BWD 506 C.R.O. - Repair - All But Dead, Will it Survive

[BTO]

BWD 506 C.R.O. - Repair - All But Dead, Will it Survive

Hi Guys...
SUMMARY :
I've been doing electronics for decades, repaired a lot of stuff, Had limited experience on Scope repair but i'm ok.
I have a BWD-506 that isn't working (Better Description Below), my suspicion is that this probably won't be too hard to fix although this post is more for sharing than anything else as i don't see
too many BWD Repair posts here.

HARDWARE :
BWD-506
Vintage : Approx 1970's
Single Channel
15MHz Bandwidth

THE PROBLEM :
Bought it around 2 weeks ago, Fired it up, I had a trace. and the power light (Which is integrated into the intensity knob) turn on Solid Red.
it worked for 2 days.
Now i have no trace but the red light still appears to come on when powered on (Mains Lead only, no Power Switch). Photo 3
Interestingly.. When turning it off some manner of trace appears (i'm feeding in a 1Khz , 2V Sine Wave)  Photo 4

Anyway.. Let's get this sucker open and see if we can bring this back from the dead.  WILL IT SURVIVE ?

Feel free to brainstorm,  it's on the bench now, More updates coming soon let me just get the housing off

Already tried turning all the knobs, Intensity, focus, and all that , Vertical and Hor Position. to no avail.
More updates soon

[BTO]

REMOVING THE HOUSING...
Removing the housing was SUPRISINGLY not too difficult. My first thought was
- Hmmm no screws anywhere... Weird !
- Second thought.. Perhaps i need to remove the 4 screws on the front CRT screen protector and all the buttons and knobs ?    NO, DON'T DO THAT !
- 3rd Thought... Could it be as simple as the feet on the bottom (Photo's 5 and 6)  That was it !

- Then Tried to take off top cover, didn't go, I had to remove the Plastic strap (Photo's 7 and
in a nutshell to get to the guts is 4 flat heads on the bottom and 2 from the top. Nice and simple

Next Post, Looking at the internals

[BTO]

SOMEONE FORGOT THEIR CHEWING GUM IN THE SCOPE

Before i get into it, There is this sucker in the picture which looks like we found FREE CANDY, Someone from the 70's forgot to take their gum with them after repairing the scope.
But seriously... What the hell is this thing ?  LOL
A Resistor of some nature ??  From top to bottom  Red, Yellow, Black, Blue (in 50 years, I've never seen one of these)

Next Post (Probably spread over a few posts due to larger file sizes, Let's have a browse of the internals

ALMOST FORGOT :  The Service Manual.
this looks like it'll be a good resource
https://www.eevblog.com/forum/testgear/bwd-product-catalogues-(vintage)/
THANK YOU SEAN0118

[BTO]

A LOOK AT THE BOTTOM SIDE
After taking off the rubber legs with 4 flathead screws i was able to remove the bottom housing to reveal the following
- First thought.... Lots of dust!
- I'm wondering if the Orange and Yellow wires are bodge wires
- THE SMALL BROWN BOARD seems to be connected to the TRIGGER CONTROLS and the TIME/CM (Horizontal Knob)
- THE METALLICA BOARD with the screws in it seems to be connected to the Vertical VOLTS/CM (Vertical Knob)
- Other than the Monster Transformer, Nothing more to see here for now

[BTO]

A LOOK AT THERIGHT SIDE INTERNALS
After removing the top cover with only 2 Flathead screws we get Photo 12 (top View)

Upon opening, THIS IS INTERESTING, (Photo 14) we have a board.
- First, there are those 4 brown cigaratte things, They all seem to be labelled with a "D" , Can i assume they are diodes ? there's 4 of them so... Rectification perhaps ?

