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Took the Plunge - GoldStar OS-7020 Oscilloscope

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Whales:
Holy crapola thankyou Brumby.  I have the 7040A model (40MHz) and I was coming here to say I've never had any luck finding schems.

olafwagner: DC drifts on mine slowly as it warms up, but apart from that it's in perfect working order.  Shoot me if you want photos or measurements of any parts.

EDIT: previous topic RE my scope

Brumby:

--- Quote from: rsjsouza on January 17, 2020, 01:32:49 am ---Brumby, can you try to upload to K04bb site? I have downloaded very high resolution scans there (600dpi grayscale for schematics) and the site is a very precious aggregator of manuals.

P.S. I am on my cellphone, otherwise I would do it.

--- End quote ---

Done.

I changed the file name to:

Goldstar_OS7020_Oscilloscope_Service_Manual.pdf

... to follow the preferred convention for that site.

olafwagner:

OK, I have started digging into this further now that I have the service manual (many thanks again Brumby, it is perfect).

1. Power Supply
I checked all the low voltage rails, and they are all within spec. Now that I received my HV probe, I checked the HV rail and it seems to be a bit low (or is that high?). According to the service manual, it is supposed to fall between -1,995v to -1,805v but I am measuring -1,650v. I hope I am doing this correctly, is this a DC voltage (service manual doesn't say), and I connected the HV Probe ground terminal to chassis ground. Is this too far out of spec?

2. Working Backwards from Intensity Pot
After looking at the 'HV CRT Drive' schematic, I started at P604 Pin 4 and measured voltages. Turning the pot sweeps from -6.6mV (~0v) through to -11.76V (effectively -12v rail) so I believe the pot works correctly.

While I was scouring the internet for this service manual (prior to getting it from Brumby) I discovered that the Hitachi V-212 is a very similar scope with pretty similar schematic layout. In their service manual, they handily include some interesting scope diagrams for various points on the board and there I saw that they included a diagram for the spot between the 2 nearest transistors to the intensity pot. (I attached it below and highlight the scope diagram in question)

I immediately hooked up my 'cheapo-scope' to my board, and made an interesting discovery. When I connect 'downstream' of transistor Q621 at a point I labeled "P1" in the attached image, then I get a similar signal (is this the blanking signal?)

However, if I connect the scope to "P2" (between the 2 transistors, similar to the Hitachi scope), then I just get a constant flat 1.34v

Does that seem right?

olafwagner:
Doesn't seem the last 3 images made it, here they are...

AVGresponding:
Looks like q621 might be toast, that constant 1.34VDC is about what I'd expect to see from the bias resistors of q622, there should be a blanking signal at p2 as well as p1

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