Tektronix 7L5 Spectrum Analyzer no signal / sweep - Page 1
Forum section: electronics / repair

author: Gulftown
author_rank: Regular Contributor
timestamp: June 12, 2025, 10:01:30 pm
content: Hi,i have a Tek 7L5, hat suddenly stopped working.It only shows a flat line and the baseline clipper.The vertical position can be changed and the readout is also still working correctly.When it broke it still showed the last acquired signal but never updated again, even on free run or manual trigger mode until i switched it off.It seems like either the sweep is not working, or something in the input is broken.Does anybody have an idea?The main oscillator is working and i checked the voltages.But without an extender, this module is super complicated to diagnose, because it can't be unfolded inside the scope.Best regards. [ Specified attachment is not available ]

author: m k
author_rank: Super Contributor
timestamp: June 22, 2025, 07:23:42 pm
content: So vertical ADC seems to be working, same with clipper comparator.Is video out equally flat?

author: Gulftown
author_rank: Regular Contributor
timestamp: June 30, 2025, 09:09:00 pm
content: The video out is flat at 2dB/div and LIN Modes and shows some 60mVpp noise on 10dB/div.On the horizontal out, there is a small pulse every 4.5ms at 300mV on every Trigger Mode, but no triggering.When i set it to manual sweep and turn the knob, the output signal changes between 0 and -5V.On every other mode the signal is at 0V with the 300mV pulse.

author: m k
author_rank: Super Contributor
timestamp: July 01, 2025, 12:30:38 pm
content: No idea what are the usual problems, if any, but quite many module connections there seem to be.I guess I'd pull out extra wires as measurement points along the early signal path.Since video out is also missing the problem can be pretty early.Can't find special mode input, output is EG of diagram number 16, low left of page 213/230 of technical manual.

author: Gulftown
author_rank: Regular Contributor
timestamp: July 26, 2025, 10:28:22 am
content: Okay,i am now fairly certain, that i have two broken components.One LM339 and the 155-0056-00 U580 Sweep Control Chip.With the LM339 replaced, i got the manual sweep back and when i short the +Gate to Ground i get a normal sweep.The sweep inhibit (Pin 1 of U580) goes high after the sweep, that should reset the +Gate.Here is the schematic:https://w140.com/tekwiki/images/3/37/Tek_155-0056-00.pdfFortunately there was exactly one 155-0056-00 NOS on ebay, we will see if that fixes my unit.

author: m k
author_rank: Super Contributor
timestamp: July 26, 2025, 03:35:22 pm
content: +GATE is inverted -GATE, is the other side flipping?Generally quite simple chip, but the devil is in the details.

author: Gulftown
author_rank: Regular Contributor
timestamp: July 27, 2025, 08:31:05 am
content: No, Gate- stays low.

author: m k
author_rank: Super Contributor
timestamp: July 27, 2025, 04:39:32 pm
content: The chip is an RTL style logic chip with nuances.So you can count transistors from input to output, even number means a driver and odd an inverter.Pin 1 to 15 has 2 transistors, so pin 15 follows pin 1.To pin 14 is 1 transistor more, so it is inverted.Pin 1 transistor Q60 has a collector resistor R60.Later on output side is Q62, it is pulling down the same resistor, third is Q86 from the low corner.So when one is pulling down others become irrelevant.If you want to test the chip it's easiest when it's in controlled test circuit.But generally, if only one input is connected and its level is changing enough +/- GATE must be also changing.Trigger & slope is an exception.There floating trigger should put the whole section off.Pins 6 and 7 are inputs for 2-input NOR gate.So here floating means down, opposite to TTL.

author: Gulftown
author_rank: Regular Contributor
timestamp: August 25, 2025, 02:28:47 pm
content: So, i finally got the 155-0056-00 replacement and it fixed my problem.The 7L5 is finally fully working again.What i think happend to the instrument is, that an ESD pulse or overvoltage went into Pin B8 at the top left of the schematic.That could totally kill the LM339 and the output stage of the 155-0056-00.Funny is that, the sweep stopped, while the scope was running on my desk wirth the calibrator plugged into the input.I checked the power supply rails of the mainframe and they look fine, so i dont know where the pulse could have come from.