Author Topic: 1964 Zenith console mm2670  (Read 201 times)

SeniorSteve

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1964 Zenith console mm2670
« on: December 14, 2017, 03:16:09 PM »
Sorry for the long post, just had to show it off.

Picked up this Zenith MM2670 a couple of months ago off of CL.  Changer had issues, tuner really didn’t work and amp had one side out.  Asking price was more than I wanted to pay, but it was THE console I really wanted.  He came down on the price and even delivered it!  I gave him some cash for the delivery as it was over 30 miles away, and he had a truck.  This model is a hybrid with a tube tuner and solid state amplifier.  It hummed really bad when I checked it out.  This is the first chassis that every one of the electrolytic capacitors were bad.  The amp the tuner, EVERY chassis mounted cap was bad.  This is also one where it used germanium transistors for the power output and drivers.  Replacements could get very pricey so I thought to myself, “Why not convert to silicon”, as those type of power transistors are much less costly.  The amplifiers of this age normally had driver transformers, and this was no different.  All I had to do is to change the biasing resistors for the output stage so it works with the silicon transistors.  A little trial and error to get it right and I have the amplifier running silicon for the output and drivers.  The germanium transistors were up to about $27.00 each, and my replacements were under $3.00 each.  I thought that would be enough incentive to do the work.  I also replaced all of the electrolytic caps on the tuner chassis as well.  The tuner is quite sensitive on FM and sounds great.
One of the main reasons I liked this particular console is that the finish was basically flawless.  It has a couple of small dings, but being manufactured in 1964 it really has a beautiful finish.  I wanted to think I was done, but there was something lacking in the lower bass ranges.  Pulling one woofer I checked to see what the free air resonance was, and it was almost 70hz.  The set had wonderful upper bass, but nothing in the lower registers.  The cabinet is constructed in a way that you can easily enclose the speakers and seal them.  That is the approach I took along with new woofers.  I picked up a pair from MCM (now Newark Element 14) for under $15.00 each (plus shipping).  Using some pipe organ music for a test I was truly impressed as to what this console puts out. 
The changer is also one of the ones that Zenith made in house.  The platter is belt driven and it also has the unique “pop up” 45 RPM spindle.  This changer is supposed to be easy on records and I was able to set the tracking force to under 3 grams.  I think that’s pretty good for a changer that is over 50 years old.  This one is definitely a keeper!!

TC Chris

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Re: 1964 Zenith console mm2670
« Reply #1 on: December 14, 2017, 07:49:11 PM »
Talk some more about the germanium-to-silicon conversion, and the different bias requirements.  This is an area where my knowledge is weak.

Chris Campbell

Motorola Minion

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Re: 1964 Zenith console mm2670
« Reply #2 on: December 15, 2017, 01:59:29 PM »
That Danish-modern cabinet from Zenith was very popular for several years. I once had a 1971 model like that and it used 10" woofers and Foster horns for HF drivers. Another 1968 model I had used RCA-sourced silicon output transistors.

Great job on changing those transistors. I'm not sure why Zenith used Ge versus Si transistors, as Magnavox went right to RCA for their best stuff for the 1963 Astrosonic, silicon. It has been suggested germanium fails much the same way tube cathode emission falls off with usage.

It sure looks like a top of the line tuner, as the later expensive Zenith models used lots of buttons and lights. The record changer is also the best available that year, and later year models for that matter. What cartridge does it have?
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Dave

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Re: 1964 Zenith console mm2670
« Reply #3 on: December 15, 2017, 02:03:31 PM »
Talk some more about the germanium-to-silicon conversion, and the different bias requirements.  This is an area where my knowledge is weak.

Chris Campbell

I would like to hear about this too. I assume bias resistor values in base/bias circuit and collector/emitter load resistor would change. Did you need to alter the power supply also?
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Dave

SeniorSteve

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Re: 1964 Zenith console mm2670
« Reply #4 on: December 15, 2017, 02:22:03 PM »
I sent TC Chris a response, but couldn't figure out how to attach either a jpg or pdf to it so that response won't make sense.  If I reply here I can do that.  Here's what I said to Chris

