Kenwood KT7500

[Deleted User]

Hey guys,
I've been having some fun modifying a classic Kenwood KT7500 AM/FM tuner, following some excellent guidance from a DXTuner site. I bought the tuner from Audiogone.com and have gone over the power supply, replaced the output opamp, interstage coupling capacitors, replaced signal path resistors and experimented with different ceramic filters in the IF stage. I was listening to a local jazz station, 88.1mHz and was really amazed at how transparent and three dimensional it sounded. Once three pins sockets are put in the IF stage you can take out one filter and try another, getting a combination of filters that suit how you want the tuner to respond in both the "wide" and "narrow" setting. I also eliminated a big chunk of circuitry, eliminating the muting and volume control function.
Anyone else into older classic 60's or 70's FM tuners?
Ken

RuSsMaN

I actually just sold one of those tuners, to a gentleman in Poland of all places.

I really enjoyed it stock, but have heard it's an outstanding tuna to tweak. Have you checked out Jim River's site?

http://fmtunerinfo.com/

They have a bunch of data on tweaking the 7500, among many others. Here's a quick link:

http://www.fmtunerinfo.com/DIY.html#KT-7500

Nice to see the post, I'd love to see a picture with the hood popped, and your mods! I'm actually headed to Jim's house tonight to dig some tunes on his rigs, and see what he's done recently. If you'd like me to pass a message along, I can have him call you directly to discuss the mods.

Cheers,
Russ

TroyD

This may be untrue but from my perspective, it would seem that tuners is one area of hifi gear that vintage gear would seem superior to new gear.......it's just an observation that I've made.

I'd be interested to see what those who really know something about the subject think.

BDT

organ

Sounds cool Ken. I don't know anything about vintage Kenwood stuff but I'm just wondering which op-amp you think is a good one to improve the sound? I'm thinking about upgrading the ones in my cheapo DVD player. I played around with the 741 a lot for my labs at school but the prof said the 741 are crap op-amps.

Maurice

[Deleted User]

Hey Russ and Organ,
It was the article you reference that got me started, excellent instructions! Both Jim and Bob are amazing in the level of detail on all of the tuners they mention. I'm not sure if it the same person, but William Ammons has made a small circuit board that allows two ceramic filters to be put in place of a single one.
Organ, I found that the BB OPA2132 sounded really good, I've added a 5k resistor from the negative supply to both the right and left outputs of the op-amp. This will bias it in a class A output and eliminates the need for the output coupling caps.
I'm going to do the AC modifications, suggested in the article, this weekend, hopefully. I say, hopefully, because we're getting a new kitchen put in on Monday and we have to take out the old cabinets. We have a 1938 house and the kitchen has seen better days.
Have fun, guys!
p.s. please say, "Thanks!" to Jim, his article reminds me of the older Audio Amateur ones.
Take care, Ken

radkrisdoc

Ken,
Its all good about the sound, but what about RF performance? Did you look into the tuner's alignment and inspect the tuning condenser for any mechanical "play"? I dont remember more than that about the tuning condenser, but I think I read about it on www.classic-audio.com, the marantz section. Its just something that might need correction cos there's at least 20 years of use.
You can also open the front-end of the FM tuner and see what RF transistors it uses and upgrade them to low noise designs available these days. The first transistor that forms the antenna input circuit is all important. Change it and you will get better noise and sensitivity performance.
Vintage tuners have easy upgradeability but the tuning capacitor is source for a lot of tuning instability. Apart from that, I think the Technics ST-9030 is the best bet (cost vs. performance). Awesome RF performance plus a probable easy audio DIY upgrade (though I dont know much about that. Will know in a few months).
A nice addition would be a used FM signal sleuth from Magnum Dynalabs. Have fun!

organ

Ken,
Thanks for the info. I'll look for that op amp next time I go to the electronic parts store. I'll get the resistors as well to get the Class A operation.

Maurice

gidrah

Originally posted by TroyD
This may be untrue but from my perspective, it would seem that tuners is one area of hifi gear that vintage gear would seem superior to new gear.......it's just an observation that I've made.
BDT

You'll notice I deleted the part about "those who really know".

