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Montgomery wards 3996A power led


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#1 baddboybill

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Posted 16 July 2019 - 11:37 AM

So I’m working on my Montgomery Ward Boomboxes and for some reason the power LED stays on when Boomboxes powered off. Nothing else is powered on as I have checked micro switches in the deck as well. According to scheme of similar Boombox the silver ST858 the transistor Q851 measurements are as follows
B- 0.74v
E-0v
C-0.4v

I’ve tried to adjust VR851 with very minor dimming of led. Anyone have any ideas of what can be problem. The collector should be 0vAttached File  B5EF1E41-51EE-4643-99EE-F18974A4B92F.png   325.95K   0 downloadsAttached File  E8988BDA-7084-4118-8E5F-064665A2970E.png   313.04K   1 downloads

#2 caution

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Posted 16 July 2019 - 03:07 PM

Can you post the entire sheet so we can see where the wires are going from the transistor/LED?



#3 baddboybill

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Posted 16 July 2019 - 03:39 PM

So I lowered the VR851 all way down and measurements are spot on
B-0.70
C-0
E-0

Here is full pageAttached File  8F21C5F5-9225-47AE-8224-159A1FD60FBF.png   348.01K   5 downloads

Attached File  6527DAB6-735C-4272-8752-48E602097A97.png   358.92K   3 downloads

#4 caution

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Posted 16 July 2019 - 03:42 PM

Bill, can you post the manual here somewhere as the original PDF, that screenshot is too blurry to read



#5 baddboybill

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Posted 16 July 2019 - 03:44 PM

It’s hard to get clear page but the scheme for silver ST858 is on Boomboxery manuals

https://download1157...e Selector).pdf

#6 caution

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Posted 16 July 2019 - 03:57 PM

Perfect!

It's clear you're getting current flow through the LED, so something upstream is connected when it shouldn't be, such as the power switch or lamp switch.



#7 baddboybill

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Posted 16 July 2019 - 04:20 PM

https://download1157...e Selector).pdf

Perfect!
It's clear you're getting current flow through the LED, so something upstream is connected when it shouldn't be, such as the power switch or lamp switch.

I checked and nothing seems to be connected. Checked all the micro deck switches to because of the sleep mode. Also the tuner light only powers up when I turn box on...The only power I have when unit is shut off position is the LED. No amp, tuner or deck powered up in off or sleep mode. So strange!!!

There is section in manual about adjusting the power led with the VR851 but does not shut LED off.Attached File  72865EFB-42D3-4FD6-8055-6D9AF9E2B3F2.png   382.68K   3 downloads

#8 baddboybill

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Posted 17 July 2019 - 05:37 AM

So I tested with batteries and it seems the led stays off when powered off but when powered on radio barely working. Fm stereo barely comes in and barely any sound. Have not checked voltage coming from supply but planning on doing that

#9 caution

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Posted 17 July 2019 - 11:11 AM

The faulty LED may be the result of a different issue - is the low signal/sound still there if you disconnect the LED?



#10 baddboybill

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Posted 17 July 2019 - 12:51 PM

The faulty LED may be the result of a different issue - is the low signal/sound still there if you disconnect the LED?


Low signal and sound only when using batteries the Boombox is fine when plugged in. I don’t think led is faulty.

#11 caution

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Posted 18 July 2019 - 06:03 PM

So signal levels are fine, sound levels are fine, and the LED is fine on AC? Everything is fine?



#12 baddboybill

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Posted 18 July 2019 - 08:32 PM

So signal levels are fine, sound levels are fine, and the LED is fine on AC? Everything is fine?


Everything working on AC but led wont go off when you shut power switch to off. Boombox powers off but led stays lit

#13 Fatdog

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Posted 19 July 2019 - 12:20 PM

Maybe it's a visual indicator that is plugged in to AC power?  :blink: :dunce: :huh: :hmmm:



#14 caution

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Posted 19 July 2019 - 03:05 PM

There's a partial short somewhere. Could be on a PCB, could be a part. Electrolytic caps can fail and start to short. Although it seems to work well on AC, it could be there's enough power to overcome the partial short and work close to nominal, but when on batteries, the power isn't as strong, and it struggles to provide full power since so much of it is being redirected to ground through the partial short.

