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Beomaster 5500 Intermittent Noise at Certain Volume Levels

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TrackerJack
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TrackerJack posted on Fri, Feb 1 2019 11:22 PM

Greetings!  New silver member here.  I'm working on a friend's Beomaster 5500, Beocord 5500, and Beolab Penta Type 6633 components.  This is my first encounter with B&O equipment repair although I'm comfortable working around electronics.

Problem: The Beomaster 5500, with or without input source signals, will emit a hiss/hum though both Penta speakers when the volume level is at 26 through 30, then quiet at 32 through 42.  The noise returns between volume level 44 through 50.  It sounds like an FM off-station static with 60Hz low-level hum. 

Visual Observation: Resistors R226 and R426 are visibly discolored due to high heat and should be replaced.  The board area under resistor R34 shows signs of high heat but the resistor is nominal and the discoloration may be leftover from a previous repair.

Additional Background:  One of the Penta speakers was activating its protection circuitry and would not operate.  Troubleshooting revealed a burned open resistor R106 on the output from the amp to the drivers.  This probably indicates a very large high-frequency signal was present.  Replacing the resistor cured the problem.  The circuit in the Penta is similar to the circuits in the Beomaster where R226 and R426 are currently overheated.

Question 1: Has anyone encountered this intermittent volume noise problem before?  I went through the forums and did not see this issue addressed. 

Question 2: I also notice that the rear heat sink gets very hot which from what I've read indicates the need for the idle current to be adjusted (the cooling fan appears to operate normally).  I downloaded the Service Manual from this site but it only has schematics, no parts callout or maintenance procedures.  Is there a procedure for the idle current adjustment in another manual or could someone share the procedure here, please?

Question 3: While scanning the forum posts on the Beomaster 5500 I noticed a lithium battery mentioned a couple of times.  I do not see this shown on the schematic nor have I seen it in the unit.  Is there, in fact, a lithium battery in this unit and if so, where is it located?

Thank you for your help.

Answered (Verified) Verified Answer

solderon29
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U.K.West Midlands
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Verified by TrackerJack

It's possible for the output stage to become unstable and act like an oscillator even.This can result in "spontaneous combustion/destruction" in severe cases.

This might explain the odd noises and the overheating too,but it's also very common for the quiescent (idling) current presets in the output stage to become intermittent due to oxidisation.

I imagine that other enthusiasts here will concur,that it would be best to replace the preset's and the electrolytic capacitors in the output stage as a matter of course,due to the age of the unit anyway.

There are also some troublesome capacitors in the pre-amp,that can contribute to this sort of problem.

The manual onsite for the Beomaster 6500 show's the set up procedure for the output stage which is virtually the same as the '5500.

The lithium battery is housed inside the control processor module,which is the enclosed metal box at the front of the unit.The battery is there to provide a back up supply for the processor in case of power outage.

It's not a rechargeable battery,and is soldered to the pcb.They are readily available still.

Regards,

Nick

TrackerJack
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THANK YOU, Nick!  I'll incorporate your suggestions and report back what the actual issue/issues were so others may benefit.

TrackerJack
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Beomaster 5500 Issue Resolved: 

I took Nick's advice and using my Sencore LC77 Auto-Z Capacitor - Inductor Analyzer, removed and tested every electrolytic in the Beomaster 5500.  As expected there were several bad electrolytic caps on the Output Amp & Power Supply board...11 of the small caps had failed their tests but all small electrolytics on the circuit board were replaced.  Burned resistors R231 and R431 were also replaced with the same values but larger 1 watt sized parts to prevent any future burning.  The idle current presets R226 and R426 were also replaced with new components.

Per an entry in Beocentral's Notebook regarding the Beomaster 5500, a 100nF cap was installed across the B-E junction of 2TR8 (BC557B). https://beocentral.com/notebook/beomaster-5500-3/

All small electrolytic caps in the pre-amp and tuner boards, as well as the cooling fan board, were removed, tested, and replaced.

The lithium battery was good and was not replaced.

The unit was powered up and adjusted per the Beomaster 6500 instructions and adjusted perfectly.  The previously temperature-of-the-sun heatsink now runs cool to luke-warm during normal operation.  The Beomaster 5500 works flawlessly now when reinstalled in the system with its newly repaired Penta 3 speakers.

Beolab Penta 3 Speaker Issue Resolved:

The rotted mid-range foam surrounds in both Penta 3 speakers were replaced with new foam surrounds.

Both Penta 3 speakers had burned 2.2-ohm resistors (R106) on the outputs to the crossover/driver section, likely from the huge amount of noise coming from the Beomaster 5500.  On one Penta, R106 was burned open and the protection circuitry kicked in to save the unit from self-destruction.  On the other Penta, R106 overheated and changed its value from 2.2-ohms to over 200K ohms which still allowed the unit to continue to function without kicking in the protection circuitry.  Both small wattage resistors were changed but using larger wattage resistors to prevent any future burning should a noise issue present itself again.  

One Penta's Input Level, Mode, and Bass Extension slide switches broke after 25+ years and would move freely back & forth without any switching action actually taking place.  I disassembled the Control Board and either glued the small plastic nubs that were still in the case or fabricated new ones and glued them into place.  Very tedious work but it was successful and the switches function like new again.

The Pentas were reassembled and tested with the freshly repaired Beomaster 5500 and functioned like brand new.

