MC Preamp no gain when DC Power Applied

[pulsed_power]

With no B+
RI & LI = 75mv pp
RO & LO = 2v pp

With B+ and no change in signal value from signal generator
RI & LI = 3mv pp
RO & LO = 300mv pp

How can a amp circuit create gain with no B+ and then when B+ is applied the gain drops to such low levels?

[TimFox]

When asking about such an apparently absurd result, it behooves you to post more details about your measurement.
1.  How do you connect the generator to RI/LI?
2.  How do you measure the voltage at RI/LI?
3.  What frequency do you measure at, and is the output at the same frequency (i.e., no important noise or oscillation)?
4.  How do you measure the voltage at RO/LO?  Are your voltage measurements true differential, or does it ground LO?

[pulsed_power]

When asking about such an apparently absurd result, it behooves you to post more details about your measurement.

1.  How do you connect the generator to RI/LI? BNC to RCA to inputs
2.  How do you measure the voltage at RI/LI? Scope
3.  What frequency do you measure at, and is the output at the same frequency (i.e., no important noise or oscillation)? 1khz
4.  How do you measure the voltage at RO/LO?  Are your voltage measurements true differential, or does it ground LO? scope

[pulsed_power]

More photos

[pulsed_power]

With power on

[pulsed_power]

Input is shown on channel 1 TOP
output is shown on channel 2 Bottom

[pulsed_power]

75mv in and 2v out with no B+

[TimFox]

Is that a x10 probe measuring the output?
The screen-indicated scale on the CRT indicates the voltage with a x1 input, not having scaled with probe connection.
Therefore, your output may be 10 times higher than the value shown on the scope.

[pulsed_power]

Set on X1

[pulsed_power]

The only change taking place is the B+ being turned off and on

[pulsed_power]

The function of this board is to take the low output voltage of a Moving Coil to the level of a Moving Magnet Cartridge

[pulsed_power]

Spec sheet

[TimFox]

Those specs are incomplete.  What voltage gain do you expect (apparently flat over audio range, with RIAA in the following MM preamp)?
I don't know what is causing your apparent high gain with no DC applied, but I would keep looking for a problem.
Without a transformer, you cannot have a voltage gain > 1 with no DC applied.
For example, what are you using for a generator, what is its output impedance, what happens when you connect the generator directly to your probe without the circuit?
What do you see at the output of the amplifier with no generator, DC on and off?

By the way, using a grounded-base input circuit for a low-Z MC cartridge will work, but there is no improvement in noise performance over a grounded-emitter (normal) amplifier.
The equivalent input noise voltages of a CB and CE amplifier for a given transistor and operating point are equal.
Your circuit is essentially a feedback inverter, with a low-frequency gain determined by R467 and R463 (approximately), and has R467 (180\$\Omega\$) in series with the MC source, degrading its thermal noise.

[pulsed_power]

Those specs are incomplete.  What voltage gain do you expect (apparently flat over audio range, with RIAA in the following MM preamp)? MC is 300uV/10ohm, looks to me like the MM input is 3mV/50k
I don't know what is causing your apparent high gain with no DC applied, but I would keep looking for a problem.
Without a transformer, you cannot have a voltage gain > 1 with no DC applied. It is defying logic at all levels
For example, what are you using for a generator, what is its output impedance, what happens when you connect the generator directly to your probe without the circuit? During those measurements it was at 2v
What do you see at the output of the amplifier with no generator, DC on and off? Zero

By the way, using a grounded-base input circuit for a low-Z MC cartridge will work, but there is no improvement in noise performance over a grounded-emitter (normal) amplifier.
The equivalent input noise voltages of a CB and CE amplifier for a given transistor and operating point are equal.
Your circuit is essentially a feedback inverter, with a low-frequency gain determined by R467 and R463 (approximately), and has R467 (180Ω
) in series with the MC source, degrading its thermal noise. I wish I understood that in greater detail

[TimFox]

The logical answer to the values you report with B+ off is that you have somehow connected 2 V of 1 kHz to the output, and see 75 mV at the input due to the resistor ratios.
Your reported 75 mV is roughly explained by the resistors R467 = 3900\$\Omega\$ in series with R463 = 180\$\Omega\$ feeding R461 = 220\$\Omega\$
(3900+180+220)/(220) = 19.6;  2000 mV / 19.6 = 102 mV
Of course, with the power off the other impedances/resistances around that network are not well determined, but that ratio looks suspiciously close to me.

By the way, your photo obscures the output impedance label on the generator panel:  is it 600 or 50\$\Omega\$ , or something else?
600\$\Omega\$ will make a difference, given the low input impedance of your amplifier (roughly 100\$\Omega\$).

[pulsed_power]

I have thought about that

[pulsed_power]

Internal wiring photo

[wasedadoc]

The logical answer to the values you report with B+ off is that you have somehow connected 2 V of 1 kHz to the output, and see 75 mV at the input due to the resistor ratios.

I agree.

On the 6 pin connector the two outer pins are outputs inputs, the two centre are power. The remaining two are inputs outputs.

Edit: Corrected after looking at the photos using a desktop PC with large screen instead of a diminutive phone. 

[pulsed_power]

The logical answer to the values you report with B+ off is that you have somehow connected 2 V of 1 kHz to the output, and see 75 mV at the input due to the resistor ratios.

I agree.

On the 6 pin connector the two outer pins are outputs, the two centre are power. The remaining two are inputs.

This is where a sketch is worth a thousand words.

[TimFox]

If you think you have connected the generator to the input properly:  with the power off,  take the probe (from the other CRO input) and move it to the circuit board and see what's on the junction of R461 and C461.

[TimFox]

The logical answer to the values you report with B+ off is that you have somehow connected 2 V of 1 kHz to the output, and see 75 mV at the input due to the resistor ratios.

I agree.

On the 6 pin connector the two outer pins are outputs, the two centre are power. The remaining two are inputs.

On the conductor side of the circuit board, the outer pins are labeled LI and RI (inputs) and the intermediate pins are labeled LO and RO (outputs).
Tracing the circuit to see if those labels are correct is left as an exercise for the reader.

[pulsed_power]

R461 doesn't junction to C461

[TimFox]

It does on the wiring diagram you posted at the top of this thread., at node "LI".
Your excerpt has two parts labeled "R463".
Do you have better documentation?
If not, you have to trace the circuit yourself.

[pulsed_power]

It does on the wiring diagram you posted at the top of this thread., at node "LI".
Your excerpt has two parts labeled "R463".
Do you have good documentation?

Those damn Yamaha schematics are very inconsistent from page to page. I have the high res version from Analog Alley.

[pulsed_power]

It does on the wiring diagram you posted at the top of this thread., at node "LI".
Your excerpt has two parts labeled "R463".
Do you have better documentation?
If not, you have to trace the circuit yourself.

Please also see reply #15