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| Topic | Replies | Views | Last post | |
|---|---|---|---|---|
| Bench supply hums at low load New | 31 | 1,204 | Today, 08:47 PM by anselm | |
| Calibrating a moving-coil meter without a standard Solved | 18 | 942 | Today, 05:30 PM by m.teague | |
| Signal generator drifts for the first hour Awaiting parts | 7 | 265 | 13 Mar 2005, 09:14 PM by tobias_k | |
| Mains isolation: read before you ask Locked | 0 | 6,318 | 02 Jan 2005, 10:00 AM by m.teague | |
| Replacement probes for an old valve voltmeter | 4 | 187 | 11 Mar 2005, 02:41 PM by dovecote | |
Started by quietfuse on 14 Mar 2005. 31 replies, 1,204 views. New
I have a twin 0 to 30 V bench supply from the early seventies on the bench. Above about half an amp it is quiet. Below that the transformer hums loudly enough to hear across the room, and there is 40 mV of ripple on the output.
Rectifier diodes test fine. The reservoir capacitor is original and reads 3,900 uF against a marked 4,700. Is that enough to explain it, or should I be looking at the regulator?
One fault at a time.
quietfuse wrote: The reservoir capacitor is original and reads 3,900 uF against a marked 4,700.
Capacitance is only half of it. Measure the series resistance as well; a thirty year old electrolytic can read nearly full value and still be useless. I would replace it before touching anything else.
Hum that gets worse at light load usually points at the regulator hunting, not the transformer. Check the small compensation capacitor across the error amplifier while you are in there.
Put the model number in the title, please.
Same supply here, same fault. On mine it was the minimum load resistor across the output, which had gone open. Without it the series pass transistor cannot regulate down at low current and the loop sings at mains frequency.
It is a 2 W wirewound part under the rear terminal strip. Discharge the reservoir before you reach in.
Are you sure you want to delete post 32 by anselm? This cannot be undone.
In the topic Bench supply hums at low load. Posting as quietfuse.
Signed in as quietfuse. Member since August 2003. Last visit: 14 Mar 2005, 07:02 PM.
| Subscribed topic | Replies | Last post | |
|---|---|---|---|
| Bench supply hums at low load New | 31 | Today, 08:47 PM by anselm | |
| Which lacquer for a walnut cabinet? Sticky | 62 | 13 Mar 2005, 04:18 PM by pendle | |
| Calibrating a moving-coil meter without a standard Solved | 18 | Today, 05:30 PM by m.teague | |
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