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This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
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.
Hi,
for Q1: To avoid influence of the Penta's i would check first whether the noise problem still exists or not. This way you could determine the component causing the issue.
for Q2: Normally those amplifiers are set to 11mV across the Emitter resistors. This way the heatsinks should stay cool. But replace the trimpots in advance!
for Q3: the lithium battery is seated in the metal can between the front plate and the main chassis. In this metal case is the microprocessor.
I hope this helps a bit :)
Ralph-Marcus
Thank you, Ralph. Your suggestions certainly helped resolve my issues. As I had posted this question on the General Discussion & Questions forum and also received an answer there from Nick (solderon29), I'm going to post what I did to resolve the issues here in case someone else can benefit. Here's what I ended up doing to repair this system:
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 previous 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.