Author Topic: Magnavox Berkeley - CR198 chassis restoration  (Read 409 times)

Motorola Minion

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Magnavox Berkeley - CR198 chassis restoration
« on: May 12, 2017, 12:44:43 PM »
This receiver chassis is used in several models including the Hepplewhite. I bought a Berkeley model 134B over a year ago and much time passed because the base of the cabinet was missing. The electronic restoration was delayed while selecting wood and researching methods of recreating the trim profile with correct dimensions.

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Dave

Motorola Minion

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Re: Magnavox Berkeley - CR198 chassis restoration
« Reply #1 on: May 15, 2017, 01:53:18 PM »
I have a second CR198 receiver chassis which I plan to put into a Mayfair model 139 cabinet using a single 12" PM speaker, so I am trying to be as thorough as possible restoring this one for the Berkeley.

The first picture is after a 100% capacitor replacement, not counting the mica caps which remain. This receiver sounded just fine as I warmed it up before the alignment and connecting a long-wire external antenna brought in quite a bit of daytime reception on SW. 

The second picture shows the bench arrangement. IF and RF alignment for the AM-SW receiver chassis was a bit off, so following the procedure in the manual posted in downloads made a big difference. I prefer not to use "modern" (1970s and later) equipment as the Magnavox procedure recommends. Using signal generator and VTVM to peak the IF at 455 Kc and the 600+1400 AM RF plus the 15 Mc shortwave, as I'm fortunate to have a 1947 Hickok 277 generator that is calibrated and works quite well. 

A bit too much signal with 400 hz modulation AND volume caused some feedback, so following conventions, the generator output was reduced to just above the background noise and the volume as high as possible. The VTVM was used, rather than a digital voltmeter, across the speaker voice coil to show a few volts AC level as each IF transformer is peaked. repeating back and forth until no improvement can be made.

The FM chassis alignment, due to it being the "early FM tuner" was also possible with an AM signal generator because it uses a limiter and discriminator rather than a ratio detector. More on that later. I want to see if I can get a scope picture of the +/- 75kHz bandpass response using a sweep generator, more important on an FM receiver IF.
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TC Chris

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Re: Magnavox Berkeley - CR198 chassis restoration
« Reply #2 on: May 15, 2017, 10:43:31 PM »
You added the fuse and what looks like an inrush current limiter?

Chris Campbell

AlexanderMartin

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Re: Magnavox Berkeley - CR198 chassis restoration
« Reply #3 on: May 16, 2017, 03:32:53 PM »
You added the fuse and what looks like an inrush current limiter?

Chris Campbell
Looks like it. I don't blame him, 6L6s are pricy and those power transformers aren't easy to obtain if something goes wrong. I'll be doing the same thing.

TC Chris

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Re: Magnavox Berkeley - CR198 chassis restoration
« Reply #4 on: May 16, 2017, 08:49:04 PM »
My Wurlitzer 1100 jukebox uses 6L6s, but instead of a slow-start feature, the amp has a start-up relay that pumps extra voltage to the filaments for a fast warm-up.  The idea was that when you put your nickel (!) in the machine, you didn't want to wait for the music to start.  I've toyed with the idea of always-on filaments at a low voltage. 

Chris Campbell

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Re: Magnavox Berkeley - CR198 chassis restoration
« Reply #5 on: May 19, 2017, 10:03:13 AM »
I had a minor setback, after I noticed the 6U5 eye tube was dark. It turned out that adjusting the slider to put the top of it close to the dial glass, placed stress on it and it broke at some point. I have only one left, but none for the second chassis. The FM chassis has one also, which is nice an bright.

I am ready to put the speakers, record changer and receiver chassis into the cabinet. The finish still needs attention but Im ready to play it while I attend to that detail.

The FM chassis will need to stay out for now though its IF stage is peaked at 10.7 mc. To finish the alignment, the discriminator transformer needs to be zeroed and then peaked to the same voltage at the extents of the frequency deviation.

My frequency counter is not functioning, so I have to get a digital scanner to calibrate my not-so-accurate generator evenly at 10.625, 10.7, 10.775 MC. 
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Dave

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Re: Magnavox Berkeley - CR198 chassis restoration
« Reply #6 on: May 22, 2017, 03:20:04 PM »
Bummer about the busted tube.  :'(
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AlexanderMartin

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Re: Magnavox Berkeley - CR198 chassis restoration
« Reply #7 on: May 22, 2017, 05:42:17 PM »
My frequency counter is not functioning, so I have to get a digital scanner to calibrate my not-so-accurate generator evenly at 10.625, 10.7, 10.775 MC.

Get an HP frequency generator and never worry about that again my man.

Motorola Minion

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Re: Magnavox Berkeley - CR198 chassis restoration
« Reply #8 on: June 19, 2017, 10:32:37 AM »
After a few listening events with this receiver, I have noticed a few things that bear mentioning.
1. The FM chassis (CR192) is a fine addition except for its Foster-Seeley discriminator. After making the adjustments as per the service manual, with an AM signal varied from 10.625 to 10.775, the on-station FM reception was pretty fussy. You have to tune right and left of not-so-strong stations to minimize the distortion. I needed to make some by-ear adjustment to the top and bottom on the discriminator. This was early FM and a ratio detector is a huge improvement.
2. The AM-SW is a great receiver with a built-in loop. I was able to find a station with music and noted how, even with a weak daytime AM signal, it sounded almost as good as FM. Finding a SW station is easy but a few feet of external antenna is needed to minimize the fading common to the 31 meter and similar bands.
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Dave

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Re: Magnavox Berkeley - CR198 chassis restoration
« Reply #9 on: June 19, 2017, 02:29:56 PM »
My frequency counter is not functioning, so I have to get a digital scanner to calibrate my not-so-accurate generator evenly at 10.625, 10.7, 10.775 MC.

Get an HP frequency generator and never worry about that again my man.

I agree, just need the $$$. My B+K 1801 freq counter has a working nixie tube display, but it does not read properly.
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TC Chris

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Re: Magnavox Berkeley - CR198 chassis restoration
« Reply #10 on: June 19, 2017, 07:54:00 PM »

I agree, just need the $$$. My B+K 1801 freq counter has a working nixie tube display, but it does not read properly.
[/quote]


There maybe  a generational thing going on here.  I'm the child of Depression-era parents, and so I learned early on about making do and being thrifty (but also spending money on things that really are important).  If you always wait until you can afford just the right device, tool, or toy, you'll never have much fun.  So improvise and make do, and either you'll get the job done and have fun, or you'll learn what you really need to spend money on.

I did a lot of loaded bike touring (tent, sleeping bag, Svea cookstove, etc.) on a $110 ten-speed bike--not exactly top of the line.  And that was after I straightened the frame with a car jack following a collision with a car.  It wasn't a perfect setup by most standards, but I had lots of fun.  If I had waited until I could afford a fancy touring bike I would have missed the experiences.

The sleeping bag cost more than the bike did.  One very cold night at Stone Mt., GA in a crummy sleeping bag made me vow that the first thing I'd buy when I had money was a good sleeping bag.  And I did.  It's a fancy down one, highly rated at the time, and it has kept me warm and happy through many cold nights since 1971 or so. 

It's always good to have a list of things we'd like to have.  That gives some incentive to be frugal with unnecessary expenses so we can be free-spending on other things.  But meanwhile, do the best you can with what's available.

Chris Campbell