Could use some help on a +B Power Supply problem in a Kenwood KR-5600

[tusntuk2]

I'm restoring a Kenwood KR-5600 that was in working condition, at least I know the FM and the Tape inputs were working, and I started with the power supply. The KR-5600 has a history of heat problems with its design, so I boosted the clumped resistors to 3 watts and the caps to UPW 100/50v. There was some slight solder-trace burning, so I folded over leads to strengthen connections. The amp passed the DBT test, the relay clicked on, the FM and Tape inputs (Audio amp) work fine, but there is no Phono or FM mute working.

I checked the PS pcb, its a 2630-10 model, Pins 3 and 4 supplying the +/- B voltage for the Pre-amp at 18v, but here is the puzzle - Pin 3 is +12.7v, and Pin 4 is -22.3v. Now, the main rail tested at +/- 40.2V, so maybe the 22V on Pin4 is OK, but what happened to Pin 3? The schematic shows this -B to go to both the Phono Preamp and the Muting circuit, and neither of them work. I'm hoping to find the problem.

The Phono Preamp has two IC's. The Muting circuit has 2 or 3 FET's. I am not an engineer to understand what the off voltage does to these circuits, but it must play some part.

Can I disconnect Pins 3 and 4 in the Power Supply and then power up the PS board to measure these pins? Without other consequences, I mean! That would identify the Power Supply as the culprit.
Is there a way to test IC's? The Preamp board is difficult to extricate, so I'm hoping I don't have to move it.

Any other ideas? Thanks a lot for chipping in!
Jim

 

[HWKI]

Not familiar with 2630-10 board. I assume you meant
1730-10.

Did you only make edits to power supply?
Did you get pre readings prior to edits?
What components did you touch/replace?

You said that the main rails now read +/-40V. What did they read prior to edits?

Both pre rails(pin 3/4)are setup by identical voltage divider.

Those are good clues on phono/muting , but before you start suspecting the op amp or muting . I would get the pwr supply in order.

you can remove those wires to see if they are loading down the supply rail. . But first i would go through all the edits you made. Reversed caps, check R2/R4 values.

 

[tusntuk2]

Not familiar with 2630-10 board. I assume you meant
1730-10.

Thank you for helping! - You are right, it is a 1730-10, the 2630-10 is the type of muting circuit. I've been having to check three or four s/m's I have to understand my confused receiver, so now I'm the confused one!

Did you only make edits to power supply?
Did you get pre readings prior to edits?
What components did you touch/replace?

I only made edits to the power supply, both boards. All electrolytics, both diodes on the top board, all 1W resistors, the bridge rectifier, the main filters, and the 2N1419 regulator.
Main rails were just over +/- 40v before the edit, but there was slight damage to a couple of traces underneath the hot resistors, so the pre-readings on Pin 3 and 4 were jumpy. I could not see this problem under the top board until I pulled the pcb to change parts. I had planned on rebuilding the board, so I just went ahead with the upgrade. That little pcb is a bit tricky to get out and work on. I did not test the functions of muting circuit or phono before the edit, I don't use them and it did not occur to me.

Thanks for your support, I'll pull those wires and test Pins 3 and 4 naked.

 

[tusntuk2]

Well, my work all checked out OK. Now - that might have been the quickest fix I ever did. When I disconnected Pin 3 and 4 I got -17.91v on 4 (spec is -18v), and 0.0 (!) on 3 - and the only part that could have failed was a small electrolytic I had just installed. Turns out it was bad, so, in went a new one. Now there is +17.91v on 3, and -17.91 on 4. (spec is 18v) Perfect! Even better, both the FM muting circuit and the Phono Preamp started working correctly as well. YAY!

Sure appreciate the help! I'll remember to do more "as is" testing before doing the edits next time. Good tip! Thank you!

 

[HWKI]

When I disconnected Pin 3 and 4 I got -17.91v on 4 (spec is -18v), and 0.0 (!) on 3 -

Pin 3 was always suspect based on your initial readings. Best to review the components first and measure all pertinent voltages/currents before diving in.

Good to hear you upsized those resistor dividers . For the size, the power rating is too low.

Show some pics of your unit. Ive never worked on a 5600. I’m mostly familiar with the larger KENWOOD models. Those smaller units can be difficult to work on . Lots of short wires and tight working space.


Glad all is well!

 

[tusntuk2]

You sure are right about being difficult to work on for all wires and no space!
Here's some pics -
View media item 29768Power supply pcb is in two parts, one board back to back with the other. The cluster of blue caps and green resistors next to the tranny is the top half. This board is prone to heat failure. One unit I repaired had 1/2" hole burned clean through! The pcb with the big caps and rectifier is upside down under this one.
Looking underneath, here's the big cans:
View media item 29769Here's the replacements:
View media item 29767and here's the reconstruction - I didn't worry about pretty alignment, I tried to create the most air space around the parts.
View media item 29766And here's the finished product:
View media item 29770Now it plays real nice and doesn't get anywhere near as warm as before.

Once again, thanks!

 

[HWKI]

Sweet looking unit! Those voltage divider Res. are putting off a lot of pwr/heat. What Wattage were you able to bump them up to? Kenwood did something similar on the 9600. However, there is enough room to move them off board and to wire-wound.

Nice job!

 

[tusntuk2]

I used 3 watt Vishay metal film (pro3000 series). Industrial type, fireproof. Lots bigger than the 1 watt green wire wounds that were in there. I also used a TIP31 regulator with a heat sink. I'm glad everything fit!

Thanks for the kind words.

 

[HWKI]

I noticed the finned heat sink. A KSC2073 or MJE15032G would of fit there as well. Any decent 220 package.