Harman-Kardon F500 FM Tuner Restoration


Joseph Strickland
 

I have been working on a Harman-Kardon F500 FM tuner from about 1961 or so. This unit was paired with a H-K A-500 amplifier which I am also working on.

Early on I discovered that the Discriminator Transformer was suffering from the Silver Mica disease. It would pop and whatever station you were listening to would drop significantly in volume. You could also wiggle the transformer and the volume would do the same thing. I did manage to disassemble and repair the transformer. The Limiter Plate coil uses a 22pFd cap to resonate and the coupling cap between the primary and the secondary winding is a 39pFd. Those values were measured with a capacitance meter I have. I replaced both and reassembled the transformer and did an IF alignment. There were a few tubes that were weak and I replaced those. That operation made a significant improvement in sensitivity.

I rebuilt the electrolytic can capacitor in the F500 FM tuner. That worked out well. There were no significant difficulties in doing so. I also replaced a number of electrolytic capacitors in the audio feeding to the MX500 multiplex decoder. The MX500 already was exhibiting difficulties. There was an audible howl as long as the stereo mode was selected with the front slide switch. I replaced C1 (.01@400V), C2 (2mFd@25V), C4 (2mFd@300V), C5 (2mFd@25V) and C22 (4mFd@350V). While I was replacing the capacitors in the MX500, I discovered that there were parts installed by the factory that were not shown on the MX500 schematic that came with the tuner and its companion MX500 decoder. I had already installed a better 12AX7 and 6BL8 in the decoder. The 12AX7 is used as a composite signal amplifier and there is a combination bandpass for the L-R subcarrier in the first LC component GLCOM5390 shown in the decoder schematic. The 19khz along with a somewhat reduced level of composite audio is fed from the output of the second 12AX7 to the triode stage of the 6BL8 which amplifies the 19kHz pilot tone that is embedded in the composite signal. The signal passes through C17 which is only a 100pFd coupling cap and pretty much removes a lot of the L+R audio from the 6BL8 triode 19kHz amplifier circuit. In the plate circuit of that triode stage of the 6BL8 there is a .002mF coupling capacitor C18 followed by a19kHz coil, GLCOM5391 that peaks the 19kHz signal going to the signal grid of the 38kHz pentode oscillator that creates the 38kHz signal fed to the diode decoder matrix . That coil has a tap that is part of the cathode circuit of the 38kHz oscillator. The 19kHz amplifier stage serves to keep the 38kHz oscillator on frequency and in phase.

The added parts showed up first in the cathode stage of the second half of the 12AX7 composite amplifier. There was a 65mH coil with a 100pFd N150 capacitor in parallel with it between R6 (1.2K ohm resistor). I unplugged the 6BN6 Limiter tube to stop audio from stations while I fed audio from an audio generator to see what that LC combination was doing. It was providing a peak at around 19kHz while passing the L+R audio, 19kHz pilot tone and the L-R subcarrier frequencies. There was another LC circuit added at the cathode of the triode 19kHz amplifier of the 6BL8 tube. That circuit consisted of a .001mF disc ceramic capacitor and a 60mH inductor. Both the 65mH and 60mH coils are non-adjustable fixed value coils with a ferrite core encapsulated in a red/brown coating of some sort. After replacement of all the capacitors in the MPX decoder that were electrolytic types I was able to stop the audible howl I had been hearing by adjustment of GLCOM5391. There is a very narrow range where that adjustment stops the howl and allows stereo sound with decent stereo separation.
 
Unfortunately every station broadcasting stereo exhibits distortion on percussion sounds or pronouncement of "P", "T" or "S" sounds when the tuner is set to stereo mode. If monophonic sound is selected with the front mode switch, the monophonic audio is free of distortion. Cymbals, snare drums etc. sound like breaking glass in stereo mode.

The F500 is Serial Number 4171189 and its MX500 is Serial Number 4650798. I believe the multiplex decoder was produced later than the rest of the tuner and installed some time after the tuner had been purchased. Based on the fact that the official schematic provided by the Harman-Kardon factory did not match how the MX500 was actually built, I believe that H-K was experiencing some problems with these MX500 decoders. There are no factory instructions on how the MX500 is to be aligned in the manual on the MX500. The F-50XK that I have (and built from kit form in the early 1960s) has a completely different schematic and parts list from the MX500. The only area that is the same are the RC networks that follow the detected L and R audio to be fed to the integrated audio amplifier driving speakers or earphones. The detected stereo audio sounds fine until cymbals or share drums are in music or when people speak and P, T or S are uttered.

