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 Post subject: Scope clock question
PostPosted: Jul Fri 31, 2020 6:52 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20855
Location: Warner Robins, GA
I have a low end Heath solid state scope built as a kit that I gutted and installed a Dutchtronix scope clock board in along with a more reliable deflection amp board as the original (produced a superior image due to higher bandwidth) was always needing some sort of adjustment or repair and was dependent on the line voltage as none of the main DC voltages were regulated.

The clock board supply came from the only available winding which is 42Vrms. It consisted of a 300 ohm series resistor to the clock board power input.

The clock board has a diode, 100uF cap and 7805 regulator. I added a 2200uF 25V cap in parallel to the 100uF cap for better filtering. The 300 ohm resistor is so that with the clock board on the B+ doesn't go above the 25 volts the cap is rated at as with no load the B+ would be 60Vdc which would damage the 7805.

I recently replaced the backup battery with a 15F cap charged by three series diodes and a 10 ohm resistor. Takes a good 20 minutes to reach +3.3Vdc.

Anyways I noticed that when the cap was initially charging (draws around 150mA initially) the +5V was around +3.2Vdc.

So I decided to rebuild the supply.

I used a diode, 5600uF 80V cap and a TL783 high voltage adjustable regulator set for 16Vdc. That then feeds the clock board. I removed the 2200uF cap initially.

I slowly discharged the 15F cap so I could see how the power supply worked with the cap being completely discharged.

It worked great, but I noticed some slight image swimming which from my experience when rebuilding that clock is indicative of a grounding issue.

I made sure the AC and ground wiring to the rectifier and cap were 16 gauge to be sure there was minimal voltage drop. Used about 1' for each wire. The output from the regulator to the clock board is maybe a 20 gauge wire.

I did reinstall the 2200uF cap and that seemed to help and it seems like the swimming is less now that the 15F cap is nearly fully charged.

Now the supplies for the CRT power are well filtered and the -HV is regulated with a string of zeners. The deflection voltage I drew up a modification where the voltage was doubled and regulated with zeners after a dropping resistor, but I am not sure if I added that yet as I didn't want to open the other side of the scope where the main scope supplies are given there is nearly -2KV on that side.

What could cause the slight image swimming?


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 Post subject: Re: Scope clock question
PostPosted: Aug Tue 04, 2020 5:25 am 
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Joined: Jan Thu 09, 2020 6:46 am
Posts: 78
Location: Lexington, KY, USA
That is still a really a very high input voltage to a 7805. I would put a 7812 in front of it.
Try small .1uf on the power output.


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 Post subject: Re: Scope clock question
PostPosted: Aug Tue 04, 2020 5:44 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20855
Location: Warner Robins, GA
I could change the value of the resistor that sets the voltage, but it does seem to work fine as is.


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