A tuner repair, or how I managed to shoot myself in the foot yet again

Ray Gianelli

Super Member
Several years ago a friend brought by 2 Kenwood KR-9600's. According to him, one had one channel out and the other had FM issues. The one with a channel out had a bad TA-200W output IC that appears to be NLA, so I focused on the other one.

The FM didn't pick up any stations, so I figured I'd just swap the tuner. Still nothing, so I set it aside to focus on the backlog piling up around my bench.

Fast forward several years. I now have a spectrum analyzer and an AM/FM stereo signal generator. My friend said he has someone who wants to buy a KR-9600, and asked me to revisit it.

I injected a an unmodulated signal into the antenna terminal from the generator. I took a look at the IF on the main board, and I could see the IF signal, but still couldn't hear anything. I started checking the op amp outputs after the ceramic filters, and saw good signal out of both op amp outputs, but nothing going into the FM IF chip. Problem turned out to be a bad ceramic filter, which had a lead rotted off of it. I replaced it with one from the donor unit, and now got a few stations, but nowhere near what I should have gotten.

I took the tuner I had pulled out of the unit I'm fixing and put it on the bench. I hooked it up to my bench power supply, signal generator and spectrum analyzer. The IF coming out of the tuner was way higher than what I was seeing when measuring the one in the receiver. I put it back in, and all was good.

The moral of the story is that unless you have a way to see what you're doing, you may make thing worse instead of better. Once I had the necessary test equipment it was relatively easy to see what was wrong.

Here's pics of the old tuner on the bench. I didn't take any pics of the other one, and it's already with its (delighted) new owner. The old tuner had to have the signal from the generator cranked up to pass any signal.20180325_141258.jpg

20180325_141252.jpg
 
Nothing better than good tools and knowing how to use them.
Isn't that the truth! I'm in the process of accumulating all the test equipment and tools I think I'll need now, so that when I retire and have less income I can supplement it with repair work. I won't need much, which is good because repair work isn't very lucrative. But it is something I enjoy immensely. I get a real kick out of going into a piece of equipment with hundreds of parts and finding the defective part or parts.
 
Last edited:
That sounds a lot like the opposite of shooting one's self in the foot, to me.
I appreciate the kind words.

Thing is I compounded the trouble, by replacing a good tuner with one of unknown functionality... which with my luck, turned out to defective. So I had 2 issues instead of the original single trouble.

Having the ability to inject a signal and trace it through is what was needed.
 
Congrats on getting everything going again. If you dig yourself into a hole, at least you can build a ladder to get out!
Must be nice to have a spectrum analyzer in your tool box, what did you get?
 
Congrats on getting everything going again. If you dig yourself into a hole, at least you can build a ladder to get out!
Must be nice to have a spectrum analyzer in your tool box, what did you get?
Picked up an HP 8950L. 9Khz to 1.8 Ghz. Paid around $500 IIRC. New offerings from Rigol and the like have really driven prices down on these. If you have the real estate on your bench there are excellent values to be had in older, high quality test equipment.

Not something I need too often, but there's no substitute for an SA when signal tracing RF.
 
Since you have the proper tools and the knowledge, have you done any experiments with upgrading the ceramic filters to better ones?

The FM DX listeners use narrower bandwidth (230 kHz all the way down to 130 kHz) to gain selectivity; this hammers the fidelity, of course.

But some of them have upgraded the correct filters with ones having steeper rolloff, because the original filters had larger tolerance. Back in the day this is understandable. Manufacturing techniques have greatly improved. I've seen claims that many of the filters sold through the big catalog houses have had the accurate ones cherry picked by distributors and sold at a premium, only leaving more out of tolerance units.

I don't have any direct experience either way. I do know that modern ceramic filters are quite good compared to the older ones so the argument is plausible.
 
Since you have the proper tools and the knowledge, have you done any experiments with upgrading the ceramic filters to better ones?

The FM DX listeners use narrower bandwidth (230 kHz all the way down to 130 kHz) to gain selectivity; this hammers the fidelity, of course.

But some of them have upgraded the correct filters with ones having steeper rolloff, because the original filters had larger tolerance. Back in the day this is understandable. Manufacturing techniques have greatly improved. I've seen claims that many of the filters sold through the big catalog houses have had the accurate ones cherry picked by distributors and sold at a premium, only leaving more out of tolerance units.

I don't have any direct experience either way. I do know that modern ceramic filters are quite good compared to the older ones so the argument is plausible.
I have not. Read about it with interest, but I really don't even have a tuner of my own these days.

Next time I find myself with a tuner and some time I may try this. I'll start a new thread and report my findings. May not be anytime soon though; got a backlog of work, and I don't even take on repair of items belonging to others. I buy items for repair, so there's no deadline or keeping folks waiting.

Thanks for bringing it up!
 
It seems like an interesting tweak.

I haven't had the time or equipment to test the results, but I think with a socket it should be a doable experiment.
 
Back
Top Bottom