frustrating power supply - Page 1
Forum section: electronics / repair

author: liviux
author_rank: Contributor
timestamp: March 23, 2016, 04:26:18 pm
content: HI, and hello for the first timeIn this pdf :http://www.jblproservice.com/pdf/PRX%20Series/PRX%20Series%20Amp%20Schematics.pdfis the schematic of the frustrating power supply (last page)In the 220v rectified sectio (up-right corder ) there is a voltage divider made by r610 and r612 24k 75k. the output goes to one series transistor witch sould output 30v and after that into a 15 v section with 3 series transistors in parallel.the thing is i am not getting 30 v under load but i get around 13 v slighty variating. and becouse at the output of the 15v section i have 12-14 v the tl494 does not start switching.if i feed from my bench power supply, the input of the 15v section with 30v i get a nice 15.6 at the ouput with a consumption of aroung 40-50 ma.please people help me understand whats going on ...i.ve checked and rechecked everything. i tried to start the main powersupply with +30v control feeded externally and it works. it just does not want to give me the jumpstart voltage because after the main switching starts i have a winding wich feeds the 15v section.sorry my englishliviux

author: Anks
author_rank: Frequent Contributor
timestamp: March 23, 2016, 04:52:21 pm
content: I would guess you have a weak resistor the breaks down when any amount of load is placed the n it but then I would expect it to get hot. I'll have a better look when I get off my train

author: liviux
author_rank: Contributor
timestamp: March 23, 2016, 05:02:25 pm
content: Hi, thanks.Original r610 24 k 2w was 26.something and chenged it with 22k 3 w olso tried 27k 3 w slighly better with 22k.

author: Anks
author_rank: Frequent Contributor
timestamp: March 23, 2016, 05:28:18 pm
content: Well I've just fixed one of these and if the supply to the switcher is not taking to much current I would say you have a bad voltage dvider. I may be talking out my arse as I'm commuting at the moment.

author: liviux
author_rank: Contributor
timestamp: March 24, 2016, 09:35:55 am
content: anyone ?

author: mij59
author_rank: Frequent Contributor
timestamp: March 24, 2016, 09:48:18 am
content: Did you check the resistors R656, R655, R644, transistor Q611 and zener D606 ?Disconnect one lead of the resistor to test it.

author: JFJ
author_rank: Regular Contributor
timestamp: March 24, 2016, 12:24:00 pm
content: Quote from: liviux on March 23, 2016, 04:26:18 pmA possible cause of that could be a failed diode in the RBV3506 bridge rectifier (D601) - thereby, only producing half-wave rectification. If that were the case, the loaded 30 V line would show a 50 Hz ripple (instead of the 100 Hz ripple expected from full-wave rectification) when observed with an oscilloscope.

author: liviux
author_rank: Contributor
timestamp: March 24, 2016, 01:06:47 pm
content: Quote from: JFJ on March 24, 2016, 12:24:00 pmHmm good ideea, i will check it .Thank you !

author: liviux
author_rank: Contributor
timestamp: March 24, 2016, 01:08:51 pm
content: Quote from: mij59 on March 24, 2016, 09:48:18 amYes checked them, i even drived the circuit with 30 v from outside and with bias on the base it opens and works ok, same 40ish ma

author: mij59
author_rank: Frequent Contributor
timestamp: March 24, 2016, 06:16:09 pm
content: Maybe D610 has a higher internal resistance, this is not necessary detectable with the diode test of the multi meter, or a bad cap e.g. C634.

author: liviux
author_rank: Contributor
timestamp: March 24, 2016, 11:19:00 pm
content: Quote from: mij59 on March 24, 2016, 06:16:09 pmChecked the obvious... I tried with d610 disconnected (since it provides power after the power supply starts (from the winding of the main switching transformer) and olso with the electrolitic cap changed.I am inclined to think the posibility of the bridge rectifier being the culprit ... but not yet checked.

author: liviux
author_rank: Contributor
timestamp: March 25, 2016, 06:19:44 am
content: i just finished searhing again, bridge is measuring fine.i want to give more info...R610 (24k) si getting hot and the resulted voltage is variating slightly. is 40 ma too much for that voltage divider ? should i try to lower the value ? but it seems not a good idea.

author: mij59
author_rank: Frequent Contributor
timestamp: March 25, 2016, 06:34:36 am
content: Quote from: liviux on March 25, 2016, 06:19:44 amThe start up power supply is not the problem, something is dragging the +15V Control down, or not enough voltage is generated by T601 at start up.Maybe the IGBT's or the driver circuit.Edit: low transformer voltage due to bad capacitors C612, C613 C631.

author: EPTech
author_rank: Regular Contributor
timestamp: March 25, 2016, 09:55:47 am
content: Hi there,You are right about the operation. The supply uses the divider and the circuit around Q610 to provide an initial power supply for the TL494 and the gate driver. 12-14V should be enough for the TL494 to start up. Do not be fooled by the 15V. It is not supposed to be a fixed voltage. The 17V zener simply prevents the 15V from going any higher than +- 15.8V. This voltage also serves as a feed-back voltage for the supply through the auxiliary primary winding.So when the TL494 receives its 12-14V you should see some start-up behavior in the form of low duty-cycle pulses being generated on the supply. Be careful in measuring on the primary side, not to cause any connections from live to ground. Plug the supply into an isolated mains or use a high voltage differential probe on the scope when trying to probe the primary side.If you are not getting these pulses, the TL494 maybe wasted but it may also be the secondary size is over-loaded or shorted. The shutdown of the amp (power-up timer in previous page) should prevent any high currents from being drawn as long as the supply is powering up. If a high current is drawn or the secondary is shorted, the feed-back loop mey not be able to close.

author: liviux
author_rank: Contributor
timestamp: March 25, 2016, 10:21:25 am
content: Quote from: EPTech on March 25, 2016, 09:55:47 amHi ! And thanks for the reply.I have to correct you...in this situation the tl494 is not using any kind of voltage feedback from the winding, but it gets power from it after igbts are starting to switch( and shuts off the q610 trough q611 wich shorts the zenner)Tl494 only receives a fixed 250khz from the generator in the low side(to be in synch with the class d operation i presume.If i power the tl494 and fan7380 combo with external power they begin to work after 14 v and stop working under 12...so with external 15 v (floating) the main power supply starts up and working (though some transformer switching noise is present but maybe thats how it works) so...whats the deal ? It gets me ...i finished repairing the class d part and digital part and now stuck on the powe supply ...ugly business.Edit: after reading tl494 and fan7380 sheets the 494 vcc is said to be from 7 to 40 v and fan7380 is from 10 to 20 so my 12 v should be ok ...hmmLiviu

author: EPTech
author_rank: Regular Contributor
timestamp: March 25, 2016, 10:40:19 am
content: Yes indeed, I have seen that.They use a fixed 250kHz instead of a jittery clock like used in many supplies for better EMC performance. A jittery clock could induce unwanted audio artefacts.Note that besides powering the control circuit, the 15V control voltage, after being divided by R613 and R614, is also compared to Vref by the TL494.I believe you when you say the unit starts working at 14V and stops below 12V but that is part of the feed-back scheme they use. The switch noise you hear is the TL494 trying to restart the supply but being unable because there is not enough voltage coming from the auxiliary primary winding to create the feed-back loop. May be something as stupid as C634 not being able to store the energy. But my first guess was excessive load on the secondary side.In SMPS everybody is a suspect.