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Heathkit AR-29

Agreed with BinaryMike, much of the knowledge I have from being bench tech back in the 70's is getting into its 4th decade. While I fixed a few Heathkit receivers mostly due to the builder not following directions, anything a poster does to make it easy to jog those memory cells will go a long way toward inviting me to take the time to reply.

Cheers,

David
 
Ok, ok. I get the picture. I had presumed that anyone interested in answering would have the manual in hand. My mistake. I do realize that there are some real audio gurus out there who might be interested.

"jog those memory cells" Yup, there myself for sure.

Anyway, yes, I will measure the voltages and post those voltages that appear under mono and stereo conditions on the new MPX chip's stereo light mode indicator pin. That way, if someone knows offhand which switching transistor operates at those voltages, we are then off to the races, hopefully. Should be able to use the existing circuit, with just a new transistor, I think.

Then, the next problem would be to devise a small circuit to power the separation adjustment circuit, about 0.4 volts which the original MC1305 IC pin 9 supplied, which will be disconnected from the new NTE722 IC because of having to use pin 9 for the B+, bridged from the old IC pin 8 B+ circuit board location.

Maybe a simple zener diode regulator circuit?

Thanks for any input. I do appreciate it. I love my old AR-29, and cannot stand the idea of throwing it out, just because some darn IC is no longer available.

Del
 
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This is interesting because, in addition to two AR-29s, I also bought a couple AA-29 amplifiers and an AJ-29 Tuner.

A couple of years ago the right channel out of the AJ-29 suddently became much weaker than normal and I concluded the MC1305 chip had gone on the fritz.

I communicated with a guy and traded an SN16852 quadraphonic decoder (he needed one for his Heathkit universal decoder) for an LM1305 (National Semiconductor) and installed it in the AJ-29. It works just fine and the tuner calibrated right up except the multiplex coil adjustments didn't seem to make any difference and I didn't investigate any further since the tuner seems to be at least the equal of my AR-29s in performance.

I guess I kind of surmised that the N.S. chip didn't need the optimization or is already internally optimized.

I have no idea where he got the LM1305 but maybe with some sleuthing, you could locate one.

And I think some of you guys were a little prickish to the new guy. Welcome to AK Del.

Doug
 
No joy in AR-29 land. Installed the NTE 722 IC in lieu of the MC1305, removed the components for the separation adj circuit, as pin nine on the new IC is bridged from socket 8 for the new 12v input. Left the Q301 installed (someone suggested disconnecting it's base lead, tried that, but no change). Q301 is the stereo light switch.

So, no input for the sep adj. circuit. Found that it is difficult to come up with a small circuit using 12v to get the 0.4v required for that circuit, as zener circuits for instance don't supply below about 1.2v or so I guess. Must be another way.

Went through the recalibration/adjustment sequences, so that the signal and tune meters are adjusted correctly, can't do anything with the 19 and 38 khz adj checks.

What happens now is that when I turn the receiver on from stone cold in the morning, I get what appears to be slightly degraded stereo sound and the stereo light comes on. After several minutes, however, the stereo light goes out, and the signal becomes a stronger monaural. From then on, it is always mono until the next morning. Will have to remember to try measuring the voltage of pin six with the stereo signal before it switches to mono. Reads about 11.5v-12.0v at mono. Should read about 0.6 with stereo and the MC1305 IC, but who knows what it really is with the NTE722.

Anyone have ideas?

thanks,

Del
 
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this probably won't help at all - but the warm up effect suggests an elderly passive component (capacitor or resistor) is changing value as it... warms up. Got a can of "Freeze-Mist" component chiller? Start shpritzing stuff and see if you can force it back to its cold operating condition... and you might be on to something...
 
Actually, the connection to pin 9 of the MC1305 is not a supply voltage to the separation circuit but is just what the DC level is at that point with a typical signal and the separation control adjusted to a typical value.

The separation control works on some transistors in the chip itself to improve the detection level and thus, separation of the two channels. It appears to me that the NTE722 does not require this control and should work fine without it or its associated circuitry.

Actually looking at the schematic and the pinouts of the two chips, it seems the NTE722 should work fine as the crucial pins are the same and the 722 doesn't seem to need some of the voltages of the MC1305.

It does seem that there may be another problem as Mark suggested. It's always a crap shoot substituting chips, though, unless you have very specific information. But, the fact that the operation is changing as the unit warms up suggests a thermal problem with a component.

The 722 isn't getting hot, is it?

Another thought, though. If you put the original chip in there, even though there is no stereo, does the operation stay consistent as it warms up? If so, it indicates a problem with how the 722 is working in that circuit.

