Kenwood KA-7300 - unsuccessful refurb.

[62vauxhall]

Perhaps how I carried out the measurements was incorrect. Because of it's small size the foil side diagram is hard for me to read even with a magnifier. Instead, I used the schematic to locate the resistor "ends" and transistor legs of which your test points were midway between. I got the same readings at both ends so I thought my method was viable.

In order to see it properly, I'll take the manual to a copy centre. Maybe I can scan it and print an enlargement?

EDIT: I am unsure what you mean by wrong reading and different reading. Would you mind clarifying?

 

[Northwinds]

Man brother, I admire you sticking to this project. If it was me, they would have indeed been recycled and the faceplates and knobs in my box of parts LOL. I also admire EW for sticking through this also

 

[gort69]

EDIT: I am unsure what you mean by wrong reading and different reading. Would you mind clarifying?

I took that mean that all of the greens should measure the same voltage and all of the reds should measure the same voltage, even if they aren't correct.

 

[62vauxhall]

Ah, I see. I'll revisit this (tonight if possible) to see if readings are consistant with yesterday's.

 

[62vauxhall]

NW:
No kidding. I wish I'd tried out the amp first, but no - for some reason I had to be cautious this time. If I'd have know, the switches, knobs & face plate would have been on e-bay.

I'll say the gamble is over when EW does lose patience.

 

[JimmyEJames]

NW:
No kidding. I wish I'd tried out the amp first, but no - for some reason I had to be cautious this time. If I'd have know, the switches, knobs & face plate would have been on e-bay.

I'll say the gamble is over when EW does lose patience.

Hindsight always 20/20 and I NOT giving grief but have no idea how not checking for basic function is being cautious?
I am lost can not follow did you ever get it to come out of protection EVER?
When you did the VERY FIRST headphone listen did relay click then?
I NOT trying to get in middle now this all you and Mr. EchoWars I just thinking if you install problem or not and maybe can go back?
Go find $2 speakers for testing anything better than nothing
Hope you get it solved!
Sorry
JJ

 

[62vauxhall]

You'll note that the 'groups' are all on the same electrical potential...there's no reason for them to be different. Wrong reading, I'll buy. Different reading, nope.

Power down, discharge the caps, and get a resistance reading to prove that those 'groups' are essentially an equal-potential point.

Prior to attempting a resistance reading, I re-measured the voltages and this time there is improved consistency. I am not going to speculate why I only got +.574 volts at the top red left point the first time, However, I still obtained only -12.5 volts at the right green point.

top red left: +.9.2 volts
top red right: +9.3 volts

left green: -15.0 volts
middle green: -15 volts
right green: -12.5 volts

bottom red left: +9.2 volts
bottom red right: +9.2 volts

This may or may not be worth mentioning but as I re-measured bottom red left, I sampled voltage at both ends of Ri15. The side opposite to that which read -9.2 volts was -.6 volts and grasping that leg with a mini-grabber produced radio reception.

 

[EchoWars]

Power down and do a resistance measurement between all the green points. They all need to be the same. If not, you need to find out why.

Might also try to measure the 'printed' resistors and see if they are close to what they need to be. The printed resistors are all shown on the schematic with a line under them.

Lastly, I think it would be a good idea to summarize the problems you are having in as specific a manner as possible, and what has been done to the unit to address it.

 

[62vauxhall]

I made those resistance measurements twice, last night and again this morning so I'm reasonably sure they are accurate. Not good, if they are supposed to be the same.

Left green to mid green: 47K Ohms
Mid green to left green: ~ 1 Ohm
Left green to right green: 47K Ohms

Checking each printed resistor will take longer than I can devote on a weeknight since there are only 6 hours between the time I arrive home and get up for for work. I will purse this on the weekend provided I am not offered any additional work.

Strictly concerning the control board. what I did so far was change every electrolytic capacitor (including the non-polarized ones) and swapped out the transistors. I don't have my notes with me, they're at home, but replacement NPN transistors were 2SA970 and NPN: 2SC2240.

