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Model 18 Scope Troubleshooting

An AK member pm'd me this info!
I had a tech look at my 18 last year. He didn't identify the cause of the pop when FM is selected, but below is what he concluded. Let me know if you figure it out!

"The popping when FM is selected is caused by the 200 volt switching on the selector switch. Have not determined what is causing the popping.

Two lines are switched 200 volts to the MPX Matrix/decoder and to the MPX oscillator. Will have to isolate the lines to see which board is causing the popping. I have disconnected the 200 v supply from the switch and the popping is gone. I can leave the selector on FM or any selection and hook and unhook the 200v to the switch and simulate switching on these leads and it pops."
 
Fhamre said:
Can you tell which board is making the pop sound? My hearing is shot and I can't tell!
This sound should not happen. It suggests an issue. Just not sure where to look.
Click to expand...

Agree that the popping must be indicative of an issue. Given that these receivers when new cost about $4500 in today's dollars when adjusted for inflation, Marantz never would have allowed the popping when the units were brand new on a high end receiver of this quality.

The fact that the issue is being observed across multiple units suggests a common problem that develops when the receivers age. It's interesting that the issue began for you after you adjusted the mpx oscillator. Any ideas on cause?
 
What's most interesting is that the pop occurs on switching in and out of FM.

The 200V is for neon lamps that are used for the stereo and mute indicators and also used to light neon lamps that act as resistive opto isolators. From what I have read they can degrade over time. One of the service builtins call for jumpering out the stereo photo switch, LDR602. This was do to intermittent operation.

I am not so sure that this was not an issue from day 1... There is indication of other "POPS" called out in the service bulletins related to the low filter.
 
What I should have said is that the resistive part of the "resistive opto isolator" can degrade over time. Apparently the resistance can change by 10% per year.

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Regarding the intermittent scope issues I'm experiencing, I've uploaded a brief video: http://tinypic.com/r/352jh4p/9 Click the image to get the video to play and note the scope behavior when I power the unit down.

The scope trace will appear and disappear at random when unit is in operation. When the scope trace goes away, the heater is still on and the overall CRT still has a glow (but the scope trace is missing). I can rotate the vertical and horizontal pots back and forth and it doesn't resurrect the scope. I also depress the push buttons -- still no fix. When I power down the unit, the trace does reappear. It seems to descend from above, as per the video. Any thoughts on what the issue is?
 
Have you replaced the scope board transistors?
That looks like the typical issue that new transistors should remedy.
 
no i haven't replaced the scope transistors. are there 4 on the scope board? were there any issues when installing your replacements?

what about the zener diodes? could they also cause an intermittent scope?
 
4 -TIP 47's will do the job you just need to pay attention to lead orientation and insulating the 220 package from the mount. My picture shows the insulator I used plus the round mica from the existing transistors and I used the white plastic insulator they had used as well.
 
I noticed that I can get the trace to reappear if I power down, and then quickly power it back up as split second later. Is that still consistent with leaky transistors?

Do you have any pics of after the install of the new transistors? And are the transistors you selected exact replacements for the original?
 
Fhamre said:
So are you suggesting the two neon lamps get replaced?
Click to expand...

monkboughtlunch said:
Or are you suggesting jumpering out the stereo photo switch, LDR602?
Click to expand...

It's not the neon lamps that degrade over time. It's the photocell portion of LDR501, LDR601 and LDR602.

There are 5 neon lamps, 2 that you can see (stereo and mute) and 3 that you can't see that are part of LDR501, LDR601 and LDR602.

Each of the LDR's is a resistive opto isolator. A neon lamp shines on a photocell. When the photocell is illuminated it's resistance should drop. There is also a time factor involved. As the part ages the resistance change is not as great and the time it takes to make the change increases.

My thinking is that the resistive opto isolators are simply way beyond their expiration date and their characteristics are no longer meeting the requirement of the circuit. They either don't switch fast enough or they don't change resistance enough.
 
Are there new aftermarket resistive opto isolators available to substitute and test the theory? By the way the pop comes out of both channels equally on mine.
 
There are modern parts that do this sort of thing but rather than use a neon tube they use an LED so you can't just drop them in.

A friend of mine that's a long time musician knew all about them, they are used in guitar amps for the tremolo effect. He said that the part is referred to as a "roach" and they fail due to age. He said that you can make your own by joining a neon lamp and photocell with shrink tube.

Looking at the manual there are two different parts used in the 18.
LDR501 has a neon tube with a single photocell

LDR601 and LDR602 have a single neon tube with 2 photocells.

Potential replacements...
This is a single
This looks like a dual unit

The problem is that the parts do not have data sheets.... One site that I looked at stated that they tried many combinations of lamps and photocells to get the sound right in the tremolo circuit and the formula was a secret. The on/off decay time creates part of the tremolo effect. In this circuit I suspect that it's not that important.


The simple thing to do would be to measure the voltage that drives the neon lamp, find appropriate neon indicator lamps and heat shrink a photocell to it. It's not clear what the spec of the photocell is but the parts are so cheap that you could try a couple combinations.

Of course this could be a rabbit hole and has nothing to do with the problem. I kind of suspect that it is the problem though because the parts are known to degrade over time and the parts have a decay time associated with them. The decay time may be what limits or eliminates the pop.
 
Some great info and incite.
Not sure I am the guy to research and create a solution. I'm in Canada and with our dollar these days. well lets just say it sucks. Plus a lot of outfits won't ship north.
That said I will continue to do research. That is cheap.
The pop on my unit can be heard without speakers attached. I just can't tell which board. My guess the MPX oscillator board where that lamp is installed.
My issue did not arise until after I did a few tweaks to my 18. Which included adjusting the DC offset,bias and tweaking the tuning capacitor FM front endC114 and L108.
 
Mine popped before and after DC offset and bias adjustments. This points to the issue being elsewhere. If you readjust the tuning capacitor FM front endC114 and L108 back to where they were previously, does the popping go away?
 
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They are pretty much back where they were. I found I was not able to tweak the front end to be able to get the dial indicator correct(needed to move over an inch)so I ended up physically moving the indicator. I marked C114 and L108 before adjusting so I believe I got it back very close to where they were.
Other than that my phono 2 does not work and perhaps my attempts at troubleshooting and repairing the issue play a role. Not sure.
Does yours make an loud pop even without speakers attached?
Can you tell which board its coming from?
 
If i turn the speaker switch to off and then rotate the selector to FM then tape and them FM, I don't hear the pop. But, if the speaker selector is on main and the volume is turned all the way down, I can still hear the pop through the speakers. So, regardless if the volume is up or down, the pop occurs when the selector switch is turned to FM.
 
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