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Author Topic: Calling all switching power supply experts  (Read 315 times)
KA0ERZ
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Posts: 18




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« on: November 26, 2004, 10:21:50 PM »

Short story is that my Alinco DM-330MV went up in smoke, cause unknown, I replaced the obvious burnt parts and several surrounding parts.  Now all lights and meter works, voltage is adjustable as it should be, voltage under no load is 13.8, voltage under load (1-2 amps)drops to 6-8 volts and the "protect" light comes on momentarily then goes off.  The schematic is available on the alinco website.  Any ideas or know where there is a alinco users group or repair info on these supplies?

Thanks
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K1CJS
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« Reply #1 on: November 27, 2004, 04:17:30 AM »

Sounds like there is a bad part in the regulator circuit, possibly the regulator transistor itself.  It would be fine under no-load condition, but as soon as some kind of load were introduced, the part would malfunction.  Also, if the circuits were overstressed by the burn-out, some of the other seemingly good parts may have changed their value.

Not to question your repair, but are you sure the parts you replaced were all good, and were the exact replacement parts needed?  Sometimes a simple accidental wrong value substitution may happen--with the results you specify.

Anyway, good luck.
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N6AJR
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« Reply #2 on: November 27, 2004, 12:43:27 PM »

awwwwwww,  ya let out the factory smoke.. now you have to spend lots of money to find the part with no smoke.. replacing factory smoke is expensive....Smiley
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KA5N
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« Reply #3 on: November 27, 2004, 01:43:38 PM »

Repairing switching power supplies is not an easy proposition.  Many of the parts are extremly critical especially the diodes many of which will be fast recovery (or ultra fast recovery)types.  Standard rectifier diodes are not substitutes.  Regulation is accomplished by the circuit varying the width (duty cycle) of the switched primary current.  In most supplies this is done by an IC.  Since you got the supply to put out voltage at the correct level I suspect that you might have a bad transformer or the input voltage is too low.  If the transformer is bad it will get hot even at a very light load.  Check the input voltage as you vary the load.
Allen
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N2MU
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Posts: 10




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« Reply #4 on: November 27, 2004, 09:14:31 PM »

I had the same problem with my power supply about 2 years ago.  If you had a problem with D4, D5 or D6 - check C29 and C30 too.  I don’t recall specifics of my problem, but I believe that this portion of the circuit provides feedback control for regulation and current limiting.  Some of the circuitry is brutal on the electrolytic caps because of the high frequency of operation and the internal resistance of the caps.  An easy check is to touch the heads of the caps while the supply is idling at full voltage and see if they are hot.  Be careful though they can get hot when their internal resistance is too high.  If you have to replace any caps see if you can find higher temp caps. (105 degrees C instead of 85C)  

For convenience to other readers, here is the link to the schematic info for Alinco products.
 http://www.alinco.com/pdf.files/
73  Hank  N2MU   ...-.-   ..
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WA1RNE
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« Reply #5 on: November 28, 2004, 10:12:28 AM »

 
 It would help if you provided the list of parts you replaced including their designations from the schematic.

 For now, I would look start with a few checks in the output circuit, then the HV switching areas:


 Output side:

 1) Check the (6) 0.1 ohm shunt resistors R50-R56. If one or more are open or have increased in value from being orverloaded, the overcurrent shutdown circuit will trigger too quickly - at lower currents like you are describing. A good tip off will be to set the meter to read current and see if it reads a much higher value - like 10-20 amperes when the supply is loaded @ only 1-2 amps.

 2) Check the DC resistance of inductor L3;  it should be very low, like 0.05 ohms tops.
 
 HV side:  *** Careful, this section is not isolated from the line, so it's common ground "floats" above earth ground potential.
 
 3) If the blown components you replaced were associated with the high voltage switching circuit, I would also check thermistors TH1 and TH2. When the supply has been off for several minutes, these devices increase in resistance and act as an "automatic" means of slowly charging the filter capacitors on power up to protect the input bridge rectifiers. After the filter capacitors are charged and the supply begins drawing current, they decrease their resistance to a lower value - like 1 ohm or so.  If they are blown and their resistance stays high, there will not be enough primary side voltage to support an increasing load due to their IR drop and the supply will go out of regulation.  