I may watch for a MX600 multiplex decoder and see if that type decoder may work better. Another alternative would be to install one of Bob's LM4500 MPX decoders. I did that with my old F-50XK tuner which is the kit version of the F500. That solved a number of problems with the F-50XK. The F-50XK also had developed a problem with its Discriminator Transformer (the coupling cap between the Limiter plate coil and the secondary shorted and took out the 1K B+ resistor feeding the Limiter plate circuit). I suspect that many of the older H-K tuners including the Citation III tuner (which uses the same exact Discriminator transformer) begin to develop problems with their Discriminator Transformer. It may be a common problem with them.

Joe
KC5LY


Joseph Strickland
 

I thought it would also be good to provide a schematic of the basic tuner.


newaag
 

It sounds like the stereo problem is either high frequency distortion, or a circuit ringing with transient signals.
If you can, I would test the detector output response up to 100 kHz to see how flat it is.
And also if possible, test the stereo adapter circuit separately with a composite signal from another tuner, or test generator. And test the tuner mpx out signal with another adapter.
The match of the tuner to stereo adapter on these old units, in some cases, was not the best. This was because the tuners were built and sold before the stereo standard was finalized. The tuner designers made guesses as to what the winning stereo standard would be, designed for that, and those circuits turned out to be poor choices.
As you stated, there were many circuit revisions done on the fly with these early adapters. Most eventually settled on the design used by later Scott and Fisher stereo tube tuners.

Perhaps I’m biased, but I prefer using solid state stereo adapters with tube tuners.
To my ears, the tube warmth is not diminished through a solid-state adapter.
Bob


Joseph Strickland
 

Bob;
You are so right about these early stereo decoder circuits and their mating to the associated RF.IF/Ratio Detector or Discriminator circuitry! I still have my old H-K F-50XK kit version of basically the same FM tuner with the exception that it did not have the IF squelch circuitry and used a MPX decoder that had a 4-diode matrix instead of the 2-diode matrix of this F500 unit. I did discover that the MX600 decoder that came out later than the MX500 decoder had the exact parts that had been added by the H-K factory to the MX500 decoder. The 4-diode matrix which was in the F-50XK tuner worked better than the one in this F500 but its stereo indicator circuit (a NE2 neon lamp) was always flaky and did not fire consistently. At times the neon would oscillate and feed annoying noise into the detected stereo audio!

I finally bought one of the LM4500 boards and chip and installed it in the F-50XK and that arrangement works flawlessly and with excellent separation. I even installed a variable control to allow me to reduce separation if dealing with less than ideal signal level. Blending the two channels with that simple variable control reduces the noise level in the audio to more acceptable levels instead of using the original switch arrangement that did a fixed amount of blend.

Since the tuner does have a weak 12AX7 that is part of the feed to the MX500 circuitry, I have two of those on order. If that takes care of the distortion fine, but if not I may be looking to build up another LM4500 decoder. That decoder works so well it is silly not to use one.

Do we know if more of the boards for the LM4500 will become available?

Joe


Joseph Strickland
 

I forgot to mention that I do have one of the Fisher 300 MPX signal generators to generate a good stereo FM signal that I can use to check FM stereo tuners with. Thanks to Dave Gillespie of the AudioKarma.org website and the Fisher forum there, I have the complete factory manual for calibration of the Fisher generator. I went through that process a couple of times making small improvements in its operation. Most of the issues were just some corrosion on a few tube sockets and variable controls that needed some Deoxit treatment. Now it works quite well.

Tomorrow I plan to be restuffing can electrolytics in the H-K A-500 that came with the F500 tuner.

Joe


newaag
 

On Mon, May 4, 2020 at 03:00 PM, Joseph Strickland wrote:
Do we know if more of the boards for the LM4500 will become available?

