gillies@p.cs.uiuc.edu (01/12/89)
I am trying to debug a 1966 Sherwood Receiver. This is a high-quality
all-transistor 140-watt receiver that has 1% distortion at its rated
output. If I can get it working, I want to give it to my sister.
THE PROBLEM: On all input settings (AM, FM, Phono, etc), the left
channel plays about 40Db more quietly than the right channel. If I
turn the balance control all the way to the left, they play evenly,
but the left channel has a tremendous amount of background noise/hiss.
I know the receiver is the problem (the speakers work fine with
another receiver).
There is a mono switch, and in this mode, both channels work
perfectly, although slightly quieter than in the stereo mode.
CURRENT STATUS: Using my trusty VOM, I've done the following things:
- Cleaned the balance control switch and tested its resistance. It
seems to be working perfectly, and fairly. Checked the volume control,
which seems to be working correctly.
- Checked all the transistors in-circuit, and the Vbe voltage drop is .6
volts, as you would expect with a working transistor.
- Checked the socketed power transistors out-of-circuit, and they seem
to have reasonable resistances, and similar transistors have similar
resistances.
- Swapped nearly all the power transistors between channels (except
perhaps the 4 massive heat-sinked output transistors, which were
checked out-of-circuit but (perhaps?) not swapped), and the problem
didn't move to the other channel.
- Done a careful visual inspection of all components for solder
bridges, cracked/melted packages, etc.
- Tested DC voltages on the power-output boards of each channel, and
the preamp board. I couldn't find any asymmetry.
I am clueless about the problem. I hoped to find something obviously
wrong in either the preamp or the power amp boards. But I found
nothing. Now I'm toying with the idea that the power supply may be
inadequete on one channel (far fetched, I know). Are there other
simple tests I can perform on this receiver?
If not, then is the next step to take it to the university, and
connect it to an oscilloscope and a function generator, and try to
trace the signal through its guts?
Thanks for all your help,
Don Gillies, Dept. of Computer Science, University of Illinois
1304 W. Springfield, Urbana, Ill 61801
ARPA: gillies@cs.uiuc.edu UUCP: {uunet,harvard}!uiucdcs!gilliesdmt@ptsfa.PacBell.COM (Dave Turner) (01/14/89)
In article <74000002@p.cs.uiuc.edu> gillies@p.cs.uiuc.edu writes: > >I am trying to debug a 1966 Sherwood Receiver. This is a high-quality > >THE PROBLEM: On all input settings (AM, FM, Phono, etc), the left >channel plays about 40Db more quietly than the right channel. If I >There is a mono switch, and in this mode, both channels work >perfectly, although slightly quieter than in the stereo mode. > Check the coupling capacitors in the signal path. Since both channels are approximately equal in mono, check the circuitry in the preamp portion that precedes the stereo/mono switch. I have had similar troubles with other stereos from that era. A Heathkit (AA-21 ? I think) had a bad capacitor in a fancy pc couplet that had a bunch of resistors and capacitors in a flat package with about 10 pins. -- Dave Turner 415/542-1299 {att,bellcore,sun,ames,pyramid}!pacbell!dmt