Help Needed - BK Precision 2160 / Elenco S-1360 Oscilloscope Repair

[ilcaccillo]

Hello
My oscilloscope is in desperate need of repair.
I work mainly in pro audio electronics and have no experience in oscilloscopes repair. So it would be great to have some of you help, if you can spare the time.




There's diferent BK Precision 2160 models, theres the 2160, 2160A and the 2160C.
I guess my unit it the 2160, as I was able to get some service docs for the 2160A and the PCB's layouts and component numbering are different.

The unit powers up, but there's only a fixed Dot in the middle of the screen.
I guess that means that both the Horizontal and Vertical circuits are not working properly.

I would like to start by checking the PSU voltage rails but without the schematics or service manual it's a bit hard since I dont see any test points or voltages silkscreened on the PCB's.

Does anyone have the Service Manual for the  BK Precision Model 2160?

Tried to contact BK Precision, Elenco, Pintek . Also I searched quite a bit on the Web for service manual/servicing info and schematics for these models and had no luck.

I guess I will have to try to measure the voltage rails using the 2160A schematic hoping that the schematic is similar.

Any tips and thoughts are welcome.

Thank you

[tautech]

My oscilloscope is in desperate need of repair.


Any tips and thoughts are welcome.

There maybe other models in this series that have different bandwidths yet their construction is similar so try this one for a 2120.
http://elektrotanya.com/b_k_2120_oscilloscope_sch.pdf/download.html

Wait for the "processing" to change to "Get Manual"

[ilcaccillo]

My oscilloscope is in desperate need of repair.


Any tips and thoughts are welcome.

There maybe other models in this series that have different bandwidths yet their construction is similar so try this one for a 2120.
http://elektrotanya.com/b_k_2120_oscilloscope_sch.pdf/download.html

Wait for the "processing" to change to "Get Manual"

Thank you so much I got that manual and also the BK 2125 Service Manual that is available on the same site and the PSU's dont match my unit.
I also got a Schematic for the Pintek PS-605 and is doesn't match my unit but it matches the BK 2160A. (this PSU uses 7812, 7912 and 7805 regulators)

I was able to get the Service Manual from BK Precision for the "2160" unit and although it's an 1993 schematic it also doesn't match my unit. This a nice  and detailed Service Manual,

I got the Service Manual for the older Elenco S1360 and finally the PSU schematic corresponds to my Oscilloscope. Unfortunately this Service Manual is the worst of the bunch I got. No parts Lists, some small mistakes, Schematics dont show values for components and theres no parts list to identify them. Anyway it's much better than nothing so with this I was able to start tracing.

[ilcaccillo]

here is the PSU Schematic:



I Measured all the voltages at the transformer secondary and every value is spot on.

Replaced all the PSU Electrolytic caps with new Panasonic 105º

Replaced BD901 and BD902 Bridge rectifiers

Replaced IC's U901 and U902, these are MC4558 opamps

Unsoldered Q901, Q902,Q903 and Q904 and verify them with my Atas DCA Pro, all the transistores measured good. Soldered them back on

Tested all the PSU diodes, all tested well

Problems:

After the BD901 regulator I'm measuring +-15v instead of +-18.
Collector of Q902 I'm measuring +1.7v instead of +12v.
Collector of Q903  I'm measuring -0.9v instead of -12v.

All the other circuit boards are disconnected from the PSU board.

Any idea of what might be causing this?
the only thing that I didnt had time to do today was measuring the resistors one by one because I have to lift one leg up, visually they look fine.

theres one strange thing with the DB901 bridgr rectifier. I measure it with the + and - legs in the air so it was not connected to the rest of the circuit. I have 16VAC coming from the transformer, after BD901 I'm measuring + and - 14VDC, thats really strange shouldn't the votage after rectification go up by an 1.414 ratio? I changed the Bridge Rectifier for a new one and the same happens.

thanks

 

[tautech]

Problems:

After the BD901 regulator I'm measuring +-15v instead of +-18.
Collector of Q902 I'm measuring +1.7v instead of +12v.
Collector of Q903  I'm measuring -0.9v instead of -12v.

Double check transistor pinouts and C902/3 polarities are correct.
Check P901 is completely home and making contact on all pins.

[oldway]

Seems to be an overload on the +12V rail.

[ilcaccillo]

Double check transistor pinouts and C902/3 polarities are correct.

Checked, Correct

Check P901 is completely home and making contact on all pins.

Do you mean Q901?
if so checked

[tautech]

Check P901 is completely home and making contact on all pins.

