Please help identify this variable resistor

[73's de Edd]

Thir hotrodjohn7 . . . . . . . .ohmiGawd . . I theem to have developed a lisp . . .

If you will look through your pictures I believe that you that you will see that resistor R2 is not any too happy with the AC line connections scheme either !
What I seem to be seeing here is the inter connection of two HOT AC line devices .
And you drew the short straw on your odds and are having a 60 VAC or the full 120 volts potential being between the two units, and your pot is burning up, when swung to one end of its adjustment range .
Give us the model of the olde time Monkey Warts phono, so that we can research to see if it's design is being a HOT chassis model also .
ISOLATION is supposed to be taken care of by your BLUE circled transformer . . . . .UNLESS . . . . you have a common chassis connection between two HOT chassis units, and just happened
to have gotten one of the NONPOLARIZED A.C. plugs into the wall socket 180 degrees off.

We can see that your unit, with its 4 non-polarized AC outlets is being a HOT chassis unit, we just need to know about the record player or anything else,plugged into an A.C. outlet that you're trying to tie into the unit .

73's de Edd

 

[hotrodjohn71]

Hi 73's de Edd, Thank you for your reply. Wow, youre right, that R2 resistor does look fried. The one in the photo below circled in blue is the R1 resistor and it seems normal. Its on the functioning light string circuit.
The serial # of the unit is 010879-D-45475
The model # is GMJ-2920

 

[73's de Edd]

Sir hotrodjohn7 . . . . . . .

With that rapid of a comeback . . .your secondary . . .nom du plume . . .should additionally be JumpingJackFlash.

That unit turns out to be of circa 1977 vintage and would be expected to be solid state design and isolated from the AC line with a transformer.
FILL IN:
You have the " Light Organ " unit plugged into the AC line and up to 4 light strings plugged into the organ units A.C. receptacles.
Now the question is . . . . . . if this is inside of the unit and being an integral unit of the stock MW unit or has it been added to the unit by someone in the past ?.

In the case of the innards being all factory stock, MW would have made the AC connection within the unit and ONLY the MW unit has to be pluuged into the wall AC outlet.

I'm trying to ascertain if any other connection to ANOTHER piece of equipmet might be introducing a HOT AC source into the unit.
OTHERWISE, our derived problem is being within the stock MW unit, yet to be delved into.

Those pots ore supposed to ONLY have a feeble .5---2volt audio signal across them. SOMEHOW, you now seem to have some seeee---rious voltage across that pot, to cause a burning of the internal resistance element when being NEAR one . . . .low resistance . . . .extreme of its range.
BTW, I would expect the other pot to be acting in the same manner, had it been adjusted

73's de Edd.

 

[hotrodjohn71]

Hi 73's de Edd,
Thank you and thank you for your response. I guess I obces on projects some time.
All of the components are factory stock from MW as far as I can see. The Light Organ unit is attached behind the unit in what looks like a factory cut-out hole.

The one good channel does actually work well and as intended. The lights flash according to the intensity of the music and the intensity pot will increase or decrease the intensity very well.

The stereo /tape / record player setup in the console does have its own transformer.
I drew a rudimentary diagram of the path of power and the light organ unit. Also for fun I checked that burnt resistor. Its still showing 10K ohms.

 

[dorke]

You are correct,the circuit is not complicated .
The item you have encircled in blue is an audio isolation transformer.
There are 2 identical channels each has it's own SCR( red).
See a typical circuit diagram below (3 channels).

Could you please try to produce a circuit diagram by following the connections on the board?


Two important note:
1. This board doesn't look very safe by construction.
2. The soldering work isn't something to write home about,cleaning and removing solder-splashes is needed .
3. The problem could be a short circuit on the bad channel or a bad SCR.
Do you have a DMM?

 

[73's de Edd]

Sir hotrodjohn71 . . . .


Since I came in at the top of the 8th inning, a full reread of earlier text was in order.

The only relevance to carry forward was the use of an INSULATED shaft on the pot was in order. Since your fingers would touch it on any adjustments. Shaft type was of minor relevance since no knob was involved, just a finger gripping of the shaft.

