Model 18 Scope Troubleshooting

[bktheking]

Do you have a pic of this opto coupler?

 

[Fhamre]

Here they are ,note LDR602 is missing and has been jumpered(yellow). LDR501 and LDR601 are the square aluminium guys.

 

[bktheking]

Let me check what I have.

 

[monkboughtlunch]

LDR602 is present on my unit.

 

[bktheking]

No extra board here , though I had one

 

[Fhamre]

My issue may not even be a light cell. Interesting that I am missing one.
I wonder if other units used the light cell. I don't have any early solid state parts units but I'll check.
Thx for looking.

 

[monkboughtlunch]

Does the Marantz 10B use the same optocouplers as the 18? If found this old thread: http://www.audioasylum.com/cgi/vt.mpl?f=tuner&m=8300

The last post says: "I believe Richard Modafferi uses Vactrols in his 10B repairs, according to an Audio Classics repair sheet someone posted a while back."

Here's a place that sells Vactrols but not sure what the part number would be to find the proper replacement or specs for the ones in the 18: http://store.synthrotek.com/Vactrol

Any ideas?

 

[monkboughtlunch]

Ok, this post on AK by forum member "TheBasilisk" is about his thoughts on why a Marantz 1515 receiver selector switch exhibited pops. Perhaps some of this analysis about the 1515 receiver applies to the Model 18? Thoughts? See below post. http://www.audiokarma.org/forums/index.php?threads/marantz-1515-pop.674122/

The thread is called "Marantz 1515 pop"

Original post (statement of problem) by Raccoon1400:

There is a pop when the input selector is changed. It occurs when changing between any two inputs other than between aux 1 and 2. It also occurs when powering the unit off. It occurs in both channels. What could cause this?

Feedback from TheBasilisk:

Okay solved this one (well pretty much!)...

The input selector switch does actually include a muting function, and ironically that's what is causing the problem. Part of the switch (SV01-1) links pins 4/5 (and sometimes 3) momentarily as the selector is moved between click stops. Exact operation is a little vague in the schematic, but essentially when this happens, the voltage on pin 4 drops, subsequently turning QM03 off, which turns QM04 on, which thus grounds voltage on JM04, causing current to flow from pin 14 on each of the ICs Q701 Q702, activating the muting function. The 26V on JM04 / pin 14 is supplied by the IC, hence current flows 'from' not 'too' the chip.

CM03 provides a short start up delay at switch on, R754/755 act as current limiters (trigger current is only around 0.2mA, must not exceed 1mA), and I imagine QM02 allows the muting circuit to be activated by other factors such as thermal / electronic overload (?)

If you remove RM07 the switch muting will be disabled, and this actually helps, leaving (in my case) only a pop switching to AM radio, which is now a result of the AM circuitry being activated, and would be the pop that the muting circuit is designed to mute!

Best I can tell, the pop is caused by the muting circuit activating too 'suddenly' - ie QM04 flicking like a switch. CM02 seems to provide a brief delay to 'soften' the switching (observed on QM03 base) however collector voltage still rises suddenly. At this point I can't say where / why the fault occurs, however adding a 22uF capacitor between QM03 collector and ground slows the rise slightly and therefore activates the mute without causing a pop.

I did replace the existing electrolytics in the circuit, and although CM03 was pretty dried up, the new caps made no difference to the actual operation.

The compromise is the mute now activates a little slow, and if you flick the switch from FM to AM quickly you will beat it and hear a pop. If you go slower, you won't. Also if you are switching across from phono or aux the muting will trigger before you reach AM and then also no pop I did try a 10uF but this seemed too quick and still popped.

I gotta call this solution 'good enough' - I get a bit pedantic about making stuff work properly, but there's only so much time I want to put in to the beast!

 

[txturbo]

Here they are ,note LDR602 is missing and has been jumpered(yellow). LDR501 and LDR601 are the square aluminium guys.
View attachment 678347

The reason yours is missing is because there is a service bulletin that covers that modification. #69-3/18-3 MODIFICATION OF STEREO PHOTOSWITCH

Without retyping the entire bulletin it basically states that the photo switch can become intermittent so they jumper it out.

 

[monkboughtlunch]

Found this Syd Smith quote about the Marantz 10b optocouplers in the VTV vol 5 (interview conducted by Mike Zuccaro).

 

[txturbo]

I am nearly certain that the photoswitch LDR501 on one of my 18's is weak because I can't get the receiver to mute properly between stations.

