FM tuner interstation muting operation

[paulsherwin]

I've acquired a 1980s Harman/Kardon TU610 tuner from Nigel (vinylspinner) which appears to have interstation muting stuck permanently on. This is just a simple entry level analogue tuner, nothing fancy. Unfortunately a circuit diagram isn't available, and I'm not sure how these circuits actually work, so I'm floundering about a bit.

Can anyone point me towards information that explains how these circuits work in general using discrete components? I know there will be some muting transistors in the audio signal path which somehow get switched on and off according to the received signal strength, but that's my limit.

I'm not actually working on the HK at present - this is just to get me up to speed so I know where to start looking.

[Nuvistor]

I seem to remember the Hacker Soverign range had ISM, circuits in the Service Data. It would give you an idea of their design.
From memory I think they took the sense voltage from the ratio detector across the AM rejection capacitor, this will rise with the signal strength.
Frank

[mhennessy]

Often they pick up a feed from the discriminator.

The Hacker sets were more sophisticated - these took a separate feed of the IF signal as it left the second IFT, and fed that into a transistor amplifier and tuned circuit. The output from that was AM detected, and a BC148 was switched on or off accordingly. When saturated, the BC148 removed the DC bias for the final IF transistor, and the audio was muted.

The advantage of doing it this way is that you don't need to manipulate the baseband audio signal directly, which does carry a risk of introducing distortion. But the cost of the coil (a special job) must have been significant...

Some sets use ICs in the IF amplifier that have a dedicated RSSI (received signal strength indicator) output (e.g. NE604), and these are engineered to be reasonably "linear". I used quotes because the outputs are usually logarithmic, so you can easily have a display calibrated in dBs. These result in a much more meaningful display, as discriminator-fed metering is a bit "all or nothing" because of the action of IF limiting. Of course, these can also feed the ISM function...

[Nuvistor]

My memory was wrong, not the first time, on the Hacker ISM. Still the RP25 would be a good start to familiarise with the technique.

[Radio Wrangler]

There are two methods.

The simple one is a comparator looking at the S-meter drive voltage that's made in most tuner IF/demod chips. They usually hav a chain of detectors giving a roughly logarithmic law. Simple thresholds can trigger switching to mono and then full muting of the audio outputs. Basic tuners just bang a pair of transistors in place to short the audio outputs after the decoder.

A more complex one relies on speech and music having a falling level with increasing frequency, while white noise from FM demodulation of limited RF noise is fairly flat. These units have two detectors, one looking at high audio frequencies, one at lower frequencies. If there isn't enough difference in the detector outputs, muting is activated. Some firms have got quite artful at how little hardware they need to do this.

David

[paulsherwin]

I've now confirmed a muting fault by removing both 2N2603 muting transistors. The tuner works normally apart from the muting. I'd like to fix this properly, but really am having trouble working out how the transistors are controlled without a circuit diagram. The LED signal meter works as it should, and the tuning scale has a nice gimmick whereby the background colour changes when a station is tuned in. I'd expect these to be driven by the same mechanism as the muting.

Of course, these things are supposed to sound better with the muting transistors removed

Thanks for all suggestions so far.

[Nuvistor]

Is this web site any use, note I know nothing about the site so just use with caution.

http://www.circuit-encyclopedia.com/...-517483-0.html

[paulsherwin]

I have the appropriate service manual, but bizarrely it doesn't include a circuit diagram, just disassembly and alignment instructions. I've never come across this before.

[mhennessy]

You might need to do a bit of reverse-engineering to determine the relevant part of the schematic. It's something I have to do all the time - I did an entire sound desk a couple of years back, so this ought to be pretty straightforward.

Obviously, between the RSSI signal - in whatever form it exists - and the muting transistors will be some processing. Perhaps only a transistor or two to provide a comparator function, or to shift a voltage level, or to invert a logic state. Or perhaps an op-amp? Whatever it is, it won't be complicated. Well, perhaps a little, because it almost certainly will also take a mute input from a mains-fail circuit to avoid noises at power up/down (which of course could be where the fault actually is). But it ought to be entirely feasible to decode the circuit with patience.

Perhaps just a few really good close-up photos might be enough for someone to decode it. Or you could send it to me - I like a challenge

[G8HQP Dave]

Some FM IF/discriminator chips had two sources of mute: insufficient signal strength (from RSI circuit), signal off-tune (window comparator running from discriminator output).

Another possibility is that the mute transistors handle switch-on thumps, while the IF chip does the rest of the muting. My DIY tuner (with Hitachi CA3089 look-alike chip) mutes nicely between signals but outputs some bumps on switching on or off, as the LO sweeps up the band.

[paulsherwin]

Originally Posted by mhennessy

You might need to do a bit of reverse-engineering to determine the relevant part of the schematic. It's something I have to do all the time - I did an entire sound desk a couple of years back, so this ought to be pretty straightforward.

It's all very well for you to say that, but I'm just a bumbling tinkerer at the limits of my competence

I'll start a new thread specific to the tuner in a day or two. I'll take some high res pictures and post the block diagrams from the service manual, and perhaps somebody will see something obvious that I've missed.

I'm trying to keep this in perspective. The tuner will have zero value even when fully working, as basic analogue tuners like this are ten a penny now. It's perfectly possible to continue using it as it is without any muting. However, I'd like to get to the bottom of things if I can.

[Radio Wrangler]

The block diagram should give rather a lot away. The first thing to do is to see which devices the designers chose. Mostly it's going to be the CA3089 or some equivalent IF amp/discriminator and a CA3090 or MC1310 stereo decoder and then a couple of dual gate mosfets for the front end.

Life gets more interesting quickly for anything off of the beaten track. I Bought the Sony 730ES from Nigel, and that's a pretty involved beastie with its PLL demod and a lot of extra IF stages. Fortunately it's going nicely and I have found a full manual.

Usually the applications note circuits from the chip makers give you a bit of a head start on these things.

A guy on Ebay is touting a CD of many H/K manuals and he's posted a thumbnail of what looks to be your missing diagram, so I think they're out there, but sixteen bucks seems expensive compared to the tuner itself.

There may be a squelch adjustment pot somewhere.

David

[paulsherwin]

There is indeed a mute level pot, but this did nothing when I gave it a shot of Servisol and twiddled it. The mute on/off switch also did nothing. Detailed discussion of this specific tuner is OT in this thread though - as I said, I'll start a new thread soon for that purpose.

[Radio Wrangler]

Identify the IF chip and have a look at the datasheet. Some have internal circuitry for muting which could be separate from the S-meter, or downstream of it.

David

[Andrewausfa]

Paul, HiFi Engine have a service bulletin 4004 with 'a revised circuit for signal strength meter and FM Mute control'. It's for units after s/n 2601 if that helps.

Andrew

[Synchrodyne]

The attached, from Ambit Catalogue #1, might provide some general background on muting derived from FM ICs in the CA3089 family.

Cheers,

[Synchrodyne]

Also, for general background, the Quad FM3 muting circuit is worth a study. It used discrete transistors to provide both noise muting and deviation muting, circuitry for the latter overlapping with that for its twin-lamp tuning indicator.

Cheers,

[paulsherwin]

New repair specific thread started here: https://www.vintage-radio.net/forum/...d.php?t=116692