Author Topic: Astrosonic R214 Re-cap, then discovered Channel Two is dead...  (Read 185 times)

Phillip Colwart

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New to this forum, and am impressed with the wisdom, experience and knowledge here!

Re-capped my Astrosonic with the R214 chassis.  Overall volume was low with very little bass response, so I replaced capacitors on the preamp board and the two speaker coupling capacitors, as well as the caps/resistors on the crossover board.

When I pulled the connectors from the chassis for removal, I noticed both the brown (ch. 1 speakers) and orange (ch. 2) speaker leads were connected to Chassis Channel 1.

Repairs were performed and the chassis was mounted and everything plugged back in the way I found it. Naturally, both sets of speakers were much, much louder than before the recap.

I consulted the speaker hookup chart and confirmed the orange lead should be on ch.2 chassis output.  Well, channel 2 is dead.

Going to pull the chassis and have a look around.  Guess I will start with checking the voltage input and output of the two transformers on the amp board.

If they both check out okay, where else should I look?  The Selector switch will also be checked for continuity just in case.  Anything else?

Years ago, my Dad had someone service the instrument.  He changed the speaker output caps, and I am guessing here, when that did not bring back the dead Channel 2, he put the orange wire on Channel 1 with the brown wire so that all speakers would play.  I never suspected this while using the console since inheriting it six years ago.

Thanks for helping me bring back Ch. 2!

Phillip Colwart

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #1 on: July 08, 2019, 08:44:04 AM »
Broke out the good, ol' oscilloscope and traced an audio signal to the preamp board. Found two d.e.a.d. resistors there.  Gonna replace all twelve resistors on the preamp pcb. May as well look through the resistors on the amp as well.

"While I'm at it,"  looking into the Stereo indicator light circuit.

And how can I get the AM section working again? 

So many questions, looking for advice here. Thanks!

chazglenn3

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #2 on: July 08, 2019, 02:57:37 PM »
Welcome! I'm of no use to help with your issues, but we have a section here specifically for the AstroSonic Magnavox consoles.  Perhaps a moderator could move this thread?
Charles
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Phillip Colwart

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #3 on: July 09, 2019, 12:01:41 AM »
Charles, thank you for responding.

I posted here because of the description:     Solid State Consoles:  Discuss repair of solid state console electronics, speakers, etc.

Here to discuss the repair of my solid state console.  :)


electra225

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #4 on: July 09, 2019, 07:32:20 AM »
Welcome aboard!  I can't help you much with solid state, but it sounds to me like you are on the right track.  Good luck!
I don't need Google.  My wife says she knows everything.

Motorola Minion

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #5 on: July 09, 2019, 07:45:02 AM »
Glad to have you here! I think I can help you, especially since you got this far ;)

I have found a cause of a dead channel to be in the output driver circuit on occasion, a bad transistor (Q13 and Q18 in R214) that drives a coupling transformer that splits the phase fed to output transistors.

This little driver transistor usually has a heat sink on it in the 20 and 30 watt chassis, but the lower powered chassis can benefit from adding one, they run hot and cause shot noise in the output.

First check the voltages on those output transistors against the R214 schematic in downloads " service manuals and schematics-consoles", 5th one down. The voltage from the -36 volts, with equal amounts dropped across both output transistors, with the bottom transistor having emitter to chassis ground.
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Dave

Phillip Colwart

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #6 on: July 09, 2019, 05:57:26 PM »
Dave,

Thank you so much - that was it, Q18.  Values:  E  -1.2v  B  -1.4v  C  -26.6v

Compared to Q13:  E  -11.0v  B  -11.0v C  -24.4v

Schematic calls for: E  -8.0v  B  -8.1v C  -19.0v

Might as well replace both, as well as all the resistors on the driver board.

Q13 and Q18 are the 35P1 PNP transistor.  I consulted a transistor substitution chart and found NTE102. Parts Express has 'em.

Can you confirm this substitution?

It will be so exciting to fire up this instrument, fully working like new, for my big brother - I remember the day he and Dad carried this console up the two flights of steps to our 2nd floor apartment when I was in kindergarten.


SeniorSteve

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #7 on: July 09, 2019, 07:35:00 PM »
I have an old ECG reference book from 1989 and the NTE and ECG replacement parts were essentially the same (different manufacturers at the time).  The reference shows a ECG102 (or NTE102) as the replacement for the 35P1 Magnavox part.  It's a PNP germanium transistor suitable for audio driver.  If you have approximately half of your supply voltage (-17v) at the negative side of the audio coupling capacitor (C27) for the speaker, the output stage should be ok.  With these circuits it isn't going to be exact.

