• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Stereotech 1200 - Hum in both channels

S

special_20

Active Member
I have a Stereotech 1200. Over the winter it developed a hum on both the left and right channels. Hum is present:
  • with no sources connected
  • on multiple outlets and house electrical circuits
  • on multiple sets of speakers and wires
  • regardless of 2 prong plug orientation (again tested on multiple outlets and house electrical circuits)

DC offset is at 20mv (right) and 16mv (left). Note: procedure for and settings for DC offset and specs for DC offset are absent from manual; reading seem "within normal limits" per the DC offset sticky. There are no DC bias adjustments.

Hum seems generally volume dependent. Note: strangely on some positions of the volume pot, the hum seems to decrease and nearly go away. I did deoxit and thoroughly work the pot with no change in symptom (may be failing/degrading traces?).

I suspected it was the power supply issue. I have recapped both power supply boards and replaced filter caps (with the exception of a 470uf/80v which is still en route). All caps were same rated uf value as originals with upped voltage ratings. I fired up unit and tested with recap of each board. Hum is still there...

What are other potential culprits of 60hz humm? Power supply really seems to be the only "shared" source for a hum.

Is it possible for this to be transistors or diodes?

Any tips for checking dodgy grounds? Most grounds are soldered connections to case and look intact and are still shiny.

I have taken some readings on both boards. Board 04-4470 does seem to be showing some odd low voltages. Amp however seems to be functioning nominally.

upload_2019-7-5_17-38-56.png
 
Register to hide this ad
You may want to check your resistors and diodes. I restored my tube preamp, after which I had a 60Hz hum in one channel. The issue was narrowed down to the power supply board, which has been recapped. I proceeded to check all the diodes, which checked out ok. I found a couple of bad wire traces which I jumpered. I also found four resistors, two of which did not match the schematic resistance rating and the other two tested open. I replaced all four with properly rated metal films. I decided to test the preamp and it is apparent the hum has significantly reduced in volume. I plan on removing the board again as there were still additional resistors which did not read as they should.
 
Make sure everything is grounded. Make sure all solder connections are shiny. Make sure all the screws are tightened.

When you say it happened in winter, I can only assume humidity (or lack of) and/or moisture based cause. Could be a noisy diode or rectifier. If one diode opened on a bridge, it could cause hum.
 
special_20 said:
What are other potential culprits of 60hz humm?
Click to expand...
Did you measure/confirm that ot's 60Hz mains AC hum?
Could it be 120Hz rectified AC hum.

60Hz hum could be power supply components, ground connections, poor wire layout,,,
120Hz could be regulated power supply, cap,,,,

Volume dependent suggest hum is entering before the vol pot, both channels suggest power supply...
 
mbz said:
Did you measure/confirm that ot's 60Hz mains AC hum?
Could it be 120Hz rectified AC hum.

60Hz hum could be power supply components, ground connections, poor wire layout,,,
120Hz could be regulated power supply, cap,,,,

Volume dependent suggest hum is entering before the vol pot, both channels suggest power supply...
Click to expand...

as for frequency, I don't have a scope to scientifically confirm. I did compare the hum on the speakers to a 60 and 120hz tones on my tone generator app. I'd say it's a 120hz hum.

I have recapped the power supply boards and main filter caps already (with one exception).

I did do one additional step in trouble shooting this morning. I did pull the pre-amp//amp jumpers. With pre-amp removed, no hum. I think we can rule out the main amp power supply, filter caps and amp board. Seems like a pre-amp related problem. I'll follow Avionics suggestion of checking some of the resistors here this weekend.
 
You said you are still waiting on the 470 uF/80 volt cap. I wouldn't be surprised if replacing that gets rid of the hum. That would also help explain why the voltages are low in that section.
 
special_20 said:
Will Check as yes, low voltage was suspicious.
Click to expand...

R831: Nominal value: 68 ohms. Measured value: 66 ohms
R832: Nominal value: 5.6 kohms. Measured value 5.6 kohms

Diodes D831 and D832 test good.

Transistor TR831 tests good/not shorted.

I reflowed all new capacitors.

I also reflowed the back of the pins to make sure I'm getting good readings. Still seeing -55v at points 2 and 3. Getting ~29v at pin 6

As noted above, I did pull jumpers to separate amp and pre-amp. With amp removed from circuit, no hum. So more evidence problem is pre-amp related.
 
Ylli said:
You said you are still waiting on the 470 uF/80 volt cap. I wouldn't be surprised if replacing that gets rid of the hum. That would also help explain why the voltages are low in that section.
Click to expand...

Missed it on my original parts order somehow. Replacement is on order but will be a few more days. It's always that one part you don't have...
 
JOOC, put your voltmeter lead back on the + side of the 470 uF cap (board pin 1). Still see + 50 volts? Now switch your voltmeter to the AC range - what do you read?
 
Ylli said:
Yup, that's your problem. Should have probably <1 VAC at that point.
Click to expand...

Finally got time to put in a replacement for C853. Replacing this cap seems to have fixed the hum! I had to get a little creative as replacement was much smaller than original. I ended up zip tying it to the main power supply caps are relocating the ground. Build quality on this unit continues to impress me after buttoning it back up. All smiles here in Minnesota.
 
You must log in or register to reply here.
Back
Top Bottom