Scope CRT tube blury till warmed up - can it be repaired - Page 1
Forum section: electronics / repair

author: Swake
author_rank: Frequent Contributor
timestamp: December 02, 2022, 07:33:25 pm
content: Hi,Before buying a Hameg scope I would like to receive your advice about the possibility to repair it.The CRT tube is blurry when the scope is just switched on and 'cold'. Ones warmed up, after about 20 minutes, the blurriness disappears.Is this generally repairable or more a sign of a near dead tube?

author: Gyro
author_rank: Super Contributor
timestamp: December 02, 2022, 08:08:44 pm
content: 20 minutes is a bit long for the CRT itself. It's also encouraging that the trace comes into focus rather drifting out. It could be aging resistors in the high voltage resistor chain that includes the focus control, astig etc.You didn't mention the model number, you should check that the manual and schematic are available on the web - I think most Hamegs are, but check.

author: Swake
author_rank: Frequent Contributor
timestamp: December 02, 2022, 08:40:36 pm
content: Thanks for the reply.I don't know the exact model yet. The person reported it as a 1GHz scope and purchased around the century change. So I guess it is about 20 year old. Couldn't find a 1 GHz Hameg scope from that era... So now I'm curiousMonday I'll see the scope in person and we'll know.

author: Gyro
author_rank: Super Contributor
timestamp: December 02, 2022, 09:31:42 pm
content: No, Hameg were more were more mid range, I don't think they ever made anything much above a few hundred MHz. Tread carefully, make sure of what you're getting and that you're not paying through the nose for it - try to at least get a model number out of the seller beforehand so that you can do your background checks.

author: james_s
author_rank: Super Contributor
timestamp: December 02, 2022, 10:12:51 pm
content: I's possible that it's the tube, but I would be more inclined to suspect the focus voltage or something else related is drifting.

author: Swake
author_rank: Frequent Contributor
timestamp: December 03, 2022, 09:36:07 am
content: Called-back because I don't want to spend their and my time on something I'm potentially not interested in. Soooo..... guess what....it looks like a scope, it smells like a scope and.... it is not a scope! It is a spectrum analyzer model HM5012-2. ok, makes sense now. The person offering isn't very electronics literate.Got the service manual, it contains a block diagram and a collection of schematics.Only I'm not really interested in this machine as I'm not in HF and can't really remember when I last needed such a thing.

author: inse
author_rank: Super Contributor
timestamp: December 03, 2022, 11:40:31 am
content: Ohh I like this good smell of oscilloscopes 🤓

author: james_s
author_rank: Super Contributor
timestamp: December 03, 2022, 06:43:39 pm
content: A spectrum analyzer is way more valuable than a scope, they are much harder to come by. It's worth probably an order of magnitude more than an analog scope so somebody is going to get a heck of a deal when they realize what they're looking at.

author: Paceguy
author_rank: Regular Contributor
timestamp: December 04, 2022, 07:35:31 pm
content: It could be the CRT, but I'm more inclined to think that it's a component in the CRT circuitry, a bad solder or something. Focus it when cold than check to see if something is thermo-intermittent or changes value when warmed up as it goes out of focus.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 04, 2022, 11:54:15 pm
content: A one GHz Scope is quite the thing and takes expensive probes. Not very common.A one GHz Spectrum Analyzer is more of a common unit. Lots of these in multiple brands.The higher freq SAs like the one GHz one usually do not do KHz freq, And if they do go down to that, they don't usually do it well.

author: CaptDon
author_rank: Super Contributor
timestamp: December 05, 2022, 01:26:10 pm
content: Drift in the focus divider resistor chain (and often complete failures) is very common in older scopes as are leaky high voltage capacitors. I suspect as one other poster noted, if you adjust for good focus at turn-on it will probably drift out of focus as it warms up. At some point with age you will reach the adjustment limit at one end or the other and still won't get into the proper focus range. Look for resistors that seem overheated or resistors that seem to have a waxy or oily look to them. Be careful as the focus grid can be 400 to 1000 volts referenced to chassis ground.

author: james_s
author_rank: Super Contributor
timestamp: December 05, 2022, 08:27:34 pm
content: Quote from: Wallace Gasiewicz on December 04, 2022, 11:54:15 pmI missed that it was supposedly a 1GHz scope, I saw Hammeg and assumed it was something like a 20-40 MHz analog scope. 1GHz analog scopes did exist but the only ones I'm aware of were sampling scopes that were very specialized. I think the fastest conventional analog scope I've seen was 400MHz.

