Hameg HM412-4 recapping issue

[argintviu]

Hi all,

I have recently found a cheap Hameg HM412-4 oscilloscope on a local auction website and decided to buy it. It only had a missing knob and a broken BNC connector (which was an easy fix), otherwise it seems to be in full working order.

Since it is pretty old (made in 1979), I decided to clean it and replace all electrolytics before putting it to use. This however seems easier said than done. On the timebase switch board, there are 3 capacitors which control the sweep rate. On the schematic, the first capacitor is 24,4 nF 1% polystyrene (C610). The calibration of the timebase is done on the 50 us/cm range which uses this capacitor. The other 2 caps are 2,2uF film (C611) and 2 x 100 uF electrolytics in parallel (C612 + C613).

The issue I have is that the schematic does not correspond with the board in reality. C611 has only 2,2uF on the schematic but on the board it has some correction capacitors in parallel with it (220nF + 22nF) which increase its capacitance to 2,2 + 0,22 + 0,022 = 2,442 uF (C610 scaled 100 times?). But I don't plan to change these, only trying to understand the logic behind their usage.

The problem is at the third capacitor which is made from C612 in parallel with C613. These two are both 100uF electrolytics which don't seem to look too good. Under them, there are another 2 correction caps in parallel but this time tantalum, each 10 uF. So they all add up to 100uF + 100uF + 10uF + 10uF = 220uF. But shouldn't this value be 244uF so that it's 10000 times bigger than C611?

Here's a table with what the caps measured:

Designation  |  Value in schematic   |  Measured with DIY cap meter |  Measured with UT61E
C610            |  24,4 nF                    |  24.07 nF                                  |  24.15 nF
C611            |  2,2 uF                      |  2,429 uF                                  |  2,445 uF
C612+C613 |  200 uF                      |  220.3 uF                                  | 223.62 uF

So these are my main questions:

1. Am I right to say that the values of these 3 sweep caps are each 100 times bigger than the previous? E.g. 0,0244 uF (C610) -> 0.244 uF (C611 + correction) -> 240 uF (C612 + C613 + correction). If so, the schematic is incomplete because they don't account for the correction caps, right?

2. Why did they use electrolytics in a timing circuit? From what I know they have horrible capacitance stability. Could they be exchanged with a more chemically stable type?

Thank you!

[David Hess]

1. Am I right to say that the values of these 3 sweep caps are each 100 times bigger than the previous? E.g. 0,0244 uF (C610) -> 0.244 uF (C611 + correction) -> 240 uF (C612 + C613 + correction). If so, the schematic is incomplete because they don't account for the correction caps, right?

Maybe the capacitors were originally intended to be matched sets so no trimmer capacitors were needed.  Or they just did not bother including the trimmer capacitors in the service documentation.

2. Why did they use electrolytics in a timing circuit? From what I know they have horrible capacitance stability. Could they be exchanged with a more chemically stable type?

Are you sure they are aluminum electrolytic capacitors and not sealed solid or wet tantalum capacitors?  Aluminum electrolytics would be very unusual for horizontal sweep timing.

In any event, I would not mess with any of the timing capacitors unless they have failed.

[argintviu]

Thanks for the input.

Maybe the capacitors were originally intended to be matched sets so no trimmer capacitors were needed.  Or they just did not bother including the trimmer capacitors in the service documentation.

Maybe you are right. In this version (HM412-4), they probably should have matched the caps. I did some more digging and it seems that in the subsequent version of this scope, the HM412-5, they did draw the trimmer caps in the schematic (see attachment), however they still didn't specify the exact capacitance value that they should trim to. One mistery solved, another one to go.

Are you sure they are aluminum electrolytic capacitors and not sealed solid or wet tantalum capacitors?  Aluminum electrolytics would be very unusual for horizontal sweep timing.

As a matter of fact, I am not 100% sure. I assumed their type based on the external shape. I have attached 2 pictures of them, one with the arrangement they were in and one with a close up. Unfortunately there was no info to be found on this "CE KX" series, except from a vague "High heat resistance" classification. These 2 are the only from this series & manufacturer in the scope.

In any event, I would not mess with any of the timing capacitors unless they have failed.

That's a very good advice. The thing is that they are 40 years old, all this time must have put a dent in their original parameters anyway. The ESR looks good but I can't wrap my head around this 24.4nF -> 2,44 uF and then 220uF. The last value breaks the symmetry if I may say so. Shouldn't they add up to 244.4 uF and they simply went bad?  .

[David Hess]

Maybe the capacitors were originally intended to be matched sets so no trimmer capacitors were needed.  Or they just did not bother including the trimmer capacitors in the service documentation.

Maybe you are right. In this version (HM412-4), they probably should have matched the caps. I did some more digging and it seems that in the subsequent version of this scope, the HM412-5, they did draw the trimmer caps in the schematic (see attachment), however they still didn't specify the exact capacitance value that they should trim to. One mistery solved, another one to go.

The absolute values are less important than matching between them in decade steps.  Tektronix made their own capacitors and graded them into sets.

Are you sure they are aluminum electrolytic capacitors and not sealed solid or wet tantalum capacitors?  Aluminum electrolytics would be very unusual for horizontal sweep timing.

As a matter of fact, I am not 100% sure. I assumed their type based on the external shape. I have attached 2 pictures of them, one with the arrangement they were in and one with a close up. Unfortunately there was no info to be found on this "CE KX" series, except from a vague "High heat resistance" classification. These 2 are the only from this series & manufacturer in the scope.

They do look like aluminum electrolytic capacitors.  High heat resistance goes along with low leakage and is the time of part I would expect.

In any event, I would not mess with any of the timing capacitors unless they have failed.

That's a very good advice. The thing is that they are 40 years old, all this time must have put a dent in their original parameters anyway. The ESR looks good but I can't wrap my head around this 24.4nF -> 2,44 uF and then 220uF. The last value breaks the symmetry if I may say so. Shouldn't they add up to 244.4 uF and they simply went bad?  .

What matters is the horizontal calibration.