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A&R T21 Tuner - variable output level fault

Operajulian

Operajulian

Julian
In celebration of Victoria Day here in Canada - and a day off from work, I thought I would dig out a T21 tuner that I have been meaning to work a bit. It was one I bought cheap years ago and has been sitting in a box in dry storage.

I changed over the transformer ( for 110v ) and cleaned the volume pot of the rear. Yes, it is still getting the 24v power to the circuit board.

Connected it to a working A60 and the volume from the tuner keeps pulsing in "waves". It is generally very quiet ( even with the T21 volume up high ) and then bursts through from time to time in no apparent frequency.

[ The A60 works fine with another tuner ]

I know enough about tuners to know that most of the servicing work is beyond me. Certainly I cannot and would not touch any of the alignment.

Does any Pink Fisher have an idea of what might be going on ?
Or is a "parts only special" ?

Much appreciated,
Julian
 
Does it sound ok when you hear it? If yes, tuner section likely OK. These have a variable output adjustment pot on the rear - tried twiddling that? I have a circuit diagram if there isn’t one on the net somewhere, PM me if you’d like a copy.
 
Thanks Dowser

I have the T21 circuit diagrams. It's nothing to to with the variable output volume pot on the rear. That's all checking out OK.
It's somewhere else.......darn
 
Sounds like it is muting. Many tuners disable mute in mono.
It is common for the 10.7 MHz detector coil slug to need adjusting to get zero dc across the coil.
Do not touch the tuner head.
 
Thanks davidsrsb

Is it fixable by a specialist and reasonable cost ? Too much and it has a parts special future for it.
 
Can you have a look at the schematic. The IF detector is probably a CA3189.
It is the IC nearest the tuner head can. There is a metal can tuning coil next to it with locking gunge on the core.
Do not try to adjust the core with a metal screwdriver, it is very brittle.

Does the mono/mute button have any effect?
Does the AFC button have any effect?
What is the tuning meter doing when these surges of volume happen?
 
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davidsrsb said:
Can you have a look at the schematic. The IF detector is probably a CA3189.
It is the IC nearest the tuner head can. There is a metal can tuning coil next to it with locking gunge on the core.
Do not try to adjust the core with a metal screwdriver, it is very brittle.

Does the mono/mute button have any effect?
Does the AFC button have any effect?
What is the tuning meter doing when these surges of volume happen?
Click to expand...


David: Thanks for your replies. The IF detector in the A&R T21 is indeded a CA3189 chip.
There is no mute button on the tuner. The Mono button has no effect and the LED's that make up the tuning display don't change except sometimes the Stereo signal led drops out.
I did try a couple of things this morning.
Cleaning the variable resistors that make up the tuning presets did seem to make it more consistent and less variable volume levels though it still cut out from time to time. I am hoping that perhaps a bit more careful cleaning will help.

The very odd thing though was whenever my hand was directly above the metal tuning coil - not touching it - just a few inches above - down went the volume. Yes there was an aerial attached. I have never noticed anything like this before when opening them up for a look.

Let's keep in touch.

Off to hear the Kronos Quartet this evening so it may be a day or two before I get back to it.
Take care
Julian
 
It might be something as a simple as dry joint or failing component.

I had similar on a similar vintage Rotel tuner, where it would just mute for no reason after about 10 minutes of being on. It turned out to be a solder joint to small ceramic disc capacitor.

I found it almost by accident by carefully prodding components in turn, with an insulated probe.

Sometimes freezer spray on components can pin point similar intermittent issues.
 
There is a pair of jfets near the output, that mute the output with the gates tied together. You can check the gate voltage as the audio comes and goes.
 
Thanks gentlemen for your tips and tricks. I shall try them in due course.

I have other T21 and a Quad FM3 so no real rush to fix them. Just brain changing hobby from the day job.

- Julian
 
I was surprised to see the fairly expensive CA3189 used with a single tuned detector. Many tuners of that era and class used a double tuned coil, which gets significantly better distortion figures
 
Thanks for the advice. Did a bit more cleaning, changed the transistors at the output just in case and the tant at C302. Seems to be holding the signal & volume at stable levels.

So on the next adventure. Opened up the next T21 bought cheap and inside is a very unusual home made addition.
Photos at my Flickr stream here: https://www.flickr.com/photos/158915456@N07/?

Its coming from the power supplies / muting circuit area. I have not powered up just yet ......

Ideas / thoughts as to what is going on.
Cheers
Julian
 
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Thanks - any ideas why a previous owner would have added this ?
I will give it a quick try and see if it improves anything - otherwise, time for removal......

Julian
 
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