Beogram 4500 CD — heavily distorted sound

[sverreh]

My Beogram CD 4500 plays the CDs, but the sound is heavily distorted. The nature of the distortion can be heard in the  attached video. The CD plays flawlessly in my Beocenter 9500.

So far I have

— replaced the capacitors with the kit from beoparts-shop

— gone through the service program, no errors

— inspected the other capacitors, no visible bulging or leaks

I would appreciate any advice on further steps in identifying the root cause.

Thanks in advance.

Sverre

[sverreh]

One more test: The optical out is working fine, no distortion and crisp, fine sound. Is there any known issues witin the Decoder PCB 5  or the Connection PCB 11?

(The video referred to in my previous post was apparently not acceptded by the site software)

[Mark-sf]

You haven’t mention what component/system it is connected to. If both channels have the same distortion you may be overdriving the input to your receiver as the diigital side sounds fine and once the signal leaves the DAC chip it flows in discrete L/R circuits which are unlikely to both distort unless you have problems with either the +15v or -15v supplies to their opamps.

[sverreh]

Thank you for your response and suggestion. Currently the analog out is connected to the AUX- input on my Beocenter 9500. The fault occurred when it was connected to Beomaster 4500. The digital output was checked by connection to a Sony DTC 55 ES DAT player.

I could have described the distortion more accurately in my initial posting: It is more like white(ish) noise, with the tune barely audible behind the noise.

From a thread in the archived forum, I found a member who had solved a similar issue by replacing four 100 μF capacitors and a burnt resistor in the opamp. I have ordered the parts, and will report back when they have been replaced.

[Mark-sf]

There are actually 2 dual opamps IC10 and 11. You may be referring to the 100 μF caps on the 15v -/+ rails that help smooth the DC. However, their failure, assuming they opened versus shorted, would not cause white noise in my experience. It is also not likely that both channels would fail together since each has its own dual opamp.  If in fact both channels exhibit the same white noise your problem is likely in the 15v dual power supply section.  This regulation was done discretely back then and is where I would start.

[sverreh]

OK, thanks once again. I´ll try to follow your advice. The burnt resistor is R121, connected to 15 V.

[Mark-sf]

If R121 burned, I would replace IC 10 & 11 as they are connected to the same resister. I would also change out C103 & 107 which I believe you are doing.

[sverreh]

Solved.

R 121 was burnt and replaced. Measured value was 24 kΩ, should have been 68 Ω. I could not find a 0.14 W resistor, but replaced it with 0.25 W. The alternative would have been 0.125W. Should I continue hunting for a68 Ω 0.14 W resistor, or is a slightly larger specified one OK?

One of the caps was shorted, C107. The other three — C103, 106 and 108 measured around 80-90 μF  and slightly high ESR out of cirquit. I replaced all four of them.

I took your advice and acquired replacement ICs, but decided to test the player without replacing them. Analog sound was back again:) Would it be wise to change the ICs even if the player is working fine for now?

 

 

 

[Mark-sf]

Since, you found a shorted cap replaced all of them, it is likely the culprit for the burnt resistor. OpAmps usually work fine or simply fail by their nature.  If the sound is now clean without noise and distortion, I would just button it up and enjoy.

[sverreh]

Thank you for your assistance in particular, and the knowledgable Beoworld ccommunity in general for your assistance. I rely on the wealth of knowledge in this forum to maintain my B&O treasures.

Sincere regards,

Sverre

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