Heathkit AC-11 Multiplex Adaptor

oldradiotvnc

New Member
What kind of experience have any of you had with this device? I have two of these that I have gotten to decode FM stereo from multiplex outputs of tube tuners quite well- for a while. I believe I have recapped one, just about totally, and the other one I think I only replaced suspect coupling caps but the electrolytic filters seem to be good. I have one major problem that both of them are doing, though. After a while, the alignment of the 19 KHz oscillator seems to go out, and one has to readjust the coil in that circuit. The 19 KHz. signal loses its "lock" with the pilot signal, becoming out of phase, and the demodulated result is a "warbling" or some other oscillating sound in the output. I have tried other 12AU7 tubes as well. Do you guys think I just keep on trying bad tubes, even if they are NOS, or are there some kind of other parts I should be looking at that maybe some of you found if you restored any of these? Do any of you have one of these working well without drifting off so much? Let me know, thanks.
 
What you describe is the classic sound of the 38 kHz oscillator losing its lock to the 19 kHz pilot signal. Casual readjustment of the 38 kHz coil can sometimes restore decent operation -- but with a 50/50 chance that the channels are reversed. It really needs to be placed on a scope and driven with a good FM stereo generator to find out were the loss of signal lock is happening. Can you provide a schematic?

Dave
 
Thermal drift of the local oscillator is a common trouble in old (tube based) multiplex decoders. The AC-11 uses a 19 KHz synchronized local oscillator built around V2/T2 which is then doubled by T1 to recover the 38 KHz synched signal needed for stereo demodulation. There are very few components affecting the frequency of the 19 KHz oscillator (T2/V2/C8,C9,C10) you should definitely check C9 (0.01µF) around the oscillator coil and replace it (if needed) with a capacitor of same technology and t° coefficient as the original part. This is very important to help stabilize the oscillator frequency against temperature changes. If you have already changed this capacitor, better to put the old one back (if not defective). To pre-adjust the 19 KHz (T2) oscillator first put it in free running mode by shorting V2A grid (pin 2) to ground and monitoring its frequency with a digital counter loosely coupled (a few turns of wire around T2 should be enough to pick up the signal ) : the frequency must be adjusted to precisely 19.000,00 Hz and should not drift too much. Then remove the short and proceed to a complete re-alignement using a good Multiplex generator. You should also check C7 : if it opens there will be no sync signal at V2A grid to lock the 19 KHz oscillator and the symptoms will be the same. (warbling sound, phased or poor stereo,etc...) Finally, it must be noted that many cheap old tube stereo decoders suffered from poor thermal stability and some drift was allways present: this is the reason why you often found old HEATHKIT's and EICO's MPX decoders with heavily perforated or missing covers. It is recommended to align the set only after thermal equilibrium is attained. ( 15 to 30 minutes with cover in place).
Hope the above informations may help you...
 
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Sorry to hijack and revive this old thread but...:

To Tubologic:

I also have two of the Heathkit's multiplexers and one of them exhibit the same behavior as the OP. When you say "To pre-adjust the 19 KHz (T2) oscillator first put it in free running mode by shorting V2A grid (pin 2) to ground and monitoring its frequency with a digital counter loosely coupled (a few turns of wire around T2 should be enough to pick up the signal )", I guess the adapter should be on only with no signal going through?

I have an Heathkit frequency counter, should I just run wrap the cable around T2 or could I connect it to another point in the circuit?

In my case, I've tried to replace C9 with no improvements, meaning it is ok first but drifts after half an hour and should be readjusted.

Here's the thread I've made a few months ago (with good advices too should I add): http://www.audiokarma.org/forums/showthread.php?t=397820

Strangely, I have one with original coils and another one I've replaced the 19Kc coil with a junk part (coming from a dead Knight MPX tuner). Guess what, the Franken-Knight-Heathkit works as it should (after recap obviously) and the other one drifts and there's even some crackling in one channel (after a recap too).

Thanks.

Patrice
 
Yes, there should be no signal connected to the adaptor when adjusting the 19KHz oscillator in free running mode. It's better to have a loose coupling to the oscillator coil because a direct connection may load and detune the circuit. Some frequency counters are not sensitive enough for this purpose but you can sometimes pick up a stronger signal by putting a ungrounded tube shield around the oscillator tube (V2) and connecting it to the hot side (input) of your f-counter. Use a 10 sec. gate time on your counter for better resolution. A freezing spray may be useful for detecting t° sensitive parts. I suspect that part of this problem is structural as I've never encountered an AC-11 without some oscillator drift. Maybe your "Franken-Knight-Heathkit" solved this problem and is the only stable AC-11 ever produced ?
 
Thanks, I appreciate. Once I go to the cottage (where resides my second Heathkit), I'll begin the investigation and troubleshooting.

