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Mac 1700 Need Help

daveyh

daveyh

Super Member
I have a Mac 1700 that I am working on. Today I can't get any sound out of the speakers. Before working on it, it did not work but on one channel. The left. The right channel was very, very faint.

Yesterday I thought I had it fixed. It was working in both channels for several hours. I even turned it off for 30 minutes and turned it on again and it worked. Now this morning when I turned the unit on there in no sound out of either channel. :mad:

Here is what has been done:

I replaced the 4 output transistors with 4 new ones. I used 2N3055. Before I did this I pulled, checked and re-greased all the output drivers. Q25, 26, 31, 32, 33 and 34. I also pulled Q39 and did the same. They all seems to be in spec. I also replaced R222 and R223 resistors, the two BP caps and the C163/164 caps on the 043-821 output board. On the pre-amp boards I replaced the two larger capacitors on each board. On the output amp board I replaced C118, C119, C114 and C115. I also checked all the small transistors and resistors on each of all boards.

I did notice that the SBS-1 and SBS-2 located on the 043-821 output board are 2N4992. I do not know how to check these. My PEAK Atlas Pro transistor checker says shorted but I do not know if it is able to read this type. Are these the correct replacements? If so what is the correct way to check these? Am I even getting close? I am Lost.
 
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2N4992 is a bilateral switch ..A1 A2 and gate ... not sure how it works without looking at data sheet .
it could be on until its switched off or other way round and latching . so maybe short A1 or A2 to gate turns it off ..
best read data sheet to be sure .
 
The bilateral switch or triac is part of the crowbar circuit which causes the rail fuses to blow if excessive DC is seen at the output.

Do you have rail voltages? How about the other supply voltages needed to power the various circuits?
 
Trying to trace this down with a DMM. I put my DMM on beep and attached the black probe on the chassis and touched each T0-3 outside case. These output transistors are made by ST. Three do not beep but one does beep while touching it with the red probe. Like it has a ground short. Is this correct?

I also checked all the driver outputs and they do not beep when touching the outside case. The one in question is Q36. I also pulled all the output transistors again and checked them they are all still OK. I even changed out Q36 with a new one and it did not help. I am going to trace the beep or short down in the morning. Am I on the right path?
 
c_dk said:
The bilateral switch or triac is part of the crowbar circuit which causes the rail fuses to blow if excessive DC is seen at the output.

Do you have rail voltages? How about the other supply voltages needed to power the various circuits?
Click to expand...

It does not blow any fuses. They are all intact. I have it on a DBT now.
 
Hey Guys! Look what I found.

Mac_1700_screw_long.jpg

Someone put an extra long screw in the positive speaker wire mount and when you screwed the wire in it was touching the chassis. Removed it and no shorts and it seems to work OK in both channels.

I did notice when I hook my meter up to the speaker terminals with no load the DC offset is 11mV in the right channel and 29mV in the left channel. There is no VR mounted on the board but just a fix resistor so I can't adjust it. What is the correct voltage? Can I add a VR to the board or should I? What is the procedure for doing this if it needs to be done?

Thanks!!
 
petehall347 said:
i shouldn't worry about the small offsets .
Click to expand...

Sounds good to me. Thanks again Pete for your response and help.

I love these Mac's. They sound so strong. Just a little turn of the volume knob and your in business. Been playing now for awhile and all seems OK. Outputs are cool to the touch and the laser pointer reads around 89 degrees each side on the collectors. I'll let it play most of the day and then check it again in the morning.

Happy New Year!!!!!!!!!!
 
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