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Old 27th June 2013, 12:53 PM   #1
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Default Broken Sony F808ES - Please help fixin'?

I bought a '93 SONY F808ES 120 wpc integrated amp that needs repair. It is built like a tank and weighs in at 53 lbs. There is a sticker indicating it was a refurbished unit when originally purchased. I bought it very cheap, for repairing because it is supposed to be one of Sony's best integrated w/ their best phono stage. Sorry I don't have interior photos, but there's plenty on the web.

It doesn't output any dc at the speaker terminals, which is a good thing.I cleaned all the pots and switches w/ deoxit and adjusted dc bias. This is the limit of my testing skill.

The sound is anemic with no body or bass, and plays only quietly with no drive or volume. When operating stereo/mono switch the sound becomes barely audible in mono while buzzing in the speakers. After playing in stereo mode for about 5 minutes the sound gets very quiet and overrun with hum in the speakers, which is louder than the music playing. A visual inspection of the inside shows nothing burned, bloated, or out of the ordinary, but it obviously has some issues.

There is no power amp input to test the power amp alone, but there is a preamp out. I tried the preamp output and the same issues exist with another power amp, so I'm guessing it is isolated to the preamp. I'm not tech, but I will pull the main preamp board and examine it more closely, especially concerning the components related to the mono/stereo switch. Can I check capacitors, diodes, and resistors while in place on the circuit board?

A little basic advice on testing the components would be appreciated. Here is the SM.
Sony TA-F808ES | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine

Last edited by mjatabor; 27th June 2013 at 01:40 PM. Reason: grammar and sentence structure
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Old 27th June 2013, 04:39 PM   #2
Mooly is offline Mooly  United Kingdom
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Before diving in deep I would first check more basic things... it sounds a strange fault (both channels affected... hum etc) to be a genuine component failure, or at least I wouldn't immediately assume that.

Check for damage and breaks in the PCB (hairline cracks due to impact damage) around any "sticky out bits" such as pots and switches. It should be fairly easy to see where its all going wrong with a scope and simply tracing a line level signal end to end.
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Old 27th June 2013, 07:54 PM   #3
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I removed and studied the control and tone circuit boards mounted on the front panel, and they look fine. I also see why it was a refurbished unit. There are some circuit traces on the control board that were bad and they were touched up w/ solder. I really studied this board and every thing seems fine.

I also cleaned the cheap stereo/mono and rumble filter w/ deoxit. They are spring loaded push button switched and now they don't come out w/o pulling on them, like there is no lubrication in them anymore.

I will dig deeper and look at the input circuit board tomorrow.
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Old 27th June 2013, 07:57 PM   #4
ilimzn is offline ilimzn  Croatia
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I would suspect two things - one is an intermittent ground line somewhere, and the other would be elecrolytic caps. Not the main ones, but ancillary ones in the PSU filter stages of the main amp, AND - very important, any audio electrolytic caps in various places. This amp may still have the Elna Duorex audio caps, which are prone to 'sudden death' failure and leakage, with various strange effects.
The service manual for this amp should be available on the web for free. Note that in some versions of the manual the bias setting procedure had an error (assumed measuring a voltage drop across one vs two source resistors of the output transistors) which ended up in a very high bias, and lots of heat, as well as somewhat dead sound (but nothing like your problem). Properly adjusted, it's one of the best integrated amps I know of, and believe me I've listened to plenty (never mind being under the hood of them too :P ).

The general setup of the amp is a high-gain power amp with passive front end and a separate phono pre. Not sure off-hand if it has a 'direct' input (it's predecessors did), this provides a signal directly to the volume pot, omitting every other input switching component, and the volume pot is directly at the input of the main amplifier. Testing it through that may offer more clues. Also, try out both speaker outputs, we might be dealing with two problems.
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Old 27th June 2013, 09:42 PM   #5
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I notice the hum is persistent all the time and gets louder as the volume is turned up. The music sometime conceals it but its alway there as you can tell on an input w/o any source connected.

I see two pairs of large caps. Which should I check?
Here is a photo of inside from the web.

I have the manual from hifiengine, but not much of a tech.
Attached Images
File Type: jpg inside by others.jpg (91.8 KB, 153 views)
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Old 28th June 2013, 08:48 PM   #6
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Ok, I tested the smaller pair of the electrolytic caps with my Fluke 179 meter and they test 8200uf as stated. I tried testing the larger 15,000uf caps in the center of the amp, but my meters auto range only goes to 10,000uf and it reads out of range. How can I test them and/or how can I test for leakage?
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Old 29th June 2013, 06:34 PM   #7
jaycee is offline jaycee  United Kingdom
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The correct tool to do this is an ESR meter. Another way is to use an oscilloscope and check for excessive ripple on the supply lines.

However you can safely assume with the age of the amplifier, that the capacitors require renewal. Basically, if it's an electrolytic capacitor, replace it. Renew all of the small ones first, then check.

Also, have you tried Source Direct turned on ? The preamp in this amp is actually passive, except for the tone controls, and the phono preamp. For line inputs, with Source Direct on, the signal is routed straight to the power amp via the volume pot.
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Old 29th June 2013, 06:46 PM   #8
jaycee is offline jaycee  United Kingdom
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My gut tells me C320/C370 on the tone control board could cause issues if it was leaky, whether source direct is on or not. Worth a shot to replace those first.
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Old 29th June 2013, 06:54 PM   #9
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You create two thread with same problem I see
Testing and locating filter capacitor?

Show us what is AC voltage from the transformer & DC voltage on the rails?
if possible monitor DC voltage when first turned on until that strange things happen, maybe voltage drop or not?
Thank you, I wish you happy
peace & G B U
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Old 29th June 2013, 08:42 PM   #10
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I will look into these caps next. What makes you think so?
I'm no expert but I don't think it is the power supply, because filter caps read 'out of range' on my meter, so I think it is beyond the 10,000uf max range.

Reassemble everything, rechecked dc bias and this is what I observe for sure.
Hum on all line input unless no line-in connections at all, then no hum. No hum on phono stage. Anemic sound any input.

John Bali,
I'm a novice so where do I check AC voltage and DC on the rails? Thanks
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