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Mac 1700 restore

rczetli

rczetli

New Member
Hello! I am a relatively new member of AudioKarma and I need some help from the experts.

I recently purchased a MAC 1700 receiver with a blown output stage. All 4 output transistors tested bad and several base to emitter resistors were fried. I am going to completely rebuild both output stages and do some work on the power supply to start but I have a few questions for the Mac gurus here. The output transistors are part number: 132-070 and they cross to a 2N5303. I believe I read on this group that an MJ15003 would work well and is readily available. Is this correct?

There are also diodes on each of the output transistors marked part #: 070-031 and they cross to a 1N4820. They test ok but two of them are badly charred from the fire after the failure. Do you know what these would cross to in current production diodes?

I was thinking about replacing the diodes in the main power supply bridge. They are part number 070-041 and cross to Westinghouse 398B. I can not find any specs on the Westinghouse diodes. Does anyone know what a good replacement would be? Would any of the fast recovery diodes benefit this older design?

Finally, any other suggestions from anyone experienced with the 1700? It appears most of the coupling caps in the signal path are Mylar so for now I don’t plan on replacing them. I will probably replace the few ceramics caps that are on the driver board and preamp boards. Power supply caps test ok and according to the shop that was using this receiver there was no hum at all before failure but I may replace them anyway considering their age.

Thanks in advance for your input and assistance!
 
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2N5303 is available at Mouser. You may want to check the specs of it against 15003.

It looks like 070-031 is just an ordinary diode so 1N400X should be fine there. Same with 070-041.

Some of those Mylar caps will have corroded leads and the corrosion seems to go up inside the cap. I had some that were corroded on mine and when I tried to move them out of the way, they broke. Might want to check them out.

I would replace all of the 'lytic caps underneath including the ones on the driver boards just for peace of mind.

Also, it's easier to figue out what you're referring to if you id the parts based on the schematic part rather than the part number.

Good luck with your 1700!

Murray
 
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Mac 1700 Restore Thanks!

Thanks Murray.

I ordered the output transistors from Mouser along with all of the electrolytics. I checked the other caps and they look and test fine. This unit was kept in a showroom since it was new and used almost daily until the outputs went so it is in super clean condition. They used it for testing and for background music. I am technically the first person to purchase this unit....40 years after it was made! Kind of cool.

Thanks for your help. Parts should be in next week so I will post some pictures when I have it completed.

Rob
 
The mylar caps are mechanically unstable, the contact at each end is pressed in place, almost any tension on the leads can break the contact, enough and the lead will pull off. If a lollipop mylar shows any cracks in the paint - replace it....
 
Thanks for the information Terry.

What brand and type of cap do you recommend to replace the original mylars that will sound good and fit?

Thanks again!
Rob
 
Thanks for the pointer on the Mylar caps, Terry. Something *else* to add to the list of stuff to look at on the1700.

Rob, be sure to check the transistors and tubes in the tuner. Those little 6DS4 Nuvistor tubes are DARNED hard to find these days, and pretty expensive when you do!
 
What do the mylar caps look like? Did a Google search for lollipop mylar caps and got nothing of use. What do they look like? Could a problem with one of these caps cause one channel to kick in or out with a loud pop as the receiver is being used? Thermal expansion, unstable contact?
 
This is a pre amp board on a '1700. The ones with the yellow on top. I don't think these cause as much trouble as the ones underneath the chassis.

Murray
 

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re intermittent pops

FWIW I had somwwhat of a similar experience with my MX114 tuner pre and one day while it was in use I used the eraser end of a pencil and gently tapped on sections of each circuit board. Sure enough it turned out to be a solder joint that I remelted.
 
I am currently working on a model 1700 that has a failed Q31 transistor with an emitter to base short. The OEM part number is 132-521. Do any of you know of a 2Nnnnn type number that will substitute for the original? Terry DeWick was my go-to person for such information, but since he is no longer with us, that source is not available. There is a companion Q33 OEM part number 132-527 that drives the other output transistor which I would like to replace too if anyone knows a replacement.

Thanks in advance!
 
audmod01 said:
I am currently working on a model 1700 that has a failed Q31 transistor with an emitter to base short. The OEM part number is 132-521. Do any of you know of a 2Nnnnn type number that will substitute for the original? Terry DeWick was my go-to person for such information, but since he is no longer with us, that source is not available. There is a companion Q33 OEM part number 132-527 that drives the other output transistor which I would like to replace too if anyone knows a replacement.

Thanks in advance!
Click to expand...
This thread may help: https://audiokarma.org/forums/index.php?threads/mc50-fix.422579/
 
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