Do you mean Q901?
if so checked

No.

Your schematic shows an interconnect P901 between the mains transformer and the bridges.
Check continuity.

[ilcaccillo]

Thanks,
Yes theres continuity between all the the transformer secondaries and the pcb pins. All the BR are receiving the correct AC voltages

[ilcaccillo]

Hello I was not getting anywhere with the +-12V rails problem so I decided to convert this rails to use the 7812 and 7912 regulators like in the updated revisions of this PSU.

here is the present PSU schematic



The +-12 rails are spot on now.
Voltages on the other 2 rails are a bit lower than the correct value

I'm measuring 117V instead of 145V and 3.6V instead of 5V

[ilcaccillo]

HV Generator Circuit Schematic

[tautech]

Hello I was not getting anywhere with the +-12V rails problem so I decided to convert this rails to use the 7812 and 7912 regulators like in the updated revisions of this PSU.

here is the present PSU schematic



The +-12 rails are spot on now.
Voltages on the other 2 rails are a bit lower than the correct value

I'm measuring 117V instead of 145V and 3.6V instead of 5V

Fix those rails too ^^^^

Something is strange with the "High Voltage Rectifier" of the Second Anode Supply (the one that has the suction cap and 12Kv), this rectifier visually is a Black rectangular plastic box with some sort of Red epoxy potting material.
As soon as I power the unit this box makes some crackling noise, when I turn the power off the box still crackles for 1 to 3 seconds and then stops.


What might be the cause of the crackling noise? arching inside the rectifier box? Excessive voltage on the rectifiers input?
The rectifier is shot?

CRT still works even with this noise going on, although I only turn it on for some seconds because Im afraid at the moment that I might be damaging something else.

Maybe, but until the other rails are correct overlook it for now.

Another thread that may be of interest:
https://www.eevblog.com/forum/repair/tek-7704-pops-and-clicks-image-compressing-for-a-moment/
Check all the links within. 

[ilcaccillo]

The +-12 rails are spot on now.
Voltages on the other 2 rails are a bit lower than the correct value

I'm measuring 117V instead of 145V and 3.6V instead of 5V

Fix those rails too ^^^^

Thanks,
will do that first.

I checked it already, Transistors measure good, all caps replaced, all resistors replaced, OPamps replaced, Diodes replaced, and still the output voltages are lower than the schematic values. I guess I have to change resistor values to increase voltage.

Could you help me in letting me know in both rails what resistors could I change the value so that I could bring voltages up?

[ilcaccillo]

I would like to ask if changing the resistors R909 in the 145V  rail and the R970 in the 5V rail would allow to vary the output voltage?

Thank you

[ilcaccillo]

Hello ,
could someone help me out?

thank you

[tautech]

I would like to ask if changing the resistors R909 in the 145V  rail and the R970 in the 5V rail would allow to vary the output voltage?

Thank you

Yes of course.

Here's some study of how to do it: 
http://people.eecs.ku.edu/~callen/501/LinearRegulators.pdf

Google: series voltage regulator using op amp for more clues.

[ilcaccillo]

Thank you so much,
will learn that.

Regards

[KG6FDM]

Hello, Sorry to hear of your oscope problem, they can be difficult.   I have several Elenco 1360 scopes and other test equipment that I will sell for giveaway prices if you have any interest contact me at 813 737 3190,  Jerry, KG6FDM

[ilcaccillo]

Hello, Sorry to hear of your oscope problem, they can be difficult.   I have several Elenco 1360 scopes and other test equipment that I will sell for giveaway prices if you have any interest contact me at 813 737 3190,  Jerry, KG6FDM

Hello Jerry, thank you so much for your offer. I located in Europe.
I will send a pm to you to see if it's possible to contact you over email

thank you so much

[ilcaccillo]

Hello everyone,
I'm still trying to fix my scope, unfortunately photobucket removed my photos from this thread, but I will try to fix the picture links using another pic hosting site.

I was finally able to fix all the voltage rails.
I was able to get a another schematic of the PSU that has the components values written (the old one didnt have) and one resitor had the worng value. Replaced that and the 145VDC rail is fine now.

So I have +12V and -12, 5V and 145V all spot on.

thank you all so much

[ilcaccillo]

Fixed the picture links in this thread

[ilcaccillo]

Going to meet a TV tech locally to see if he can provide some help in testing the CRT

[ilcaccillo]

I met the TV Tech, he had a nice HV probe. We are all to measure all the voltages above 1KV.
All those voltages were fine. Even the 12KV voltage going to the CRT suction cap.