- Joined
- Aug 23, 2023
- Messages
- 11
Hi all, Appologies in advance for the long winded post but I'm having trouble (or I think I am) with 2 PL 400 Series 2 amplifiers. I have 2 of them and they are both acting differently when I bring them up on a variac. Both had shorted output transistors (caused by apparent over tightening of the outputs and cutting through the Silipads) and both were replaced with MJ21195’s and the remaining good RCA driver transistors were left in the 2nd and 4th positions(from the transformer).One of the 400’s is an earlier production Serial # in the 24000 range and the other is in the 29000 range.
When going through the start up procedure The 29000 ser. amp doesn’t begin outputting a sine wave until around 55 VAC on the variac whereas the earlier production does at around 30 VAC as outlined in the start up procedures posted online. This is the same with or without the output transistors installed. I have replaced Q 101,105-201,205 on the PL36 board as well as the two Zener diodes and all the electrolytic caps. I have also tried new bias transistors with zero difference.
With no output devices installed I have equal clipping with 1 kz sine wave once I get to 12vpp input on both channels(both amplifiers also). When I install the output transistors I still have nothing in the way of signal output until I reach around 55 VAC on the variac but it does seem to operate correctly at that point, it seems to draw a lot of current until the signal starts whereas the earlier production amp does not.
I have 3 amp fuses in the power supplies of both and I brought this amp up to 120 VAC and have good sine wave at the outputs and once I get past around 60-65 V on the variac the DC offset settles in at 5mv and 3mv. The thing that concerns me is the difference in the start-up voltage and the Idle current also sets up differently on both amplifiers as well. On the Earlier production amp the idle current isn’t adjustable down to 350mv via reading the voltage drop across R128 and R130 but I can adjust it from 0 past 8mv on the emitter resistors(.33 ohm) to achieve 25ma as stated in the online start up procedure but on the newer production amp I can achieve 350mv across R128 and R130 which does not allow me to achieve 25ma or 8mv across the emitter resistors, it's .00 untill Signal and volume turned up otherwise I would have to get to about 500mv across R128 and R130 get there.
My question is… Is this just a difference in the production time between the two amp’s or do I have a problem? I've never had two different production runs side by side before. They are clearly different but are both PL36 boards. The older amp has the LF356 and the newer one has the LF351 IC’s. If I remember correctly these draw more current than the LF356?
Both of these 400’s can be run at to at least 100W power into 8 Ohm speaker loads with no problems or clipping read at the speaker loads and draw 2.5 amps AC. They both have almost zero DC offset , 6mv or less. Any help or advice on this would be greatly appreciated.
When going through the start up procedure The 29000 ser. amp doesn’t begin outputting a sine wave until around 55 VAC on the variac whereas the earlier production does at around 30 VAC as outlined in the start up procedures posted online. This is the same with or without the output transistors installed. I have replaced Q 101,105-201,205 on the PL36 board as well as the two Zener diodes and all the electrolytic caps. I have also tried new bias transistors with zero difference.
With no output devices installed I have equal clipping with 1 kz sine wave once I get to 12vpp input on both channels(both amplifiers also). When I install the output transistors I still have nothing in the way of signal output until I reach around 55 VAC on the variac but it does seem to operate correctly at that point, it seems to draw a lot of current until the signal starts whereas the earlier production amp does not.
I have 3 amp fuses in the power supplies of both and I brought this amp up to 120 VAC and have good sine wave at the outputs and once I get past around 60-65 V on the variac the DC offset settles in at 5mv and 3mv. The thing that concerns me is the difference in the start-up voltage and the Idle current also sets up differently on both amplifiers as well. On the Earlier production amp the idle current isn’t adjustable down to 350mv via reading the voltage drop across R128 and R130 but I can adjust it from 0 past 8mv on the emitter resistors(.33 ohm) to achieve 25ma as stated in the online start up procedure but on the newer production amp I can achieve 350mv across R128 and R130 which does not allow me to achieve 25ma or 8mv across the emitter resistors, it's .00 untill Signal and volume turned up otherwise I would have to get to about 500mv across R128 and R130 get there.
My question is… Is this just a difference in the production time between the two amp’s or do I have a problem? I've never had two different production runs side by side before. They are clearly different but are both PL36 boards. The older amp has the LF356 and the newer one has the LF351 IC’s. If I remember correctly these draw more current than the LF356?
Both of these 400’s can be run at to at least 100W power into 8 Ohm speaker loads with no problems or clipping read at the speaker loads and draw 2.5 amps AC. They both have almost zero DC offset , 6mv or less. Any help or advice on this would be greatly appreciated.
