the main amp circuit, alarm circuit, and a couple other things. The lower board is the mixing board, but gets help from this second board that the IC is on.

 

[caution]

Oh, one more thing - the board with IC401 has a two-wire cable to the main mixing board PCB1, where it goes through R138 and R139 before it connects to the function switch. The needles plug into PCB1 and route to the function switch.

What's frustrating about the schematic is that they didn't label any connectors, probably because they had no way to, because they didn't group parts based on the PCB they live on. Compactness was a priority I guess. In an attempt to make a reference sheet, I ended up with the mess below. The numbers refer to the PCB, which are labeled on my wiring diagram.

 

[Superduper]

What? Do the numbers refer to a discrete pcb? That would mean that the C100 has 23 pcb's? I've fixed a few, but that was at least 5-10 years ago and I never recalled being that many boards. Also, the repairs were almost always blown amp related so I concentrated on the main board I guess.

 

[caution]

Yeah, 23, crazy huh? Actually 24 since the tuner is number zero. Most of them are small, with a single switch/pot/LED or small number of parts though. 22 and 23 aren't PCBs, just designations for parts. But, there are two more real PCBs without a number.


0 - Tuner
1 - Primary mixing
2 - Alarm/main amplifier
3 - Lower deck audio/power/logic
4 - Secondary mixing
5 - Mixing knob/jack
6 - Right LED VU meter
7 - Left LED VU meter
8 - Transformer
9 - Tape run LED, lower deck
10 - Reverse LED, upper deck
11 - Forward LED, upper deck
12 - Edit button
13 - Headphone jack
14 - Dubbing button
15 - MSSI button
16 - Monitor jack
17 - Upper deck power/present switch/motor/LED connections
18 - Stereo/Record LEDs
19 - Sensor module
20 - Power/alarm switches/LEDs
21 - Dial bulb
22 - Lower deck mechanism (connector from solenoid/switch is J2202)
23 - Designation for connectors for chassis-mounted parts - speakers, mics, needles.
?? - Upper deck head and auto-reverse switch
?? - Tape run LED power cable tie point