Marantz 2270 Amplifier Overload Trimpots

we set the idle current and offset voltage , but there are two more trimmers for setting the power threshold but nothing in the service manual on how to adjust these last two pots,

maybe run a signal into a dummy load and adjust for 70 watts per side?

it would be easier to set them by base voltage, maybe 0.4 volts like we see on a working unit?

or calculate power with 45 volt rails, how much current will flow through the 0.2 ohm sensing resistors , how much voltage will be transmitted to the base by addition to the voltage already on the bases via the trimmers?  ie  70 watts / 90 volts =  0.78 amps  into the  0.2 ohms =  0.155 volts added to 0.40 volts = 0.555 = transistors turn on?

thanks for any help!

70W/8ohm=3A eff = 4.3A peak
for Re=0.2ohm,  Vre=Ipeak*Re=0.86V
for Vbe=0.65V and R782=1.5k, VR782=0.21V, Isense= 0.14mA, R794=Vbe/Isense=4k6

The better way is to use a scope and limit te output voltage waveform when it is greater than about 35V peak on 8ohms load.

looks like we have a voltage divider in the mix with those 6.8 k resistors off the + and - 7 volt zener string,

CJ said:

we set the idle current and offset voltage , but there are two more trimmers for setting the power threshold but nothing in the service manual on how to adjust these last two pots,

maybe run a signal into a dummy load and adjust for 70 watts per side?

it would be easier to set them by base voltage, maybe 0.4 volts like we see on a working unit?

or calculate power with 45 volt rails, how much current will flow through the 0.2 ohm sensing resistors , how much voltage will be transmitted to the base by addition to the voltage already on the bases via the trimmers?  ie  70 watts / 90 volts =  0.78 amps  into the  0.2 ohms =  0.155 volts added to 0.40 volts = 0.555 = transistors turn on?

thanks for any help!

Click to expand...

That schematic is not very easy to follow but could those be V-I limiting trims?  If they have some PS rail voltage fed in they may be protecting power devices against secondary breakdown (a product of voltage and current)... and also including some foldback for more current output when voltage across the power devices is lower.

I have no obvious idea how to optimize them and that trimpot topology gives way too much adjustment range (IMO)... between full off (zero protection) and max clamping... maybe the design was still a work in process... If the amp still works with trimpots set for max protection (wipers to junction of both trimpots) perhaps a good place to start.

JR

CJ said:

looks like we have a voltage divider in the mix with those 6.8 k resistors off the + and - 7 volt zener string,

Click to expand...

We have also NTC-R string paralleled to the trimer.
The voltage protection kicks in only if the output voltage is too high. From my experience, overcurrent protections is much more important so I always adjusted trimers this way on few 2270 and similar receivers I serviced in the past.
But, if you like to ruin your day, please download the service manual from here
http://www.opweb.de/en/model.php?id=1697
connect a high grade 6uF capacitor to the output and use yours two dual trace scopes to look into lissajous curves
(page 24 in the manual)

got a 2270 in and measured the base voltage for the clamping transistors at around 0.40 volts

marked up the schemo,

CJ said:

got a 2270 in and measured the base voltage for the clamping transistors at around 0.40 volts

marked up the schemo,

Click to expand...

That sounds about right for current limit devices below when they start limiting.

JR

some versions do not have the pots,

you can dial out differences in hfe on output devices with the pots,  i bet they started matching transistors so they could eliminate the pots,

The emitter degeneration resistors R788/R789 should force rough current sharing.

Trim may be for Vbe mismatch between H754 and H767.

or not

JR

[edit  BTW asymmetrical current limiting is not good as it can put DC into the speaker voice coil. Most designs don't trim this /edit]