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Oscilloscope Repair

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shimniok said:
Cool, thanks for the tips on the 'lytics. I posted part of the circuit somewhere above btw. Posted a link to the service manual early on...

Michael
Click to expand...

?????????
found a link to a cap meter, but no link to the oscope schematic, looked through the whole thread......

your first reply to this thread is post #18......
 
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ok, i've had a look at the schematic....

the electrolytics here are c16,17, 18, 20, 22, 25.

c16,17 are in the switching and limiter circuit, and are possible but not likely. c20 is in a current amplifier stage and that's likely. c18,22 are in a likely place to cause oscillation in the amplifier, as well as c25, which is a filter for the DC reference for the constant current source, and that one is a likely cause of the problem. i would actually go ahead and replace them all, as it's only 6 caps, which have all aged.
 
if you had an ESR tester you could check all of the caps in other sections as well.
 
Cool thanks!! That's usually the recommendation for the vintage audio stuff I am into... just replace everything in the signal path... and even PS if there are problems. I will likely suggest a recap to the guy who is getting my 3502... and my V1050F probably could use a recap too... hmm...

I really feel like I need to go learn a lot more about some of these types of circuits (constant current source, current limiter) ... google isn't helping yet...
 
a constant current source, when used in place of a load resistor in a voltage amp stage increases the gain, because it acts as a very high impedance. such an amplifier also has much lower distortion because the current source reacts linearly with the device that's doing the amplifying
 
for an excellent article on current sources and current mirrors, you can find it here:

**broken link removed**

for uses other than audio, the principles remain the same, but the circuit configuration might be a bit different.
 
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Hey guys! Just to let you know I am still alive, but due to things outside my control, I have been sooo busy and this oscilloscope is just waiting for me to fix it.. can feel it. So tomorrow, the first thing on my list is fixing my baby.. thanks again for all the help.. will let you know tomorrow about all the findings!
 
Sounds cool! If you do measure live voltages, be sure to turn off when moving probe and use mini clips to prevent blowing up a transistor like I did on a stereo I was working on :) I think that measuring AC and DC voltages from Y- back (and probably Y+ back to compare) with DC gnd should help localize the problem...

Michael
 
Right, so I opened up the scope, and first did some tests. Firstly with the power off
That board is quite intemidating, hope i didnt bite off more than I can chew.
ok, here goes.

1.

Firstly, I cant find the 680ohm resistors. I find them on the circuit diagram, but why dont they have a number, like R101 or something??? So i cant find it on the circuit board. You said on the left, about halfway, but I cant find any 3watt resistors. Only thing i find is R553 which is a big resistor, but thats not it. On the left however, I found 2 big 6.8K resistors, both 3watt. but thats not what we are looking for right?

2.

Secondly, the diodes, D6 D7 D8 D9 measures roughly 750 in forward bias and 2000 in reverse bias, which seems to be ok. D1,2,3,4,5,16,17,18,19 measure the same in forward bias and infinity in reverse, which seems ok. Struggling with transistors since I dont know which is b or c or e.

3. Then I started measuring voltages. I wasnt sure which side of the resistor to measure, so I measured both sides and if one side gave the right voltage, I assume it is correct. Am I correct?

So if you look at the schematic, these ones are ok, well they are all 0.1V lower than they should be, dont know if thats normal?

Looking at the vertical amp, bottom right, there is 120V that checked ok. bottom right theres +12VA (dont know what the VA means) but it measured 12V. Then middle right, there is a +20V and this is what I got. I measured on R91, With you looking down at the scope, with the CRT at the top and looking from the side. On the left of the resistor, I measured 29.4V (AC) and 13.7V (DC) and on the right 47.8V (AC) and 22V (DC).

Does that seem right? A high AC voltage, what does that mean?

Ok, so thats a start, the scope is on my bench, and any help will be appreciated. Thanks alot!
 
Ok, I am guessing these are the two 680Ω resistors (pink arrows)... well, they *are* 680Ω and they are sitting amongst the other components in the vertical amp section. So it is a reasonable guess I suppose.

**broken link removed**
 
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Here is my suggestion for measuring next. We need to be very methodical and we are looking for something very specific -- namely, voltage fluctuation on the Y- side of the circuit when the trace is going funky. We want to verify that voltage doesn't normally fluctuate when the trace is straight, but does fluctuate when the trace is wonky.

1. Set the o-scope to GND coupling, with the line positioned vertically so that it is a straight, flat line

2. Measure the AC and DC voltages at the GREEN and PINK points represented by the schematic below and report back each voltage at each point.

**broken link removed**

3. Now, reposition the trace's vertical position lower to were the entire trace is all wonky as shown in your videos. Maintain GND coupling.

4. Measure the AC and DC voltages at the GREEN and PINK points represented by the schematic below and report back each voltage at each point under these new conditions.

I'm suggesting that you measure at resistors because then you don't have to know which pin is which and there's less chance of frying a transistor. ;)

The more methodical you can be about measuring each condition (not wonky and wonky) and each type of voltage (AC and DC) the better so we know exactly what we're dealing with. Let's see what this gives us then we can measure other points later.

If you feel ambitious after finishing the volt measurements, you might try turning the scope off, unplugging, and measuring the ESR of C16, C17, C18 and C22 electrolytic caps (should be able to do this in-circuit). If one of the pairs of caps is off, it should be obvious. My understanding is that electrolytics are far more likely to go bad (with high ESR) than ceramics.

Sounds like all the diodes check out. Good!

Michael
 
preliminary test .... with the scope turned on, take an appropriate non-conductive object ... plastic or glass maybe .... and tap some of the electrolytics .... also some of the larger watt resistors .... watch the screen for any hint of associated movement.
 
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