TESTING DONE HERE 
(if you're a beginner and attempting this, BE VERY VERY VERY CAREFUL. this is how people die)  Wear Low Voltage Gloves if at all possible.
- Tested my Mains Power - 240V AC Confirmed
- Tested Voltage between Active and Neutral  on the input screw terminal - 240V AC Confirmed
- Tested Voltage Active to Earth - 240V Confirmed
- Tested Voltage Neutral to Earth - 240V Confirmed
So all good on the input Screw Terminal

From here i visually followed the Red and black Very carefully , it goes up the back of this board, under the metal frame bar and TO THE SWITCH (INTENSITY / POWER OFF KNOB) of course.
There are 4 Wires
Red
Black
Red/White
Black/ White

- Tested Voltage Between Red and Black (as that is coming out of the screw terminal) -240V Mains confirmed
- Tested Voltage Between Red/White and Black/White - 240V Mains Confirmed
This explains why the light is still ON.

This then runs directly to the Primary side of the Mains Transformer, I'll assume this is an AC Step Down Transformer as no rectification has occurred yet.

- Tested Primary side of the Transformer - 240V AC Confirmed

At this point i'm wondering if i have a dead C.R.T.

Next Post, Testing the Secondary of the Transformer

[John.S]

Hi BTO,

Your piece of candy is a capacitor.
I use to have a sheet with the color code, but at the moment it seems to be out for beans 
As I remember well, from top to bottom, first 2 colors represent same numbers as resistor colors, third is multiplier, also same as resistors and capacity is in pF.

John.

[BTO]

HOLD THE PHONE.... We have a problem
While looking around at the Transformer, i noticed this....

There is cap that , for some reason, is not connected to it's solder joint  (could it be that easy ?)
Let's find out

[BTO]

Hi BTO,

Your piece of candy is a capacitor.
I use to have a sheet with the color code, but at the moment it seems to be out for beans 

John.

Yeah i figured it was, it looks like a Psychadelic non polarized AC Cap.

Currently i have 2 things going on with this scope
1. I have a cap that has a broken leg
2. I don't know if my C.R.T. is buggered and i'm not sure how to directly test them.

I'm gonna solder this cap on and FINGERS CROSSED    LOL , Don't jinx it,  Tounge at the right angle and pray to the oscilloscope gods that that's all it was.
Wish me luck

[BTO]

HOLD THE PHONE.... We have a problem
While looking around at the Transformer, i noticed this....

There is cap that , for some reason, is not connected to it's solder joint  (could it be that easy ?)
Let's find out

Soldered the cap back on,
Of course it didn't work  LOL  that'd be too easy.

[BTO]

SECONDARY SIDE OF THE TRANSFORMER

well, now i'm not sure if my Transformer is faulty (if it is, i doubt i'll be getting a replacement for this)
and if it's the C.R.T. i doubt there's a replacement for that, I've already checked, can't find a damn thing.

Anyway.. that board with the cigaratte sticks
- i Assumed the Cigarettes were Diodes,   i tested them on my DMM in Diode Mode, Either they are not diodes or they are faulty diodes because in both bias directions they are O.L.

- Now it was easy to test the other end of the cables coming out of the Secondary Side of the transformer.
  on that board from bottom to top the board has
6 Cables  then a gap  then 5 Cables
The bottom 6 come from the secondary of the Transformer
but some of them go to the underside of the Mainboard on the base.

the top 5 Seem to go directly to the back of the CRT (I haven't tested these voltages yet.
From Absolute bottom of the board, going up
THE BLACK IS 0V Obvioulsy

Tested Orange / White   got -423 DC   611 AC  (i assume this is 611 AC but don't understand why i got -423 DC)

Tested White  -420V DC  (didn't get any AC on this one just a - DC Reading

Black  0V

Tested Brown / grey  0.47 AC   -0.36DC (this is suspect, i expected to get soomething here)

Tested Orange / Yellow   52.5 V AC  (This seems correct)

Tested Orange  57.5V DC (This seems correct)

At this point i'm not totally sure what to make of these figures, I'll have to consult the service manual

[BTO]

I JUST NOTICED SOMETHING ELSE

it would appear that there is a lifted pad on the bottom of the board....