I'll let you know, I may try a new supplier, should be a month or two most. The one I used was closed until things got better.
Bob


exactadave <EXACTADAVE@...>
 

Sony fm tuner   st a6b   is an fm tuner from the 70’s and just got one off ebay……....really nice spec accept for the tuning dial………….the specs state the fm tuning range is normal  89.    To 108  aprox.   The dial that this tuna has is 75-91 mghtz……….can anyone help explain…………thanks dave.   I don’t bother you people very often but this is a confusion to me.    Thanks   dave

 

Sent from Mail for Windows 10

 

From: newaag via groups.io
Sent: Monday, May 4, 2020 8:16 PM
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

On Mon, May 4, 2020 at 03:00 PM, Joseph Strickland wrote:

Do we know if more of the boards for the LM4500 will become available?


I'll let you know, I may try a new supplier, should be a month or two most. The one I used was closed until things got better.
Bob

 


Eric Benson
 

The Sony STA6B is the Japanese FM bandwidth version; check power too; you will need a 110v 50Hz to 120v 60Hz converter.  Your stations will be limited to only the lower range of US FM airwaves.


On Fri, May 8, 2020, 11:51 PM exactadave via groups.io <EXACTADAVE=AOL.COM@groups.io> wrote:

Sony fm tuner   st a6b   is an fm tuner from the 70’s and just got one off ebay……....really nice spec accept for the tuning dial………….the specs state the fm tuning range is normal  89.    To 108  aprox.   The dial that this tuna has is 75-91 mghtz……….can anyone help explain…………thanks dave.   I don’t bother you people very often but this is a confusion to me.    Thanks   dave

 

Sent from Mail for Windows 10

 

From: newaag via groups.io
Sent: Monday, May 4, 2020 8:16 PM
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

On Mon, May 4, 2020 at 03:00 PM, Joseph Strickland wrote:

Do we know if more of the boards for the LM4500 will become available?


I'll let you know, I may try a new supplier, should be a month or two most. The one I used was closed until things got better.
Bob

 


tune in
 

Dave, that tuner appears to be manufactured for the Japanese market and  most likely runs on 100 volts AC.  Check the information on the back of the tuner.  You will need a step up or down transformer for most countries outside of Japan and will only be able to access the lower end of the FM band (to 91mhz).

Tom



Sent from my U.S.Cellular© Smartphone


-------- Original message --------
From: "exactadave via groups.io" <EXACTADAVE@...>
Date: 5/8/20 11:51 PM (GMT-06:00)
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

Sony fm tuner   st a6b   is an fm tuner from the 70’s and just got one off ebay……....really nice spec accept for the tuning dial………….the specs state the fm tuning range is normal  89.    To 108  aprox.   The dial that this tuna has is 75-91 mghtz……….can anyone help explain…………thanks dave.   I don’t bother you people very often but this is a confusion to me.    Thanks   dave

 

Sent from Mail for Windows 10

 

From: newaag via groups.io
Sent: Monday, May 4, 2020 8:16 PM
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

On Mon, May 4, 2020 at 03:00 PM, Joseph Strickland wrote:

Do we know if more of the boards for the LM4500 will become available?


I'll let you know, I may try a new supplier, should be a month or two most. The one I used was closed until things got better.
Bob

 


exactadave <EXACTADAVE@...>
 

Yo know………..thank you very much………..will let the group know what I find out………….thanks again…..dave

 

Sent from Mail for Windows 10

 

From: tune in
Sent: Saturday, May 9, 2020 6:09 AM
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

Dave, that tuner appears to be manufactured for the Japanese market and  most likely runs on 100 volts AC.  Check the information on the back of the tuner.  You will need a step up or down transformer for most countries outside of Japan and will only be able to access the lower end of the FM band (to 91mhz).

 

Tom

 

 

 

Sent from my U.S.Cellular© Smartphone

 

 

-------- Original message --------

From: "exactadave via groups.io" <EXACTADAVE@...>

Date: 5/8/20 11:51 PM (GMT-06:00)

To: FMtuners@groups.io

Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

Sony fm tuner   st a6b   is an fm tuner from the 70’s and just got one off ebay……....really nice spec accept for the tuning dial………….the specs state the fm tuning range is normal  89.    To 108  aprox.   The dial that this tuna has is 75-91 mghtz……….can anyone help explain…………thanks dave.   I don’t bother you people very often but this is a confusion to me.    Thanks   dave

 

Sent from Mail for Windows 10

 

From: newaag via groups.io
Sent: Monday, May 4, 2020 8:16 PM
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

On Mon, May 4, 2020 at 03:00 PM, Joseph Strickland wrote:

Do we know if more of the boards for the LM4500 will become available?