Did you mention that the control board was double sided? I had an experience with one not long ago. It was an add on DC protection board with a suspect relay. Not knowing it was double sided or that it had "pass through connectors" in the holes, I inadvertently damaged it when I re & re'd the relay. If this control board is double sided with those pass throughs perhaps it to suffered damaged. A lot of original components were taken off and new ones mounted.

 

[JimmyEJames]

What is double sided?
JJ

 

[62vauxhall]

Power down and do a resistance measurement between all the green points. They all need to be the same. If not, you need to find out why.

Might also try to measure the 'printed' resistors and see if they are close to what they need to be. The printed resistors are all shown on the schematic with a line under them.

Lastly, I think it would be a good idea to summarize the problems you are having in as specific a manner as possible, and what has been done to the unit to address it.

Excluding radio reception when touching the volume control, the problem is substantially weak output on the left channel.

To re-cap what has been done:
Strictly concerning the control board. what I did so far was change every electrolytic capacitor (including the non-polarized ones) and swapped out the transistors. I don't have my notes with me, they're at home, but replacement PNP transistors were 2SA970 and NPN: 2SC2240.

Apologies for previously identifying 2SA970 as NPN in my last entry. Not being home between 4AM and 10PM, I frequently post using my iPhone. Consequently there are errors - either typos or Spell Correct .

To obtain values for the printed resistors, I referenced the parts list and there are 62 on the control board. Obviously there are fewer on the section of board I am addressing. However, since they are all permanently in circuit, isn't obtaining their actual resistance impossible?

 

[EchoWars]

However, since they are all permanently in circuit, isn't obtaining their actual resistance impossible?

They have to make a connection between pt. A and pt. B in one form or another.

Left green to mid green: 47K Ohms

Seems to me this is by design...the value is too standard. There's either an axial resistor or a 'printed' resistor in there doing the duties.

That volume control should be grounded. To keep from scratching through the anodized knob, remove it and take a resistance measurement from the volume pot stem and the chassis.

And just because I'm helping (or trying to) doesn't mean to shift your brain in neutral. It does sound like you have a lot on your plate and this is essentially a low-probability diversion, but you need to apply yourself here a bit and use some logic to help sort this out.

 

[62vauxhall]

You are 100% correct, being able to apply one's self. Since there are just 6 hours between leaving and arriving home from work, I am unable to devote the time needed. Therefore I am abandoning this project.

EW, thanks for your willingness to help. You go above and beyond.

The amp is somewhat disassembled and I shall not be re-assembling it. To salvage some value, some parts will be posted on e-bay and the remaining carcass - to the re/cycler.

 

[rshep]

You are 100% correct, being able to apply one's self. Since there are just 6 hours between leaving and arriving home from work, I am unable to devote the time needed. Therefore I am abandoning this project.

I'm not in your shoes but only having 6 hours of rest between work shifts (if I'm reading what you're saying right) has got to take its toll after awhile. Take care of yourself. I enjoyed reading about your attempt to repair the Kenwood, and am always amazed at the help you can get on this forum. Best of luck to you.

 

[Thespeakerdude8]

I know this is an old thread, but I figured that anyone else pulling their hair out on this issue may benefit from my experience. I just repaired one of these for a customer with the same problem of a weak left channel, and it turned out to be Q14 on the tone control board had gone low gain. This is on the output of the board, both transistors (Q14 Lch Q13 Rch) are 2SA872's and had black oxide on the legs. I used KSA992, though a higher gain substitute may be more appropriate as the 2SC871/SA872 family seems to have issue in some circuits (IE, Sansui 2000x) when replaced with a lower gain device of less than 500-600hfe (current 1845s/992s are all around 350-400hfe.)
The KSA992 seems to work fine in this application with no apparent reduction in voltage gain or change in frequency response. I also replaced the 2sc1345's on the preamp board with KSC1845s as they are also known noise makers. This amp was a little bit of a pain to troubleshoot because of the printed resistors and double sided boards. I found it helpful to check and mark each printed resistor before proceeding just to verify that none had been scratched or gone bad.

Cheers