[BTO]

Update : The Pad is lifted, However it seems connected to a resistor on the other side regardless and it seems solid so i'll leave this alone for now

[BTO]

I'm off to bed...
So far i've had a quick look at the schematic,
1. it doesn't appear that i'm getting the correct voltages off the Secondary side of the transformer
2. The C.R.T. doesn't seem to be showings signs of life

Does anyone know how to check if i have power going into the C.R.T. (Admittedly, this is something i have never needed to check in my life)

[BlownUpCapacitor]

Does anyone know how to check if i have power going into the C.R.T.

Do you have a HV DC meter? Like a BK precision HV-44? Or something similar that can safely measure voltages above 20kV DC? If so, proccess is simple for checking anode voltage. Disconnect anode cap or plug, and use the meter to measure the voltage from the anode voltage source.

Also check the pre-acceleration voltage. Should be around 500v to 1kV??? I don't have the service manual for this thing, and I can't find it in the link you provided.

The simplest step would likely be to check the filament voltage. Check the two filament wires and check for a voltage of 6.3v DC or AC RMS. Then check the resistance of the heater element. If it is open, the tube is garbage.

[factory]

If a trace appears when it's switched off (as mentioned in the first post), the CRT is unlikely to be dead.

Seems to use mains derived HT, with some large capacitors in the multiplier stage, be very careful here.

The stick rectifiers are almost certainly selenium, these use a pile of selenium diodes in series in each tube, the voltage drop will be much higher than your DMM can test.

David

[BTO]

Does anyone know how to check if i have power going into the C.R.T.

Do you have a HV DC meter? Like a BK precision HV-44? Or something similar that can safely measure voltages above 20kV DC? If so, proccess is simple for checking anode voltage. Disconnect anode cap or plug, and use the meter to measure the voltage from the anode voltage source.

Also check the pre-acceleration voltage. Should be around 500v to 1kV??? I don't have the service manual for this thing, and I can't find it in the link you provided.

The simplest step would likely be to check the filament voltage. Check the two filament wires and check for a voltage of 6.3v DC or AC RMS. Then check the resistance of the heater element. If it is open, the tube is garbage.

"Do you have a HV DC meter?"
No i don't,Never had a need for one.

"Or something similar that can safely measure voltages above 20kV DC"
No, i have a Fluke 117 that can measure up to 600V,  that's about it

"The simplest step would likely be to check the filament voltage. Check the two filament wires and check for a voltage of 6.3v DC or AC RMS. Then check the resistance of the heater element. If it is open, the tube is garbage."
sounds pretty simple,  How do i identify the 2 filament wires ?

[BTO]

If a trace appears when it's switched off (as mentioned in the first post), the CRT is unlikely to be dead.

Seems to use mains derived HT, with some large capacitors in the multiplier stage, be very careful here.

The stick rectifiers are almost certainly selenium, these use a pile of selenium diodes in series in each tube, the voltage drop will be much higher than your DMM can test.

David

"If a trace appears when it's switched off (as mentioned in the first post), the CRT is unlikely to be dead."
that's my suspicion, but it doesn't appear everytime when switched off , and it's not really a trace it's more like 2 small lines, Not sure what to make of it. i hope it's not dead.

"The stick rectifiers are almost certainly selenium,"
Ahh yes, that was my suspicion, 4 in that configuration is likely going to be a rectification bridge. and i had a peek at the service manual
and the schematic symbols were diodes that matched up with the component numbers on board, so yes they are definitely diodes.
Weird diodes, but still diodes

" the voltage drop will be much higher than your DMM can test. "
Interesting,   This now has my curiosity aroused.
Any idea how one would test these diodes,  Possibly voltage on the output of 2 of them or something like that , right ?
but i'd still want to know how to actually test the Vf on them . I imagine there must be a way

[BlownUpCapacitor]

"The simplest step would likely be to check the filament voltage. Check the two filament wires and check for a voltage of 6.3v DC or AC RMS. Then check the resistance of the heater element. If it is open, the tube is garbage."
sounds pretty simple,  How do i identify the 2 filament wires ?

Do you have schematics? Those will tell you. Otherwise, try to find two identical pairs of wires close to each other. 75% chance those are the filament wires. Also, check where they end up. The transformer?

[ardiesse]

I have the same model of oscilloscope.  They're pretty reliable, but there are some things that go wrong.

