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Model 18 Scope Troubleshooting

txturbo said:
The model 18 manual states:

"interstation muting circuit reduces noise to a comfortably low level while tuning from station to station"
Click to expand...

There you go, which is what i suspected out of the 3 units i've worked on and it is what I have experienced, it's not completely muted.
 
bktheking said:
Agreed, but the muting is a "non-issue", the pop could be related to a weak LDR. It may improve muting on a non muting unit.
Click to expand...

Maybe the neon portion of the LDR is weak and requires a higher than normal voltage to make it break over and conduct. So you get this high V spike ?
 
I did notice that when I first turn the 18 on and it's cold the poping sound is much less...

I am going to hit the LDR with a bit of freeze spray and see if it makes a difference.
 
HI,

Brad asked me to weigh in. I had a Model 18 that he got working. It had been fully recapped (which improved the sound, by the way). Also the driver transisters in the scope board were replaced, similar to what was done earlier in this post. The muting works as Brad describes - namely reduces the interstation noise but doesn't completely eliminate it - you can hear static and weak stations if the volume is turned up. I think that it is reducing interstation volume by 14dB as suggested earlier. I do not have a popping sound when switching to FM.

Hope that helps.
 
txturbo said:
Maybe the neon portion of the LDR is weak and requires a higher than normal voltage to make it break over and conduct. So you get this high V spike ?
Click to expand...

In addition to the muting LDR as a potential source of the pop, could the visible-to-the-eye NE2 orange neon muting indicator lamp in left rear of the unit somehow be throwing out the voltage spike? Every time my unit pops, that visible neon light flickers on.
 
txturbo said:
You will never be able to "drop in" a vactrol.

The neon lamp in the LDR works in a circuit at a much higher voltage, like 55. The vactrol uses an LED, it works at 2 volts. They both run at low current.

To get the neon lamp to initially light it requires a higher voltage then, once it starts to conduct, it drops. The LED works differently.

Think of the neon lamp as a 55V zener diode, with current limiting resistors installed in the circuit. In the case of the muting LDR that would be R519 and R520. You could put in a zener diode to replace the neon lamp. One at say, 50V in series with the LED of the vactrol. But I am not sure about the rest of the circuit and it's current. IMHO... It's simply not worth the effort.

If we can't take the LDR apart it's still easy to simply shrink tube a neon lamp to a photo resistor.
Click to expand...

Found this in the 2014 Audio Classics catalog. Vactrol part numbers for Marantz 10b and Mcintosh MR71 tuners:
-----------
Vactrol

VTL5C1 Optocoupler MR71 10B 6.00 EA

VTL5C4 Optocoupler MR71 10B 6.00 EA
------------

Here's a link to the catalog (see page 19):
http://www.audioclassics.com/catalog/Audio_Classics_2014.pdf
 
For the Mcintosh MR71 tuner, I found a thread in which Terry Dewick recommended a VTL5C9 LED Vactrol as a replacement with "a with a 1K 1/2W resistor in series with the diode." Would such and approach work for the Model 18?

http://www.audiokarma.org/forums/index.php?threads/mr-71-in-stereo-mode-output-is-mono.414342/

dewickt said:
For the opto a VTL5C9 by Perkinelmer with a 1K 1/2W resistor in series with the diode, observe its polarity with + lead to the +6V supply.
Attached is the separation mod.
Click to expand...

Some additional discussion here: http://www.audioasylum.com/cgi/t.mpl?f=vintage&m=151539
 
Update.....

I received a bag of goodies from Mouser and started to experiment.

First I pulled out the LDR from the mute circuit. The photoresistor section was basically wide open. With the LDR removed the popping still occurs.

Next I installed a 65V .3ma lamp in the LDR circuit. It appears to be about as bright as the stand alone mute indicator. I also tried a 65v .7ma lamp, it looks about the same, it's just longer.

I then installed a new photocell. When I place it near the lamp it mutes a but not fully. If I use a white LED flashlight it mutes to the point that you can't really hear anything unless you really crank up the volume dramatically, then you can faintly hear something but it's orders of magnitude better. I can do a better check with my audio meter and get an actual DB reduction.

Unfortunately it did not solve the popping problem.
 
hirscwi said:
HI,

I do not have a popping sound when switching to FM.

Hope that helps.
Click to expand...

Hi hirscwi,

What serial range is your 18 that doesn't pop? My "popping" unit is a 7000 series. I wonder if some of the OEM parts suppliers (caps, diodes, resistors) varied over the production run and some serial number ranges are more prone to popping than others.
 
monkboughtlunch said:
Could the visible NE2 muting neon lamp in the rear corner be causing the pop?
Click to expand...

maybe...

It could just be a noisy transistor or out of spec resistor. I'll keep hunting.

I am also going to see if I can get an led to work on the mute circuit using a zener to drop most of the voltage.
 
Interesting. These 3 serials are confirmed "poppers," and were likely produced within a two or three month time span, perhaps a common component prone to failure.
  • 6769
  • 7009
  • 7025
The production run was about 7000 units (serial numbers 1000 ~ 8000).
 
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