Analog BK 1500 Scope Repair - Page 1
Forum section: electronics / repair

author: xeonon
author_rank: Contributor
timestamp: August 26, 2015, 07:48:32 pm
content: Hey guys, so I am now the proud owner of a beauty of a classic scope. Works great in XY mode, but for some reason the sweep doesn't change no matter what timebase I've got selected. I'm opening it up now, but other than the usual cleaning of the contacts what should I look for. Below is the calibration point and oscilifun for the XY test. Lovely little scope if I can get it working 100%. Also ironically, it actually seems to be close in calibration so far. Hard to test with no sweep.

author: tautech
author_rank: Super Contributor
timestamp: August 26, 2015, 08:18:52 pm
content: It does look nice and tidy.Now keep the beam Intensity down so you don't get damage to the display Phosphors.A failed Sweep generator is not too uncommon, but you'll probably need another scope to diagnose.Grab the Tek troubleshooting pdf from the Repair Docs and Links thread and do some study.

author: xeonon
author_rank: Contributor
timestamp: August 26, 2015, 08:30:39 pm
content: Will do. If nothing else I've got a good little XY scope. The intensity was a bit high in the pic... But that's mainly the camera. Phones are not made for pictures lol

author: tautech
author_rank: Super Contributor
timestamp: August 26, 2015, 08:34:24 pm
content: I see you have another thread on this CRO:https://www.eevblog.com/forum/testgear/bk-precision-1500-100-mhz-oscilloscope/Can you post or provide a link to the pdf BK sent you? Was it a schematic?We then maybe can advise test points for you to check.

author: xeonon
author_rank: Contributor
timestamp: August 27, 2015, 06:51:23 am
content: They sent the calibration guide in PDF. It's below if you want to download it. I sent in a request for the schematic and manual, but we will see if they send one.

author: MarkL
author_rank: Unknown
timestamp: August 28, 2015, 02:31:52 pm
content: I used to have a BK 1590A many years ago. On the outside it looked identical to your 1500 and it had the brightest screen I've ever seen on a scope. It was also very finicky getting it properly calibrated, as I recall.Perhaps this is stating the obvious, but in your one photo you have single sweep activated. Did you try AUTO sweep mode?

author: xeonon
author_rank: Contributor
timestamp: August 30, 2015, 04:52:50 am
content: Nice on the 1590A, I am actually looking at one now. Far as I can tell the difference is very slight between the two. The sweep generator is dead on this unit. I was testing the single shot but it doesn't trigger correctly as it will not reset. If it's on auto, not normal but only auto, you can watch the triggered light come on as normal. If my assumption is correct I can grab this 1590A and swap out the sweep. On a related note, I do have a 100% working Tek 2245A on the way. But this BK has won my heart so I'm still going to fix it.

author: tautech
author_rank: Super Contributor
timestamp: August 30, 2015, 05:04:42 am
content: Quote from: xeonon on August 30, 2015, 04:52:50 amWith another scope it should be quite straight forward to fix and quite possible no calibration will be needed.

author: xeonon
author_rank: Contributor
timestamp: August 30, 2015, 05:06:41 am
content: Yea, it's very hard to tell which problem is the root one. Trigger seems strange and so does sweep. I do know I don't have the 29V at the sweep gen board, but it looks ok.

author: tautech
author_rank: Super Contributor
timestamp: August 30, 2015, 05:13:48 am
content: Unless you can get a triangular waveform output at the sweep you will get a display like this one you linked:

author: xeonon
author_rank: Contributor
timestamp: August 30, 2015, 05:28:36 am
content: I attached the schematic for the trig/sweep board if it helps.

author: mij59
author_rank: Frequent Contributor
timestamp: August 30, 2015, 05:29:55 am
content: Got the Trio Kenwood CS2110 look almost identical to BK 1500 except for the 50 Ohm input option.I can replicate the screenshot when in X-Y mode, do the horizontal display buttons work ?Its strange that the single and reset led of the trigger mode are lit.

