SM-200 monitor scope

[chris54]

Hello and thanks for the time to read and reply.

I have into possession of a amature radio scope (I am a ham operator) SM-220 and the problem I am having is with the DC balance circuit and trying to figure it out the service manual is not very helpfull. I am going to attach if I can a copy of the service manual. So if anyone and help thanks

 

[mneary]

What is the "problem" with the DC balance control?

The purpose of this control is to maintain the trace in the same location as the gain is changed, when there is no input. The most vulnerable component here is Q102 but you wouldn't be able to calibrate the gain if Q102 were bad. Poor DC offset control would also be caused by Q103 with lesser impact on gain. Note that Q102 and 103 are supposed to be sort of matched. Since they handle the input RF directly, they can't be just any JFET, either.

 

[chris54]

That is the problem the trace will not stay centered whan the V gain is rotated. so I guess I can try to get a matched pair of jfets. What do you think.

 

[chris54]

If I input an rf signal into the vertical input could I trace it through the circuit

 

[bizz]

I would change C109 and C111 first. IT's alot easier to get two caps than two matched FET's. Sometimes they read good on capacitance but they are leaky, especially when they get old. If you have new FET's then change the FET's too. Sometimes it is more rewarding to fix things than use them when they do work.

 

[chris54]

I learn more by fixing things. It challenges me. Thanks for all the help. The ones called for are 25volt 10uF and I have 35volt 10uF what do you think. Thanks

 

[mneary]

Have you followed the procedure (page 15, right side) for adjusting the DC balance? Is there no adjustment point that satisfies the procedure?

 

[bizz]

I would use new 25 volt ones that are the same size as the originals. You also have a verticle balance pot you can try to adjust. You should check transistors 104 and 105. Power supply ripple also causes alot of problems so check both outputs from the power supply and see if any capacitors are getting warm after it's been on a while. It could also be just a solder joint that went bad. Look at everything carefully with a good light.

 

[mneary]

35 volt caps should be fine if they fit into the space.

 

[Nigel Goodwin]

I learn more by fixing things. It challenges me. Thanks for all the help. The ones called for are 25volt 10uF and I have 35volt 10uF what do you think. Thanks

If they will fit they will be even better than the originals.

 

[chris54]

Have you followed the procedure (page 15, right side) for adjusting the DC balance? Is there no adjustment point that satisfies the procedure?

Yes I have tried that thanks

 

[mneary]

What happened? The actual response will tell us a lot about the actual bad device!

 

[chris54]

Ok update changed the c111 and c109 still same problem. I have ordered the replacment transistors and should get them tommorrow. Reading the voltages at q104 & q103 would there be similar voltages on each of these. Drain Gate Source. ?

 

[chris54]

What happened? The actual response will tell us a lot about the actual bad device!

When you turn the V Gain the trace goes up and off the display. I have tried the adjusment shown in the service manual but still no good.

 

[bizz]

Just because a capacitor has a higher working voltage than the orignal one doesn't make it better to use. The quality of the part is what counts, especially with capacitors. It is hard to get quality parts today. Does Kenwood still exist? If they do maybe they will sell you a whole new old stock circuit board. Then you will have plenty of spare parts to keep your monitor going for many years. Soon there will be no more CRT's but you will have one.

 

[chris54]

Just because a capacitor has a higher working voltage than the orignal one doesn't make it better to use. The quality of the part is what counts, especially with capacitors. It is hard to get quality parts today. Does Kenwood still exist? If they do maybe they will sell you a whole new old stock circuit board. Then you will have plenty of spare parts to keep your monitor going for many years. Soon there will be no more CRT's but you will have one.

I tried kenwood and unfortunally the parts have been discontinued. I tried that route first. Thanks again for all the help.

 

[bizz]

Capacitors stress out everything when they start going bad so you have to change them. I hope when you get the transistors that your monitor works. Sometimes you have to change alot of parts to find the bad one. The power supply voltages should be checked out because all the other circuits are dependant on it. Be carefull around the high voltage for the CRT.

 

[chris54]

Ok checking the votages at the junction of R128,129 and R131,132 when the scope is centered there is a volatge differance of 3.5 volts. 73.1 volts at the R131,132 and 69to 70.1 volts at the R128,129 would 3 volts be in range

 

[mneary]

Ok checking the votages at the junction of R128,129 and R131,132 when the scope is centered there is a volatge differance of 3.5 volts. 73.1 volts at the R131,132 and 69to 70.1 volts at the R128,129 would 3 volts be in range

That sounds good.

What are the voltages at the ends of VR101 (a) when the pot is centered (b) when the beam is centered?

 

[bizz]

On my scope the DC balance specification is + or - 2 volts for the vertical deflection amp and at zero deflection the vertical plates are at approximately +45 volts to ground and the horozontal plates are at approximately +55V. This is just a regular 30Mhz scope not a monitor type. Keep checking all the parts and you will find the bad one.