Your final drawing nailed the fact that this problem was relevant to being within the unit, with no additional possibility of introduction of HOT external AC into the unit.

Now inspect. . . . . .vewy-vewy cawefuwwy . . .a la Elmer Fudd . . . . that my supplied marked up reference of your PCB board sides has the top pic showing YELLOW flow arrows.

Normally those arrows directions would be reversed, since full audio would be across the extreme outside of the BLUE pot and and your selected lower audio being taken off from the center wiper control and being fed over to the gate of the BLUE SCR-1.

In that manner the audio volume undulations provide a varying gate drive to that SCR gate to make it vary its Anode-Cathode conductance to those coincidences and illuminate the light string accordingly.

I think that your SCR-1 has broken down to its gate . . .somewhat seriously . . . and the present condition is a strong*** AC voltage now flowing down the YELLOW arrow path direction and into the pots wiper terminal .

And THEN, if you rotate the pot clockwise, that potential 10K of the pot would progressively decrease in resistance, until the resistance is low enough for “Smoke City” -----“Arcville” which would start occurring at a low resistance.

If you peg the control, then R2 should start getting uncomfortable . . .as you can see that it has . . . already . . . . . by its discolored center belly ring.

If you had the full 120VAC potential back feeding through that R2 resistor only ~12 ma of current passage would discolor it as you see it.

Looks like you now might need to power down the unit from AC power and take DVM in its LOW ohms function and take initial comparative resistance readings between Gate to cathode and then anode of the units good GREEN SCR-2's unit.down lower on the PCB.
Then test SCR-1 to see if it might actually have anode or cathode directly shorted to the gate terminal.
Otherwise just compare like readings between the two units.

You say
SCR1590` and also a Motorola stamp then `7913`

We have to watch out for gate sensitivity and I will see what that SCR1590 crosses into as a newer sub.
And indeed . . . .a manufacture date of the 13th week of 1979.

Feed back time . . . . .

ADDENDA:
I see that in the interim time of getting my image hosted, that Sir Dorke has come on and given some information.
One difference in the supplied schematic is that it is a frequency selective feeding of three frequency spectrums . . . . .All---Low--High frequencies . . . . to three SCR's.
As being compared to an actual board foil path "reading" of your circuitry .
Yours has full frequency range coverage of A and B to two lighting strings and is responding to audio input voltage peaks.

MARKUP:

73’s de Edd

 

[dorke]

ADDENDA:
I see that in the interim time of getting my image hosted, that Sir Dorke has come on and given some information.
One difference in the supplied schematic is that it is a frequency selective feeding of three frequency spectrums . . . . .All---Low--High frequencies . . . . to three SCR's.
As being compared to an actual board foil path "reading" of your circuitry .
Yours has full frequency range coverage of A and B to two lighting strings and is responding to audio input voltage peaks.



73’s de Edd

.

The 2 gates are driven by different R-C networks, hence it is a 2 spectrum device.
Besides,
If it where a single spectrum device there would be no need for 2 SCRs ,one would do the job

 

[hotrodjohn71]

Hi dorke, Thank you for your reply. I agree that the circuit board is quite rudamentary, although carefully studying it and doing some resistance tests (with my minimal electronic skills) I dont believe there to be any splash-over solder shorts. I will try to recreate a circut diagram.

Since the one channel does work, that would indicate to my minimal electronic knowledge that the audio isolation transformer is probaby okay. Infact I have run the unit with both colored light strings hooked up to the one good channel and it has worked well. I could probably leave it that way, but id really like to get that other channel working.
Seems like that SCR is the only possible bad link in the circuit.
What is a DMM?
Thanks

 

[dorke]

A DMM is short for Digital Multi-Meter a tester for voltage/current/resistance etc.

Yes,the audio transformer is fine.
The SCR is the main suspect(assuming no leakage through board "dirt").
I would start with cleaning the board with rubbing-alcohol and cotton-swabs.

