• Please note that there are a few updates and clarifications made in the Audiokarma Rules, mostly relating to advertising and the addition of the new "Paying it Forward" & "Giving back" forums in the AudioKarma Audio Marketplace section.

Kenwood KA-7300 Power Supply Issue

G

Gravis

New Member
Dear Members,

I'm struggling with asymmetrical AC output voltage on power transformers which works in dual mode configuration. Transformer feeding left channel generates 36V AC while the other feeding right channel generates 40V AC. After rectifier left channel gets 44V of DC voltage while right one 59V! Left channel power amplifier works with offset ca. 20-25mV while right one 18-20mV. However the radiator of DARLIGTON TA80W for right channel is much hotter that the left one so definitely 59V is issue here. On top of it when switching off the power offset voltage on both channels raises quickly up to 5V and the goes down slowly below 1V. Then further drop of offset is in very very small steps (few mV /s).

Any clue and help will be kindly appreciated.
 
Register to hide this ad
Hi,
it would help if you specify in which points of the schematics you measured the voltages.
The transformer secondary should measure 33-0-33 VAC, that is 66VAC if you place the voltmeter probes in point 1-3 (transformer T1) and 6-8 (transformer T2).



ka7300.jpg
 
Dear Claneg,

Thank you for your quick response. T1 1-3 = 81V, T2 6-8 = 71V. Please notice that I'm from Poland so power network voltage is 220-240V from the socket.
 
T1 secondary is too high: consider to check if the problem is in the transformer itself or the primary connections. I would disconnect the DK1-2 from the PCB, and recheck the voltages 1-3 and 6-8. If they are the same, probably the transformers are OK. If not, I would check the transformers primary connections and the voltage selector switches. Your version should be the "W type".


ka7300-2.jpg
 
Definitely yours is a W TYPE. With power off, check if the switches need a cleaning (measure resistance). Also check that the primary cables are connected as in the picture (W type).
 
After disconecting dk1, 2 I got 76VAC on both transformers. It’s handly possible they are both faulty in exactly the same degree.
 
So you get 76 Vac instead of 66 Vac: that is possible since there's no load!
What you get from this measurement, is that both transformers acts equal: I think they are both good.
Nexth is to check what happens after the bridges with partial load: lift one leg of Rk1-2 and anode of Dk3. Doing so, you are just powering the power amp section: measure how much volts are on point 2-4 and 7-9. Should be around 100 Vdc.
 
Dear Claneg,

You don’t how much I appreciate your kind support. I will follow your intructions but first I need to rebuild soldering on pcb as I damaged pads when removing bridges. They were sticking strongly to the pins and it was very hard to unsolder them safely. For the moment I’m happy to know that transformers are both fine. Have a great evening.
 
So I started to prepare more working area for resoldering bridges by removing R1 & R2. There are installed originally 270 & 180ohm while in SM there are 560 & 330ohm respectively.
 
When I switched off amplifier I still measure constant 10VDC on pins 2-4 and 7-9. Seems like main filter capacitors can be faulty. I did already full recap of power supply but w/o filters.
 
I resoldered Dk3 and kicked on amplifier, Relay clicked on quickly and after that I measured again on pins 2-4 100VDC and on pond 7-9 106VDC. I assume protection circuit is not an issue. Then I resoldered Rk1 & Rk2 and old problems of assymetrical AC/DC came back. On top of it I measured 27VDC on pin 12 and -32VDC on pin 15. Can Rk2 causing it?
 
You were right, I replaced Zeners and now I got symmetrical 25-26VDC. I disconnected again Rk1 & Rk2 to cut off 24V section. Checked again VDC after the bridges and it came back to acceptable values +/-50VDC at pins 2-4 while +/-52VDC at pins 7-9. Then I reconnected Dk3 in order to verify measurements at protector section. Voltages measured:

Dk3: anode 38 VAC; cathode 31 VDC (should be 19 VDC acc. to SM)
Dk4: anode 12 VDC: cathode 31 VDC same as Dk3
Dk7: anode: 52 VDC, canthode 52 VDC

Qk1: B 0,2 VDC, E 0 VDC, C 12,8 VDC (seems all OK to me)
Qk2: B 1,9 VDC, E 0 VDC, C 52 VDC

Now the question is how it is possible that 52VDC is measured at Qk2 C while SM manual states 1V. It is exactly the same voltage value as measured at pin 9. I suspected Dk7 so I removed it and measured but seems to be ok.
 
Ummmm, looks like Dk7 failed short. What voltage do you measure at Dk7 terminals (not refered to ground)? Should be around 24Vdc, that is the relay voltage.
In the thread I posted before, they suggest to change Qk2 (prone to failure).
 
You must log in or register to reply here.
Back
Top Bottom