Mac 1700 distortion when balance on middle

omri617

Super Member
Hello guys
I recapped this 1700 and recently it distortes on Right channel .
The issue is that it happens only when Balance is on the middle where should be.
When I turn it to the same right channel , destortion vanishes.
I lubricated and deoxited the balance pot- no luck .
Any help would be greatly appreciated .
Thanks
 
Apparently , the destortion happens doesn't matter where balance is . It's only heard more clearly when balance is on the middle.
I measures DC offset , it's pretty good 0.002V both channelsZ
 
You are going to have to determine if it is in the pre or amp section. Without a scope to monitor for the distorted waveform you will need to rig up a way to send the pre sections output....l to r and r to l.
 
The 1700 does not have power in / pre out , so I'm cluelessZ
Any ideas what can help me ?
 
Yes, I know there are no pre outs......you need to figue out a way to monitor the pre out.......

With a scope I would attach a probe to pin 6 (red wire I believe) of the amp boards and see what the wave form looks like. If I was in the field with no test gear I would swap the pin 6 wires left for right and see if the distotion follows.

I assume you have a schematic other than the one in the Akdatabase which does not indicate pin numbers, if using the database schematic you will need to infer the input pin finding the input resistor that is first in circuit.
 
Oooh wow nice lead , will do that . Till now I reflowed the pre board - no luck.
There is some darkening on one of the Lollipop caps on the bottom of the pre board - soon will be In front of PC and check the manual and edit it's part number .
Thank you .
 
I would suspect a noisy transistor long before I would suspect a cap....

No guesses until you determine where it is happening.
 
C_dk -
I followed your lead and switching between the power boards using pin 6 (you were correct - red wire) I can say 100% it's Right Power Amp problem.
Now reflowing the solders until you maybe can throw me another lead?
Thanks again
 
All solders were reflowed , no luck .
I counted 4 transistors on each Amp Board :

q21 , q23 - 132-029 - Is 2SA970BL / KSA992 a good sub?
q27 - 132-032 - Is MPSA55 a good sub?
q29 - 132-021 - Is MPSA05 a good sub?

How do I get data & replacement alternatives for them?
 
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I would guess the differential pair.....I do not have my cross references here at the house but i am quite confident the 970 were the subs, so yes the pnp a992 should work well. I believe the pin out is different so be careful when you install the ksa992 if they are readily available.

The sentry monitor transistors seldom get noisy....usually just burned up when the amp has a catastrophic failure.....I have used those transistors as subs.
 
So the deferential pair are the ones gonna be checked .
I swapped between the other pair (q27 with q28 & q29 with q30) and no luck again so
Your call may be the answer. will do it later on today.
 
C_dk ,
I swapped all 4 transistors between amps - the problem still exists on the same right channel. So ... It's not pre-amp , it's not a transistor , all electrolytic caps are new .
What is the next suspect ?:scratch2:
 
Bummer !!!!
By mistake I made a short between the signal red wire lead PIN#6 with the pin next to it PIN#3 . A small blast happened and since then the channel is REALLY distorted !
I swapped between SBS 1 / 2 but no luck ..
 
Although Im the only one writing right now :) but I'm happy to say that the problem was a blown fuse . apparently when the short happened , it blew F2 .
 
I swapped between the L/R channels the 3 chassis mounted driver baby transistors no luck .
Swapped also the output transistors ! Nothing changed :(
Any help would be really appreciated .
Thanks
 
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.
 
I'm afraid I'm not familiar with the "Crowbar Circuit" , what is that?
Does each amplifier board has it's ground ? or it's common?
 
I'd sure recommend a dim bulb tester to help avoid more accidents - they happen to all of us.
 
Strange.. the defective Right channel sounds louder like it has more gain and of course still distorted.
Checked all grounds and cleaned the rivet reflowed everything , still no luck.
 
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