Tektronix 2215, pulsing/chirping noise, no trace.

[lordvader88]

Good idea, it's 12AM though so I will try it tomorrow. I'll keep you updated. However, I don't understand how there could be a problem with the rest of the scope if the prereg board doesn't output anything (I later realized I needed to check the output voltage with the board's common, indicated in the service manual, that shows the output is 0V, perhaps a few mV)

Was it around 12AM when this started ? It could be the scope is haunted somehow. Anything from the shipping truck killed someone with it onboard, to someone killed at the factory where the screws were made, or someone used it to murder someone, who knows.

[GoodCrossing]

Alright, I've tinkered with it some more and I feel like I am getting close. I measured the caps and checked some for shorts and they seemed fine. The diodes also seem fine. I decided to take a look at the Q933 MOSFET, and decided to check for a short there, as I hadn't checked for a short after the rectifier. Lo and behold, 0 Ohms to ground at the source. Suspicious! I started checking all components going to ground thru the FET, and none of them were shorted after desoldering them. About halfway through this, though, I decided to take a shortcut. (Pardon me.) I fed in 5V limited to 1A with my PSU from the source to ground, and checked which component got hot. And it was T933! Unfortunately I ran out of time so I'll desolder and check the transformer when I get back to the scope on Friday. I also suspect U920 might be dead, so if I can buy a replacement at my local components store I'll get one just in case.

https://www.ebay.com/itm/233180863827

I will probably get that board. I wanted to troubleshoot mine a bit more before I did, though.

[Old Printer]

Qservice are good people to work with from my experience. I would grab the board for $25, looks like you already have.

[GoodCrossing]

No, I actually didn't buy it; it looks like the seller pulled the listing actually. I was speaking with them about my board, and I asked if they would include the thermistors for a different price, or if they could confirm it was working before removing them; as the listing was classified as for parts/not working. They told me they had all components in the board in stock so it would make more sense to buy those instead of the entire board to repair my scope. I sent them the part number of the transformer, and they haven't replied since. After seeing your post I noticed they pulled the listing, which is kind of a bummer because the only place I've found selling the part I need charges quite a bit for shipping so it ended up being better to buy this one.

[Old Printer]

There are a lot of the 22XX series scopes out there, though maybe not in Spain. I would look to see if your board was used in another of the 2XXX series scopes, that would give you more to choose from looking for a donor scope. I have a 2225 and found a parts scope for $25 that I could pickup without shipping. Even if you found a board that was similar you may be able to rob some components.

[james_s]

I had a 2215 with something loose inside the CRT. Occasionally it would sit there chirping refusing to start if that loose bit landed in the wrong place.

[pbarton]

https://www.ebay.com/itm/323857754896    Posts to UK ONLY.!
I don't mind buying it and forwarding it to GoodCrossing in Spain.

[GoodCrossing]

Oh, that'd be great. I'll be contacting the seller to make sure it works and will get back to you.

[GoodCrossing]

Well I thought I would do a final update after getting the board.

Replacing the board obviously fixed the issue but I took a moment to compare some measurements on the new and old boards, which pretty much confirmed T933 was internally shorted to ground. Still, replacing the board was easier and cheaper than replacing that component as it was more expensive than the board @pbarton found. So, the noise was probably caused by the board resetting after detecting the short to ground, and the transformer turning off and on again quickly.

[jdragoset]

Really good luck to find a reasonably priced, working A18 board.

[pbarton]

Good to know that the replacement board fixed the problem. Well done.

[james_s]

That's surprising, I don't recall ever encountering a bad SMPS transformer. You might look closely and see if you can find a short between the windings and the core or something where the leads come out.

[jdragoset]

It does seem that the T-933 ferrite core is very likely non-conductive and not likely to allow a short to ground from any leads.
CR 913 feeds back "run power" to the PWM operation, check it is not opened or shorted
R 933, 0.2 ohm, feeds back current to the PWM, not to exceed 1 amp (200 mV across R 933).
Crow bar Q 935 (SCR), R 934 (3 ohm) and VR 935 (51 volt zener diode) trip and cause excess current, putting the circuit into a repeat start (chirp) mode if 51 volts is produced or the zener diode or Q 935 are shorted.
I've just refurbed one of these with the FET (Q 933) gate-to-source shorted.
Replaced the FET and R 934 (burned with no voltage regulation) and it now runs a 90W incandescent bulb at 40.6 volts through a variac.
It will repeatedly chirp, trying to start if the variac is less than 70% of 120VAC.
These can be easily tested out of their scope, attach a small piece of Aluminum plate to Q 933 for a heatsink.
 

[jdragoset]

Got one that chirps the same way, flex the A 18 board corner and normal operation resumes.
Flex/relax problem repeats, start trying to narrow it down, 1 amp fuse blows, remove A 18 board, test and find gate-source leakage in Q 933 and crow bar fires when AC voltage in is raised above 60.
Checking T 933 winding 9,10, the one that is negative line bridge rectifier (pseudo ground) tied to one side and earth ground through C 932.
That resistance measures 0.4 ohms, whereas the similar (in wire gauge) 6, 7 winding shows 2.2 ohms.
The winding is not used as the grounded shield is installed and there is no C 932 installed.
I have other 2215's and will check that winding resistance on at least one.
If it turns out to be shorted, I have two options, since all tektronix 120-1439-00 seem to have been exhausted:
1) dis-assemble, count turns, measure wire gauge, rewind, eliminating the 9, 10 open circuit winding as not needed.
2) got two of these cheap, NEW MAGNETEK TRIAD TRANSFORMER F-40X 115V VOLT 26.8V.C.T@1A AMP,
Wire the 120 VAC in parallel fed by P1, P2 to A18 one full secondary plus the second to the center tap to 
fast-on 1, 2 on the A 18 board, a 10 ohm  resistor or choke (AC Power Choke, ( D-37-N) 0,6 H 0,51 А, made in USSR , NOS. Lot 1pcs) between T 933, 1 and Q 933, D.
Already checked the scope with a 40 volt bench supply for several days.
I would leave the crow bar circuit and 1 amp fuse to blow if the supply goes over-voltage.
this arrangement can also have the primaries wired in series for 240 volt operation.
Internally mounted, this would add about 4 pounds to the weight, but nowhere near the 12 pound weight of the Sorensen 40-0.75 supply, not to mention only a single lift.
Anyone with T 933 winding turn-count /wire gauge would be greatly appreciated.
Keep the old dogs running!
 

[jdragoset]

Tested T 933 winding resistances on a working A-18 board:
1-2, 2.2 ohms
4-5, 1.8 ohms
6-7, 2.2 ohms
9-10, 0.4 ohms
Did find numerous cracked/poor looking solder joints so re-soldered all and cleaned flux with IPA.
I had already replaced all of the Rifa type and power supply electrolytic caps.
Scope now works fine.
All low voltage power supply values in spec'.