[notwist]

TP14 (collector of TR7) measures -4.8 on a cold start but it starts dropping voltage immediately. When the voltage drops below -2.3 the audio also drops out.

The drop-out of the audio is much faster now. Just 2 minutes before it hits -2.3 and the audio drops out.

Bert

[notwist]

Problem persists on the Tape 2 channel. Just wanted to check because Tape 1 has a different layout in the circuit. But no, the problem is on both tape inputs and by extension all inputs.

This all reminds me really badly off that Revox machine I restored where a single damn lamp caused the entire system to mute. Took me hours to realise that was the failure origin. Of course, that was before the google years (which I would loved to have in that era).

Bert

[notwist]

As to the change of caps to film. I could not sleep so I took that time to do something worthwhile. I changed the caps in the muting circuit of the channel that I did not desolder the fet (108) back to electrolytics.

Effect: no change.

Bert

[notwist]

I connected my trusty Philips cassettedeck and can confirm that the audio drop out is also on Tape 1.

In all cases AFC is in the off position.

Bert

[notwist]

Hmm, I think I found 5C15 which appears to be a stray tantalum capacitor (failed to spot this one as I changed all tantalums). That one would have a higher value, likely 10uF. Can you confirm this?

TP14 seems to be behind the pre-amp board. I can take it out but perhaps you can confirm the location?

Bert

[notwist]

Can’t seem to find 5C15, but only the 2.2 and 1 uF capacitors are in film. Everything from 3.3 and above is Vishay electrolytic. When I read the schema the parts corresponding with 5 don’t have a capacitor (but I can’t seem to find C15, will look further).

The lamps all work fine. They are all the correct size and voltage as far as I remember (I inherited a bucket of these lamps). But in the interest of completeness will check them again of course. Might have made a mistake.

Let me check what voltage is provided on TP14. Just a moment ;-).

Problem isn’t an audio provider but the rca-din cable. I have recently cleaned my workspace and as we all know stuff you don’t regularly use gets hidden somewhere. But I remember I have an old Philips cassettedeck I restored and that is DIN so let me check that old beastie.

The FM signal is strong, we live 300 meters away from a big broadcast antenna. Can receive stations here on just a thin piece of wire. But I have an antenna in the beomaster (just a regular dipole). I can try another antenna if necessary.

Bert

 

[notwist]

So after a long time I think we can conclude it is the muting circuit. For the record, the other channel is still muted (only desoldered TR208). This is on the FM tuner. Will check the situation with a cdplayer or external tuner on that input.

Conclusion: after some time the mute circuit is triggered. But in a ‘delayed’ manner as the audio dies out gradually instead of immediately. Cold machine = working fine, issue appears after 20 minutes on a cold machine start.

Bert

 

[notwist]

 

Hmmm. Success?!?!?

Okay, I totally understand your point. It is just that I’m loaded with work and trying to ‘do’ my personal machine in between all tasks. Not having a clear working point makes it hard to use the time available in a constructive manner.

You are correct about the voltages but there are just too many points to test. I checked them all and there are no deviations to what is expected in the main voltage rail. Doesn’t mean downstream is problem free but the main, basic voltage providers are working as I expect them to.

Nevertheless, I was able to desolder two of the TR208 fet legs, it is no longer in circuit. I used an angled smd soldering iron and high grade flux to wick those legs under the metal bracket. It took some effort but in the end I was successful.

Now that channel is working problem free. At least the tuner has been running for 1 hour now without a hitch. I will now test if it still works on a higher volume (this test was on a low volume, about 2 on the slider). Will up to 4.

That confirms your hunch that the muting circuit is involved.

Question is, what the hell is actually going on. I would expect this not to be a failure per-se, but perhaps some sort of fine-tuning? Hard to say from the service manual.

Bert

[notwist]

The thermistor (R40) reads 50ohms. When I place my industrial worklight over it (that is pretty hot) it slowly, very slowly increases to 52-53 ohms. That seems like a very shallow response, the lamp is much warmer than the heatsink has ever gotten.

Unsure if this thermistor is still doing its work. Unsure whether the ohms reading is good. Nor, if it is indeed broken what replacement is suitable (any PTC thermistor?).

Bert

[notwist]

Hmmmm. I checked 208 / 108 and it is very difficult to desolder those fets. They are under the long metal bracket and to get to them I would need to remove the pre-amp and that bracket. Which is far from easy as I recall.

I also find it a bit hard to think that that circuit would fail for both channels. Not impossible  but highly unlikely that both channels would suffer the same ‘fate’ due to a failure. Capacitors were all changed into film caps. Leakage is possible but all audio, aside from the massive drop in volume, is still the same (balance, treble, etc). I would expect a different ‘effect’ on each channel. I also still don’t understand why the muting circuit would activate, what triggers it? Is is a certain voltage threshold?

