Heathkit SO-4552 oscilloscope help needed

[wayne55]

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 are getting very hot with the unit turned on. I just wondered if anyone here could give any pointers on what I might check or had run into a similar problem?

Thanks,
Wayne

[tlentzsr]

Re: 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 are getting very hot with the unit turned on. I just wondered if anyone here could give any pointers on what I might check or had run into a similar problem?

Thanks,
Wayne

As there is a glow at the base of the CRT then the low voltage fillament is working ok and probably emitting electrons as it should from the strontium oxide coating on the cathode. Evidently there is an issue with the high voltage section. If you had high voltage but no diflection voltage I would expect that there would be a bright spot in the middle of the screen but you did not mention that so it also appears deflection voltage loss is probably also not the problem. I tend to suspect that the high voltage diode is at issue and may have gone bad. If you can find the part it should not be expensive to replace. Testing it can sometimes be tricky though as even a new one may show a very high ohms value when tested with a meter so simply replacing it may be in order. You mention that when failure occurred the brightness faded away. That would also be expected if the HV diode failed since the excitation of the electrons hitting the phosphorous screen creates a brightness that does not die immediately upon loss of high voltage. This persistance of the phosphorous to glow is occurring for a short time after high voltage loss. If the scope is very old you might have a hard time finding an exact replacement for the diode. If this is the case you might scrounge a HV diode from an old microwave oven and get the scope working again. Just be sure to make sure the power is off of any microwave you take it from and that you carefully discharge all capacitors/components in it before sticking your paws in there. Hard to believe but a couple of years ago there was an electrical engineering student at Southern Illinois University in Carbondale, Illinois that was killed by electrocution in his apartment while doing some repair to his microwave. I would have thought he knew better.

[wayne55]

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 zener diodes, replaced or checked all capacitors with a capacitance and esr meter, checked all resistors and cannot find the problem. That's why I think it may even be a short in the transformer.

[Jack Crow]

Re: Heathkit SO-4552 oscilloscope help needed

Wayne,
Try some easy ones first.

Did you turn up the brightness?

That voltage should be near 1000 volts so be real careful poking around .

Dim the room lights and see if you can catch a glow on the screen.
Is the beam deflected one way or another?
Is the scope in a strange mode, like waiting for a trigger to sweep? Or in X vs Y?.

If all else fails, set the brightness to minimum and leave it on all night.
The emitting surface in the CRT may need to cook for a while.

Just an idea.

Jack Crow

[Caleb]

Re: Heathkit SO-4552 oscilloscope help needed

I have an old IO-105 I got from a salvage yard. Mine had a blown bc-137 transistor supplying the +15v, a shorted zener in the bias circuit for the +15. Replacing those got it working but it would intermittently shut off for no apparent reason, I poked around a bit and had a cold solder joint on the horizontal board as well. Do you have the manual for your machine? I would guess you have something dragging down your voltages rather than a short in your transformer.

[wayne55]

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 diode that checked good, but I replaced anyway. There are 4 bipolar transistors that check good that I also replaced anyway. There are two 8 pin chips that I replaced also, thinking that would fix it, as I had replaced everything else, but still no luck.

Thanks for your reply.

[wayne55]

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.

[budm]

Re: Heathkit SO-4552 oscilloscope help needed

OK, the +/- 12V linear regulators also feed the switching power supply (I can see voltage doubler circuit on the secondary side of the T960) that produce the high voltage (I cannot see the rest of the circuit to the right of the page) which has self osc circuit driving the trAnsformer T960 by Q950. You can lift the one leg of L962, L961 to see if you will get the +/-12v back to normal voltage. A lot of these old scope also uses a lot of Tantalum cap for DC filter, they will usually fail in shorted out mode.

[wayne55]

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?

[Caleb]

Re: Heathkit SO-4552 oscilloscope help needed

A short beyond the transistors will also cause them to get very hot as they try to source the current. On mine the supply for the 15v comes off the same transformer leg as the 30v supply, it is just knocked down by the zeners and fed into the transistor network. Have you tried removing Q902, Q903 and Q904 one at a time and see if you can isolate the trouble to a specific area of the circuit? What dc voltage do you get when testing across the filter cap c902?

[budm]

Re: Heathkit SO-4552 oscilloscope help needed

You are reading 29v (depends on AC line voltage) output of the whole winding (it has center tap for ground ref, so you need to measure with ref to GND), it is center tap transformer feeding full wave rectifiers for +/- supply so the ouput after rectification (ceter tap is GND) = 14.5 V x 1.414 = 20.5VDC for the + supply, -20.5VDC for the - supply.

[wayne55]

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.

[wayne55]

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 are still good. The only thing that is removed is the BD237 transistor. It tests bad on my little transistor checker. My question now is, will replacing the transistor fix the problem or did a capacitor or diode blow the transistor? Or like in a previous post, should I replace the diodes even though they check good? C951, C952, C959, C960, and C958 are ceramic capacitors. C954 and C955 are film. C953 is electrolytic. I guess I could pull them and check for shorts and their capacitance values. On the electrolytic, I can check esr also.

[budm]

Re: Heathkit SO-4552 oscilloscope help needed

If the transistor Q950 is removed (as in post 8 that I had suggested to lift the legs of the inductor which feed the Q950 to isolate Q950 circuit from the +/-12v power supplies), when you put the diodes back in, the Diode will not have any affaect since the transistor is not in the circuit to drive that HV transformer. You should check C960 which is the filter cap for 2KV power supply output.
You can have bad HV transformer that caused the transistor to fail, or the transistor may have just fail.

[wayne55]

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 on my meter may not be enough as I now understand it. So I guess I should try to replace the diodes anyway when I replace the transistor. I will check and/or replace that capacitor.

The ohms readings for the HV transformer check with what's on the schematic.

Thanks

[budm]

Re: Heathkit SO-4552 oscilloscope help needed

You need ringer tester for checking the transformer, DMM will only show DC resistance of the winding, it not show shorted turn of the transformer winding.

[wayne55]

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.

[popeye]

Re: Heathkit SO-4552 oscilloscope help needed

Hi Wayne

if you want to check to see if the HT supply is correct You can remove the transister then connect an domestic light bulb across the ht connect the + side to the collector of the transister and the - side to the secondery ground switch on if thsaat is ok then replace the transister with a new one makeing sure to replace the heatsink compound not to much just a thin covering power on by a variac you should find it will burst into life at about 60% if you do not have a variac just switch on if it blows the transister again it will most likely be the line out-put transformer will need replaceing as well as the transister.Hope this helps.

POPEYE

[budm]

Re: Heathkit SO-4552 oscilloscope help needed

"You can remove the transister then connect an domestic light bulb across the ht connect the + side to the collector of the transister and the - side to the secondery" Can you clarify the above statement, it is a little confusing.

[wayne55]

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.