McIntosh MC60 Pulses

AA6U

AA6U

Active Member
I'm working on a McIntosh MC60 (model with the neon bulbs for voltage regulation) that is putting out a short, low pulse every 4-5 seconds. It has been completely rebuilt and worked great until one of these events caused it to start pulsing (not sure which):

Event One- One of the 5U4 tubes shorted and blew the fuse. After both were replaced, the owner thinks the pulse may have started.

Event Two- The owner switched out the 6550 output tubes and he installed one without the pin locator on the bottom of the tube incorrectly and blew the 220 ohm 1W screen resistor for that tube as well as the fuse. I replaced that resistor and tested all of the components around both output tubes and they test fine as well as the other resistors.

During a pulse, the neon lights voltage regulators go completely dark but come back on quickly. Also, the grid bias voltage jumps around and the B+ voltage jumps a little during a pulse.

With no audio input and the output tubes installed, the pulse sounds like a soft bump in the speaker. With audio playing, the volume decreases moderately for a split second during the pulse.

The pulse occurs with or without output tubes installed, with a dummy load or a speaker connected to the output, and with or without the 12AU7, 12BH7 or second 12AX7 installed. I have checked all of the filter caps and they seem to be fine, short of replacing them all. The output transformer measures the correct ohms.

I installed a different set of tested driver tubes, and it behaved the same.

As a possible solution, I replaced C4, the filter capacitor in the voltage regulator circuit, and C5, the bias filter capacitor, but neither solved the problem. I've also checked various resistors and everything seems to be in spec.

The audio sounds great. It's just that it pulses every 4-5 seconds. I've asked some of my tech friends and they are perplexed also.

Does anyone have any thoughts or suggestions on what's causing this and how to solve it?
 

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have you replaced the neons themselves? I've had NE2 bulbs get funny with a lot of hours and require extra voltage to strike or they just get intermittent and flicker for no reason.

these are being used basically the same as gas regulator tube, besides dud bulbs, the only thing that comes to mind that would make them drop out is insufficient voltage, or insufficient current. Might be worth checking R9, R10, and C14 for issues if the plate voltage at V1B is good.
 
The fact that the Ne lamps goes out is an indication of the problem.
Neon lamps needs a certain current to keep glowing, if below that current the ionization withing goes out
and the lamp becomes high resistive. In connection with the capacitor C14 it becomes a "flasher"

If voltage is 350 Volt at R10 then the explanation is that R10 has changed in value or that one of the Ne lamps
has aged . Reducing R10 somewhat will cure the problem.

A better solution is to replace the Ne lamps with zener diodes,recalculation of circuit values, the main
goal is to have the grid of V1B at 120 Volt.
 
One additional point, if the bulbs are aged and in need of replacement, there's a big difference between NE2 and NE2H bulbs; the NE2H drawing more than twice the current of the NE2. Aged neon bulbs also sometimes don't strike properly in a dark environment, so beware of old stock NE2s.

The zener mod is the most reliable long-term solution.
 
zeners are more reliable, but don't look as cool as a part that glows. If they are buried inside though I can't really make a good argument for the visual aspect.
 
Thanks for the analysis/recommendations. a zener seems to be the consensus. While the neon bulbs are cool, they are not critical because they are enclosed in the chassis and can't be seen. What wattage zener should I use?
 
Neon bulbs certainly do go stupid on a regular basis, but we only notice this when they are used for purposes other than lighting,haha. Replacing those bulbs with Zeners is definitely the way to go for a long-term solution.

I have an old Fluke 407D high voltage power supply that I have dragged out of retirement and am starting restoration on. It uses a bunch of NE2E & NE83 neons throughout the circuitry, and they have all degraded to the point of causing the unit to become very erratic and unstable. Well, in addition to all of the extremely drifted carbon composition resistors, dead electrolytics and leaky bumblebee caps :rolleyes:
 
well in my opinion redesigning the circuit to get those gimmicky NE2's out of there is the real solution. I can't believe McIntosh stooped to such silloy levels just to sell an otherwise excellent product.....but then that is just me!
 
NE2s were effective regulators. Same principle as the VR75, VR150, et.al. Zeners weren't really a thing when the 240 was designed in the late 50s, and even then, it took some time before they became reliable.

Not a bad design at all, given the era. However we can do better now, as with many things.
 
w3jn said:
Not a bad design at all, given the era.
Click to expand...
Absolutely agree. My first ham transmitter used a string of NE2s to regulate the screen supply for a 1625. Neon life was the last thing I ever had to worry about. :) As for doing better now, I'm not sure that's true. This amplifier is decades old, and I've seen a lot of Zeners that didn't last that long. If it was mine, I'd just buy new neons from Digikey and see if that fixes the problem.

Jack
 
Just throwing in a new neon really sounds easy until you actually try doing it...there are several specs associated with using a neon lamp in this kind of application. You need to meet most of them in order for the circuit to work as intended, if at all. Been there, done that.

Unless you can source an unopened box of old stock neon bulbs, made by a reputable manufacturer, you never know what you're getting. Like most modern incandescent bulbs, todays neons are all made in china, and suffer the usual chinese issue: They slap whatever number on whatever garbage they have that kinda-sorta looks like that part number, and ship them out. Specs?? What specs??
 
gadget73 said:
probably so someone wouldn't freak out about evil solid state parts installed in their tube amp.
Click to expand...
If you were replying to me with that comment...no not at all. I've designed, rebuilt and repaired countless pieces of tube audio gear in my day... In my vast experience in this field circuits that employ gimmicky stuff like NE2 or insert whatever goofy new fangled reinvent the wheel modern component you want that very same part is either what fails and causes some normally reliable part to fail... These silly things are just not required to have a lovely real sounding and very reliable piece of tube gear... They are the result of bored engineers trying to get there names up in lights reinventing something that was designed to perfection 8 decades ago...
 
arts said:
Just throwing in a new neon really sounds easy until you actually try doing it...there are several specs associated with using a neon lamp in this kind of application.
Click to expand...
You can download a manual (below) that explains the different types of NE2 and the various suffixes. Even without that, it's pretty simple to select one that will work in this particular circuit. Last time I looked, Digikey was stocking all the main types.

Using and Understanding Miniature Neon Lamps
VCC Neon Indicator Lamps

Jack
 
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arts said:
Just throwing in a new neon really sounds easy until you actually try doing it...there are several specs associated with using a neon lamp in this kind of application. You need to meet most of them in order for the circuit to work as intended, if at all. Been there, done that.

Unless you can source an unopened box of old stock neon bulbs, made by a reputable manufacturer, you never know what you're getting. Like most modern incandescent bulbs, todays neons are all made in china, and suffer the usual chinese issue: They slap whatever number on whatever garbage they have that kinda-sorta looks like that part number, and ship them out. Specs?? What specs??
Click to expand...
They don`t need no stinkin` specs !
 
arts said:
Unless you can source an unopened box of old stock neon bulbs, made by a reputable manufacturer, you never know what you're getting. Like most modern incandescent bulbs, todays neons are all made in china, and suffer the usual chinese issue: They slap whatever number on whatever garbage they have that kinda-sorta looks like that part number, and ship them out. Specs?? What specs??
Click to expand...
You can find NOS NE2 bulbs. But, that's just putting the weak link back into the circuit.

Right? :idea:
 
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I agree on not using neon bulbs. I'm going to put in zeners in both MC60s I'm working on (the amp with the problem and its twin that doesn't have a problem- yet). The zeners are supposed to arrive today.
 
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