So perhaps the muting circuit is activating, but I just have a hard time wrapping my head around that. Muting is instant, not a dying own effect. That dying down of the audio when the machine is ‘warm’ is what is bothering me, that appears to be of something gradually failing over time.

I just can’t wrap my head around it. I don’t have any clear paths I would take. I can blindly yank stuff out of the machine but that is just nonsense.

Sorry for the rambling but this is such an odd issue that I just don’t know where to start. I’ve already done the basics and those voltages and amps all check out.

Bert

 

 

[notwist]

Work and a giant backlog caught up with me the past days. I hope I will have some spare time to check more on the machine, it is too nice to let it turn to dust.

I did reflow many more joints but that didn’t do anything.

I would describe the issue as if a balloon is slowly emptying. It is as if ‘something’ is losing charge. When I power down, wait 5 minutes I get sound back but it dies of within 1 minute. If I wait a night and power it on it will give me sound for 20 minutes. Then the sound will die off slowly (it takes about 20 seconds to die to almost inaudible).

Although the muting circuit sounds promising, the fact that it is such an odd symptom makes me think if it is not a capacitor high in the system that is somehow losing charge. The volume never dies off completely, I can use the volume control to make it ‘louder’.

It feels as if something stops working for the most part after some time. It works when the machine is cold. It stops working for the most part when the machine ‘warmer’.

Bert

[notwist]

I tried finding intel in the service manual on the muting circuit but alas I can’t find any detailed information. At least not in my manual. This all reminds me of a Revox machine I have where the failure of one light bulb in the muting circuit would silence the entire machine. Good luck finding the source of that issue.

Can anyone point me to a service manual or description / circuit so I might learn about the muting?

Bert

[notwist]

I resoldered most of the contact points and terminals but this did not help.

I did a cursory review of voltages but they all seem to be OK. Nothing stands out as being faulty.

There is no heavy load or current draw. I’ve put the machine on my dim bulb tester as a quick method of this. I don’t think the voltage regulator is failing but I can install a new one if that is prudent. Should have some lying around.

I was able to trace the audio using my tracer. The audio is fine, clear and loud until TR100 / TR200 after which it drops some 80% in volume.

As pointed out, the muting circuit might be active. That would, IMHO, be a good explanation as to why both channels fail at the same time. Question is, why is the muting circuit active? What is the trigger?

Bert

[notwist]

Hi all,

good leads, today I had some spare time to reflow the major connection points. I should have done this before because of the known issues with weak solder (not brittle or broken) joints on these machines.

I’ve got nothing connected to the machine at the moment so I can only describe the issue from the FM tuner. Please let me find some DIN to RCA cables and check the issue with another input.

I will check the voltages this evening, should have the time.
Will also desolder the thermistor, if that is broken it might explain the issue as well. Then again, the heatsinks are not even lukewarm, bias etc is still ok. Will use my reflow station to heat it slightly to see the drop / rise in resistance.

If the 15v rail is failing, my question would be why? I’ve not often seen a failure on a power rail without some serious damage. And it would also stay broken, not degrade with time?

Thanks!

Bert

 

[notwist]

Hi all,

• When I serviced the machine all output transistors were replenished with new washers and thermal compound
• There is no hum at all and the filter caps register as fine (outside of circuit)
• Sorry, no FLIR camera but nothing is getting really hot on the board aside from the transistor with the giant ‘star’ heatsink (which I would expect to get hot). But it doesn’t seem to be failing and a failure here would be apocalypse on the power supply
• There is one (1) component which I think is an old thermistor. It is in the middle of the heatsink. On inspection I saw that it’s very long legs were touching. That is not good. But I don’t know how to test the bugger.

I will indeed check the soldering again. I don’t see any cracked solder but i’ve experienced tired soldering on almost all B&O machines. Problem is that this is very hard to see. Doing a complete reflow of the main board is a total pain to say the least.

Bert

[notwist]

Hmmm, opened it up and it seems I did not change the main filter caps. But they test allright on my ESR meter. Also I can’t see them failing at the same time with the same issue.

Bert

[notwist]

Hi Martin,

The whole pcb was reflowed and all connections cleaned. So it appears to be a faulty IC. Damn. That pcb is a crime to work on 🙁

Bert

[notwist]

Hi Martin,

Are you referring to the ad7110?

How would I proceed to determine the actual cause?  Just janking ics and replacing them seems a bit off. How do we determine what broke?

Regards,

Bert

[notwist]

Hi all,

sorry for the late reply. I have an eye disease and sadly I am sudenly almost blind.

I was able to repair the Beocenter by dismantling the entire cassette mechanism and relubing it all. Now with the replacement transistors it is all working fine again. It is likely to be my final repair 🙁

Many thanks for all the help,

Bert

[notwist]

Also, I am wondering what the hell actually happened. The deck was playing fine for some 30 minutes. Then it started to slow down and eventually stopped, causing a cascade blowing the two transistors and burning the resistor. I have this hunch that more is going on here than just a bit of lubrication.

Bert

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