Beocord 5500 Cassette Deck Issue Resolved:

The distorted drawer rollers were replaced with new rollers purchased from Martin Olsen at Beoparts.com (THANK YOU MARTIN).  While in the unit the belts were also replaced.  The spindles were disassembled, cleaned, and lubricated.  The drive motor had a dead-spot which caused intermittent start-up.  The higher current draw of the motor overheated resistor R65 and also caused the protection circuit to kick in.  I disassembled, cleaned, lubricated, and reassembled the motor.  The drive torque was abundantly better after doing that.  R65 was replaced with the same value but a larger 1-watt sized resistor.  The tape deck was cleaned, reassembled, and tested.  All functions performed nominally.

 

 

All Replies

solderon29
Top 150 Contributor
U.K.West Midlands
679 Posts
OFFLINE
Gold Member
Verified by TrackerJack

It's possible for the output stage to become unstable and act like an oscillator even.This can result in "spontaneous combustion/destruction" in severe cases.

This might explain the odd noises and the overheating too,but it's also very common for the quiescent (idling) current presets in the output stage to become intermittent due to oxidisation.

I imagine that other enthusiasts here will concur,that it would be best to replace the preset's and the electrolytic capacitors in the output stage as a matter of course,due to the age of the unit anyway.

There are also some troublesome capacitors in the pre-amp,that can contribute to this sort of problem.

The manual onsite for the Beomaster 6500 show's the set up procedure for the output stage which is virtually the same as the '5500.

The lithium battery is housed inside the control processor module,which is the enclosed metal box at the front of the unit.The battery is there to provide a back up supply for the processor in case of power outage.

It's not a rechargeable battery,and is soldered to the pcb.They are readily available still.

Regards,

Nick

TrackerJack
Not Ranked
5 Posts
OFFLINE
Silver Member
Verified by TrackerJack

THANK YOU, Nick!  I'll incorporate your suggestions and report back what the actual issue/issues were so others may benefit.

TrackerJack
Not Ranked
5 Posts
OFFLINE
Silver Member

Beomaster 5500 Issue Resolved: 

I took Nick's advice and using my Sencore LC77 Auto-Z Capacitor - Inductor Analyzer, removed and tested every electrolytic in the Beomaster 5500.  As expected there were several bad electrolytic caps on the Output Amp & Power Supply board...11 of the small caps had failed their tests but all small electrolytics on the circuit board were replaced.  Burned resistors R231 and R431 were also replaced with the same values but larger 1 watt sized parts to prevent any future burning.  The idle current presets R226 and R426 were also replaced with new components.

Per an entry in Beocentral's Notebook regarding the Beomaster 5500, a 100nF cap was installed across the B-E junction of 2TR8 (BC557B). https://beocentral.com/notebook/beomaster-5500-3/

All small electrolytic caps in the pre-amp and tuner boards, as well as the cooling fan board, were removed, tested, and replaced.

The lithium battery was good and was not replaced.

The unit was powered up and adjusted per the Beomaster 6500 instructions and adjusted perfectly.  The previously temperature-of-the-sun heatsink now runs cool to luke-warm during normal operation.  The Beomaster 5500 works flawlessly now when reinstalled in the system with its newly repaired Penta 3 speakers.

Beolab Penta 3 Speaker Issue Resolved:

The rotted mid-range foam surrounds in both Penta 3 speakers were replaced with new foam surrounds.

Both Penta 3 speakers had burned 2.2-ohm resistors (R106) on the outputs to the crossover/driver section, likely from the huge amount of noise coming from the Beomaster 5500.  On one Penta, R106 was burned open and the protection circuitry kicked in to save the unit from self-destruction.  On the other Penta, R106 overheated and changed its value from 2.2-ohms to over 200K ohms which still allowed the unit to continue to function without kicking in the protection circuitry.  Both small wattage resistors were changed but using larger wattage resistors to prevent any future burning should a noise issue present itself again.  

One Penta's Input Level, Mode, and Bass Extension slide switches broke after 25+ years and would move freely back & forth without any switching action actually taking place.  I disassembled the Control Board and either glued the small plastic nubs that were still in the case or fabricated new ones and glued them into place.  Very tedious work but it was successful and the switches function like new again.

The Pentas were reassembled and tested with the freshly repaired Beomaster 5500 and functioned like brand new.

Beocord 5500 Cassette Deck Issue Resolved:

The distorted drawer rollers were replaced with new rollers purchased from Martin Olsen at Beoparts.com (THANK YOU MARTIN).  While in the unit the belts were also replaced.  The spindles were disassembled, cleaned, and lubricated.  The drive motor had a dead-spot which caused intermittent start-up.  The higher current draw of the motor overheated resistor R65 and also caused the protection circuit to kick in.  I disassembled, cleaned, lubricated, and reassembled the motor.  The drive torque was abundantly better after doing that.  R65 was replaced with the same value but a larger 1-watt sized resistor.  The tape deck was cleaned, reassembled, and tested.  All functions performed nominally.

 

 

Dillen
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Copenhagen / Denmark
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Dillen replied on Tue, Feb 26 2019 6:48 AM

Good job!

One comment though, it's not a good idea to upgrade resistors wattage to keep them from burning.
Some resistors are designed to act as fuses and burn as a means of "controlled handling" of things gone wrong.
If currents are high enough something WILL burn. If the resistor has been changed to resist the load then something else
will burn instead, and it may not be as easily controlled (fire hazard) - or repaired.
Resistors working in positions like this are often (but not always) marked with a small triangle with an exclamation point in the
schematics (safety component) and MUST be replaced with one of the same type (typically non-flammable) and same rating.

In speaker crossovers it's not uncommon to see resistors darkened from heat.
This is not necessarily a sign of something very bad (and often the resistor will be fine for future use), it's merely a sign of a speaker
having lived a loud life.
If the resistor has burned to the point or losing continuity, it will most likely have saved a tweeter.
If, when you replace the resistor, you "upgrade" the resistors wattage, next time the tweeter will burn instead.

Martin

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