Mac 1700 distortion when balance on middle

c_dk said:
Without a ocsiliscope and probe to find the point where the distortion starts these are just guesses.....

Since you are swapping parts have you swapped the crowbar circuit.....how about the resistors? I have had issues in the past with a bad ground from the (-) of the amp output to chassis ground which is by a rivet if I remember correctly.
Click to expand...

Hi .. Finally I got a scope , and started using it.. it appears that the distortion appears on LOW volume. when I turn the volume higher , distortion dissappears.
checked on the pre-amp wires that go to the power amp boards - very clean sine wave.
any help would be appreciated.
 
With the unit off and unplugged, measure resistors in circuit comparing the readings between the good channel and bad channel. You can't expect to get accurate readings of the resistors, but I've found the culprit this way in amps that had one good channel and one distorted channel. If you get an odd reading on the bad channel you can then lift one leg of that resistor and check it "out of circuit" so to speak.
 
- I replaced differential transistors with 2sa970.
- I followed the signal path and found out that I get a messy unstable sine reading on R152 (Right channel Amp Board) .
- Took R152 out of circuit and measured 1.1K (instead of 1K)
- Replaced with new 1K resistor - still distorting and unstable reading .
(of course it reflects on all components afterwards)

confused.
 
Are you saying the distortion goes away as you roll the balance control towards L or R. Does the distortion fade as you roll the control or just go away.

If it fades I would expect a faulty ground path from the speaker ground to the driver boards.......I had a MAC1900 that because of a lifted speaker ground had minimal output when the balance was in the middle.......it was spooky weird so I still remember it from 30 years ago.

Is there ripple on the supplies after q39...the voltage regulator.
 
This receiver is bringing me a lot of headache

I get ONLY +23Vdc instead of the +37 supply
Should I be worried about transformer?
 
So both the - and + 37 volt supplies are down? I would suspect the main filter caps and rectifier....

How was this converted to 220? I see hat the primaries are a double winding but I do not know if putting them in series is the correct conversion......Patrice, I am sure you have done hundreds of conversions...what is correct?
 
The -37 is fine . I figured it out and +37 is back to normal again - when the fuses are out of circuit in monitor board it gives the correct measure . I think this means maybe some short on transistors maybe on the monitor board . Will check them and seeZ
Thanks
 
Last edited:
Bit of progress :
When Output transistors are pulled out , the -37/+37 supplies are spot on.
Checked the transistors out of circuit and they are NOT SHORTED. they are fine.
Maybe something that follows them is the cause of this?
 
Could be a driver....which side pulls it down. If you swap the transistors side to side what happens.

I would not be surprised if you need 4 new mj15003 outputs. Don't just do one side.....
 
After a long time , Coming back to deal with this MAC 1700 ,
I found out I mixed all driver transistors together ! I have 4 NPN transistors : 2 which need to be 132-515 (Q25,Q26)
2 which need to be 132-521 (Q31,Q32)

Other than barely seen color marks on them I can't figure out which one is what
Anyone can please confirm which color is what or can give me an idea how to be sure,
Thanks
 
Let me get this straight.....you are hoping either Terry or myself has buried in a drawer somewhere some original made for McIntosh Motorola driver transistors so you can match the colors? Why not just check for the NPN vs PNP with your ohmmeter? I am sure I might have some around but after 30 years.......
 
Just to show how sick I am I do have 3 original TO66 transistors......023 (521) has brown dot, 024 (528) red dot, 028 (515) orange dot. I believe the 028/132-515 is the class A amp others are respective NPN/PNP drivers. 132-028/515 is a 2N3738 NPN, 132-023/521 is a 2N3767 NPN and 132-024/528 is a 2N3741 PNP.
 
Done installing the driver transistors !
I found out that the thermal cut-outs (S9/S10) wires were disconnected... It feeds from AC directly so I assume the polarity doesn't matter?
 
Correct. There is a proper name for these devices that escapes me at this time. However I was happy to see in a past email from a major parts supplier that Honeywell was making these for those of you that have a bypassed open one in one of your Mac amps.
 
Thanks done .
A little question here also :
This thick white wire comes from R187/R189 intercross , does it connect to this point on the photo ?
 

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You will have to confirm via the schematic. It's termination is not in my memory file.
 
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