If you have a DMM you should test the resistance(without power!) between the Gate and Anode of the SCR in both directions (and with the lamp chains disconnected).
You can compare the results with the other(good) channel SCR .
There should be very high resistance (theoretically infinity).

 

[hotrodjohn71]

Hi 73's de Edd, Thank you for your reply and your time and advice.

I agree about the replacment pot needing the insulated plastic shaft. That style is what I ordered on the good advice of Minder (earlier in the thread) . although I smoked the new one, atleast I have a model # and the search for that particular part is over thanks to the advice of the guys on the thread (which I am very grateful)
But as for that SCR - that is very hard to locate, or even find a cross ref.. Ive googled the # SCR1590, also a stamping of a motorola logo and '7913' date code, and many derivatives of the Part # with 'motorola' and 'cross reference'
I like to consider myself good at locating hard to find parts when restoring some old item, but not so with this SCR. I called Digikey and inquired about a crossrefrence, and a couple other bigger electronics suppliers and they had nothing.
I looked through a couple Mototola technical PDF guides online, one from 1965 and one from 1975, but not too sure about what I was looking at in those books.
If I cannot find the specs for this SCR1590 so I can cross it over to an equivelant unit, would you hazzard a guess on what the parameters of the SCR1590 are? (shot in the dark?) I am willing to bet dollars to donuts (whatever that means) that that SCR failed.

After reading your last post several times, I think I am really understanding this. I think you nailed it very well.And that pathway you indicated with the yellow dots is the very path I was tracing with my eyes as I was also tracing the pathway of the good circuit and comparing it to the bad circuit. You guys are actually making this understandable.

The way you also explain the overheating resistor from the failure of the Gate on that SCR and the other consequences of current going through that pot make sense also. I thank the Lord that a fire never occured previously when the system was plugged in long before I decided to restore it.

The DVM resistance testing of the one good SCR to the bad SCR will require me do learn a bit more as Im a little unsure on how to do that.

Thank you for deciphering the date code on my SCR.

 

[hotrodjohn71]

Hi dorke, Thank you for clairifying the terminology. Yes I do have a DMM, silly me

Question, when testing the resistance on the SCR's will it matter that they are connected to the circuit board? in other words, since they are soldered to the board, could the other circuitry in the board being connected to the SCR's by way of the traces on the board cause incorrect readings on the resistance test as opposed to testing the scr's removed from the board and without connection?
I will do the cotton swabs and rubbing alcohol cleaning also.

 

[dorke]

That SCR1590 is indeed "untraceable".
It shouldn't worry you at all,a "proper" replacement device can be found easily.

We can guide you with that later on.
It probably should be a 200V or so device,
power handling my very and depends on the power of the light chain,can you estimate it?
The gate sensitivity is a bit more difficult ,it may be measured in a few ways.

So,do you have a DMM?
If you do ,post it's photo please.
so we can guide you if needed.

 

[hotrodjohn71]

Hi dorke,
My DMM is a cheapo but seems to work well.
The 2 light strings have (4) c7 light bulbs each. Each of the two channels of the unit runs its own string of four light bulbs in parallel (that is each bulb is supplied with power and if one burns out, the others remain on) if thats the definition of parallel. I did take intro to electronics in high school in 1988.
Each bulb is probably 5 or 7 watts, just like a night light bulb.

 

[dorke]

O,k
So we have a load of 4*7w=28W hence a 1A or higher device should be fine.

Yes,
The resistance measurement is effected by the wiring of the board and other devices connected to the SCR.
But,we can still measure and have conclusions.

Here we go:
1. Disconnect the devices(audio,light organ,and physically the light chains from the outlets.
2. Put both channels potentiometers at maximum sensitivity.
3. Put the DMM in 20Kohm range(as shown below). The probes should be connected for ohm measurement.
3. Touch both DMM probes you should get zero ohms.
4. Measure resistance between the Gate of the SCR and the 2 plug chain outlet (both points! ),what do you get?
5. Do #4 for the other channel,what do you get?