I'll let you know, I may try a new supplier, should be a month or two most. The one I used was closed until things got better.
Bob

 

 


tune in
 

Dave,

If you're looking to receive the full US FM band you'll need a different tuner. Ebay's return policy should allow you to send it back if you decide that tuner won't meet your needs. 

Tom



Sent from my U.S.Cellular© Smartphone


-------- Original message --------
From: "exactadave via groups.io" <EXACTADAVE@...>
Date: 5/9/20 11:20 AM (GMT-06:00)
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

Yo know………..thank you very much………..will let the group know what I find out………….thanks again…..dave

 

Sent from Mail for Windows 10

 

From: tune in
Sent: Saturday, May 9, 2020 6:09 AM
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

Dave, that tuner appears to be manufactured for the Japanese market and  most likely runs on 100 volts AC.  Check the information on the back of the tuner.  You will need a step up or down transformer for most countries outside of Japan and will only be able to access the lower end of the FM band (to 91mhz).

 

Tom

 

 

 

Sent from my U.S.Cellular© Smartphone

 

 

-------- Original message --------

From: "exactadave via groups.io" <EXACTADAVE@...>

Date: 5/8/20 11:51 PM (GMT-06:00)

To: FMtuners@groups.io

Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

Sony fm tuner   st a6b   is an fm tuner from the 70’s and just got one off ebay……....really nice spec accept for the tuning dial………….the specs state the fm tuning range is normal  89.    To 108  aprox.   The dial that this tuna has is 75-91 mghtz……….can anyone help explain…………thanks dave.   I don’t bother you people very often but this is a confusion to me.    Thanks   dave

 

Sent from Mail for Windows 10

 

From: newaag via groups.io
Sent: Monday, May 4, 2020 8:16 PM
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

On Mon, May 4, 2020 at 03:00 PM, Joseph Strickland wrote:

Do we know if more of the boards for the LM4500 will become available?


I'll let you know, I may try a new supplier, should be a month or two most. The one I used was closed until things got better.
Bob

 

 


exactadave <EXACTADAVE@...>
 

Thank you tom.     You are right……………ebay good this way

 

Sent from Mail for Windows 10

 

From: tune in
Sent: Saturday, May 9, 2020 11:55 AM
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

Dave,

 

If you're looking to receive the full US FM band you'll need a different tuner. Ebay's return policy should allow you to send it back if you decide that tuner won't meet your needs. 

 

Tom

 

 

 

Sent from my U.S.Cellular© Smartphone

 

 

-------- Original message --------

From: "exactadave via groups.io" <EXACTADAVE@...>

Date: 5/9/20 11:20 AM (GMT-06:00)

To: FMtuners@groups.io

Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

Yo know………..thank you very much………..will let the group know what I find out………….thanks again…..dave

 

Sent from Mail for Windows 10

 

From: tune in
Sent: Saturday, May 9, 2020 6:09 AM
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

Dave, that tuner appears to be manufactured for the Japanese market and  most likely runs on 100 volts AC.  Check the information on the back of the tuner.  You will need a step up or down transformer for most countries outside of Japan and will only be able to access the lower end of the FM band (to 91mhz).

 

Tom

 

 

 

Sent from my U.S.Cellular© Smartphone

 

 

-------- Original message --------

From: "exactadave via groups.io" <EXACTADAVE@...>

Date: 5/8/20 11:51 PM (GMT-06:00)

To: FMtuners@groups.io

Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

Sony fm tuner   st a6b   is an fm tuner from the 70’s and just got one off ebay……....really nice spec accept for the tuning dial………….the specs state the fm tuning range is normal  89.    To 108  aprox.   The dial that this tuna has is 75-91 mghtz……….can anyone help explain…………thanks dave.   I don’t bother you people very often but this is a confusion to me.    Thanks   dave

 

Sent from Mail for Windows 10

 

From: newaag via groups.io
Sent: Monday, May 4, 2020 8:16 PM
To: FMtuners@groups.io
Subject: Re: [FMtuners] Harman-Kardon F500 FM Tuner Restoration

 

On Mon, May 4, 2020 at 03:00 PM, Joseph Strickland wrote:

Do we know if more of the boards for the LM4500 will become available?


I'll let you know, I may try a new supplier, should be a month or two most. The one I used was closed until things got better.
Bob