I've noticed this too. I don't think FM will die for a very long time, but the dedication from most manufacturers just doesn't seem to be there anymore. Sure there is M'D', but that seems about it. It seems like anybody that wants good FM reception (compared to 60 presets of static with the occasional sterile sound music) are going M'D' or vintage.

Have you tried tuning in local TV signals on a newer set? It's a shame what they try to pass of as a tuner since the abundance of cable.

Sidenote - I recently acquired a very sweet Pioneer receiver that had the tuner updated/modded. The guy that did the work even offered to come bring his filters by my house and "dial it in" for me for best reception. Heck, I don't even have an antenna connected to it and it sounds okay.

danger boy

Ken,

A few months ago i found a minty Pioneer TX-9500II tuner on ebay. got it for really good price.

I didn't ever think that a classic tuner from 1977 could sound so good. It's stock... no mods that i know of have been done to this one. I would like to do some mods.... but i'm not real good with things like that.

I read a lot of good things about the Kenwood 7500 tuner. It's a fave of alot of FM enthusists.

I just had my Pioneer serviced.... it needed a good cleaning and a few of the lights needed to be replaced. Other than that it's in great shape. It puts to shame current modern day tuners that's for sure.

al

[Deleted User]

Hello,
Thanks for all of the comments and suggestions on the tuner. I have a Heathkit FM generator and the operations manual and I'm going to try re-aligning the Kenwood after all the mod work is done. I tried the circuit board that allowed a second ceramic filter to be used in the wide mode. I didn't have a great deal of time to listen, but I believe I prefered the single Murata 180 filter. But I want to try putting it in the narrow section first filter position. I'm trying to get a DC jazz station 89.3mHz that is near a local college station 88.9mHz that is just up the street when I live. So I need a sharp narrow filter grouping with low loss.
As always, guys, have fun!
Ken

danger boy

Ken,

Does your Kenwood have a muting switch? that may help with the close station tuning.

Jed Leland

Here is a photo of some of the changes to the tuner's power supply. Since the tuner uses the unit's chassis as the grounding structure it's important to place, as much as possible, of the AC section above the chassis. The AC transformer is lifted off of the frame using rubber grommets. The AC fuse and other AC wiring is removed from the main circuit board and kept isolated. I used an IEC type chassis socket that has some AC line filtering built-in. There is also a "snubber" circuit added to the AC prior to the AC transformer.

Jed Leland

Here is a photo of the complete tuner interior. It's important to route the connecting wires, getting them as far away, as possible, from the audio signals. The wires that lead to the front panel on/off switch are carrying AC, I had thought about placing some shielding braid over them, but didn't have any that was the correct diameter. I had also considered using a shielded cable for the connection, then grounding the shield. If anyone else does a similar project and decides to shield any AC wires, let me know what happens. The other modifications deal with re-routing the final audio signal. In order to provide interstation muting and volume control Kenwood routes the audio signal through additional circuitry that accomplishes this. However, in terms of transparency and detail these all involve reducing the performance. You can see some twisted pairs of Kimber speaker wire (I've used Kimber wire twisted together in an electric drill as a favorite connecting wire for many years) going from the output IC to the output RCA plugs. The output IC is changed to a more modern version operating in class A mode. And all of the interstage coupling caps have been upgraded to Black Gate higher values to improve bass definition. Also, power supply rectification has been changed to fast recovery diodes and the power supply filter caps have been increased. One of the caps has to be carefully positioned, it is very near where the dial string cord goes parallel to the inside of the front plate.

Jed Leland

Here is a photo of the underside of the tuner. This is where the output IC mounting socket comes through the bottom of the circuit board. One with slightly longer pins is used, this gives you the ability to add de-coupling caps to the IC's negative and positive power supply. You should also be able to see the 5k resitors that bias the IC into class A operation. What you're doing is taking a small amount of the positive power supply voltage and giving it to the IC's output. In effect turning the IC "on" all of the time, operating it in class A mode. The reason for doing this is that solid state circuits are really untra fast switches, turning on and off rapidly. Well, nothing's perfect and one section of the switch is not completely off when the other section starts turning on. Class A is kind of "fooling" the circuit into conducting all of the time. There is some concern about the additional heat generated inside the IC, I couldn't find a heat sink the correct size to cool it off. Using the "finger on the part" method of temperature dectecting it doesn't seem excessively warm. But, I bought two ICs just in case.