Oh, BTW, could the 38Kc be verified the same way (with FC)?

Patrice
 
Just out of curiosity, have you checked the resistors for being in spec? Old resistors tend to drift, and that could possibly throw things off alignment. If you really want to check them, hook up the ohm meter and warm the resistors with a hair drier or something. If they change value significantly, it may be worth replacing them.
 
Just out of curiosity, have you checked the resistors for being in spec? Old resistors tend to drift, and that could possibly throw things off alignment. If you really want to check them, hook up the ohm meter and warm the resistors with a hair drier or something. If they change value significantly, it may be worth replacing them.

All the resistors were already replaced... But maybe one of them is defective.
 
Possible. May be worth confirming they are all in tolerance.

and yes I think the 38kc can be verified with a counter, though its possible that it's frequency will depend on the exact frequency of the 19kc tone. I know I was able to verify it when I was playing with my C-M MPX unit, but I don't know a bunch about these, and I know nothing about the Heath unit specifically.
 
A counter can to used to verify the accuracy of the regenerated 38 kHz sub-carrier frequency, but frequency itself is not the only critical issue: If the phase of the sub-carrier is reversed, it will swap the resulting L and R outputs. The phase can easily be reversed through improper adjustment of the 38 kHz transformer, although overall performance can still appear to be otherwise normal.

Dave
 
A counter can to used to verify the accuracy of the regenerated 38 kHz sub-carrier frequency, but frequency itself is not the only critical issue: If the phase of the sub-carrier is reversed, it will swap the resulting L and R outputs. The phase can easily be reversed through improper adjustment of the 38 kHz transformer, although overall performance can still appear to be otherwise normal.

Dave

This is correct: the regenerated 38KHz subcarrier phase is very critical and must be maintained within +/- 3 degrees between main and subchannel to achieve 30 dB of separation. A perfect 180° phase reversal result in a L/R channel inversion which is not too serious as far as the separation is maintained, (there's about 50% chances that L/R channels are reversed at some point in the chain between the broadcast studio player and your playback system at home) but in between these two limits the channel separation is severely compromised (L "leaking" into R, and conversely), or even absent (no stereo decoding). Accurate alignment can only be done with a good multiplex generator with MPX modulated RF feeding the tuner because it is the phase response of the tuner-adaptor combination which is the most important.
A poorly aligned or insufficient bandwidth discriminator in the tuner will also result in poor separation or total lack of stereo. Add the thermal drift problems typical of tube circuits to the picture and you'll get a fair idea about how difficult it is to maintain good channel separation.
Who said it was easy ?
 
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oh, and something else worth mentioning, depending on the design and where you hook things up. The counter can sometimes load an oscillator circuit down enough to shift it's output frequency, so you may have a situation where its adjusted with the counter connected, but with it unhooked things are wrong. I like to use the scope as a buffer / amplifier for the counter for this reason. A 10x probe should give enough isolation, but if it doesn't and you don't have a 100x probe, a 100k resistor in series with a 10x probe will generally do the trick.
 
Hey, thanks to all: a lot of things to read again and digest correctly before taking the plunge.

About the phase shift: I obtained a reversed LR by tuning the 19Kc coil too.

Admitting I tune the 19Kc and 38Kc coils to exactly their values, it is not 100% guaranteed the MPX will work as it should, correct?

Gladly, it is just a hobby for me...

BTW, a Lafayette LT-220 was added to the family last summer and soon, an Eico MX-99 will find a way to my home. Once I'll be able to align one of my multiplexers (it's why I've choosed the intermittent functionning one, the AC-11), I guess I'll be good to restore the other ones to specs.
 
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If the 38kHz subcarrier has it's phase reversed for any reason -- be it an improperly adjusted 38 kHz transformer, or changing the ultimate phase of the pilot signal, the end result will be the same. Also, simply "peaking" the 19 and 38 kHz transformers is no guarantee of proper, or optimum performance either. As Tubologic pointed out, no more than a 3 degree error can be allowed to achieve proper separation. As the phase error increases from this specification, separation starts to deteriorate real quick.

If you are trying to restore any decoder to "specs", then a good multiplex generator is a must, and again as Tubologic points out -- one that can actually transmit a full composite stereo multiplex signal to the tuner/adaptor combination in use is the only kind to consider. This will also require a good scope of at least 10 mHz bandwidth, and preferably, much more. Any error in phase response in the scope used due to limited internal bandwidth will only act to introduce separation error into the decoder alignment process.

There are modern generators you can get rather readily. I have three generators from the period: A rack mounted HH Scott 830 unit -- but it has no FM carrier generator in it, a Heathkit -- but it only has one balanced modulator circuit (which prevents simultaneous L and R modulation), and a Fisher 300, which does it all. Is it any wonder their tuner/decoder combinations were some of the very best produced back in the day?