 

[hotrodjohn71]

Okay, thank you.
one real quick question first. How do I determine which pin is the Gate. Would it be on the far right looking at the side of the SCR that has the part # on it?

Also, setting the potentiometers at maximum sensitivity, does that mean full resistance, or no resistance.
Thanks

 

[dorke]

Yes,
That is the gate like you described and the drawing shows.
Max sensitivity is the pot. at full resistance ,should be the same as " max light intensity" on the panel.

Please add this check
6. Measure resistance between the Gate of the SCR and the mains cable (both points! ) when the switch is set to on, do it for both channels,what do you get?

 

[73's de Edd]

Sir hotrodjohn71 . . . . .


The thing that positively identifies that unit is the one-of-a-kind distinctive case that it uses. That was a definite Motorola thing.

I pulled out my 1965 Motorola Thyrister Hand book . . . as they had not initially named them as SCR’s back at day one of its birth, in the in the mid 1950’s.

At your units time frame of 1977 you would have had that unit available as being a 2N6240 in a mil-spec unit / aerospace version , while the industrial / consumer version was a C106D.

After the infamous " 1999---200X great Motorola-ON Semi-Fairchild-Littlefuse Debacle" of their discrete components manufacturing sites, looks like you can still buy the unit as a C106D Litlefuse unit now. ***

This unit would have the required gate sensitivity, but double the breakdown voltage.

It’s all at a mere 66 centavos per copy . . . . baby cakes !

BTW . . .what are the capacitances of the GREEN dipped polyester caps ?

The “fat” C1 cap favors selection of the low frequencies, while C2 favors the mid ranges
of the two sampled channels.

Here is the correct basing / connections data on the installed units.



One SCR Source: ***

https://www.digikey.com/product-detail/en/littelfuse-inc/C106DG/C106DGOS-ND/918418



73’s de Edd

 

[hotrodjohn71]

Thank you guys for your replies. I am blown away by your kindness and willingness to help me in this endeavor.
I regret that I havent gotten back to my workbench to check the components, but hope to soon.

dorke, thank you for your instructions which I will follow soon and report back with the figures. The switch that you mentioned in #6 only controls one plug in outlet on the unit. It is for an auxillary light string that this stereo system apparently never came with. Its not part of the channel 1 and 2 colored light setup, but I will switch it to on when I check the resistances as you mentioned in item 6.

73's de Edd, Thanks for going through all of that trouble to find the current # for that SCR. I am amazed that you have that Motorola book and even moreso that you would pull it off the shelf to help me possibly solve this repair.

I have a question, Ive been studying the workings of these SCR's but cant quite seem to wrap my mind around how they work.

https://electronicspost.com/draw-and-explain-the-v-i-characteristics-of-an-scr/

My question is that you mentioned on the SCR you show above with the Digikey link, you said it has twice the breakdown voltage of the original unit. Will that be noticably different from the original unit in the performance of the colored bulbs?

I have orded another potentiometer and one of those SCR's from digikey and they are both on the way. very affordable

 

[dorke]

In this case the switch position has no influence on the measurement.

The breakdown voltage is the point at which the SCR will fail( be destroyed) if reversed biased.
It will be reversed biased since it is connected(A-K) to AC.
So,In principal, the higher this voltage the better.Operation will be fine.

The simplest way to understand an SCR is to think of it as a diode(A-K) with a control pin (G-for gate).
For the diode to conduct there should be a sufficient amount of current to be supplied momentarily to the gate.
If that is done the diode will conduct as long as it is forward biased (A-K) even if ,the gate current is removed.
It is a bit like a latching relay.

There are many clips on YouTube for SCR (Thrystor) in various levels ,look at this one.
If you still have questions ,we are here.
For an in dept understanding look at the 1972 GE SCR manual

 

[hevans1944]

look at the 1972 GE SCR manual

Many thanks for this link! I haven't seen this extremely valuable manual for many years! My wife does not understand why I insist on keeping "obsolete" references like this around. If I don't hide them carefully, they wind up in the trash when she "tidies up" the house.