BTW, proofreading my post alarmed me how many times I used the word "seem". :D

Doug
 
Doug G. said:
... [technical stuff that I didn't understand :-P]...

It does seem that there may be another problem as Mark suggested. It's ...always a crap shoot substituting chips, though, unless you have very specific information. But, the fact that the operation is changing as the unit warms up suggests a thermal problem with a component.

The 722 isn't getting hot, is it?

Another thought, though. If you put the original chip in there, even though there is no stereo, does the operation stay consistent as it warms up? If so, it indicates a problem with how the 722 is working in that circuit.
Click to expand...

Good question and good idea!

BTW, proofreading my post alarmed me how many times I used the word "seem". :D
Click to expand...

How unseemly :-P
 
Thanks for the replies. Doug's comments make sense. The 722 does remain close to ambient room temperature at all times, measured just above at ~80F.

There appears to be no difference in operation between the two IC's. The 722 IC outputs mono signals just as the 1305 had. The stereo light no longer comes on from stone cold. The readings at the pins of the 722 IC read as follows (with and without the base of Q301 stereo light switch disconnected), from 1 through 14 (Heathkit voltage values in parentheses):

1 12 (11.6)
2 2.8 (0.0)
3 3.1 (3.0)
4 2.6 (1.5)
5 12 (2.5)
6 0.3 (0.6)
7 0.0 (0.0)
8 12 (old B+)
9 12 (new B+)
10 4.5 (3.9)
11 4.5 (9.5)
12 4.5 (9.5)
13 4.5 (3.9)
14 0.0 (0.0)

Certainly, pins 4, 5, 6, 11, and 12 seem to be out of whack compared to the Heathkit manual voltage chart. Pins 8 and 9 will of course read 12v as input due to the bridge.

Maybe now the IC is blown. In handling it, everything was properly grounded, including myself, lol, but maybe having the base of Q301 connected eventually did something to the IC. This IC never did output stereo reproduction, however, except maybe for the apparent several times at the beginning after the installation of the 722 when the stereo light came on. I will pull the Q301 to check it out.

Also, have to admit, with the Heathkit diagnostic chart suggesting that amplifier coils L301, L302, and L303 might be a problem for having no stereo reproduction and no light. Don't how to check them, except that the solder connections are sound, or even if they are replaceable now. I do have access to another of the 722 IC, so may try again when I get it.

All of the other voltage readings on the Multiplexer board as shown in the manual are close to those required.

I'm beginning to think that restoration of the stereo reproduction may be a lost cause on this receiver. There may be something fundamentally wrong with the multiplexer board.

thanks,

Del
 
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Of course, the ideal thing to do is see if the composite FM waveform is actually reaching the multiplex board from the AM-FM IF circuit board at pin J3-6 of the multiplex board (pin 3 of IC302) with an oscilloscope because without that happening, you have no chance of getting stereo.

If that waveform is there, you at least know the problem is on the multiplex board itself. If any components in the 19kHz or 38kHz tuners (coil circuits) are bad, that will kill stereo since IC302 will not be able to decode the channels. In fact, that's what I suspect is happening and your original MC1305 may still be good which would be nice. \:^)

I doubt that anything other than a short or overvoltage would ruin the new chip. Of course, you may want to ensure twelve volts is getting to the chip too.

Doug
 
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Alas, I have no access to an oscilloscope, so I cannot check that composite FM signal from the AM-FM-IF board; however, it makes sense. Maybe I'll have to find someone locally who has a scope to check that signal.

I have checked all of the voltage readings for that particular AM-FM board, and all are correct, as are the relevant voltages on the multiplexer board. Also checked all soldering and the AM-FM circuit board itself for breaks, leaky capacitors, etc. Also checked the continuity of the L301 amplifier coil, and found nothing wrong there. I have never touched any of the amplifier coils, have assumed that they have remained tuned as from the factory. The voltage readouts at the pins of the 722 IC are as I reported in the last message, with the exception that pin 6 reads 11v, as the base of Q301 is connected right now, as I said.

Did discover that if pin 6 is shorted to pin 7 of the NTE 722, the stereo light does stay on continually. Pin 6, as it does read 11v with the base of Q301 connected, is then supplying power to pin 7, which is otherwise reading 0v. Pin 7 is connected to the light circuit through the collector of Q301.
 
