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Model 18 Scope Troubleshooting

It's more complicated because you have to consider the other parts of the circuit that might be impacted. You might be able to do it on a small circuit board with surface mount parts and still fit in the same space. The problem is that the original circuit is slow. If you replace it with really fast parts then it might not eliminate the pop.

It looks like the resistor for the neon lamp is provided externally.

I think the easy thing to do would be to simply try replacing the part with a cell and lamp that are shrink tubed together. I have a bread board that I can wire up to the 18, then I can just sub parts till I find a combo that works.
 
txturbo said:
It's more complicated because you have to consider the other parts of the circuit that might be impacted. You might be able to do it on a small circuit board with surface mount parts and still fit in the same space. The problem is that the original circuit is slow. If you replace it with really fast parts then it might not eliminate the pop.

It looks like the resistor for the neon lamp is provided externally.

I think the easy thing to do would be to simply try replacing the part with a cell and lamp that are shrink tubed together. I have a bread board that I can wire up to the 18, then I can just sub parts till I find a combo that works.
Click to expand...

Makes sense!
 
monkboughtlunch said:
BTW, I found some traffic online saying that Audio Classics is using vactrols for the Marantz 10B. I wonder if the 10B specs (neon lamp and cell) are the same for the muting function in Model 18's LDR. I found this datasheet for a Vactrol LDR (not clear if this is the one folks are using in the 10B) which uses LEDs: http://www.excelitas.com/downloads/DTS_vtl5c3c4.pdf
Click to expand...

Maybe they substituted it in for the mute function on the tuner, but the 10B is not a pre amp and has no switch over pop to contend with. I am not sure what would happen with the speed..
 
FYI. Here's the info from Audio Asylum posted by Richard Links back in 2005 about using Vactrols in the 10B for the muting switch: http://www.audioasylum.com/cgi/t.mpl?f=tuner&m=3377

Quote from Links below:
----------------------------------------------------
"I have three Marantz 10-B's.

Every so often, they all exhibit this problem, as do most of the 10-B's which are owned by my vintage collecting friends.

The problem may have one or more causes.

The most common one is that one of the LDR modules (light-dependant-resistor) may be failing. The originals all fail in due time and need replacement. There are some people who have offered replacement modules. I recently spotted some for auction on eBay, as a matter of fact.

There are three such modules in the tuner and they come in two basic varieties. Depending upon when your tuner was manufactured, they can take the form of cylindrical housings or in later production versions, come in long vertical rectangular housings. They perform silent switching functions and often fail. The two-channel switch consists of single neon pilot lamp with a flanking cadmium disulfide photocell on either side, in a sealed container.

The other one is a mono switch, consisting of a single neon lamp and a single CdS cell. It serves as a muting control switch.

Not to sound ridiculous, but the basic components inside these things are actually available right off the shelf in most Radio Shack stores! Or, you must somehow figure out the requirements for the "dark and light" cycle of the CdS cell and work out something appropriate for the given resistance. There used to be a company called ECG VACTEC which manufactured them, but I doubt that they still do. They called their products VACTROLS, in the old days."
 
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FYI. Looks like the ECG Vactec Vactrols (that Richard Link referenced as being defunct) are now made by Excelitas Technologies. Here's their official web page: http://www.excelitas.com/Pages/Product/Analog-Optoisolators-and-Optocouplers.aspx

Their web page has datasheets for all their LDRs. If somone has a 10B that has used a Vactrol, and if they could identify the part number, that could help folks potentially find a drop in replacement that might work in the 18 for the muting LDR (since the 18 tuner is supposed to have been based on the 10B).
  • The VTL5Cx family, which includes the popular VTL5C2, VTL5C3, VTL5C3/2 and VTL5C4. Originally made by EG&G Vactec, later part of Perkin Elmer, now Excelitas Technologies.
 
I've worked on 2 18's and I now own a 20b- the muting on all 3 isn't like solid state muting, it drastically lowers the inter station noise but it never completely blanks it out, could be by design.
 
I found a Vactrol Application Note that recommends LDR part # VTL5C4 (see where I added the red arrow) for MUTING in the reference design below. This appears to be the same part that Newark sells for $7.65.

http://www.newark.com/excelitas-tec...CMP=AFF-CJ-3696791-Newark_element14&source=CJ

Below the App Note I uploaded a comparison of the characteristics of all Vactrol parts / part numbers for reference. Perhaps after txturbo measures characteristics on the breadboard of a functioning replacement, we can compare with the components below to see if a drop in replacement exists (which could mean folks might not have to saw the original LDRs open and restuff them if we are lucky).

vactrolmuteappnote.jpg


vactrolcompare.jpg
 
bktheking said:
I've worked on 2 18's and I now own a 20b- the muting on all 3 isn't like solid state muting, it drastically lowers the inter station noise but it never completely blanks it out, could be by design.
Click to expand...

Since LDRs in this era of Marantz components are known to degrade, how do you know the mute is working as designed across your 3 units? Have you replaced the LDRs or are they original?

If the muting circuit in the 18 is identical to the approach in the 10B (can someone confirm?), does anyone have a 10B with new LDRs installed for muting? If so, how well does the mute suppress interstation noise with new LDR?
 
I emailed the gent who was referred in the post above, i'll see what he has to say about the pop and muting and these optocouplers- we may be chasing a unicorn here.
 
I would be happy if there was a solution to make the muting like later solid state, I think an entire circuit redesign would be in order eliminating the optocouplers and substituting a chip.
 
If the $7.65 Vactrol LDR part # VTL5C4 (available at Newark) ends up working as substitute it could last decades since it uses LED instead of neon.
 
monkboughtlunch said:
I found a Vactrol Application Note that recommends LDR part # VTL5C4 (see where I added the red arrow) for MUTING in the reference design below. This appears to be the same part that Newark sells for $7.65.

http://www.newark.com/excelitas-tec...CMP=AFF-CJ-3696791-Newark_element14&source=CJ

Below the App Note I uploaded a comparison of the characteristics of all Vactrol parts / part numbers for reference. Perhaps after txturbo measures characteristics on the breadboard of a functioning replacement, we can compare with the components below to see if a drop in replacement exists (which could mean folks might not have to saw the original LDRs open and restuff them if we are lucky).
Click to expand...

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.
 
Have a read

http://www.silabs.com/Marcom Documents/Resources/FMTutorial.pdf

Page 10 shows a graph- from everything i've read the circuit designed with LDR's is called "soft muting" and is designed to attenuate the fm signal by 14db in the example, it does not completely cancel the signal. It can be attenuated by as much as 100db but that's a much later design using LED's and different circuit's. You have to remember this is 1968 design and not 2016.
 
bktheking said:
Have a read

http://www.silabs.com/Marcom Documents/Resources/FMTutorial.pdf

Page 10 shows a graph- from everything i've read the circuit designed with LDR's is called "soft muting" and is designed to attenuate the fm signal by 14db in the example, it does not completely cancel the signal. It can be attenuated by as much as 100db but that's a much later design using LED's and different circuit's. You have to remember this is 1968 design and not 2016.
Click to expand...

Interesting....
 
The model 18 manual states:

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