Dave
 
Well, an Heathkit IG-37 is coming home soon but I realize that I'll have to refurb all my testing gear prior to use it (Heathkit IG-37, Heathkit IM-2410 frequency counter, an Eico 360 sig gen, a Stark sig gen, a small 70's o'scope and an old tubed Telequipment o'scope. Also, an Eico VTVM...

Dave, could I guess the Heathkit stereo gen you own is in fact an IG-37?

Patrice

**So far, the AC-11 is less prone to drifting since I've replaced the 12AU7 associated with the 19Kc circuitry but it's not perfect... It was paired with an old Arkay FM-8 this time. The MX-99 I just acquired seems to work as it should... except that its output is really hot (it overdrives the input of a little tubed console, in mono mode and stereo mode. Maybe the big 50 uf electrolytics at the output?).
 
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Not to engage in thread necroambulation, but I just came across this thread while looking into MPX units for a non-MPX tuner. I believe I have an explanation for the problem AC-11 issues, and thought I would share that in case it is helpful.

Note: this hypothesis is speculative as I do not have an AC-11, so I have no way to verify this. I hope someone with the unit performs this test. But I suspect my hypothesis explains the cause of the drift and poor functioning.

IF cans are typically hybrid creatures, containing:
(a) a coil with a tuning slug
(b) a silver-mica capacitor at the base. The capacitor is not encased and is created using a sheet of mica silvered on two sides.​

Over time electrochemistry resulting from potential difference plus moisture causes silver ions to migrate through the mica; this is the infamous "silver-mica disease". The migration ruins the capacitor, and may even bridge the primary and secondary. The leakage of B+ to the secondary eventually becomes a short, but prior to that point the leakage couples all manner of noise from primary to secondary. In an radio it sounds like crashing waves of static. In addition, sulfidation of the silver—silver sulfide is an insulator, unlike silver oxide which is a conductor—creates a poor contact and the capacitance continually varies as the frequency cycles. The effect in old radios and TVs is called "scintillation". Corrosion also causes impedance changes which results in frequency drift as the LC circuit changes its resonance point since the circuit is now RLC instead of LC, and the R is likely frequency dependent.

I suspect the replacement coil above described either did not have a defective capacitor or, more likely, had one less defective.

The solution is to remove the capacitor in the can and replace it with an external capacitor. Modern silver-mica capacitors use a silver alloy which does not migrate. I don't know how much better temperature stability would be obtained with SM vs. a modern PP.

Many times the capacitor value is undocumented, and the reported fix is to experiment with various values until the LC resonance point is hit. This surprises me because resonance occurs when XC(f) = XL(f), so if L is known and f (resonance point) is known, then XL is known and consequently XC must be known, and C can thus be calculated. I suspect a variable capacitor would be the easiest way to do the experimental calculation. In any event, once determined the new capacitor can then be soldered underneath the can.

In any event, I believe that deterioration of the capacitor in the IF can would explain the lack of frequency stability and other poor functioning.

The poor reputation for the AC-11 may have something to do with IF can issues.

I hope someone with an AC-11 will be able to verify this hypothesis and report back.
 
The Heath AC-11 does not utilize conventional IF type cans with the internal silver/mica caps. Rather, it uses “open” type slug tuned coils with external dipped silver/mica caps soldered to terminals on the coil former.

The mediocre performance of the AC-11 is due primarily to a mediocre circuit design combined with a poor choice of user accessible controls and an alignment procedure in the manual that ignores the critical nature of 38KHz phase. Heath corrected some of these issues in later versions of the basic AC-11 decoder circuit they incorporated into their FM stereo tuners but stable performance was never a characteristic of this design.
 
Do you know if those dipped capacitors are phenolic case with wax coating? If these are phenolic jackets, those permit moisture and this causes silver-mica disease. That would 100% account for the temperature drift I above described.
 
The critical mica caps in the two AC-11s I’ve encountered are coated with a maroon plastic material, probably epoxy like more modern versions. Definitely not phenolic or wax impregnated “durez”.

Frequency instability isn't the only problem with the AC-11: the basic design is mediocre.
 
Fair enough. This would not be the first time Heathkit cut corners.

I've seen discussions that many of the early MPX units suffered from silver-mica disease in the cans and this caused thermal instability and sloppy decoding.

Beyond poor design, do you know what causes the temperature drift?

Everything I've read about LC oscillators, resonators, and tank circuits suggests that a key factor in stability is the capacitor's thermal stability, which is why mica was used. Corrosion may have been a bigger problem in the older radios. I have a MPX decoder and a tabletop radio both of which need to have every single can disassembled to remove the deteriorated capacitors in the cans, which is how I came to the issue.
 
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