Line Signal Muting

Line Signal Muting

While studying my Sony TC-K61 cassette deck, I was intrigued how the muting transistor action works? After sketching out a similar circuit of my own to analyse and assemble, I think I may now understand how this muting is achieved.

Here is Sony's muting circuit for the TC-K61 cassette deck for both recording and at Line Out - marked in green.

After sketching out my own generalised circuit, I had some components easy-to-hand, and so put this simple circuit together.

Here we have - an external signal generator shown to the left with its own internal resistance of around 600 ohms. Then I just added a simple, non-specific RC network to simulate general external circuitry. After that there is a load resistance of 20,000ohms. An oscilloscope was connected across the load RL.

The muting transistor here is simply a S9014 NPN which is about to be switched 'on'. A sine wave signal voltage of 1Khz across RL was set to 2 volts peak-to-peak with no interference from the S9014. Once I had switch the S9014 'on', the output completely muted. 

The results ..

VL (before muting) = 2v.
VL (during muting) = 5mv.

This gave me an effective reduction of 20Log(0.005/2) or -52dB.

So why does this work?
Firstly, note that - the transmission line is effectively dc decoupled. Secondly, the S9014 is not experiencing any external one-directional electric field (hence voltage) to motivate charges across the collector-base-emitter junctions. 

So how does forward biasing the base-emitter junction result in an efficient -52dB muting reduction?

My theory: Both base-emitter, and base-collector junctions are forced into forward biasing modes.

There is no, or very little dc current running through RL, but since both forward biased junctions are now offering a conduction path, and in particular to an ac signal - it is this ac signal voltage component that gets shunted through both junctions as both junction slope resistances 𝚫V/𝚫I (thanks to biasing) are very small.

Effectively, the source ac signal (component only) voltage drop, almost entirely occurs across the 600Ω, and 670Ω+j/𝛚C impedances since the slope resistances (collector-base, base-emitter) are so low in comparison. It's basically potential divider law, the ac signal voltage at the two junctions is 'lost' earlier across the said impedances. *

(To understand slope resistance 𝚫V/𝚫I or slope conductance 𝚫I/𝚫V in this context, you'll need to study transistor NPN junction characteristics, and in particular the Ic vs Vbe (or even Ie vs Vbe) curve which are not always illustrated in datasheets) *

Shunting of the ac signal works in both directions, only one direction shown above.

Can we expect a forward biased base-emitter junction to offer a better conduction path than that of a forward biased base-collector junction?, probably, but I haven't investigated this.

I noted that during this electrical state, there was a very small dc potential across RL (collector-emitter) of 6.9mV, and again 6.9mV when I completely removed the circuit to the left of the S9014. This suggests that while one junction (probably base-emitter) was approximately 0.7v (forward biased), the other PN junction was at around 0.7v-0.0069v; effectively the same!

One final note - efficient muting was also realised when I reversed both emitter and collector.

23/03/2020.

2nd revision, 24/3/2020.
** 3rd revision: 20/03/2024

 

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I also later simulated the circuit in https://www.falstad.com/circuit/circuitjs.html

The results were different concerning muting ability - this simulation was less efficient for some reason?, but the remaining analysis was reasonably accurate. You can see that both PN junctions were effectively forward biased and the slope resistance very low.

 
25/03/2020.

Recording/Playback Level Instability

Sony TC-K61 Recording/Playback Level Instability

For some time now, I have noticed that the deck's ability to record and playback at stable levels is questionable. Quite often I would have to recalibrate the playback levels (via 400Hz Dolby Level ref tape), and re-configure the record levels for my Maxell UR tapes. These random deviations from the target values were not small, but in the order of 2dB-4dB.

Frequently, it was the left channel that began to vary with time - was an hfe or hFE (ac and dc current gain) transistor parameter drifting as the deck warms up in the playback circuit? - if so, which one? Often small variations in hfe in a circuit with negative feedback have little effect. However, with reference to the playback circuit below - are the signal inverting transistors Q104/Q204 exhibiting problems?

New Components: So far I have replaced every capacitor in the audio and control boards, including all bias frequency generator capacitors. The bias frequency is stable at 100Khz, and generates around 30v (peak-to-peak) for Type I tapes at the record head terminals. Type II setting yields 34v, and about 65v for Type IV tapes.

Signal Path Relay: I even changed the Record/Replay Head signal relay to eliminate any possibility of a potentially and slowly oxidizing contact?

Old G2V-2 relay replaced with OMRON G5V-2. Note: old caps were still in circuit at the time of this image.

However, this full capacitor and relay replacement did not cure the problem, so what was it?

Playback Circuit: It was only recently that I began to study the playback circuit which consists of three transistors in an apparent Current Shunt negative feedback configuration - not Voltage Shunt as I originally thought? They are: 2SA836 (PNP), and the 2SC1345 (NPN). These are low power, low noise-figure (NF) transistors, which are no longer made.

There may be several schematic errors in the service manual?
Example: above Q103 and Q203 there are a biasing resistors in circuit, not capacitors.

2SC1345 Replacement: After searching the  internet for suitable replacements, I decided to 'chance' a KSC1845 Fairchild transistor which I had in stock. And which offered similar specifications - in particular dc hFE characteristics. As mentioned above, the basic purpose of Q104/Q204 is to act as a signal inverter, current-shunt negative feedback is later fed from Q105/205 to Q103/203.

From voltage measurements, the default 2SC1345 dc quiescent collector current for Q104/Q204 was approximately 310uA. Replacing both Left/Right channel 2SC1345 transistors with KSC1845s worked well - producing the precisely the same quiescent collector currents!!! Wow!

Board Modifications: Note - I have reversed the record and playback level potentiometers, re-soldering them so they are easily accessible. The 'new' KSC1845 NPN replacements are marked in red. (long pins)

Assessment: Currently, the replay amp is working well. After playback and record calibrations, the TC-K61 seems to have stabilized for both left and right channels.

Time will tell if I have found the source of the problem, fingers crossed!

Article subject to revisions, and correction of mistakes. 18/03/2020.
+10th minor revision, 31/03/2020.


26/03/2020: I have now replaced the Q103/203 and Q105/205 (2SA836 PNP) with Fairchild KSA922FBU PNP transistors, and recalibrated the TC-K61 - the deck's playback circuit is very stable! The dc quiescent collector currents Ic, and Vce quiescent voltages differed from the original 2SA836 configuration by between 2% to less than 5%. Virtually identical - good!

Finally, the buffer amplifier at the input line-in stage appears to be a simple emitter-follower configuration. The old (but not faulty) 2SC1345E was replaced with a Fairchild KSA1845FCK NPN transistor.

 The TC-K61 is working superbly well for Type I tapes!