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 Post subject: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite Fixed!
PostPosted: Aug Sat 14, 2021 1:17 am 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 2008
Location: Great Bend KS
Customer brings this in for blows fuses at turn on. Simple repair, shorted dual rectifier diode in power supply. But noticed stereo light doesn't work. Lamp is good, troubleshooting the fm stereo circuitry is getting me nowhere. Set uses a MC1307P IC for stereo demod, all voltages check like the data for the IC says except pin 6 is zero. Electrolytics changed. But the schematic shows pin 6 of the IC is driving the stereo lamp, but in actuality it's pin 14 on the pcb, pin 6 is blank, not connected. I get zero volts on either pin whether on fm stereo station or not. Also the schematic shows one lamp driver transistor but actual circuitry has two, and the pictorial of the service manual shows two transistors but the actual set only has one. I know, it's a design change. But I would like to know what kind of signal I need to get the lamp to light on a FM stereo station. I'm thinking if I knew exactly how the fm stereo circuit is supposed to work, the 19kc and 38kc signals and pilot signals are getting lost in my mind any more! Is there an online refresher course somewhere?! I don't want to diddle the alignment yet!


Last edited by SteveT on Aug Mon 23, 2021 8:47 pm, edited 1 time in total.

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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite
PostPosted: Aug Sun 15, 2021 3:25 am 
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SteveT wrote:
But the schematic shows pin 6 of the IC is driving the stereo lamp, but in actuality it's pin 14 on the pcb, pin 6 is blank, not connected. I get zero volts on either pin whether on fm stereo station or not. Also the schematic shows one lamp driver transistor but actual circuitry has two, and the pictorial of the service manual shows two transistors but the actual set only has one.
On the MC1307P there are two possible lamp outputs. Pin 14 goes High when the Stereo Pilot signal is detected. While pin 6 is an "Open Collector" output which pulls toward ground when the Pilot signal is detected. So If you put an Ohm meter on pin 6 you should see a low resistance when stereo is present (it never goes high in voltage because it isn't connected to any voltage). See data sheets on the MC1307 here https://elcodis.com/parts/6014751/MC130 ... #datasheet

The whole stereo system depends on sensing the 19 KHz pilot signal which is in turn doubled to 38 KHz to mix with the received signal to demodulate the Left-minus-Right channel. It is detection of the Pilot 19 KHz signal that causes the Stereo light to be lighted either by 14 going high voltage or 6 going low resistance.

So the MC1307 could be bad or the 19 KHz tuned circuits or the 38 KHz tuned circuits may be tuned off frequency. There is no real reason for the tuned circuits to be off frequency unless someone was in there with a "golden screwdriver" (let's tighten these up to make things better).

Here is another thread that may shed some light on the subject (pun intended)... viewtopic.php?t=375988 Apparently the NTE722 is a replacement for the now discontinued MC1307P.

Curtis Eickerman

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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite
PostPosted: Aug Sun 15, 2021 4:38 am 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 2008
Location: Great Bend KS
Thank you so much for the info! Gives me the courage to get back into this thing!


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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite
PostPosted: Aug Sun 15, 2021 12:35 pm 
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I've seen plenty of those chips go bad, but usually, someone has tried an "alignment" with nothing more than a metal screwdriver and a wish. Might want to run through one before changing too many parts ... unfortunately to do it right requires either a good FM stereo generator, or a lot of prior experience and some luck. I've done it both ways.

You don't sound like you've done this too many times, so I'd resist the urge to twiddle adjustments just yet. But that said, most of the time touching up the alignment restores good stereo most of the time. Can you tell if the thing is actually receiving stereo by listening? That might help to limit your troubleshooting to just the lamp driver system. In other words, if it's receiving and processing stereo, and it's just the lamp that's not working, you've eliminated a lot of fussing.

Maybe a local shop, if there is one, will let you use their FM generator long enough to get your FM stereo working.

Regarding your differences between the schematic and your unit, that makes things a little tougher of course, since you can't really trust what you see on the schematic anymore. Try to find the correct version, or revision, that matches what you've got.

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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite
PostPosted: Aug Sun 15, 2021 2:00 pm 
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Joined: Apr Wed 01, 2020 7:22 pm
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Check to see if it is actually producing a stereo FM signal. If it is only producing a mono output, it may be a discriminator problem and the signal for the stereo light might not be present.

