PL 700 ii Strange Bias and Offset !! ??
- Thread starter Greg
- Start date
- Tags bias offset phase linear phase linear 700 series 2 pl700ii
So after letting the PL 700 II sit idle all day I powered up again to test it did glitch and repeat conditions as stated above .0001 VDC Bias volts and held 28 VDC as Offset volts on the speaker out put terminals then after about 1 minute I cycled the power switch and all is fine again !!
I find the idle sit time to represent a CAP issue maybe ?
Noted and confirmed 14.7 VDC and 14.8 Vdc across each of the 15 VDC zener diodes
Im going to guess at replacing the caps on the PL 36 quick and easy Not sure how to catch the issue ?
Other than absolute failure how does one find the issue
I find the idle sit time to represent a CAP issue maybe ?
Noted and confirmed 14.7 VDC and 14.8 Vdc across each of the 15 VDC zener diodes
Im going to guess at replacing the caps on the PL 36 quick and easy Not sure how to catch the issue ?
Other than absolute failure how does one find the issue
So a quick Update after retesting about 8 times over the last few days since last post I cannot replicate the issue of 28 VDC Offset on both channels and near zero Bias voltages both channels !! So I am going to replace the electrolytic Caps on the PL 36 board and the OP amps with a pair of socketed LF356N s ...
this is a copy paste from IC supplier
Description
The LF351N are high speed JFET input single operational amplifiers which are incorporating well matched, high voltage JFET and bipolar transistors in a monolithic integrated circuit. The device needs a low supply current and still maintains a large gain bandwidth product and a quick slew rate. It’s pin compatible with the standard quality LM741 and it uses the same offset voltage adjustment circuit system. This feature of it permits the designers to right away upgrade the overall performance of the prevailing LM741. The LF351N has a low noise and offset voltage drift, but for applications where these needs are critical then instead of using LF351N , the LF356 is employed. Also ,if maximum supply current is important, then LF351N is better choice. It can handle a maximum power supply voltage of ±18V.It has a operating temperature ranging from 0℃ to +70℃ and storage temperature range of -65℃ to +150℃.The input voltage range of this device is ±15V . It is available in 8-DIP package.
Description
The LF351N are high speed JFET input single operational amplifiers which are incorporating well matched, high voltage JFET and bipolar transistors in a monolithic integrated circuit. The device needs a low supply current and still maintains a large gain bandwidth product and a quick slew rate. It’s pin compatible with the standard quality LM741 and it uses the same offset voltage adjustment circuit system. This feature of it permits the designers to right away upgrade the overall performance of the prevailing LM741. The LF351N has a low noise and offset voltage drift, but for applications where these needs are critical then instead of using LF351N , the LF356 is employed. Also ,if maximum supply current is important, then LF351N is better choice. It can handle a maximum power supply voltage of ±18V.It has a operating temperature ranging from 0℃ to +70℃ and storage temperature range of -65℃ to +150℃.The input voltage range of this device is ±15V . It is available in 8-DIP package.
See the attached service bulletin and think twice about the 356. Don't even consider dropping in one of the newer audio grade opamps. Check R101 (7.5K/2W) on each channel. I've seen several of thes open up = full DC on ouput. Check the ripple frequency and ripple voltage on the DC caps. This should be done with a true RMS meter.
I do Have a scope i just got a tektronix TDS 1002B and have a audio signal generator on order for some more testing