Adventures With A 500-C

[dcgillespie]

OK. Basically what we will be doing is to take a mono interconnect, and plug one end into a Tape Monitor input of one channel -- either one, it doesn't make any difference, and turn the Tape Monitor switch ON.

Now, ideally, you will take a clip lead and clip one lead to the center pin of the other end of the interconnect, and then tape the interconnect/clip lead, not only so they don't come apart, but also so that the ground sleeve of the interconnect doesn't touch anything. Alternatively, if your careful, your can just use the pin of the RCA plug at the end of the interconnect, but attaching a clip lead would be safer.

If this operation is successful, you should then be able to turn the unit on, advance the volume considerably, and touch the clip on the free end of the clip lead with your finger (or alternatively, touch the tip of the RCA plug). You should hear a substantial hum. If this is so, then all is good.

Now, with the unit upside down, bottom cover off, turned on, turned to FM AUTO, tuned in to a known stereo station, and volume control turned to 12:00 noon, we will CAREFULLY be using the free end of the clip lead to probe different points in the unit and listen for the result.

First up, touch the clip to Test Point 4 on the schematic, and let me know the result!

Dave

 

[notdigital]

K, looks doable. I'll probably get to this tomorrow nite, if not, next day or so. I'll let u know.

Right now, I think I'm gonna pour me one and boob out :thmbsp:

Thanks Dave!

 

[notdigital]

OK, I'll confess: I couldn't wait to get to this so before the family arrives.....

(and for the benefit of those coming after me who might encounter the same/similar problem, I'll document this procedure as best I could.)

I put together the cable using a mono RCA and test lead;

Test Point 4 is located on the 5 lug terminal strip just north of Z5. It is the second lug from the left where the L28 choke meets C-60 and the black wire running to the MPX.

Inserting the RCA jack into either TAPE MON left or right, the corresponding speaker comes alive!!!

Observations: Volume was set WAAYYY below 12 o'clock because at that position, it is much too loud. Also, when RCA is inserted into left TAPE MON, immediate buzz, even before touching test clip. Right TAPE MON is relatively quiet. The left channel is the very weak channel.

 

[dcgillespie]

Not -- Good job. OK. This proves that the basic tuner section of the unit is working fine. I'm sorry that the volume was too loud -- I was just trying to get to a setting that would provide enough gain for the signal tracing effort to work, without causing the unit to produce full power! Different points we test will have different potential signal levels present, so use what setting is best for the scenario. Maybe starting low and then advancing the volume with each test might be best.

Before we go further -- this should produce the same signal volume (for a given volume control setting) whether you used the left tape monitor input and left speaker, or right tape monitor and speaker. Is this true? The channel with the interconnect test lead inserted would be expected to be sensitive to hum when the test clip was not connected to anything in the circuit.

Dave

 

[notdigital]

Before we go further -- this should produce the same signal volume (for a given volume control setting) whether you used the left tape monitor input and left speaker, or right tape monitor and speaker. Is this true?

Dave

It's as true as my wife is pissed at me for doing this **** instead of getting ready for her folks

I'll try to sneak in step 2 after my daughter's soccer game which, to some, is today's number one priority

 

[notdigital]

Another observation:

Those reverb jumpers......:scratch2:

I pull the jumpers out of my 800-c and it kills the FM. I pull them out of the sick 500-c and I still get that weak sound but sound nonetheless. When I trace the wires from the jumpers a few end up at the switches where I can see where one was repaired.

Is it possible that on/more of the switches are shorted causing this effect? Would a mis-wiring of the switches have the same practical affect as if the jumpers were removed? If that is the case then I guess I'd have to check out the schematic which, to me, looks like some plopped a bunch of spaghetti strands down on a piece of paper!

:scratch2:

 

[dcgillespie]

Not -- if rewiring has been done, then of course anything and everything is suspect.

Faint signal with the jumpers removed is not uncommon, and nothing to pursue. However, speaking of switches, the FM Filter Switch is your next port of call.

With the unit set up exactly as described before (but use your volume setting), then attach your "probe" to the first one, and then the other of the center two contacts of this switch. Let me know!

Dave

 

[notdigital]

D: I've attached a pic of the HIGH FILTER switch and the LOW FILTR switch. I thought those black sleeves looked out of place so I pulled them back to reveal the repair to the LOW FILTER switch.. Apparently, it has been replaced.

So, looking at the pic, the left switch is HIGH, the right is LOW.

The center lugs of the HIGH are jumpered together. The LOW ("repaired") switch is not.

Which do you want me to test?

 

[dcgillespie]

Not -- looking at the schematic for the 500C, there is an "FM Filter Switch". It is shown as a slide switch just like the ones you showed a pic of -- except I am not talking about the audio High and Low filter switches.

Let me know!

Dave

 

[notdigital]

OK, found it. On the schematic it is called the "FM Filter Switch" and it shows C-60, a 560pf cap across the "on" position; on the face plate it is labeled MPX Filter.

Touching center contacts of switch in turn: left speaker comes alive when in Left Tape Mon; same in right speaker when in right Tape Mon. Equal volume.

 

[dcgillespie]

OK. Good job again. Hope you warmed things over with the wife!

You have now proven that the tuner, MPX, and Auto FM matrix board are all functioning properly. The ONLY thing between the signal that appears at the MPX Filter switch and the input to the line amplifier proper, is the selector switch.