Here is how I approached the issue, I know germanium transistors have a forward voltage drop of about .1 volts to turn on the transistor.  Silicon devices have a forward voltage drop of about .6 - .7 volts.  This is what needs to be adjusted in the output circuit.
   The bias circuit is just a voltage divider.  I'll use the left channel for parts identification, but the same parts would be used in the right channel.  The divider consists of R132 (this resistor has the current of both channels running through it), R123, R120, R121, and R122.  I totaled the resistance (doubled R132) and then figured out the current running through them.  The original setup had 61ma running through the divider.  The voltage across resistors R120 and R122 (2.2 ohm) was determined to be about .13 volts.  The 470 ohm resistors were running close to their wattage, so I decided to replace all except R132 as it was a wirewound resistor.  Several of the resistors had drifted in value.
   I decided that since the new devices are silicon, I won't need as much bias current through the divider and changed the resistors accordingly.  I went with about 45 ma through the divider.  When I received the transistors I measured the forward voltage between emitter and base and it was around .55 volts for both transistors.  With that in mind I figured the resistors (R120 and R122) should be about 12 ohms for a voltage of .55 with 45 ma running through it.  After wiring it up with the new transistors and resistors it sounded terrible at low volume.  I knew right away I didn't have R120 and R122 correct.  Not enough voltage to turn on the output transistors at idle.  I changed them to 18 ohms and the sound was great, no distortion.  However after running the amp for awhile I decided it was running too warm, the output transistors were conducting too much at idle.   The next step was to install 15 ohm resistors for R120 and R122.  No distortion and the amplifier ran cool, barely warm to the touch on the output transistors.  I checked the voltage drop across the emitter resistors R124 and R125 and calculated that the current through the transistors was around 50 ma, enough to stop the crossover distortion and low enough for them to run cool.  Most of the early solid state were designed this way using the interstage transformers.  It wouldn't matter what brand then

R132 no change
Changes
R123 (R131)  150ohm 2 watt changed to 300 ohm 2 watt
R121 (R134)  470 ohm 2 watt changed to 620 ohm 2 watt
R120 (R133)  2.2 ohm 1 watt changed to 15 ohm 1 watt
R122 (R135)  2.2 ohm 1 watt changed to 15 ohm 1 watt

output transistors I used were 2N3792 which I purchased at Newark (they were the least expensive).  I used these because they were the same case style.  I also upped the output capacitor size to 2200mfd as this would be a limiting factor on the lower bass.  Filter caps for the solid state portion on the chassis were selected on size to fit a bracket that fit the original can wafer mounting insulator.  Way overkill, but there isn't a hint of hum in the speakers.  3 amp fuse was changed to 1.5 amp.

I hope this helps explain what I did, and gives you a starting point to work on any projects you have.  My bottom line was to see if this was going to work (the original output transistors were good), and because I'm not getting rid of this console I didn't care if it isn't "authentic".  It's just updated to newer type of transistors.  If I really wanted to go all out, I would change out the germanium transistors in the tuner chassis, but it works so well now, I decided to leave well enough alone.

I have included two pictures, the one is a scan from the Sam's , and the second is a simplification of one channel of the output stage.  I rearranged how it is layed out to make it easier to follow.  Let me know if you have questions.  No engineer here, just an enthusiast!

Steve   :D

SeniorSteve

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Re: 1964 Zenith console mm2670
« Reply #5 on: December 15, 2017, 03:18:34 PM »
Hello Motorola Minion
In response to what cartridge is in the changer, it's a Zenith branded ceramic cartridge  part 142-167.  I did have a test changer with a magnetic cartridge (an old KLH unit) and that had a slightly better sound to it when using the preamp into the tape input.  It wasn't enough to warrant any changes though.

My recollection of silicon output transistors didn't appear until the late 60's.  Just about all of the manufacturers used germanium transistors whether it was Fisher, KLH, Packard Bell, RCA, Zenith etc.  If the stereo was a hybrid (tube front end and solid state amp) it was almost always using germanium transistors.  It seems like any time they used the driver transformer and PNP outputs, they were germanium.  When silicon transistors came out they were NPN type transistors.  That's when the manufacturers started using DC coupled amps and took out the transformers (and many speakers too, unfortunately). 

Motorola Minion

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Re: 1964 Zenith console mm2670
« Reply #6 on: January 26, 2018, 04:30:01 PM »
Very interesting those were germanium in Zeniths. The .1 to .3 forward bias voltage gives that away.
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Dave