BeoGram CD5500 bridge rectifier diodes are very hot

[Wilhelm Tell]

Hi all,

I’ve been messing a bit around with a CD5500 that I got for cheap. I love the DAC and sound, so I’d like to add it to my audio stack.
I’ve now added nice RCA plugs and I’m now also able to use my mobile phone as a remote control via an ESP32 web server and the datalink80 interface.
I’m now in the process of recapping.
One of the things I noticed that the ripple on C13 (4700uF) is significant with 1.4V. It’s not terrible, but it is something to investigate.

I then noticed that the rectifier diodes (D11-D14) before C13 are getting insanely hot. I think at least 80degC already after 1 or 2 minutes of playing. The soldering spots of those diodes also don’t look like they will survive many more playing hours.
A simple solution would be to replace those diodes with Schottky diodes, which should be a very big reduction of energy cooked off in the diodes, however, this energy will then be cooked off by the LDOs, which have a heat sink.

Still, i don’t feel comfortable with this. Calculating back from the ripple on C13, I estimate the current drawn here is somewhere in the direction of 1A, which seems quite normal for what is connected to it. 1A on those diodes (1N5401) seems quite modest and do not explain why those things are getting so hot. An explanation could be that the transformer has become very ‘soft’, and that bad inductances stress the diodes around the reverse point an exceptional lot.

So, a few questions:

• What are the experiences with recapping of C13 and the ripple voltage? Is my 1.4V anywhere in range?
• anyone also noticed the very hot diodes, and what did help?
• anyone replaced with Schottky, and what are the experiences?
• are bad transformers a thing?

thanks.

[Wilhelm Tell]

No one any hints or experiences?

[Mark-sf]

I’d start with replacing C17 as that is too much ripple for a 13vDC line. Are the diodes mounted above the the board? If not then that heat is not normal. How much power is the who player drawing when playing? It should be no more that 20-25W.

[Dillen]

Schottky diodes would make no difference to speak of.
And they have no place in power supply rectifying anyways, that’s not what they are meant for – or good for.

Check the filter capacitors – also any smaller ones sitting in the analogue circuits.

Martin

[Wilhelm Tell]

I’ve done a full high quality recap now, except for the servo PCB.

Diodes are still getting hot. The ripple remains about 1.4V on the big C13 4700uF cap. Doing some basic calculations indicate that the current consumption of the CD player powered from this cap should be about 0.6A, which I think is not a lot, given that the MCUs, the SAA7210, SAA7220 and TDA1541 are all running from this cap.
This 0.6A would mean an average current of 0.3A per diode, which should probably only heat them up very little.

• This reply was modified 2 weeks, 4 days ago by Wilhelm Tell.

[Wilhelm Tell]

Mark-sf wrote:

Are the diodes mounted above the the board?

No, they are touching the PCB. Is there a copper layer acting as a heat sink?

[Wilhelm Tell]

Dillen wrote:

Schottky diodes would make no difference to speak of.
And they have no place in power supply rectifying anyways, that’s not what they are meant for – or good for.

Well, Schottky diodes would have 200mV less voltage drop, resulting in much less energy burned off in the diodes. However, this then obviously shifts to the power regulators, but these are on a heatsink and are currently not as hot as the diodes.

I’m still a puzzled though, because my estimate for the forward current is only about 0.3A. An explanation could be that the reverse current is very high, maybe because of aging. However, I’d not expect this to happen to all of them, or at least not in the same rate, but my finger says they are all about the same temperature.

[Dillen]

Put to work with any current to speak of – as in rectifier circuits, Schottky diodes can become quite unreliable, and they
will lose their Schottky properties.
They are meant for much finer workings, high-speed switching etc. That’s where they are fantastic.

Martin

• This reply was modified 2 weeks, 3 days ago by Dillen.

[Wilhelm Tell]

On D11 I measure this voltage over it. It has this weird notch.
D12 has it a smaller notch. D13 and D14 don’t show it.

[Author]

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