• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Troubleshooting: I have isolated a problem resistor, but the issue lies somewhere down stream

M

mikeabbages

New Member
I'm new to hifi repair and have become really engrossed with it's puzzles.

I found a Technics SA-101 on the street last week and took it home out of curiosity. I need to give it a deeper clean but I don't think that's where my issues lie at the moment.

When I got the amp, the right channel was dead. After a quick visual inspection, C704 was busted so I replaced it (as well as C413, because I could only order caps in a min qty of 2). I initially thought I had solved the problems but a new issue creeped in, this time on the left channel.

After 5-10 min of run time, clipping/distortion became more and more noticeable, only out of the left channel.
I had a suspicion R704 might be related to the issue because when I was cleaning the board and replacing caps, there was some noticeable discoloration (due to heat) on the board under R704. Sure enough, I touched R704 and it was burning hot to the touch!

The reason I suspect R704 to be more of a symptom than a culprit is that I had checked it's resistance with a multi meter when I was changing the caps and it was within it's 5% range.

I was wondering if anyone more seasoned than me might have some suspicions as to what is causing R704 to generate so much heat.

My next logical step I was thinking of taking was to replace all the caps touching that rail. This might be over kill, so I thought I might need a second opinion from the community before diving in.

See below an annotated schematic and an image of the discoloration under R704.

sa-101 schematic annotated.png 99kRLgK - Imgur.jpg


Thanks in advanced for your help.
 
Register to hide this ad
Welcome to AK... The fact that the resistor is mounted standing away from the board and the physical size of the resistor ( 3-5 watt size) tells me it is suppose to run hot.
 
Last edited:
avionic said:
Have you tried cleaning the switches and controls yet ?
Click to expand...
I haven't yet cleaned them. My volume pot causes a crackling in both speakers when adjusted. That was my next thing to do, but the fact that the distortion only exists in one channel got me thinking that these system switches might not be a priority.
 
mikeabbages said:
That was my next thing to do, but the fact that the distortion only exists in one channel got me thinking that these system switches might not be a priority.
Click to expand...
Clean all switches and controls then re-evaluate the situation. Most all of the controls and switches have contacts for left and right channel. If they are dirty or oxidized it will easily make one channel or the other malfunction..
 
Exercise the various switches and controls see if that has any effect on your left channels SQ issues.
 
That green resistor is a power handling resistor and is made to get warm or even hot to the touch but should never smoke or be smoking. The brown on the board looks normal from heat and doesn't look like it has had any excessive heat problems. This is why it is mounted above and not directly touching the board.

Could be dirty pots and switches but you will need to know what you are doing and know how to read a schematic and a meter.

Since it was free, this could be your learning amp/receiver.
 
Okay, so I cleaned all of my switches, pots and orifices. This worked wonders for the crackling and idle hiss. It's dead silent now!

Unfortunately, it did not solve the left channel distortion.

Measuring some units in the "main" speaker channel I get the following results

Right channel = 8 Ohms, 23V, 5A

Left channel = 4.4 Ohms, 13V, 2.6A

So left channel is essentially getting half the juice of the right amp.

This amp has an ohm rating of 4-8 ohms and I'm driving a pair of Polk S15 which are rated at 8 ohms.

What could be causing a 50% power cut to the left channel?
 
mikeabbages said:
Measuring some units in the "main" speaker channel I get the following results

Right channel = 8 Ohms, 23V, 5A

Left channel = 4.4 Ohms, 13V, 2.6A

So left channel is essentially getting half the juice of the right amp.
Click to expand...
Not clear to me what you've measured so can't confirm half power...
What components did you measure and what parameter (ohms, volts...)
 
Distorted signal can be lots of things.. Even preamp issues... If you are not used to work with electronics you might have to call in soms help for callification and confomation on your conclusions... any way... Find the services manual!!! With circuit diagram!!!

What you want to measure (using a service manual ) is the supply voltages in both poweramps
If you measure DC in your output, there is a big problem and your speaker will be humming or pop out or simply burn...any way.... thats not a good thing :-(

I gues that is not the case... SO supply voltage first... then compare voltages LOGICALY A to B channel following the Circuit Diagram.

A very usual suspect are Cap's to fail... These can "F**k-up many supply voltages...for excample in your picture... The Blue and back "cans"
If a cap fails Near a High mounted resistor usaly this R can be out value...

Keep track of what you do on one side, and if the defect side repaired then replace the same components on the other channel aswel..this is to keep them sound "the same"

Great thing you try to fix it... it is a very nice hobby!
 
You must log in or register to reply here.
Back
Top Bottom