838 went from fine to transistor radio like

loweran

Super Member
Hi All,

I restored this 838 a while back. My daughter has been using it all summer. She tends to want to listen with it half way up for long hours. Today, she was listening to it, and it something happened. She said she did not hear a pop or anything, but it just went from a nice full sound to zero bass and a very tinny, transistor radio like sound. All inputs sound this way.

I just got home and wont have a chance to get voltages till this weekend.

Any ideas what might have happened?
 
quick guess power loss to output transistors .
next guess speaker problem .
start simple and check the controls and source material . ...i was providing sounds at a recent party and a badly recorded track was playing and i thought the amp had blown up .
 
Yeah, it was playing very nicely, and then suddenly went south. I checked aux and tuner and both are the same. I worked the switches to see if it may be a switch. Those were all cleaned when I restored the receiver less than a year ago. I think I will need to get some voltages to see what is going on.

The receiver has all new transistors and caps except for the filter caps, relay, and resistors, everything has been replaced with the recommended components.
 
best i can imagine it is if output transistors lost power then the drivers would power the speakers and sound as explained . i didn't look at schem yet to see if thats possible
 
quick look says my theory is wrong unless its just one channel at fault .
check power supply first .
fresh thought is try main in to narrow things down .. i always fear the worst first .
 
pull the main pre connectors and connect an ipod or similar . whatever it is must have a vol control . start at 0 vol . if say a cd deck was plugged in there it would be at full vol .
if sound is no use then you will need another amp to feed from pre outs to an input on the other amp .
 
OK, pulled the jumpers and hooked my phone up to the power amp side. Same result. I guess that means it's in the amp section? Or maybe power supply?
 
yeah sounds like a possible PS issue. Since both channels do the same thing then it must be something common between the channels.
 
Looks like there are five fuses on the power supply board. The three at pins 1, 18, and 19 feed amp board related voltages. I bet fuse at pin 1 is blown.



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Ah. I didn't think about a fuse. I will check that first. I left the old fuses in there, so it may have been weak. Maybe I should replace them all, but I will check one by one to find where the fault may be. Hope its as simple as that. I do recall loosing a pad on Q5 on the amp. I scraped down to shiny trace and soldered to that. Does not seem like that is the issue though. This thing has been running like a champ since I finished it up in December.
 
Problem is in both channels so it's very likely a power supply problem.

I bet if you pull the jumpers again and run the pre-out signal to a different amp, you'll find issues with the preamp output as well. The fuse off pin 1 also supplies power to the tone board.

If it is a blown fuse, the question becomes "why did the fuse blow"?
 
Cool. I am going to run by the HW store and get some 1A fuses, and a 3A for pin 23 after work and we will see. Thanks for the tip.

BTW I assume these are not slow blow fuses correct?
 
OK, so I replaced all the fuses even though the old ones looked fine. No difference in sound. Then I checked the PS voltages and got the following results. I think they look OK.

Pin Voltage
1 38.16VAC
2 32.2VAC
3 32.3VAC
4 42.5V
5 -42.3V
6 0
7 -44V
8 -13.11V
9 -13.11V
10 20.8V
11 26.27V
12 30V
13 33V
14 12.06V
15 0
16 0
17 49.3V
18 39.65V
19 39.75V
20 0
21 8.5V
22 12VAC

I guess I should get the amp voltages next.
 
Well, I feel stupid. I hooked up a different speaker on a whim and it works just fine. I am going ahead and check the neutral and offset while its open. What would cause both those speakers to fail? They are ADS L500s
 
87db/1w/1m 4ohm speakers rated for 30W, and a 60W per channel receiver @ 4 ohms with the user running the volume control @ 12:00 could very well be overloading the speakers. Remember that Volume controls are not all equal (in that 12:00 doesn't usually equal 1/2 power.) It could very well be 3/4 power or more, and with these fairly inefficient speakers only rated for 30w the voice coils probably burned up on the woofers. The crossover on the tweeter probably saved them.

Get her some speakers of about 93 to 95 db/1w/1m efficiency, rated for at least 50w, and the volume control will come down for the same SPL.
 
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