If you turn it off and D852 is still getting current through it, then either the power switch has a problem completely disconnecting, or an alternate route for power has been established. Check the node the resistor for the LED (R853) is on, which also includes D851, SW16, R802, C561, IC551-1, Q801-C and SW20. IC551 can probably be ruled out since you know the amp works, but maybe check C561 to see if there's any resistance across its leads, but you'll have to pull it out to isolate it first.



#15 baddboybill

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Posted 19 July 2019 - 04:04 PM

Maybe it's a visual indicator that is plugged in to AC power? :blink: :dunce: :huh: :hmmm:

That is what I thought but Melly has same unit and says his Led is off when powered off.

There's a partial short somewhere. Could be on a PCB, could be a part. Electrolytic caps can fail and start to short. Although it seems to work well on AC, it could be there's enough power to overcome the partial short and work close to nominal, but when on batteries, the power isn't as strong, and it struggles to provide full power since so much of it is being redirected to ground through the partial short.

If you turn it off and D852 is still getting current through it, then either the power switch has a problem completely disconnecting, or an alternate route for power has been established. Check the node the resistor for the LED (R853) is on, which also includes D851, SW16, R802, C561, IC551-1, Q801-C and SW20. IC551 can probably be ruled out since you know the amp works, but maybe check C561 to see if there's any resistance across its leads, but you'll have to pull it out to isolate it first.

The Radio on DC seems to not tune in but tape deck fully works fine on DC. C561 tests good. Can’t find the switches SW16 and 20 you are speaking of? And Q801c seems to test high on voltage.

#16 caution

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Posted 19 July 2019 - 06:47 PM

Oh sorry Bill yeah that SW20 is a leaf switch on the cassette mechanism. If you look on the very left side of page 6 of the PDF (page "-9-" of the manual) with the exploded view of the deck, you'll see 604 in a big circle with SW20 right under it, pointing to the switch.



#17 baddboybill

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Posted 20 July 2019 - 05:05 AM

Oh sorry Bill yeah that SW20 is a leaf switch on the cassette mechanism. If you look on the very left side of page 6 of the PDF (page "-9-" of the manual) with the exploded view of the deck, you'll see 604 in a big circle with SW20 right under it, pointing to the switch.


I did check all leaf switches plus amp only comes on with regular power switch or when in sleep mode when pressing play. I also noticed very low voltage on this yellow wire with batteries but over 16v on AC and that is actually the power to LED. I’ve checked all diodes and they are all good. Attached File  29DD18A0-A790-4143-B5A4-C3ECEEA580EE.jpeg   62.71K   4 downloads

#18 caution

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Posted 21 July 2019 - 01:06 AM

"amp only comes on with regular power switch or when in sleep mode when pressing play"

Exactly. By the way I meant to say SW15 earlier, not SW16. The main power switch is SW15, and SW20 is the leaf power switch on the deck for sleep mode. These two switches are the only entry points for power on the node powering the power indicator LED. Somewhere else on that node, power is getting through that shouldn't be, but where?

 

It's a matter of elimination, maybe start by disconnecting main power switch SW15 completely and see if the LED is still on when you plug in AC. Then, disconnect SW20. They are on wires so it should be easy to try. If the LED is still on then you're getting power to that LED in a way that's going around both power switches.

"Q801c seems to test high on voltage."

Q801 is the voltage regulator and the collector is the input. The output, the emitter, goes all over the place. Try pulling Q801 out and see if that kills the power LED. It would be good to rule out anything bleeding through the emitter to the collector. There's a lot of stuff connected to the emitter. If you follow the wire on Q801-E, it zigzags down and to the left past the lamps PL1 and PL2, and splits left and right. To the right it goes to the other page to the VU LEDs and deck preamp. To the left, it goes to the tone controls, and to radio switch SW7A, which gives power to the tuner, its LEDs, and the rec/erase heads on the other page through marker J.

As I can see in your screen shot you've traced the yellow wires for the LED in and out of the power indicator drive PCB and over to the power supply board plug where it hops from pin 7 to pin 6 to join main power switch SW15. Then, it goes north on an orange wire to the power amp board.  if you keep tracing the node on the amp board, it first goes right away into Q801-C, R802, and an orange wire over to C561 and pin 1 of the amp, but also runs north on a trace and through a jumper wire to cable JM1, which goes to the other page into the rec/play amp PCB on J107/P107 pin 5. It runs on a trace over to J106/P106 pin 3, where it goes out on an orange wire to leaf switch SW20.

