700B meter

Michael F

Michael F

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#1
Since re assembling the amp last week, the left VU meter is acting strangely. On power up, it does no deflect as the right one does (at very low level) but “kicks in†after increasing the gain to that channel and then tracks properly in both minus 20 and 0 DB scales, and will continue to do so.
The left and right outputs are equal all the time though. Both needles move freely when amp is jostled so it does not seem to be sticking in the low end of the scale.
A White Oak LED board is installed so I'm not sure what lies between the speaker terminals and meter terminals.
TIA
 
Gepetto

Gepetto

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Michael F said:
Since re assembling the amp last week, the left VU meter is acting strangely. On power up, it does no deflect as the right one does (at very low level) but “kicks in” after increasing the gain to that channel and then tracks properly in both minus 20 and 0 DB scales, and will continue to do so.
The left and right outputs are equal all the time though. Both needles move freely when amp is jostled so it does not seem to be sticking in the low end of the scale.
A White Oak LED board is installed so I'm not sure what lies between the speaker terminals and meter terminals.
TIA
Click to expand...
A 22K and 1.8K in series along with a small signal diode in series with those 2 resistors coupled with an identical small signal clamping diode across the meter terminals when in 0dB mode.


Amp + output ---22K----1.8K----|<-------negative meter terminal
|
|
___
^
Amp - output -----------------------|---positive meter terminal


When it is in -20dB mode then a 1.8K in series along with a small signal diode in series with those 2 resistors coupled with an identical small signal clamping diode across the meter terminals


Amp + output ----------1.8K----|<-------negative meter terminal
|
|
___
^
Amp - output -----------------------|---positive meter terminal


The action of the 0/-20dB switch is to short out the 22K resistor

Clear?

Sorry HTML hates spaces so the clamp diode did not come out in the right place.
 
Last edited:
Michael F

Michael F

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Gepetto said:
A 22K and 1.8K in series along with a small signal diode in series with those 2 resistors coupled with an identical small signal clamping diode across the meter terminals when in 0dB mode.


Amp + output ---22K----1.8K----|<-------negative meter terminal
|
|
___
^
Amp - output -----------------------|---positive meter terminal


When it is in -20dB mode then a 1.8K in series along with a small signal diode in series with those 2 resistors coupled with an identical small signal clamping diode across the meter terminals


Amp + output ----------1.8K----|<-------negative meter terminal
|
|
___
^
Amp - output -----------------------|---positive meter terminal


The action of the 0/-20dB switch is to short out the 22K resistor

Clear?

Sorry HTML hates spaces so the clamp diode did not come out in the right place.
Click to expand...
:confused2::confused2::confused2::confused2::confused2::confused2:

Recipe for a Romulan cloaking device?
 
Phase Linear Phanatic

Phase Linear Phanatic

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LOL Lee! I've also had my share of that problem if you recall. Not the best meters. Fragile sometimes. You also have their age as a factor there.

Hey did you ever go about seeing what a bottom swinging 700 would look like with the 400 meters? I recall a discussion of that somewhere.........
 
Michael F

Michael F

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#14
Follow up

Replacing D52 and D53 did the trick. Both meters now track evenly in both switch positions.
If I could only remember which of those diodes were faulty.. in any case, problem solved.
 
laatsch55

laatsch55

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#17
Perhaps the bad diode came from that side?? Mike you'll love the new meter board, perks those meters right up!!
 
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