I'm Back,
Just had some time to Jump on the Kenwood KR 790. Here's the short Story:
I have another one that works except for another problem. To eliminate the Main function switch ... I took it out of the unit it was working in and stuck it in the problem one. Same Problem!!! Then I took the one that came out of the non working Reverb section and stuck it into the good unit.......It Worked!!
So Now I know I have an electronic problem. The BBD IC has been pushed out to Nov 2nd delivery. But I have both units on the bench and I can trace from the good one and then...
Mark R
Senior Member
Last Activity: 10-21-2025, 03:09 PM
Joined: 10-20-2023
Location: Egg Harbor Township NJ
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Hey Capleaker:
Just dropping by to let you know ..I had a slight detour . Lost my rear brakes on my Classic Ride. Been rushing around to get parts to do the repair. Will be back on the Kenwood soon,.
I'll keep you posted.
Regards: Mark R...Leave a comment:
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Thanks Pete,
Got the correct voltages coming in at the board @ +24 Volts DC and - 24 Volts DC. Need to Check around to make sure they are what the schematic calls for at the IC pins and other parts of the circuit. But this all changes when you turn that multi contact switch.
That's why this think is a PITA as there is no Theory of operation or service manual to follow signal flow at the different switch positions. As Capleaker says it's time to break out the scope for the umteeth time....Leave a comment:
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Tried that once but that switch has to be traced out. I'll hook up the scope and start probing again. Also check voltages in and around the circuit which I have done before. I have another BBD in route. it'll be about a week. Like you say...Sometimes the new stuff you get is garbage. if it passes audio and Mic but no reverb it won't hurt to throw another one in.
Also, I have another KR 790 that the Reverb circuit which does work ok. But that one has a dead Power L.E.D. display. That one should be an easy fix. I could put the other one up next to the bad one and compare scope traces....Leave a comment:
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Yep..sharp eye's Leaker. I'm aloud one backwards cap out of the 20 I replaced L.O.L. on that board. Anyway It checked good even being in backwards. But I installed a new one and checked it with the LC103. Fired up the unit and still ...Audio in the off position ...Next click Mic only... that works...but no reverb...Light comes on ( That's what Board #13 & 14 run from the switch) ...Then next click is supposed to be Audio and reverb and Mix. I get the reverb light but totally nothing else. That cap that was in backwards is in the delay line control feed.
I'm still leaning towards...Leave a comment:
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Ok, I took some shots of the Ebay switch which is the same as the one in the unit. Except the shaft is different. Butt he shafts are interchangeable. Also some shots of the Reverb Board. I'm going to swap out hose two diodes now. Just another shot in the dark. They did test good in circuit on diode check with my Fluke 8808A 6 digit DVM.
Here's the pic's
...Leave a comment:
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Yeah I have had it apart and it is in 4 block sections that snap together. Each Section has 4 contacts or 4 pins that prelude to the circuit board a total of 16 contacts. Each group of 4 contacts is internally the same.They all move together as you rotate they the selections.
I'm finishing up some work here on the puter, so when I'm done .I'll snap some pic's. But before I do, there are two Diodes I am going to change for S's & giggles ... I'll be back with Pic's
Thanks Leaker..L.O.L....Leave a comment:
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Hey Cap Leaker,
Still haven't had a chance to get to the KR 790. But I think that the Board # 13 & 14 Feed power to Bulb that shows the Reverb is on. I'll verify that when I get back to it...
Regards: MarkLeave a comment:
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I do Quote you. Went back and checked Notifications. It was Set to "ON DAILY" .... I changed it to "ON"...so I just got this notification. So I'm good to go.
I'm going to change out the only two diodes on the board in that area you mentioned. Also I hear Yah on new parts being bad. I have swapped some 8 legger IC's but no change. The MN3008 (BBD)was replace but as you said it could be bad. It came from ebay and was supposed to be new and it was costly. I only bought one.
I do have another KR 790 that the reverb section is working... But that one...Leave a comment:
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Hey Cap Leaker...Have to run out and pick up a Bud needs a ride. I'll check the 13 & 14 Board out's tomorrow. Also I have checked but will re check those IC's . Thanks for the guidance on that area.Leave a comment:
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I be on that in a Min or two and let your know.... What is strange ...not related ...I have all my notifications checked for replies and I'm not getting your replies.? I checked all the boxes under notifications...but not getting yours? I have been checking to see if you answered stuff. Are you getting notification when i answer your post?
Mark R...Leave a comment:
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Yep.. Both chips replaced...I think I did scope them but I can check again. Be back at yah..Gonna Grab some dinner.
Mark R...Leave a comment:
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Ok Capleaker ..here's the latest.
Tested every cap and replace a few that the LC103 said were just below -20%. 90 % of those ELNA caps were dead on after all these years. The circuit Power is only + - 24 Volts DC. So not much strane on the caps.
Every resistor was in spec hot and cold. Previously I had tested all 7 of the Transistors in that circuit. I used my B&K 520 Testor out of circuit and they showed no leakage. At that point i had checked everything in the circuit.
So, Next step "SHOTGUN". I had picked up from Flea Bay a Bag 0f 100 pc's of...Leave a comment:
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To Day Is Sunday...Day Of Rest... But not for me.... I'm going to check the rest of the resistors and caps....Long Day ahead. No Dark Goo on he components. All good there. I always remove that stuff when I see it
That is almost impossible. If you don't know what position the switch is in from the schematic you have no idea what's supposed to be on or off. The schematic I posted looks like it's in the off position??? From there you can get some idea of what's what. So in the off position, regular audio passes thru...which it does as in normal operation. And I get Audio...Leave a comment:
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If you read the post above.... I love upside down freeze spray and used it to locate that resistor. I'm about to go at it again now. Will see what else took a dump. Also I have no wifey....L.O.L. And I Do have a hot air gun. Be back ...........Leave a comment:
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Good Advice..Now an Update:
Received the Flea Bay switch. The circuit board it was on is laid out differently then the KR 790 Kenwood board as far as Circuit traces go. Even though it was from a Kenwood unit. Installed it and ...no go. So I'm assuming the original switch is good. So....I put the original switch back in. Now I tied my tried and true method. Turn the old duster can upside down...and what do you have??? Freeze Spray.
So I set the unit in the Source & Mic position where there should be audio and started with a few short burst of - 10,000 degree spray..L.O.L...Leave a comment:
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Thanks for the reply "Capleaker" The unit at 120 volt from my Sencore PR570 only gives a reading of what is stated around 62 volts on the rails. If dial up the Voltage to 126 Volts AC, the FM will come on and the rail voltage is at spec. The power transformer is weak and has rust and goo on it. I think it got wet at some point as it was stored in a Basement for years. Sort of like the TV show "Roadkill" where they find junk and make it run.
It you plug the unit into a normal AC outlet it's weak. At full audio out @ 1Khz I can get 123 watts @ 8 dummy load before...Leave a comment:
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