Looking at this a bit differently... How would the failure of the LDR's cause a pop?

a) The neon lamps, when they come on, somehow introduce the pop?

b) The photocell, when illuminated does not drop it's resistance the way it should?

c) The photocell is noisy or somehow introduces the pop?

 

[monkboughtlunch]

I am nearly certain that the photoswitch LDR501 on one of my 18's is weak because I can't get the receiver to mute properly between stations.

Have you tried adjusting the R328 muting control (see below)? I'm not sure how well the muting control performed when new. Perhaps muting worked as well as muting in a 10B when up to spec. I've adjusted this pot on my unit and it does eliminate some between station noise, but not all, so perhaps photoswitch is a culprit, and it's not performing up to spec due to degraded optocoupler? Does anyone have a 10B with new muting optocoupler? If so, how well does the muting in the 10b eliminate between station noise?

 

[txturbo]

Have you tried adjusting the R328 muting control (see below)? I'm not sure how well the muting control performed when new. Perhaps muting worked as well as muting in a 10B when up to spec. I've adjusted this pot on my unit and it does eliminate some between station noise, but not all, so perhaps photoswitch is a culprit, and it's not performing up to spec due to degraded optocoupler? Does anyone have a 10B with new muting optocoupler? If so, how well does the muting in the 10b eliminate between station noise?

Yes, I have tried many times.....

I have a model 20b, the tuner in the 18/19 is nearly identical to it. I don't remember how well it mutes. I'll check.

 

[monkboughtlunch]

Observations about my 18:

• When the selector switch is rotated to "FM" and the pop occurs, the orange muting neon lamp (located on the MPX OSC board) flickers on at the same time as the "pop" is heard
• Even if I depress the "Muting Off" push button, the orange neon lamp on the MPX OSC board still flickers on briefly at the same time as the "pop" is heard when selector is rotated to FM

This could be a coincidence, but does it tend to suggest the issue may be located on MPX OSC board and the suspect component is the sealed LDR501 optocoupler?

When new and up to spec did the LDR501 optocoupler introduce a delay that masked out the pop?

Alternatively, are there any capacitors on the MPX OSC or MPX Matrix boards that could also be causing pops due to leakage?

 

[monkboughtlunch]

Found this image online showing the interior of a Marantz 10b optocoupler:

 

[bktheking]

10b coupler is early design round body, nothing like the 18. As for the 1515 it does not have one, so the pop is something different.

 

[monkboughtlunch]

10b coupler is early design round body, nothing like the 18.

Are you saying the neon lamp and photocell in the 10b optocoupler are nothing like the 18 -- or just the shape is different?

 

[monkboughtlunch]

I have a general question about the Model Eighteen's RMS power output when driving 4 ohm speakers (such as AR-3).

The technical specifications in the Marantz owner's manual states:

• Rated continuous (RMS) power output per channel, both channels operating simultaneously, 20 Hz to 20,000 Hz......40 watts at 4 and 8 ohms / 20 watts at 16 ohms

Often one sees in tech specs from different manufacturers that the RMS output is higher when driving 4 ohm speakers vs 8 ohm.

However, with the Model Eighteen, Marantz rated the output the same for 4 and 8 ohm. Is this manufacturer rating for 4 ohm accurate -- or should the output rating for the 4 ohm be higher?

In this 1968 review in HiFi Stereo Review, the publication concluded, after testing, that the Model Eighteen delivered 78 WPC into 8-ohm loads and 132 WPC into 4-ohm loads (at clipping point).

How could the 132 WPC performance into 4-ohm loads be so far off from what Marantz published in the user's manual (only 40 WPC into 4 ohm)? If 132 WPC into 4-ohm is accurate, would the Model Eighteen not be suitable to drive the reference standard audiophile speaker of the 1960s -- the AR-3 (which AR said required at least a 25 WPC amp)?

In contrast, the Dynaco Mark III amps AR used for Grand Central Station tests in the early 1960s were 60 watt amps. So could the Eighteen not perform well with an AR-3 if Marantz's published specs are significantly conservative/ underrated?

Has anyone tried driving AR-3s or AR-3as with an Eighteen?

Here's the quote from the HiFi Stereo Review 1968 Marantz Eighteen assessment:

 

[Fhamre]

I am going to try and narrow down my pop today. Before I start I would like to know what the switch at the back inside of the 18 is for ans what setting should it be at?

 

[txturbo]

It's a cover safety switch...