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I reinstalled the removed components for the sep adj on the multiplexer board, removed the pin 8 to 9 bridge, and reinstalled the MC1305 IC. The pin voltages at the IC read exactly as given in the manual for no stereo output, ie, 11v at pin 6 for no stereo, the other pins as well matching the manual. Running a test for composite stereo output from the AM/FM/IC board, of having a no station hum with the treble turned up all the way, the mono switch makes no difference in the high frequency output, just the same constant volume usual no station buzz or hum. If there had been a change, it might have indicated that there was a composite signal.

Testing between D and E on the board (left and right signal out), with the unit off, and toggling the mono switch, the ohm meter reads13k ohms switch out to 2k ohms switch in. Normal resistance.

I think the NTE722 IC I had is kaput now, as originally, when I first installed it, the pin readings were close to the manual readings for the 1305, with pin 6 reading 11v for no stereo. Later the 722 IC readings became as listed in the earlier email, with pins 10, 11, 12, and 13 reading 4.5v, etc. Must have shorted it somehow. Another one should be arriving n the mail momentarily from a very slow outfit. Should have ordered this one from Mouser as I did the one I just fried. Mouser delivers almost immediately. Good outfit.

I'm beginning that the problems is with the AM-FM-IC board, although as I mentioned before, all the voltage readings for that board are correct, no bad solders, no breaks in the board, no leaky capacitors, and no coils, etc, have been touched, but show continuity as they should.

Well, at least I have good mono FM. AM is ok as usual.

thanks,

Del
 
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In all honesty, if I were getting signal to the multiplex board and I didn't have an oscilloscope, I think I would assume the complete composite signal was there and troubleshoot the multiplex board itself.

It sounds like either the 19 or 38kHz circuitry is malfunctioning and not allowing the decoder chip to separate the channels.

How about the two switches for those frequencies to switch between calibration and normal operation. I don't remember exactly what happens when they are switched but I would check them to be sure they are working and not intermittant.

Other than that, checking the components in the tuning circuits would be a good idea.

Doug
 
Well, you are probably correct now that I think further on it.

The 19 and 38 kHz switches both work properly. When each of them is switched to adjust, the signal meter goes to zero. The adjustments called for in the manual have absolutely no effect.

All of the L coils have continuity. Those adjustment circuits are dead.
Also, 19 kHz sensitivity R310 has zero effect on the signal meter when the 19 kHz switch is thrown for adj.

The voltages for Q305L and R do read 6.5 volts as they should. The voltages for pins J11-1, -2, -3, -4 read 5.5v.

The only thing I notice which might be anomalous is that the slug for L303 is backed out about 6 full turns from the approx positions of those of L301, L302. Almost coming out, sticking out about 1/8 inch above the tube it is in, whereas the slugs for L301, L302 are sunken in about 1/8 inch below the top of the tube. Don't know if that means anything however, as I have never been able to get the signal meter to indicate anything but zero, completely dead, when adjusting coils L301, L302, or L303, as the manual says I should with the 19 and 38 kHz switches each in their adj positions.

One last thing, the signal meter almost always read a full 5 on the scale when tuned to a station, almost never anything lower at a station. Could never get it to adjust with the 19 and 38 kHz adjustments as mentioned in the manual.

I guess I'll just have to check more components on that board.

Just did find that capacitor C309, 2 microfarad, in the part of the circuit leading to the 19 and 38 kHz switches, is most likely open according to the test from the manual. Will change that. R306 is ok. C309 changed, no difference in signal.
 
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Still no stereo. Since the 722 IC before I fried it also didn't give stereo when installed, but all the voltage reading for both the 1305 and 722 IC's and the other board voltages were as per the manual, except for the monaural signal IC pin voltage being that for monaural rather than for stereo output at the IC, I guess since I still cannot find a bad component on the Multiplexer board, I'll just live with the monaural signal. Wish I had an oscilloscope but that's the way it goes.

I don't think anyone has any other answers.
 
One thing I forgot to mention is that, circuitry connected to unused pins on the new IC will, of course, have no effect since there are no internal connections to those pins in the 722.

I'm not looking at the schematic right now but if that coil with the slug up above the windings is connected to active circuitry in the IC, that is definitely not normal. Normal adjustment should place the slug down into the coil. Of course, if that coil is connected to an unused pin on the IC, it doesn't matter.

If I remember correctly, none of those three coils had any effect when I did the adjustments on mine with the LM1305 chip in place but the chip does actually work and produces stereo just fine.

I think they just made the newer chips with the multiplex optimization internal to the IC instead of relying on external adjustments.

Doug
 
Thanks for additional info. Still trying components on the multiplex board, but no faulty components found yet. Have checked the continuity of just about every circuit board connection on both the m board and the am/fm/if board,. Nothing wrong there. Sigh.
 
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