author: mij59
author_rank: Frequent Contributor
timestamp: August 30, 2015, 05:40:47 am
content: Quote from: xeonon on August 30, 2015, 05:28:36 amStart by checking the absolute value and ripple of the power supply voltages.

author: xeonon
author_rank: Contributor
timestamp: August 30, 2015, 06:02:47 am
content: 54.7V 55V at JP4919.75V 20V4.81V 5V9.814V 10V-9.9V -10V.9mV 29V at JP41As for ripples, this is the only scope I have. As I said, the 29V rail is the problem.Edit: Also should note that HV is out of my domain. And as the scope works fine in x-y mode I shouldn't need to worry about it anyway.

author: tautech
author_rank: Super Contributor
timestamp: August 30, 2015, 07:33:53 am
content: Quote from: xeonon on August 30, 2015, 06:02:47 amAs I see those measurements 29V is not the only problem.Highlighted voltages IMHO are low too. ^^^Quick check for ripple can be made with your DMM on AC mV

author: mij59
author_rank: Frequent Contributor
timestamp: August 30, 2015, 07:55:41 am
content: Quote from: xeonon on August 30, 2015, 06:02:47 amTrace back the 29V rail, probably its one of the power supply voltages.As I recall the power supply is quite hard to get to, I replaced all the filter caps of the secondary side, some of the old caps had only half the capacity rated .

author: xeonon
author_rank: Contributor
timestamp: August 30, 2015, 08:04:49 pm
content: I know what you mean on the power supply on this one. It sits in the middle right around the crt. The HV board is on the other side of the crt. The 29v is not on that board though. On the rotary unit board is the usual calibration adjustment for the 29v rail. I attached that page as well if it helps. Correct me if I'm wrong but doesn't it take the 55v and output 29v based on the input of A VAR? I might be reading that wrong though.

author: tautech
author_rank: Super Contributor
timestamp: August 30, 2015, 08:29:34 pm
content: Quote from: xeonon on August 30, 2015, 08:04:49 pmYou are.+29 is derived from a classic linear regulator made from discrete components and powered from the 55V rail.Q2 is the pass transistor.

author: xeonon
author_rank: Contributor
timestamp: August 30, 2015, 09:21:25 pm
content: I was looking at the wrong VR. So the other board takes that and does sweep etc. That does mean I at least have a problem on this one then?

author: tautech
author_rank: Super Contributor
timestamp: August 30, 2015, 09:38:23 pm
content: Quote from: xeonon on August 30, 2015, 09:21:25 pmI'd say so.Check 55V is available a the collector of Q2 and ~7V at the emitter of Q4.VR1 is the adjustment for 29VVR2 (A VAR) is just a voltage divider not a regulator.

author: xeonon
author_rank: Contributor
timestamp: August 30, 2015, 09:57:32 pm
content: No go. Q2 had 55, and I might have mixed Q4 and Q1 up. Also might have shorted out Q1 and let out the magic smoke. So looks like I'm on my way to find some new ones

author: tautech
author_rank: Super Contributor
timestamp: August 30, 2015, 10:07:58 pm
content: Quote from: xeonon on August 30, 2015, 09:57:32 pmNow we know D2 is conducting.Often you need other means of connection for densely populated PCB's.Even the cheap little grabbers work fine and can save you some grief.

author: tautech
author_rank: Super Contributor
timestamp: August 30, 2015, 10:20:09 pm
content: Also R3 and Q1 form the 29V current limit, check the volage across R3, it should be much less than 0.6V.Anywhere near 0.6V means there is excessive load on this VR output.

author: mij59
author_rank: Frequent Contributor
timestamp: August 31, 2015, 04:05:56 am
content: Quote from: xeonon on August 30, 2015, 09:57:32 pmTransistor Q1 and Q4 are of the same type so mixing them up should not be a problem.Check for shorts of the 29V rail to ground with the continuity test of your DMM .Measure the voltage of D1 and C4 to ground.