Marshall 9200 bias and balanced phase inverter
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PelliX
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I hope you mean "contact cleaner" or something similar, you do NOT want flux in there. You won't be able to access them from the front anyway. With any luck they might be directly accessable from the back once you remove the chassis/cover.
sorry, I meant desox anyway to say that in the end I had to repair the Marshall 9200 to have to repair the 2445 wow
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PelliX
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scusate, intendevo desox comunque dire che alla fine ho dovuto riparare il Marshall 9200 per dover riparare il 2445 wow
The English was easier for me, but my Italian is *just* good enough to figure that out
(@Marcomel79 has tought me some basics).I'm sorry, probably when I wrote, Google Translator was active and I didn't realize it. I was probably distracted by the enormous disappointment of this afternoon, I opened the oscilloscope, I didn't notice any swollen capacitors, I tried to disassemble the entire front part, it was all dusty, the central potentiometer had a repair as you can see centrally in place of the var knob they put a knob with a screw. The save button was stuck so I managed to unblock it. I took the opportunity to clean the windows. I haven't noticed any improvements, in fact moving the division knob tends to interrupt the signal for a fraction of a second. Now it's just cleaner and has BBC written on it. Evidently the front board has some problems with those IDC connectors, unfortunately when you try to do good to the old stuff it only ends up getting worse, now I find myself having finally prepared next week to make my own cables with the bnc connectors. And instead I find myself in my hand a piece of iron that goes and doesn't go. What could be worse for a beginner than starting to learn to use an oscilloscope without knowing whether it works well or not? in addition to the waste of money
PelliX
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Indeed, the main big ones will probably be OK, it's the small signal smoothing caps supplying the of the logic IC's - though, as I mentioned, it might be worth getting in touch with the seller about this.
Bit dirty, indeed - I guess that's to be expected... that screw looks hacky, though!
Uhm, that's normal - at least what I saw on video was perfectly fine. Switching time or voltage divisions will always "blip" a little...
Are you sure? Those are typically quite reliable in such units as they also never really get stressed, normally. Making your own probes? Or do you mean replacing the IDC leads?
I'm sorry about the poor experience - get in touch with the seller. If he sold it in 'perfect working order', then I suggest he addresses the situation. I guess learning to repair the tool before using it has SOME value, but I see you point, of course. You didn't pay for a fix-up job, you paid for a working scope. Incidentally, there are full service manuals and schematics for these scopes, so if you go hunting for the offending cap(s) it should be fairly straightforward. If the worst comes to the worst, I myself would take that on as a fix-up, but by all means try plan A first!
I want to be superstitious, perhaps it has stabilized, probably the dexos has greased the front board a bit and now the signal interruption problem is gone, I made a few more videos, I apologize, unfortunately I get demoralized easily. Replacing the capacitors I think is not a problem, I have to understand what they are or at least look in the diagram at those that have to do with positioning adjustments.
here I took the opportunity to do some exercises, I found some disturbances at high frequencies probably and the old test cable that I am using in the signal generator
here I took the opportunity to do some exercises, I found some disturbances at high frequencies probably and the old test cable that I am using in the signal generator
PelliX
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First of all, nice work - looks like you're getting the hang of things by looking at the videos. 
Let's not mix things up - the flickering of the LED's on the front panel can be caused by ripple or unstable supply for the logic IC's. The positioning wavering that I saw looked like dirty-pot-syndrome to me...
No need to apologize, I understand the frustration. Just try to keep a level head and enjoy solving the puzzles, mate...
Let's not mix things up - the flickering of the LED's on the front panel can be caused by ripple or unstable supply for the logic IC's. The positioning wavering that I saw looked like dirty-pot-syndrome to me...
No need to apologize, I understand the frustration. Just try to keep a level head and enjoy solving the puzzles, mate...
I meant the capacitors for the drift of the vertical and horizontal positioning, the vertical drift is done with each potentiometer of each single channel to which it is dedicated, it would be strange for all the potentiometers to be dirty in sync and give the same drift. also because I noticed that the potentiometers are the sealed type. I would like to find its original central knob complete with VAR, but there is nothing online. there are many complete panels of the normal 2445 but nothing of the 2445A.
PelliX
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That drift would not be related to or indicative of a failing capacitor. Assuming the pots are ok...
...agree, it could be a dodgy common contact or such, but I'm pretty sure there's a mechanical factor involved, looking at the videos.
PelliX
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That looks more gradual and consistent though; yours was jumping when you touched the controls. Agree that those old electrolytics could do with a change, though. Might also fix the flickering of the LED's while you're at it, of course.
Maybe in the videos I made I didn't capture the moment where he was drifting alone, but it happened. Now I have to understand and learn where they are in the circuit by looking at the diagram. As far as LEDs are concerned, it is probably beyond my competence to understand what it is and why that flashing occurs.
PelliX
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Possible. As I said, either contact the seller or indeed go for the filter caps in the power supply/distribution area.
The LED's can flicker because when you switch a control, the voltage which the caps are "buffering" drops (because that "buffer" isn't actually buffering as it should). That causes the IC to jump a little. That's the simple explanation. I'm currently a little preoccupied, but a quick search on Youtube should reveal a few repairs. I think Dave from EEVblog even tore a few of these apart on camera once...
it would seem that, if we want to call it that, the problem seems to be that in the 24xx series LED flickering is a common thing. When necessary I take it as a fun pastime to replace capacitors
Tektronix 2465B oscilloscope teardown - Page 44
Tektronix 2465B oscilloscope teardown - Page 44
www.eevblog.com
in the meantime I was trying to understand which capacitors to replace, going back to the Marshall 9200 phase inverter triode from what I understood to read the wave at the anode output you need a differential type probe due to the high voltage, I assume that the balancing must be done using two channels of the oscilloscope to be able to observe the two half-waves, example: one section of the triode a1 will have a non-adjustable half-wave, while the other section a2 will have a trim pot for adjusting the inverted half-wave, having both on the oscilloscope monitor I imagine that the inverted half-wave a2 will have to be reconverted again using the appropriate function of the oscilloscope to be superimposable on the half-wave a1 in such a way as to bring the half-wave a2 perfectly synchronized with a1. This is what I mean by balance, I don't know if it's correct.
PelliX
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Short answer, yes, that's more or less how it works. Although it's not STRICTLY required to have a perfect balance there, that's the idea. You don't need a differential probe per se, but make sure you either clip in to the signal AFTER a coupling cap and/or set the AC coupling on the scope. That will filter the high voltage DC and allow the AC (signal) to pass. After all, you're not interested in the DC voltage there, you could always read that with a meter. The scope is for analyzing the AC signal. Of course, ensure that your probes and scope are rated for a value exceeding anything that you're probing. After a coupling cap you should have 0VDC (capacitors block DC but allow AC to pass), though in practice on older amps this could be a few volts (because the cap leaks a little DC). Before a coupling cap you will have whatever the HT is at there as a DC offset plus the AC signal you're looking for. Does that make sense?
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