McIntosh MR71 FM-tuner discriminator question

[Apfelmus]

Hi all,

I got hold of this nice valve tuner, very good cosmetic condition. But totally deaf.

After some fiddeling around the RF circuits I discoverd the oscillator valve was broken, one filament was out. I replaced it, and the tuner came back to life, everything else seemed to be OK.

Since I have some toys to measure around such a unit, I started to look if it was in spec. Tuning was a little off, not surprising with a new oscillator valve. So I decided to do the realignment as described in the service manual. Beginning with the IF cans, using a wobbler, then the RF part, then the discriminators. There are two, one for the AFC/Muting, one for sound - see the attached extract of the circuit diagram that came with my unit (it says "starting serial #45B00).

When rechecking for specs I found out.

• the good thing: RF and IF is precisely aligned and has better than spec sensitivity - spec says 2,5uV at the antenna for 3% of distortion.
• the BAD thing: distortion remains at something around 3% even when I ramp up the signal generator.

The spec says normal distortion is below 0,5%. I reckon this is when the limiter goes into action (beginning with 10-20dB higher RF levels). The AFC and the limiter circuits seem to act as they should. I tried and retried aligning the transformer of the AF discriminator to no avail. I can bring down distortion to something like 0,8%, but only with the the primary totally misaligned (i.e. the tuning indicator is way off centre).

The IF portion looks very well aligned. I checked with a signal generator modulated with 400Hz FM (75KHz excursion). This gives a perfectly centred spectrum at 10,7 MHz at TP1 (output of IF).

I get the same behaviour when I feed the IF directly from the signal generator (i.e. generator 10,7MHz, 400Hz FM with 75kHz deviation) at TP1 (same result if fed on the other side of R24).

I did not check components yet. No electrolytics, only ceramic caps. I will check the valve and the diodes if nothing else comes to my mind...

Is there anything else to look for, or another way to check the discriminator?

cheers
Martin

[Apfelmus]

Some news:

When I first aligned the IF it was by injecting 10,7MHz into the mixer valve, as of the alignment instructions. I made another alignment, this time by injecting 99...101MHz from my wobbler via the antenna socket. That turned out to be easier to align, the stages seemed to "naturally" align more towards 10,8MHz. When I realigned the discriminator with this setting, all of a sudden I got veeery low distortions, below 0,1% at 400Hz!

That didn't last long. Next morning it was back to 2%... maybe theres some intermittent contact somewhere? But no reaction at all if I poke around.

The diodes checked fine. Valves too, measured close to 100%. A NOS replacement (EH90) gave the same results. Cannot find any broken or otherwise suspect part. Tried to open the can of the transformer, but its not evident how to do it so I didn't try hard. I also did not check the ceramics.

With the current alignment (centered 10,78MHz), carefully positioning the bottom core of the discriminator it is possible to bring AF distortion down to 0,3-0,6%. But only for a given RF frequency, somewhere else on the scale I get 1-2% again - same story.

That does not seem normal behaviour, does it?
Anybody know how to open these cans? They seem to have a sort of latch on the side but probably also on the bottom.
I might check the ceramics if I knew which ones are critical.

Any help appreciated...

cheers
Martin

[Apfelmus]

Some more news:

I found a thread in audiokarma.org about possible Mica-capacitor failures inside the transformer cans of these tuners. That could explain why I seldom get low distortion and mostly not. There are 3 capacitors in the can but all connected to the base. I applied a small 18pF cap at each position, but nothing really changed.

The bottom slug (primary) has to be moved to the bottom end for the max output! That cannot be right. Furthermore, the output level of the second discriminator is much lower. I wonder if this is due to the 10k load instead of 47k or some other factor. See the pic of both waveforms with same V/div settings.

Attached also a screenshot of the audio analyzer. The waveforms are done with 500uV and 5uV output of the generator. Antenna input is 6dB less due to the 50-to-75 Ohms adapter. The white line (5uV) is within specs, but the green line with strong atenna signal should be much lower, below 0,5% to be in spec.

I'm a bit out of ideas right now...

cheers
Martin