I've seen that issue a few times and it only took a minor adjustment of the discriminator cores to correct the problem.

Don't "screwdriver" those cores or you may have to do a distortion alignment, or break a core and have to find a replacement, and then have to do a distortion alignment...

73...Jordan VE6ZT


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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite
PostPosted: Aug Mon 16, 2021 3:55 am 
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If the multiplex decoder circuit has a B+ feed, with a decoupling capacitor, make sure that the capacitor is not open; that will kill the stereo, and the light will not light. I had that problem once, years ago.

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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite
PostPosted: Aug Mon 16, 2021 4:32 pm 
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I used to collect a lot of Dynaco equipment in the 80's and 90's, and the FM-5 tuner model was notorious for having bad MC1307 decoders. About half of them I picked up had no stereo operation. I'm pretty sure the easy to obtain LM1307 is the same IC. Dynaco ran the lamp off of pin 1 with a sensitivity adjustment pot.

-Mark-

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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite
PostPosted: Aug Mon 16, 2021 8:36 pm 
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Jordan390a wrote:
Check to see if it is actually producing a stereo FM signal. If it is only producing a mono output, it may be a discriminator problem and the signal for the stereo light might not be present.

I've seen that issue a few times and it only took a minor adjustment of the discriminator cores to correct the problem.

Don't "screwdriver" those cores or you may have to do a distortion alignment, or break a core and have to find a replacement, and then have to do a distortion alignment...

73...Jordan VE6ZT


^^^This is the best advice I was about to give. Check (with headphones) if you have stereo reception. If not, tune at the center of the channel by ear (for strongest and cleanest sound) , disregard the center tuning indicator (if there's one) and slowly adjust the discriminator coil (usually the upper coil in the can, but not allways) until you got stereo (and the indicator lights up). Don't touch any other coil cores in the I.F chain, only the discriminator. This is a very common problem I've encounterd in many tuners/receivers. If this doesn't work you may also try adjusting the MC1307P decoder chip PLL frequency pot (if there is one) till the stereo indicator lamp ligths up and stereo reception is restored.


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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite
PostPosted: Aug Tue 17, 2021 2:35 am 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 2008
Location: Great Bend KS
Did some more checking on this thing tonight. The 19KC pilot signal is there, very strongly in fact, on a stereo station. And it appears to me that it is actually stereo playing in the headphones. Alignment cores have not been altered, takes a very tiny flat blade screwdriver type diddle stick which I don't have. Even though this set doesn't use pin 6 of the IC as the lamp driver output, there is a kind of a strong square wave at a 19KC frequency and absolutely zip at pin 14 which this set uses as the lamp driver. I guess I can assume the chip is bad, so might as well order the NTE replacement and see what happens! Thanks for all the ideas guys! Will let you know if this fixes it!


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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite Fi
PostPosted: Aug Mon 23, 2021 8:51 pm 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 2008
Location: Great Bend KS
Problem turned out to be the demod IC after all! Came in today and held my breath when I turned it on and viola! The FM stereo light came on on a stereo fm station! I guess 50 year old IC's do go bad occasionally. Thanks for all the ideas and assistance from you guys!


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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite Fi
PostPosted: Aug Mon 23, 2021 9:02 pm 
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Quote:
I guess 50 year old IC's do go bad occasionally.

Oh yes, they surely do. Fortunately you've got one that was still available. I had to retire a Pioneer RT-909 RTR deck due to an unobtanium bad system control micro. It made me sad. Especially since I couldn't charge for all the time I spent trying beyond hope to find some other issue, not the chip :(

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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite Fi
PostPosted: Aug Mon 23, 2021 9:48 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4787
Location: Boston, MA USA
In the tape recording community some are starting to favor machines of somewhat older technology manufactured with standard components and no digital controls or processing. I suspect the same will happen with hi-fi/stereo if it hasn't happened already.

-David


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 Post subject: Re: Harmon Kardon 630 Stereo, FM stereo lamp doesn't lite Fi
PostPosted: Aug Mon 23, 2021 10:11 pm 
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It has already happened, but this is one popular and high end deck, favored by the community. Most parts can still be had, or faked. Someone even had a run of 200 or so R/P heads made, and I imagine mad a lot of $

But a custom CPU….. if it’s dead, it’s dead

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