Anywhere along that path could be where the problem is, so maybe unplug one or more of those those plugs and see if it starts to turn off the LED when plugged into AC but shut off.
 



#19 baddboybill

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Posted 21 July 2019 - 09:52 AM

I disconnected the orange wire going to the Q801c with no result. The yellow wire coming from power board has 16.2v on ac and the led in off position plugged to ac has 1.62v. It has to be somewhere on the power board or the switch board.

Tested with switches unhooked and still no results. Tested the transistor again Q851 and still getting 0.04v on collector in off position which it is supposed to be 0v so I ordered a new transistor and also ordered new 3k ohm vr and going to see if maybe transistor has a short

#20 caution

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Posted 21 July 2019 - 11:46 PM

Bill I think that maybe the Wards version of this radio might have differences that we're not seeing in the Silver schematic. If you look at the L version of the ST-858L schematic Bobby has on the Boomboxery library, the Power Indicator LED isn't tied to switched power, it's tied to un-switched power, where I circled it in blue.  So, it would be on all the time as you're experiencing, and acting as a pilot light.

 

I'm suspicious the Wards version you're working on shares aspects of both the non-L Silver model, as well as the L model. Maybe some of its own too. Can you check and verify that you don't have any of those extra parts I circled in red? Which side of the power switch is the power indicator LED's resistor tied to -- switched power, or unswitched power?

 

First image is the ST-858, then the ST-858L.

 

48344230731_f4c9c138d8_o.jpg

 

 

48344427712_7080be205b_o.jpg



#21 baddboybill

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Posted 22 July 2019 - 06:49 AM

Mine doesn’t have L version parts but are you sure L version which has extra parts including a resistor on power board isn’t going to help power LED to shut off and my version acts as pilot light with ac on but not in dc mode? I know that Melly states his shuts off but not sure of his version. I actually thought in beginning it was pilot led until Melly said his LED goes off in AC mode



#22 caution

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Posted 22 July 2019 - 10:15 AM

You don't have to have those extra parts in order to have pilot light mode, that's coming from the different wiring I circled in blue

 

If your power LED is tied to switched power, then you still have a short somewhere.

If your power LED is tied to unswitched power, then you have a pilot light and there's nothing wrong with your box

 

Without a Wards service manual, it's impossible to know how your box is different than the Silver.



#23 baddboybill

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Posted 23 July 2019 - 09:11 AM

Eric mine is same model number as Melly and he said his Led only comes on with power plus if I remember the switch is tied in like 1st pic.

#24 caution

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Posted 23 July 2019 - 09:17 AM

So then you'll have to disconnect one thing at a time from that node. Keep track carefully. Plug it back in to AC between each step until the LED finally shuts off. You need to find where the unswitched power is leaking through to switched power.



#25 baddboybill

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Posted 23 July 2019 - 06:01 PM

Eric I’m sorry but you are correct about the power supply being L version so I guess the LED is pilot. Thank you for all your help

#26 caution

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Posted 24 July 2019 - 11:09 AM

Awesome!

So it's Mel's box is the one that's broke, not yours



#27 mellymelsr

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Posted 24 July 2019 - 12:29 PM

I just watched a Youtube video of a Wards 3996A. In his demonstration it is plugged in and power switch is in the off position and the LED is off. Once he powers it on the LED comes on. His functions exactly like mine.


One thing I just tried Bill...when my 3996A is plugged in but powered off the LED will illuminate if I play the cassette tape. The light goes off when I press stop while powered off.

#28 baddboybill

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Posted 24 July 2019 - 01:10 PM

Awesome!
So it's Mel's box is the one that's broke, not yours

No it’s mine. Melly has the non L model but I was able to remove diode and move jumper lead over to get it to act like power led instead per Norm!!!
 

I just watched a Youtube video of a Wards 3996A. In his demonstration it is plugged in and power switch is in the off position and the LED is off. Once he powers it on the LED comes on. His functions exactly like mine.


One thing I just tried Bill...when my 3996A is plugged in but powered off the LED will illuminate if I play the cassette tape. The light goes off when I press stop while powered off.

Thank you Melly, figured it out With the help of Norm and Eric thank you again guys appreciate it

#29 mellymelsr

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Posted 24 July 2019 - 11:33 PM

Glad you figured it out Bill! Now it's time to play some tunes and enjoy...I really like the way it sounds.