 4) Also check capacitor C16....

 5) Check for unobvious stuff, like loose grounds, smoked ground returns on or off the pc board, etc.

  Hope this helps....

 73, Chris
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KA0ERZ
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Posts: 18




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« Reply #6 on: November 28, 2004, 04:16:58 PM »

List of replaced parts:  

Obvious damage:  D7, D8 and D9.  All other parts looked normal.

Assuming damage extended in that general area, I replaced: C32,R24, D10 and D11.  The online schematic may differ from the actual board parts #'s.

Temperature:  C15 and C5, the two large ones, are exactly the same temp at idle, room temp.

               T3 at idle is 91F..... T2 is 71F (problem or does T3 take load first then T2?)

The amp meter at 1/2 amp draw and 2 amp draw indicates accurate reading.  Goes into protect mode at 2 amps, making me think current-limiting circuit.

The voltmeter is verified accurate and now will drop to 6-8 volts under 2 amp load then eith slowly rise to 13.8 under load or quickly rise to 13.8 under load.
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KA0ERZ
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Posts: 18




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« Reply #7 on: November 28, 2004, 04:17:17 PM »

List of replaced parts:  

Obvious damage:  D7, D8 and D9.  All other parts looked normal.

Assuming damage extended in that general area, I replaced: C32,R24, D10 and D11.  The online schematic may differ from the actual board parts #'s.

Temperature:  C15 and C5, the two large ones, are exactly the same temp at idle, room temp.

               T3 at idle is 91F..... T2 is 71F (problem or does T3 take load first then T2?)

The amp meter at 1/2 amp draw and 2 amp draw indicates accurate reading.  Goes into protect mode at 2 amps, making me think current-limiting circuit.

The voltmeter is verified accurate and now will drop to 6-8 volts under 2 amp load then eith slowly rise to 13.8 under load or quickly rise to 13.8 under load.
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WA1RNE
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« Reply #8 on: November 28, 2004, 06:08:25 PM »


 OK....before we get into anything more complex, on a hunch, check diode D4 with an ohmmeter to see if it's shorted.

 This diode, in series with R24 (15 ohms) allows the main DC output to connect to the housekeeping supply, probably to help get the supply stabilized on power-up. The purpose of the diode is to isolate the low current housekeeping supply (approx. 12-18 volts dc) from the output, otherwise it would attempt to drive high output load currents which it cannot do.

 But if D4 is shorted, it will cause the *housekeeping* supply to drive whatever load is connected to the main output PLUS 15 ohms. Since the housekeeping supply is a low current unregulated supply, it will finally run out of power and the supply will start shutting down. Interesting that 13.8 vdc driving 15 ohms = ~1 amp, which is approximately the current you say the supply starts to trip out at.

 If D4 is not the problem, you need to check the housekeeping supply and reference regulator for the PWM chip IC1. Use a DVM and/or a scope - which you will definitely need if you need to start looking at switching waveforms.

 *Housekeeping supply:  Check at both ends of R25 referenced to output ground. should be `12-18 vdc. check with supply running no load then with load applied.

 *Reference Regulator:  Check at junction of R18 (22k) and C24;  should be 5 vdc +/- 0.25v

 
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KA0ERZ
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Posts: 18




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« Reply #9 on: November 29, 2004, 08:58:30 PM »

D4 on my diode checker shows .464 forward biased and .971 reverse biased.  1.1k ohms in circuit.  Before I go further, is D4 good or bad?
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WA1RNE
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Posts: 823




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« Reply #10 on: November 30, 2004, 01:53:54 PM »


 There are several electrolytics and other paths that could be causing the 0.9 volt reading so I would either lift one end of D4 or one end of R24.

 While reviewing the parts you replaced again -which are all part of the housekeeping converter- there are some others that may also be suspect:

 *Q10, Q11, Q9, R38, R30, R31 and D5

 All of the parts in this circuit along with T3 make up a low power 12-18 volt multivibrator-type dc-dc converter to provide the housekeeping supply. The output is fed to IC1 via D5 and to the fan and LED's through D6.

 After that, go on to the housekeeping voltage checks as I described before.

 
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