Schematic: go to Kevin Chant's website, and look under test equipment, BWD.

The EHT supplies are mains-derived.  The rectifiers go bad.  Replace them with generic microwave oven diodes.  I used 1N4007s in series, but that's not the best way to do it.

The BF197 and BF336 output transistors in the vertical amplifiers often fail, causing the trace to go off-screen.  Equivalents in TO-92 or TO-18 cases are easily available (may even be BC108) - search datasheetcatalog for the datasheet.

And I had the uA733 (a 10-pin metal case IC) fail on mine, but fortunately at work we have lots of vintage semiconductors.

Rob

[BTO]

I have the same model of oscilloscope.  They're pretty reliable, but there are some things that go wrong.

Schematic: go to Kevin Chant's website, and look under test equipment, BWD.

The EHT supplies are mains-derived.  The rectifiers go bad.  Replace them with generic microwave oven diodes.  I used 1N4007s in series, but that's not the best way to do it.

The BF197 and BF336 output transistors in the vertical amplifiers often fail, causing the trace to go off-screen.  Equivalents in TO-92 or TO-18 cases are easily available (may even be BC108) - search datasheetcatalog for the datasheet.

And I had the uA733 (a 10-pin metal case IC) fail on mine, but fortunately at work we have lots of vintage semiconductors.

Rob

"I have the same model of oscilloscope."
that's awesome to hear... and rare. Appreciate it.

"They're pretty reliable, but there are some things that go wrong."
OK

"Schematic: go to Kevin Chant's website, and look under test equipment, BWD."
Already been there, that's how i figured out that the brown stick things were diodes.   Thanks though

"The EHT supplies are mains-derived.  The rectifiers go bad.  Replace them with generic microwave oven diodes.  I used 1N4007s in series, but that's not the best way to do it."
HANG ON... so even if it's not the best solution,  Your saying.. I could just sub in 1N4007s , At least just to test it ?

"The BF197 and BF336 output transistors in the vertical amplifiers often fail, causing the trace to go off-screen.  Equivalents in TO-92 or TO-18 cases are easily available (may even be BC108) - search datasheetcatalog for the datasheet."
Will Definitely do that,  At least you're not pointing to the Main Transformer or C.R.T.  , i was really thinking it was one of those 2
and if the Transformer, i don't know where i get another one that is tapped the same

Re the UA733 , Do you have any spares, I'm happy to get a few off you and cover the postage, Please let me know

[ardiesse]

If you are going to connect rectifier diodes in series, you need to include high-voltage-rated bleeder resistors across them to make the reverse voltage drops equal.  The example is R201-R204 across the capacitors C201-C204 in the CRT cathode bias supply.

It's less trouble to use microwave oven diodes.  If you're in Sydney, go to WES in Summer Hill.

[factory]

The circuits are here; https://www.kevinchant.com/uploads/7/1/0/8/7108231/506.pdf
Details of the CRT here; https://frank.pocnet.net/sheets/011/d/D13-27GH.pdf

BF197 is one of those lockfit transistors, lockfit transistors seem to have bad reputation.

David

[BTO]

If you are going to connect rectifier diodes in series, you need to include high-voltage-rated bleeder resistors across them to make the reverse voltage drops equal.  The example is R201-R204 across the capacitors C201-C204 in the CRT cathode bias supply.

It's less trouble to use microwave oven diodes.  If you're in Sydney, go to WES in Summer Hill.

Yeah i am in sydney, Do you mean W.E.S. Electronics ? I thought they moved from Summer Hill.
OK,  i can call them and check them out. Are you also in Sydney ?  I'm in Macquarie Fields

[BTO]

The circuits are here; https://www.kevinchant.com/uploads/7/1/0/8/7108231/506.pdf
Details of the CRT here; https://frank.pocnet.net/sheets/011/d/D13-27GH.pdf

BF197 is one of those lockfit transistors, lockfit transistors seem to have bad reputation.

David

thanks , appreciate it, i think that helps me identify those 2 centre wires in the C.R.T. as f and f
Appreciate it