You say that the Aux inputs and (I assume) phono inputs all work correctly when selected, so if that is absolutely true, then something is amiss with the contacts on that switch that engage the FM signal when selected. Follow the leads from the two MPX filter switch terminals tested, and they should ultimately connect to the selector switch at some point, in a addition to their output connections on the matrix board. Use an ohm meter to verify that terminals are in fact making proper contact when selected. You've got it on the run now!

Good luck with it!

Dave

 

[notdigital]

OK. Good job again. Hope you warmed things over with the wife!

Yeah, I've gotten a lot of practice having to do that over the years :yes:

Follow the leads from the two MPX filter switch terminals tested, and they should ultimately connect to the selector switch at some point, in a addition to their output connections on the matrix board. Use an ohm meter to verify that terminals are in fact making proper contact when selected.

I did a real cursory check of the wires coming from the MPX Filter Switch to the selector switch. Seem OK. Tomorrow I'll again trace the wiring to see if I find anything.

THANX!

OH yeah. The AUX works flawlessly. Dunno about PHONO. My LP12 is missing a cartridge and I ain't in the mood to install one and do a set-up, just to test the PHONO. Would a faulty PHONO circuit cause these problems?

 

[dcgillespie]

No, nothing in the phono circuits would cause these problems. You should be able to test the phono circuits by simply selecting them and advancing the gain to near full. If the selector switch selected them properly, you should hear the "rush" sound of the preamps with the volume elevated in both channels.

I would more suspect something with the actual selector switch, rather than the wiring to it for the FM circuits.

Dave

 

[notdigital]

I would more suspect something with the actual selector switch, rather than the wiring to it for the FM circuits.

Dave

OK, I'll zero in there.....

 

[notdigital]

OK: Two wires lead from FM FILTER SWITCH to Selector switch.

One wire goes to section ONE; the second wire goes to section THREE. On meter zero'd out on ohms; beeped on continuity setting. Touching probe to solder joints = strong sound. Touching wipers = strong sound; touching metal plate on wafer = strong sound.

I again examined ALL wipers and contacts on Selector switch = all good contact. :scratch2:

Man, by the time we figgur this out I'll be a drunkard :beer:

I bet this is going to end up as something really, really dumb :yes:

 

[dcgillespie]

Not -- I'm sure the wiring to the selector switch is good. What you want to focus on is the continuity within the selector switch itself. That is, you found the terminals where the FM stereo signal is applied to it. But you also need to follow the signal through the actual switch to make sure it is coming out of the output terminals, and actually being applied to the input of the line stage amps. It is almost surely the selector switch itself where the disconnect is happening -- not the wiring to or from it.

Dave

 

[notdigital]

BINGO!!!! After tracing the FM signal up and down and left and right and finding no break in continuity I put everything down in a combination of disgust and resignation that I'd have to send it out to someone with the right equipment to track down the break. I sat there for what seemed like forever and the jumpers kept popping into my head. Like when owners complain that they have lost sound and one of the first things that is recommended is to check the jumpers? Well, guess what? It wasn't the jumpers! But feeling I was onto something I traced those big fat wires back to the front of the unit and 4 ended up at he TAPE MONITOR switch. Upon examining the switch I saw that the contacts inside the housing were out of place. (See pic.) I pushed them back, fired her up and:

It appears that the previous owner attempted to correct the FM problem but changed the wrong switch. Additionally, the alignment seems to be off cuz the stations aren't locking very well, etc, no doubt due to making adjustments on the fly. Nevertheless, I'm content that this major issue has been identified and I can now plan for the remaining mods to finally bring this puppy up to snuff!

Thanks Dave for all your time, attention, and patience. I learned a lot from you, as always :thmbsp:

Now, does anyone have a switch they can spare????

 

[dcgillespie]

Not -- Glad you found it!

Actually, with the "signal tracer" you set up, you had everything you needed to do the job as good as any tech could have done. There are in fact many ways you can use a unit to self diagnose various problems with itself.

Interesting that it was the Tape Monitor switch that was preventing the signal from getting through. If it was not letting FM through, I'm not sure how the phono was getting through either -- unless it actually wasn't as well.

Either way, I'm glad you got to the bottom of it, and even more importantly, glad you learned a few things along the way. It will give you that much more confidence to tackle any future problems that might come up.

Congrats again on finding it!

Dave

 

[bhamham]

Way to go, NotD! and, to Dave for his expert help

Chasing down these nagging issues are the best learning experience. I've got a slider switch from a 400 parts unit but won't be at my storage for over a month - probably be the first of next year before I could get it to you. If you haven't found one by then let me know.

 

[notdigital]

Not -- Glad you found it!

Actually, with the "signal tracer" you set up, you had everything you needed to do the job as good as any tech could have done.

That sure is a handy dandy tool! I chased that signal everywhere, from within the switch and down every path I could see; and at every point I got a signal. That, and putting the switch under magnification, convinced me it had to be something else. That's when the jumpers jumped back into my head. And the rest is very happy history :yes: Thanks again for your help and guidance Dave :thmbsp:

And Bruce: Now that I know its just the switch I'll address it later as a not-so-critical item. If it turns out that I still need one down the line, I'll let you know. Grats for the gesture.