Re: Heathkit SO-4552 oscilloscope help needed
I contacted Pintek, the Taiwan maker of the scope. They said when the transformer went out, it normally took the transistor with it. Since the transistor was definitley bad, I assume that's what happened. There is a replacement transformer, but the cost of having it shipped and some kind of bank fee they charge make it cost prohibitive.
wayne55
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Last Activity: 04-14-2013, 11:04 AM
Joined: 01-29-2011
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Re: Heathkit SO-4552 oscilloscope help needed
One more thing to add, I do get the +12 volts at L162 and the -12 volts at L161 as shown on the schematic.Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
I don't have a real high voltage probe, but I have a volt-pen that lights when it is near high voltage. I use it to check an electric fence. I do not think I'm getting high voltage, because it does not light up when I place it near the -2Kv checkpoint. Maybe the high voltage transformer is the problem afterall.Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
I got the parts quicker than I thought and after replacing the bad transistor, the filter capacitor, both HV diodes and the 237 8 pin chip, I'm now getting all the correct voltages in the power section. I get the +12, -12, +5 and +145 just as it calls for. This is with all components in place. I also get the AC voltages of 6.3 and 31 that feed from the main transformer to the CRT.
The bad news is that I still get nothing on the CRT display. The CRT rear end glows but no trace appears. At least now nothing seems to be...Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
I'm sorry I'm so slow, but do you mean to take a regular filament bulb and connect leads from the bulb to the emitter and collector holes where the transistor was mounted? Then turn the power on and if the bulb lights, does that mean the transformer is good?Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
I wish Popeye could clarify that too.
Anyway, I ordered the transistor with a few extra ones along with capacitors and diodes yesterday. Maybe by middle of next week I can test it out.Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
Thanks budm for all the info.
I don't have a ring tester, although I see the Blue ring tester is not too expensive at $42 for the kit version. If it is the transformer, I guess I would be at a dead end anyway since there's probably little chance of finding a Heath replacement.
I guess I will just try to replace the transistor and hope for the best.Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
I did not lift the legs of the inductors, just the diodes in order to check them with my meter. I did remove the transistor completely. You could see a little scorching under the transistor heat sink after it was removed. After doing this, I started to get the correct voltages as previously stated. I then re- soldered the legs of just the diodes, and checked voltages again, and everything was still good. So maybe the diodes are not shorted.
Since they are high voltage diodes, just checking them with the diode function...Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
I read back through all the replies and gave it one last shot. I removed the transistor (Q950)in the HV supply and it appears to be bad. At that time I un-soldered one leg of each of the HV diodes (HD950 and HD951)and checked. The diodes appear to be good, but maybe I should replace anyway. When I removed the diode legs and the transistor, it cleared the short. Now I get the proper +12, -12, +5 and +145 volts at the W01-P01 connector. Since the diodes checked good, I re-soldered to the board and checked the voltages again. The voltages...Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
Thanks to all for taking the time to help. I thought since the transistors were getting hot that I might could fix it by replacing components in this area. If the problem was in the low voltage section, I might have had a chance. This is getting way beyond my limited ability as I have no idea where to locate a problem somewhere else. Thanks again.Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
Revisiting the idea of a transformer problem-
If the output of rectifier BD902 is 18 volts DC, shouldn't the input voltage be about 12-13 volts AC. Instead it reads about 29 volts. Since the transistors Q902 and Q903 get exceptionally hot in just a minute after turning the oscilloscope power on, would the high transformer voltage account for this?Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
Here is a schematic of the low voltage power supply. I hope it's not too hard to see. If you rotate it clockwise and zoom to about 85%, I think it is readable....Last edited by wayne55; 03-03-2013, 03:17 PM.Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
I have the manual and a schematic. As you said, there must be something dragging down or shorting in the low power supply. I have taken out and tested as best I can every component in the low power supply which supplies the +12, -12 and +5 volts, except for the resistors. I have checked the resistors in circuit and they all show their proper values. There are 7 capacitors which I removed. Even though they check good for capacitance, for esr, and for being shorted, I still replaced the 3 biggest. There are 4 zener diodes and 1 small regular...Leave a comment:
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Re: Heathkit SO-4552 oscilloscope help needed
Thanks for your reply.
I beginning to think there may be something shorted in the power supply area. There are 6 voltages I can check coming from the transformer. Two ac voltages check correctly at 6.3 and 31 volts ac. The four dc voltages are where the problem is. From the transformer they go through 2 rectifiers and 4 bipolar transistors(which get exremely hot in just a short time). These voltages are supposed to be +12,-12, +5 and +145 volts dc. I'm not getting anywhere close to that. I have desoldered and checked all...Leave a comment:
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Heathkit SO-4552 oscilloscope help needed
I purchased this scope new about 20 years ago. It has sat in a closet most of that time. The other day I took it out just to power it up and began calibrating one of the probes. The display was giving the square wave for calibration and everything was fine. After about 5 minutes, the display faded away and will not come back on. I know little about diagnosing this, but I did check the transformer voltages and they seem OK. There is also a glow at back of the CRT but no display. I don't see any leaking capacitors or broken parts on the circuit boards. However, the mosfets near the 2 rectifiers...
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