author: Swake
author_rank: Frequent Contributor
timestamp: December 05, 2022, 09:11:54 pm
content: Ended up not buying this spectrum analyzer. Asking price magically increased over night to an amount that was ridicules in my opinion: 3/4 of a new 1,5GHz Siglent. Again, I might never have a use for this thing. Sure it looks fancy on the bench. Can spend those bucks on something else

author: TimFox
author_rank: Super Contributor
timestamp: December 05, 2022, 11:20:18 pm
content: Quote from: james_s on December 05, 2022, 08:27:34 pmTektronix made two non-sampling 1 GHz analog CROs, but they were not really conventional.The 519 (from 1959) did not have vertical amplifiers, and needed 125 ohm drive directly to the vertical deflection, at 10 V/cm over a narrow height band.The 7104 (from 1978) required a channel-plate to achieve a useful display at 1 GHz, which had a finite lifetime.At least the 7104 could use a "normal" plug-in for vertical: 7A29, 50 ohm input, 10 mV to 1 V per division.When I could have used such a beast (in 1983), I found that you could not rent the 7104 since it was far too easy to damage.

author: james_s
author_rank: Super Contributor
timestamp: December 05, 2022, 11:44:30 pm
content: Quote from: TimFox on December 05, 2022, 11:20:18 pmThose sound like fascinating machines. The lack of a vertical amplifer is one way to get around the bandwidth limitations I suppose. Isn't the only limiting factor at that point overcoming the parasitic capacitance of the deflection plates? You'd need a fast horizontal sweep too but I'm too lazy to calculate just how fast.In any case neither of these extremely specialized instruments is likely to come up with a hobbyist looking for a cheap scope.

author: srb1954
author_rank: Super Contributor
timestamp: December 07, 2022, 01:20:04 am
content: Quote from: james_s on December 05, 2022, 11:44:30 pmPrior to the introduction of the 7104 Tektronix also made a direct access plug-in, the 7A21N, which gave a usable bandwidth of 1GHz with the 7904 and 7844 at the expense of a high deflection factor of 4V/cm. To achieve this bandwidth internal modifications were required on the mainframe, bypassing most of the internal amplifiers and beam switching and directly connecting the plug-in to the CRT.

author: james_s
author_rank: Super Contributor
timestamp: December 07, 2022, 01:27:19 am
content: Quote from: srb1954 on December 07, 2022, 01:20:04 amNow that you mention it, I think the manual for the 531A I had when I was a kid had mention of a modification like that, there was a removable plug in the side of the housing.

author: TimFox
author_rank: Super Contributor
timestamp: December 07, 2022, 04:25:51 am
content: Quote from: james_s on December 05, 2022, 11:44:30 pmI believe that the 519 arranged a set of vertical deflection plates into a delay line, similar to that in a "distributed amplifier", to minimize the effect of the plate-plate capacitances.

author: srb1954
author_rank: Super Contributor
timestamp: December 07, 2022, 09:10:57 am
content: Quote from: TimFox on December 07, 2022, 04:25:51 amHaving segmented vertical deflection plates tapped off a delay line also improves the high frequency response whilst maintaining a good deflection factor. This compensates for the effect of the finite speed of the electrons between gun and screen.Consider the case of a non-segmented deflection plates of a length such that an electron takes 1ns to transit the plate region. If you applied a 1GHz sine wave to those plates (and ignoring the small signal propagation delay across the plates) then the electron would experience a positive deflection for half its transit time between the plates and a negative deflection for the other half giving net zero deflection making it useless for a 1GHz scope. By using segmented plates tapped off a delay line such that the signal propagation time through the delay line matches the electron transit speed between each plate the electron always experiences the same instantaneous deflection voltage and the achieved deflection is maximised at higher frequencies.The ultimate improvement in frequency response is somewhat limited by the finite number of segments in the deflection plates but considerable improvement can be obtained in the usable bandwidth of the tube. I believe that such tubes were necessary in scopes of greater than 100MHz bandwidth.A more detailed description is provided in the Tektronix Circuit Concepts book "Cathode-ray Tubes".