Repair sencore pa81 - Page 1
Forum section: electronics / repair

author: Bargainhunter
author_rank: Regular Contributor
timestamp: April 29, 2020, 01:23:14 am
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: April 29, 2020, 01:26:13 am
content: It acts as a switch mode power supply you have 2 transformersThe IC5 aka TLO61 is the oscillatorIc2 is a switched-capacitor voltage converters, it take a positive voltage and create a negative voltage for ic3, like icl7660 icl7661 familyIn the end they created a complicated supply for nothingto 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

author: pigrew
author_rank: Frequent Contributor
timestamp: April 29, 2020, 02:45:47 am
content: Quote from: Bargainhunter on April 29, 2020, 01:23:14 am13.6V 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.Quote from: coromonadalix on April 29, 2020, 01:26:13 amThey 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).

author: coromonadalix
author_rank: Super Contributor
timestamp: April 29, 2020, 02:49:23 am
content: Youre right Pigrew i forgot the battery section my bad

author: Bargainhunter
author_rank: Regular Contributor
timestamp: April 29, 2020, 03:44:42 pm
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: April 29, 2020, 05:25:37 pm
content: 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 ?

author: Bargainhunter
author_rank: Regular Contributor
timestamp: April 29, 2020, 06:36:37 pm
content: 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.