Author Topic: Repair sencore pa81  (Read 337 times)

0 Members and 1 Guest are viewing this topic.

Offline Bargainhunter

  • Contributor
  • Posts: 46
Repair sencore pa81
« on: April 29, 2020, 01:23:14 am »
I'm working on repairing my sencore pa81.  When turned on, the DC Balance left and right light.  I do not get sound from the built-in pa81 speakers or meter indication when an amp is connected to the input dummy loads.  I started with the power circuit board. I replaced the 12v regulator and the -5 volt regulators.  I've attached the schematic of the power supply section.  Adjusting R2 varies the voltage very little.  I get 13.6v not 12v. Input is 15.3v.  Also in the upper right section of the schematic, it looks like wein bridge oscillator driving two pairs of npn & pnp transistor each with -5v and +5v regulators.  Could someone explain this circuit?  I assume it may be associated with the dc balance circuit?

Thank you for your help.
 

Offline coromonadalix

  • Super Contributor
  • ***
  • Posts: 2967
  • Country: ca
Re: Repair sencore pa81
« Reply #1 on: April 29, 2020, 01:26:13 am »
It acts as a switch mode power supply   you have 2 transformers

The IC5 aka  TLO61 is the oscillator

Ic2  is a switched-capacitor voltage converters, it take a positive voltage and create a negative voltage for ic3,   like icl7660 icl7661 family

In the end they created a complicated supply for nothing  :palm: to reduce the cost of a bigger xformer with more / added secondaries voltages, or add an isolated psu section ?

You end up with 3x +5volts, 3x -5volts and a +12volt
« Last Edit: April 29, 2020, 02:49:59 am by coromonadalix »
 

Offline pigrew

  • Frequent Contributor
  • **
  • Posts: 591
  • Country: us
Re: Repair sencore pa81
« Reply #2 on: April 29, 2020, 02:45:47 am »
I'm working on repairing my sencore pa81.  When turned on, the DC Balance left and right light.  I do not get sound from the built-in pa81 speakers or meter indication when an amp is connected to the input dummy loads.  I started with the power circuit board. I replaced the 12v regulator and the -5 volt regulators.  I've attached the schematic of the power supply section.  Adjusting R2 varies the voltage very little.  I get 13.6v not 12v. Input is 15.3v.  Also in the upper right section of the schematic, it looks like wein bridge oscillator driving two pairs of npn & pnp transistor each with -5v and +5v regulators.  Could someone explain this circuit?  I assume it may be associated with the dc balance circuit?

Thank you for your help.

13.6V sounds like the float-charge voltage of a lead-acid battery... that could very easily be correct.

I normally wouldn't use 1N4148 as a power rectifier as their max current is somewhat low (<500 mA), but maybe these are supposed to be low-power supplies? It looks like the transformer is specified for 40 mA.

As coromonadalix said, it's a higher-frequency low-ish power isolated switching converter. It makes an isolated +/- 5V for the left channel and an isolated +/- 5V for the right channel. In addition, there is a non-isolated +Vbattery, and +5 and -5.

In the end they created a complicated supply for nothing  :palm: to reduce the cost of a bigger xformer with more / added secondaries voltages, or add an isolated psu section ?
They can't just use more taps on the AC mains transformer since they want it be be powered from a +12V lead acid battery. The have to do switching in order to generate the negative voltages, and they want isolation for the L and R channels (perhaps to avoid ground loops).
 

Offline coromonadalix

  • Super Contributor
  • ***
  • Posts: 2967
  • Country: ca
Re: Repair sencore pa81
« Reply #3 on: April 29, 2020, 02:49:23 am »
Youre right Pigrew   i forgot the battery section  my bad   |O
 

Offline Bargainhunter

  • Contributor
  • Posts: 46
Re: Repair sencore pa81
« Reply #4 on: April 29, 2020, 03:44:42 pm »
Thank you for your help.
I don't have a service manual, just the parts list and schematic.
I realize now that when the schematic shows +12V it means that it connected to a 12V source, not the actual voltage.  In this case it's 13.6.

I'm not sure why I can't adjust the voltage by R2.  According to my math, I should be able to adjust the voltage from 13.75 to 16.34 but it varies little from 13.6 volts. 
Is R4/R5 affecting the control of R2?

It would be a great help if I could find a service manual showing test points with actual voltages and some sort of calibration.

Thanks again, I appreciate all your help.
 

Offline coromonadalix

  • Super Contributor
  • ***
  • Posts: 2967
  • Country: ca
Re: Repair sencore pa81
« Reply #5 on: April 29, 2020, 05:25:37 pm »
If your lm317 voltage input is not high enough  like 2 volts over the needed output voltage, maybe it can't regulate properly ?

You could try with say 14vdc 15 vdc if you have some  psu / ac to dc adaptor ??  laying around ??


R4 and R5 goes to the ac power indicator connector,  maybe a light bulb or a led ??  or a voltage signal to an mcu / controller ?  since thay are high ohms, it cant create a problem ...   unless the lm317 is toasted or get very hot ? or the battery is short or bad.

Or  passing thru R7 100ohms there is an low impedance / a short on the batt pin connector ?

have you tested   lm317 output  pin2 to ground  impedance  while everything off no battery ?
 

Offline Bargainhunter

  • Contributor
  • Posts: 46
Re: Repair sencore pa81
« Reply #6 on: April 29, 2020, 06:36:37 pm »
I have the board disconnected.  I applied +15.3 volts to pin 2 which is connected to CR1 and the negative to pin 1 which is ground. There is no load except R4 adn R5 going to ground.  I measured 2.4k to 2.6k ohms depending on the setting of R2 from pin 2 of lm317 to ground (no voltage connected).

With 15.3 voltes applied to pin 2 and pin 1 of P1, I get 13.2v out from pin 2 of LM317 with very little change when adjusting R2 (+/- .1V).  I did check R2 out of circuit and it check ok.  I replaced LM317 and C3 and C4.  When I received the unit C10 had exploded so I replaced it along with IC4.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf