HP 1220A "Auto Line" switch as well as power selection. - Page 1
Forum section: electronics / testgear

author: Nexrem
author_rank: Newbie
timestamp: May 12, 2025, 09:57:17 pm
content: Hello, I recently obtained a working HP 1220A oscilloscope. It appears to work fine (tested with my insulated variac), however I only punched it up to 220V. The manual states there are 4 settings for the power 100V, 120V, 220V and 240V. The power here in the Netherlands is about 230V. The manual states the powers are -10% +5%. Currently it's switched to 220V (that's how I got it). Should I put it on 240V just to be sure? Or should I keep it as is.On that note, mine appears to have a switch on the back without any documentation anywhere that simply says "auto line". It might be some modification from its previous owners at ASML but I'm unsure. Please see pic attached. Can someone tell me what it does?Thanks, and best regards!

author: wn1fju
author_rank: Frequent Contributor
timestamp: May 13, 2025, 12:15:24 pm
content: There is definitely no such switch on the back panel of my 1220A. Why don't you take the covers off and see where the switch wiring goes. Should be easy to figure out what the switch does.As far as the voltage setting, it probably doesn't matter. The power supplies are all linear regulators so if your line voltage is 230 and the switch is set to 220, the worst that can happen is that the regulators will dissipate a bit more heat. If the switch is set to 240, the worst that can happen is that the regulators will drop out, that is to say they don't have enough input voltage to regulate to the proper output voltage. I highly doubt that HP would design their power supplies with input voltages that close to the drop out. You could also check the DC voltage test points while you have the cabinet open to investigate that back panel switch.

author: Nexrem
author_rank: Newbie
timestamp: May 14, 2025, 09:02:02 pm
content: Boy do I have a mystery on my hands. See attached pictures

author: colorburst
author_rank: Regular Contributor
timestamp: May 14, 2025, 09:46:32 pm
content: Does the scope have a front panel control to select between normal and automatic trigger mode?My guess is they cut the auto trigger signal and put a switch in. Hence the name, auto "line."

author: Nexrem
author_rank: Newbie
timestamp: May 14, 2025, 09:53:48 pm
content: This scope does not have auto leveling, you gotta adjust the trigger level yourself.

author: colorburst
author_rank: Regular Contributor
timestamp: May 14, 2025, 09:59:39 pm
content: I'm referring to automatic sweep when no trigger event occurs, i.e. there is no input signal or your trig level is adjusted too high/too low.In normal mode, the scope would sit with a blank screen waiting for trigger. In auto mode, it would periodically force a sweep and you'd see a horizontal line or rolling signal.

author: Nexrem
author_rank: Newbie
timestamp: May 14, 2025, 11:21:57 pm
content: Um, from what I can see, not really. It has a trigger source selector: internal, external and line (the line thing refers to a TV signal synchronization line as this scope seems to be specialized for TV signals)There is a mode selector between normal and TV mode, whatever that is.

author: TimFox
author_rank: Super Contributor
timestamp: May 14, 2025, 11:38:50 pm
content: Normally, US oscilloscopes used “line” trigger source selection to mean the AC line (power) input (i.e. 50 or 60 Hz, depending on location), to facilitate looking for line-synchronous hum and ripple waveforms.Oscilloscopes for TV service often had “TV line” and “TV field” trigger sources from a sync separator circuit, since the field rate is slightly different from 60 Hz.Auto triggering, as discussed above, is a time-out circuit that generates a trigger if none occurs in a specified time.

author: Nexrem
author_rank: Newbie
timestamp: May 14, 2025, 11:43:19 pm
content: Yes, the line settings here all refer to TV lines. The time/div knob even has secondary markings for the number of lines, number of frames and so on it can sync on and display. I can for example set it to 2 frames and observe 2 frames. The syncing is done with a TV frame sync circuit.

author: colorburst
author_rank: Regular Contributor
timestamp: May 15, 2025, 12:09:10 am
content: Ok, I took a look at the manual, this does seem to be the auto trig disable feature. The service manual is here:https://www.worldradiohistory.com/Archive-Catalogs/HP/HP-1220-A-Ocilloscope-1976.pdfPage 78 shows layout of the board, the modifications in your unit are centered around U8. Page 71 shows the schematic, U8 is labeled "Auto Trig Circuit." Pin 8, which has been rerouted, is one of its outputs (Q). Another pair of wires is probably for the inverted output (Q bar); the third pair seems to be VCC and GND to provide a pull up and a pull down when the outputs are disconnected. Page 22 explains the circuit under "4-51 Bright-line Auto Operation".

author: Nexrem
author_rank: Newbie
timestamp: May 15, 2025, 04:33:25 pm
content: Quote from: colorburst on May 15, 2025, 12:09:10 amThis seems to be exactly it! Thanks a lot for figuring it out. Now I wonder who would need to disable the trigger timeout, it's a highly specific requirement.