Hi Workbenchers!

I just restored a Beomaster 6000 output stage (right channel burnout). After replacing the four main IC's and putting in new trimmers on both sides, I decided to fire it up on the bench separated from the BM using two regulated power supplies. After spending about three hours to figure out that one needs to connect the ground leads of the main power plug and the two signal plugs (i.e. P34 to P32/31) in order to have the correct voltages on the DC network of the entire circuit (oh the joys of learning something new...;-), everything seems to work properly. I adjusted the no-load and offset currents to specifications. Now each of the channels draw about 0.18A at ±50V. So far so good! Letting it run for some time, I noticed that the two power transistors on each side (TR210/110 and TR211/111) get pretty hot. All four reached about ~62±5C after 10 min, i.e. they felt quite hot to the touch. By then the temperature remained stable, though.

I was wondering if this is allright, or if I missed something??....(I did exchange all electrolyte caps, and ,remarkably, two 1/2W resistors R155/255 that were both OC and had burn marks on them - got to love ebay purchases labeled "was not able to test"...;-).

Thanks much in advance!

Rudy

180mA does sound like a lot.
I would've expected around 50mA or something like that for a reasonable idle current,
are you sure that 180 mA is correct ?

Are the amplifiers fed a signal (maybe just noise) or are the inputs grounded ?
With no signal, the output stage transistors should normally not get warmer than your hand
regardless of the amount of time they are powered on.
I would say around 30 deg C.
62 deg C is unpleasant to the touch, not good for long time idle and points towards a
too high idle current or some kind of signal is being amplified.
Have you checked for self-oscillation ? It can sometimes happen when testing an amplifier
out of its normal working condition (lack of external power decoupling etc.).

Martin

Hi Martin,

Thanks much for your (as always) valuable input! With regard to the drawn current (as measured on the two power supplies' amp meters), my BM 8000 output stage drew 100mA when bench testing it after replacing the TIPs in one channel. Here I got 180mA, but for both channels...so maybe not all that bad. I am certain that I adjusted the two trimmers to the right specs (i.e. 22mV on the test points and 0V DC on the output), but I did not hook up an oscilloscope to see if there is 'self-oscillation'. With regard to the transistor temperature: The output transistors are not getting very warm. They are just pleasantly 'hand warm'...what gets hot are the heat sink tabs of the 110/111 and 210/211 transistors (mounted on the PCB, one in the constant current source the other in the signal path before it goes to the output transistors). Thanks!

Rudy

PS.: Nothing was connected to the signal inputs except jumpers to connect the signal entry PCB ground traces of both channels to the main PCB ground connected to the power supply plug. This seemed o.k. to do since the respective pins of the plugs (currently dangling out of the receiver...;-) are directly connected when measuring resistance between the pins

I never noticed the driver transistors becoming warm like that but I may not have felt much on them.
I still feel that the current is a bit on the high side but I don't have one to compare with at present and
it might be correct.
What type of bipolar caps did you fit ?

Martin

I used TIP 141/146 that I still had lying around from my BM 8000 "fry"...They are listed as alternates for the BDV64/65s that were in there...I hope that was not stupid. Seems the TIPs run at the same temp as the BDVs that are still in the other channel...Unfortunately one cannot easily measure the currents going into each side individually...

oops...caps: I used standard aluminum electrolytics from newark.com. They seem to work nicely in my BM 8000...

In the meantime....I put the output stage back in and ran it. It seems it is working properly now...the BM sounds quite nicely. I noticed, however, that the power supply heat sink is getting pretty warm after a while. I checked the four electrolyte capacitors. They all have ~0.1R ESR, i.e. should still be o.k. (do not have the right sizes to replace them right now...). I am wondering, what is the best way to replace these strange three-negative-terminal-cans? Use axial ones and jumper the connected negative terminals?

The TIP 141/146 transistors should be fine.
I meant the bipolar electrolytic caps before.

It's quite normal for the Beomaster 6000 to run a bit warm at the
right side cooling fin where the power supply regulators are.
I use normal radial caps to replace the 4-pin caps in the power supply, fit the
positive lead to the center pad and the negative to one of the surrounding and
finally interconnect any negative pads in use.
If I remember correctly it's just a single connection on the BM6000 since most of
the pads are not connected.
You can then physically attach the cap to the board with a blob of glue or similar
to prevent the cap from vibrating and cracking its solder joints.

Martin

I ran it now for some time, and it seems things are stable...I also noted that when I run the A/C in our house the power supply heat sink is much cooler...I guess we could call that the 'Florida effect'...;-). Thanks again for your help and input!

Rudy