Sansui 800 -capacitor coupled; DC offset

Jerry's TV

Active Member
Hi all - I picked up an original condition 800 receiver. The inside looks to be very original, un-touched. It sounds great. I'm trying to decide if i should leave it alone and just enjoy it or if i should restore. I don't want to re-start the "repair vs restore" debate here, but i do have a question which might lead me that way.

My question is regarding DC offset measurement. I need to place a resistor dummy load on the speaker terminals to get the DC offset to measure ~ 0mV. When i have my test speakers connected (mid-90's cheap bookshelf speakers) the DC offset will drift to ~+10V and above. I place the 8 ohm resistive load on B speaker channel... the DC offset drops to 0mv. When i switch back to A speakers only, the DC offset will start to slowly drift up (20 min = ~1V ; 2 hours = 10V) .

Both Channels L&R will drift at about the same rate and measures about the same. When it is playing with 10V background voltage it still sounds good, no distortion. top 800.jpg 800 front.jpg 800 bottom.jpg

I suspect I am up for changing the coupling capacitors... but thought i'd ask.
thanks
 
From my experience with capacitor coupled receivers such as this DC offset should always read 0 at the speaker terminals because you are coming through a Electrolytic capacitor which should block all DC. On units such as these, there is usually a DC balance adjustment, where you are trying to balance across the 2 output transistors. Many times they are 2 NPN's, and when done properly, the point that feeds the big electrolytic is exactly 1/2 of the full DC supply feeding the collector of one of the NPN transistors. Reading DC at the speaker terminal, especially 10V seems as if that capacitor is leaking, and feeding DC through it.
 
Without any adjustments (by me) the power rail is at 51.5V (should be 54V per the schematic) and prior to the coupling cap each audio channel is 25.2v and 25.8v respectively. Well within range. There is about 100 mV of AC ripple on the main supply C1+ terminal.

My guess is the additional 8 ohm resistive load is drawing down the leakage current of the old coupling cap, keeping the speaker lines at ~ 0 mV. Without that load, the leakage current allows the voltage to rise.

I'll plan on changing the coupling caps. I'll also change the five main power supply caps while I'm at it.
I'll leave the rest unrestored for now. I'm impressed it still works after 50 years!
 
When I did my 2000 a while back the change with new caps was outstanding, bottom end now rivals anything I own.
Cap coupled Sansui's are something special but shhh don't tell anyone :)
-Lee
 
If you look up ConradH in the member list he has a very very detailed writeup of the cap coupled mods on his website that really make this series shine. Not all his upgrades would apply to the 800 but if you read between the lines you will turn that unit into one outstanding performer.
 
If you look up ConradH in the member list he has a very very detailed writeup of the cap coupled mods on his website that really make this series shine. Not all his upgrades would apply to the 800 but if you read between the lines you will turn that unit into one outstanding performer.

Thanks - I'll look at ConradH... maybe that will motivate me to move from repair to restore.
 
Capacitor Coupled Test Speakers...
I finally got to ordering and installing coupling capacitors on this unit. Once installed, no change in behavior from the original post. It turns out my test speakers are "capacitor coupled" too.They are designed to not provide a resistive load.

My only problem.... was thinking I had a problem.

My test speakers are shown in background of the OP. These are 1990s surround sound inexpensive units. I pulled them part and sketched the crossover schematic attached below. I ran a dedicated wire from the 4 ohm main speaker and when connected, the bias works as expected. i put a meter on the speakers, they show infinite resistance.

Disappointing because i had thought of keeping this unit original as built in 1968 since it was in such good shape. I have the proper Nichicon LKG caps installed on the coupling and main power, they fit and look good, so may as well keep moving forward.

FYI -- if you're testing a capacitor coupled amp... make sure you really do have a resistive load attached!
 

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Yes indeed, just like tube amplifiers, you should use a set of non-inductive load resistors during testing and adjustment.
 
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