Beomaster 5500 Intermittent Noise at Certain Volume Levels

2 replies
TR
TrackerJack
Bronze
4 posts
Posted February 1, 2019 at 23:22 GMT
#2118153

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.

DI
Dillen
BronzeModeratorFounder
12777 posts
Posted February 26, 2019 at 06:48 GMT
#2118154

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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