Heathkit AJ-1510 Tuner help needed.

[Tubologic]

Heath's AJ-1510 was one of the very first all digital synthesis PLL FM tuner ever produced, introduced in 1972. (A true historic piece) My problem is that it was mainly designed for the U.S market and can only receive ODD ending channels (like 92.3 or 106.9 MHz) and has 200 KHz frequency increment steps. Even numbered channels (like 92.4 or 106.8) can not be entered or you get a "RE-PROGRAM" error. In Europe both odd and even ending FM channels are used and of course I can't access the latter. I know AJ-1510's were sold in Europe and some circuit modification should have been designed to access even numbered channels (or 100 KHz freq.increment). I have good basic digital circuit knowledge but admit being lost in the quite complex circuits used in this tuner (preload decoder,programmable frequency divider,code converter,etc...) using over 30 digital logic I.C's ... and failed in finding some easy way to convert the tuner for European use. Hopefully there is some "AJ-1510 Expert" reading this who remember how this modification was implemented ? I know the conversion was then published in some British electronic magazine but I have no more details about it. The right person would be Mr DAVID M. THOMAS , Senior Design Engineer on the AJ-1510 Project and later Chief Engineer at HEATH Ltd. Gloucester, England but I was unable to get in touch with him so far. (I'm not even sure he is still with us). Any help to solve the above problem would be greatly appreciated.
NOTE: I have the original AJ-1510 manual/schematics and 3 tuners available for experimentation.

[stratus46]

Post a link to the schematics and someone might have a bright idea. It could even be me.

G²

[Damon Hill]

Check with the FM tuner group at Yahoo Groups:

FMtuners : High End FM Tuners

I own a semi-functional AJ-1510 and am relying on help from knowledgeable owners in that group to perform a set of modifications to improve stability and fix design flaws, as well as a general parts upgrade.

[amptramp]

Originally Posted by Damon Hill

Check with the FM tuner group at Yahoo Groups:

FMtuners : High End FM Tuners

I own a semi-functional AJ-1510 and am relying on help from knowledgeable owners in that group to perform a set of modifications to improve stability and fix design flaws, as well as a general parts upgrade.

I have written up a blurb on the AJ1510A tuner, but it is in word format. I am not sure if I can copy it into here as it is rather long. And you are right, there are several design flaws, very unusual for a Heathkit product.

It does not cover the original poster's problem of changing the tuning, but it does cover a number of flaws. If you have enough information to contact me, I can send it via e-mail.

[kevinkr]

You are welcome to post it here as a zipped attachment to a regular post. Zip files of 976K or less should post with no problem..

[amptramp]

I have never actually used the zip function before, just the unzip, but here goes:

[kevinkr]

This is great stuff - thanks for posting it here..

[Damon Hill]

I would like to understand more about how the FM demodulator works, and possible upgrades. The demodulator includes a 709 type opamp, which is ancient history, and it's suggested that as an integrator the circuit is actually incomplete. I tried once to replace the opamp with something more modern, but lacking a signal source I gave up when the change didn't work. Open to ideas there as well but mostly I'm looking for a clear explanation how it actually works.

[Tubologic]

Originally Posted by Damon Hill

I would like to understand more about how the FM demodulator works, and possible upgrades. The demodulator includes a 709 type opamp, which is ancient history, and it's suggested that as an integrator the circuit is actually incomplete. I tried once to replace the opamp with something more modern, but lacking a signal source I gave up when the change didn't work. Open to ideas there as well but mostly I'm looking for a clear explanation how it actually works.

This is a pulse (or counting) type demodulator. A basic explanation of his working can be found in the May 1973 issue of RADIO ELECTRONICS which features a 5-page article about the AJ-1510 written by the designer himself. (David M.Thomas). See attached picture. He didn't actually invented the circuit which was already know in the tube-era and sometimes called a frequency-counting detector or frequency differential counter. Just a modern implementation of an old circuit.
I tried various modern (pin compatible) opamps in this position and sometimes it worked,sometimes not (or with degraded performances). Looks many parts of the AJ-1510 circuits are critical and works "on the edge" of the (then) available technology. Replacing the 709 had very small effect on the average sound quality which is mainly limited by the MC1310 MPX decoder chip and the many switching diodes in the audio signal path. I would consider the AJ-1510 as a significant historic piece in tuner technology and a strange-looking device (conversation piece) , but performance(and sonic)wise only a average tuner. Reliability is another issue but can be improved. (see amptramp's post above for more details)

[Tubologic]

Originally Posted by amptramp

I have never actually used the zip function before, just the unzip, but here goes:

GREAT work and very useful informations. Thank you for sharing it with us.