HP1741A 'scope - flyback visible?

[RussellSher]

Hi all
I was using my HP1741A and noticed that on very slow trace settings (slow enough to follow the spot sweeping across the screen);
After the spot sweeps to end the right hand side, I can see it reverse direction and return at about the same speed (right to left). As so it continues, back and forth.
This return is a bit dimmer than the forward sweep, but still visible.
Is this a flyback trace that is not suitably blanked?
My original issue with the scope is that the beam intensity was low (at all front panel settings) and I adjusted the internal intensity limit adjust pot (see attached) to improve this.
When increasing the front panel beam intensity there is a point where the trace becomes invisible for the rest of the adjustment position. (Is this the dreaded 'double peaking', without the second peak - due to age?)
Also I can't quit identify the flyback blanking cct in the service manual - would this be part of the horizontal amplifier ?
Any thoughts?
regards
Russell

[MotorBikeLes]

Is it double (humping) peaking? it does sound a bit like that. You should not be able to see the flyback if it is correctly adjusted. I don't know this model, but possibly the preset brightness is so high (but dim if CRT is shot) that the flyback blanking is overwhelmed.
i would read the manual descriptions carefully, then do some voltages checks, but bear in mind that you will be working in a very high voltage area. Make connections with power off, and be sure you have suitable meter and leads, probably an EHT divider probe.
Les.

[PETERg0rsq]

Be very careful working on the PSU.

I had HT issues on a slightly earlier version of this scope, and it ended up taking out the heater in the tube.

The heater regulation is derived from the main HT regulation, so if the HT goes over volts, the heater will too!

Mine had a faulty capacitor in the feedback from the HT, causing leakage to ground, so the HT was running away.

Scrapped a good scope!

Also found several errors in the schematic (incorrect values on schematic, but correct in the parts list) (not the same as yours) and also circuit differences, so do not assume the schematic is the same as the circuit you are working on.

Definatly get an EHT divider probe and check HT voltages, as fault could be a dying tube!

[WME_bill]

HP1741A.
The faint beam even after adjusting the Beam Intensity sound rather like a failing tube. I had this trouble with a HP1707, and eventually had to relegate the unit for spares to keep another one going.
The Bright-up circuit for flyback blanking is quite clever and does not appear readily as such, particularly if you are used to the Tektronix system of two EHT windings, one for the grid voltage, and one for the cathode.
Here you are using the 20khz feed from the EHT oscillator.
The main output from the EHT transformer terminal 8 is rectified to give the -2295V +-50V to the cathode.
Terminal 5 produces about 150V AC (I have not yet measured the voltage on my 1741, so I am estimating it). This alternating voltage is limited positive through CR4 to the Intensity Limit, and limited negative through CR6 from the Gate Output signal (normally about 25v as shown on Gate Amplifier circuit, TP2 / waveform 4).
So you have an AC voltage varying between two limits. This is fed through the high voltage capacitor C4 and peak rectified by CR7 & CR8 to give a negative voltage standing upon the cathode voltage by CR11.
The front edge of the gate bright-up pulse at CR6/CR5 is fed through C5 to brighten up the tube quickly.
The switching from the TimeBase is fed to the Gate Amplifier and then to the Brightup.
I find it helpful with circuits like this to identify the high voltage capacitors and draw a line to separate the low voltage areas from the EHT part of the circuit.

The neon tubes (about 65v each) are to limit any voltage surges if the CRT has a flash over. They do not act as regulators.
The pair DS4/5 are situated in a low voltage <100v point. The three DS1/2/3 are at EHT, as they are beyond the line you have drawn through the high voltage capacitors. So TP4 and TP5 are both around -2300v.
If you want to measure in that area, make up a high voltage probe as suggested here in the past & calibrate it against the 120v rail elsewhere. Use with a digital voltmeter or another scope.
Be warned, don't put your Avo across the CRT grid feed, as the current it draws will make the grid go sharply positive and burn out the CRT. A 10M input DVM or scope should be alright.
If you have a second scope, look at the waveform at junction of CR6 anode and DS4/5.
Peterg0rsq advice is very helpful, as it is not immediately apparent from the manual, for the EHT voltage is set primarily to give the correct heater voltage.
wme_bill

[RussellSher]

Thanks all for the replies. I will look at the suggestions given. If I turn the intensity limit internal trimpot, then yes I can get rid of the flyback but at the expense of some dimming of the main trace. Although it is still useable. So it may well be a sign of a tiring tube, although if I enable the storage mode, then the tube does get quite bright. Also in delay mode the highlit portions ar equite bright, so maybe some internal adjusting as per the above suggestions.
73 Russell - ZS1RUS