Rowland Model 5 Power Supply Questions


2010-10-12 5:42 pm

I own a Rowland Research Model 5 amplifier which recently developed an intermittent problem. Occasionally, the unit enters into "protect" a few minutes after power up and shuts itself off.

Close examination revealed the left channel was developing around -15vdc across the speaker output terminals. This was measured with inputs and speakers disconnected.

Schematics are not available, but I was able to discern the following:

The encapsulated input (voltage gain) section for each channel has it's own power supply which supplies both + and - 90vdc. These power supplies are always energized and the power switch only energizes the output section.

When initially applying power to these 90vdc supplies, the output is only ~85vdc. This slowly climbs up to ~90vdc after 30 minutes or so...This seems abnormal.

I found the left channel -90vdc was about 2vdc lower than the positive side so I adjusted it to match.

The bias is running around 0.660-0.670 after about 30 minutes, I left it untouched.

Since the small voltage adjustment, the unit has not entered protection. I'm dubious at best that the small voltage correction actually helped.

I'm thinking about replacing the smaller electrolytic capacitors, there are only around 20 or so in the entire amplifier.

Your thoughts?



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The bias is running around 0.660-0.670 after about 30 minutes, I left it untouched.
0.660 what?
volts, amps, oranges, apples?

I too doubt that adjusting a 90Vdc supply down to 88Vdc would remove the output offset.
15V is bad and that is why the amp protection triggers.

See if you can replicate the conditions that make it misbehave.
Try a hair dryer to slightly warm up one heatsink. Try touching some components with a pencil.
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