Jed Leland

This is another photo, of the tuner, with the AM antenna connected and some additional cleaning done. I cleaned the circuit board with a Q-tip and Goo Gone, going over each small section again and again until the grunge comes off. I took out all of the rear panel components, RCA jacks, etc and polished them with Flitz metal cleaner. I added a 75 Ohm "F" connector rather than the older style screw type connector. I polished most of the interior metal work and front and rear panel. A word of caution, I believe Kenwood used a type of lacquor finish on the black back panel. When you try and polish it you begin smearing the finish. I had to use some serious elbow grease and car wax to put the original finish back on. The other very interesting aspect of this project is dealing with the IF filter section. By placing some three pin connectors in the IF filter stage you can experiment with different filters. These filters are extremely important to the way the tuner deals with the wide variety of incoming FM signals. I live near a moderate power college radio station, 88.9mHz, and wanted to receive a more distant station located in Wash. D.C. that is 89.3MHz. I have a rotating roof top Channel Master antenna (gratefully installed by Kim Jasper, many years ago). But the necessary direction of the antenna to receive the D.C. station places it close to where the incoming more powerful 88.9 signal is coming from. One of the good things about a highly diectional antenna, like the Channel Master, is that it is unsensitive to signal coming from the side and most sensitive to the direction it's pointed in. But, now with "plug and play" filter sockets I can try different combinations of filters to allow the wanted signal to be received and shape the bandwidth and response of the individual filters. Jim Ammons even makes filters available that he has carefully measured to insure best filter operation.
The tuner now sounds wonderful, extremely detailed, very spacious with fine bass defintion. On our local classical station, if it is a live recording, the audience "width" when they applaud is startling. In fact, during testing I was using a set of cables I had made from some bulk Cardas Litz type cable. Then when everything was finished I connected it in it's "regular" positon, beside the pre-amp using my standard twisted Kimber wire interconnects. I'm not completely sure, but the Kimber wire doesn't sound as good as the Cardas cable I made. Gulp, that would mean all of my cables, turntable, power amp interconnections everything would have to be changed! That's the way audio equipment works, you make one change and you find out something you didn't expect.

Jed Leland

Here is a photo of the tuner, in one of the listening periods. I keep the cabinet to the right of the rack for temporary gear. I use this space to set up whatever source is not a permanent one. If I want to use the Stellavox, for listening to tapes, this is where it goes. Or, in this case, listening to the various changes to the tuner. When I had a store, in Silver Spring, it was important to have a place customers could bring anything in to play and compare with something we might have on display. I keep a spare AC cable and RCA connections so I can bring something in a hook it up to listen for a short period of time. Plus it eliminates potential clutter of having tape decks sitting around.

adam13

Hi, I'm about to embark on this project and was hoping someone could help me with a parts list for the modification. I see where many of you have made Mod's to this unit and any help would be greatly appreciated. Thanks, Adam

Jed Leland

Hello,
Funny you should mention the Kenwood, I just got mine back from a trip to Bill Ammons and I am in audiophile heaven. I've spent the last few hours listening to our local classical station and it sounds amazing. I had done a few more modifications to it before I sent it to Bill. I added a negative voltage regulator which allowed me to adjust the negative supply voltage and eliminated the slight drift I had previously experienced. I also replaced the dual output opamp with two discrete, high end, opamps.
Then sending it to Bill for a final alignment, usually needed when you change the IF filters. He added two PCB adder boards, to the narrow section, and tried different combinations of wide band, low group delay filters until he lowered the distortion dramatically and improved channel separation. He said that the modifications to the AC power supply resulted in the highest signal-to-noise ratio of any KT7500 he has seen. That made me feel good.
To answer your question, the best source of information is www.tunerinfo.com. and I'd be happy to offer any help. The results are outstanding, in fact I think I'll do another one that I have and see if I can sell it.
Good luck with your project, Jed

mhardy6647

Kenneth Swauger wrote: »

Hey guys,
I've been having some fun modifying a classic Kenwood KT7500 AM/FM tuner...
Anyone else into older classic 60's or 70's FM tuners?
Ken

EDIT: oops, 2004 post...