Phillip Colwart

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #8 on: July 10, 2019, 11:34:42 AM »
Steve, thanks for confirming.  Yes, I saw volts on the negative side of those two caps, so output is fine. Those transistors were in spec. I appreciate your suggestion.  I'm planning to replace resistors on the preamp and the amp, as well as Q13 as Q18.

Poked around the Stereo Indicator section, gonna replace some resistors and maybe that transistor there, too. I really want to see that Stereo light illuminate!


SeniorSteve

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #9 on: July 10, 2019, 02:36:26 PM »
Phillip, I hate to see you just replacing resistors when it isn't necessary.  It's a lot of extra work.  I can see replacing resistors on some of the older tube units as those resistors have a tendency to drift high as they age.  But, on almost all of the solid state stuff I've worked on, I haven't seen them drift much unless abused by some other device (transistor, diode etc) that might have shorted.  With the voltage readings you have shown on the defective side, I believe that hasn't happened, the transistor looks like it went open (high collector voltage means no current draw through the transformer, it should be dropping 3-4 volts).  What I would look for is any resistor that has been overheated, and that should be visible.  Once you replace the driver transistor, you should hear sound from that channel.  If you still don't have any sound, here's quick tip.  You have some tape output jacks that are connected to the volume control, connect a signal source to that point and see if you get signal.  You can compare it with the other channel.  If you get sound from both sides, your problem is before that point.

Looking at the circuit, you have a "PEC" or packaged electronic component in that audio stage that sets the bias voltage on the transistor.  With the transistor out of circuit, you should see around 8-9 volts on the base connection.  Emitter connection should be very near 0 and the collector should read near the -23 volts.  There won't be any voltage drop between the blue and red wires on the driver transformer.

The stereo indicator lamp is an #1867 which is a low voltage, low current lamp (4 volt, 60 ma).  If you use a standard bulb in it, you may not see it illuminate, or very dimly.
Hope this helps.
Steve

Motorola Minion

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #10 on: July 10, 2019, 03:45:49 PM »
My Astro's stereo light barely comes on unless its a real strong signal. That is a special lamp, not 6.3v like a #47 or #1847.
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Dave

Phillip Colwart

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #11 on: July 14, 2019, 12:42:32 PM »
Steve, thank you so very much for those tips. 

When tracing an audio signal with the oscilloscope, two resistors on the drive board were found dead. Unsoldered one lead each to test, no reading. Stopped there and decided to simply replace all thirteen resistors on the drive board.

This is the first time I've gone so deep into a project like this (since I was a teenager), so I'm having fun with it.

What irks me is, I'm learning that the replacement capacitors are way too high of a voltage value for the job. This was pointed out to me on the facebook Magnavox group. Original caps of 25v or 50v value were replaced with 100v electrolytics.  The 500uF, 50 volt caps were replaced with 400 uF, 450 volt caps - because this is all Parts Express had and I was in a hurry.  So, I will find the proper caps elsewhere.

The photo shows the replacement 400 uF, 450 v caps with shrink tubing on the leads, crammed into the drive board.  ::)

Caps are not cheap. Bought some audio quality caps for some 70's high end, three-way speakers and ended up in a hurry, buying expensive caps of too high a voltage value. Gonna try to return them, unused, for credit.

Found this in a google search: "Electrolytic capacitors, when operated well below their voltage rating (a 100 V cap in a 5 V circuit, for example), can exhibit a considerably smaller capacitance than it's nameplate would lead you to believe. And, the capacitance can drop over time, too." And this: "The voltage has to do with the dielectric breakdown point, although the rating is probably far below the actual expected breakdown point. However, higher voltage caps use thicker dielectric and larger plate area, so serious under-voltage could affect actual capacitance."

Maybe it's nit-picky, maybe I can get by with the higher voltage electrolytics for now, but so far it's been a fun learning experience, and the Magnavox is so easy to work on. Reminds me of building all those P-Box kits from Radio Shack. Or the little microphone preamp that never worked right because I probably burned up the IC soldering it in (and not on a plug-in socket), or the metal detector I built with radio parts. And so on.  I do want to honor the Astrosonic, as it was a big investment made by my parents, and they are long gone. I do my own automotive work, too, so it's just my time and less than $100 to get the good, ol' family ghetto blaster back in black.  My big brother is going to be so impressed! And no, no blue tooth, only analog signals shall ever pass my "instrument."

So, I don't mind replacing resistors that are probably okay. They're inexpensive, and by the time I unsolder to check values, I could just replace 'em.   

For sure, I am leaving the radio section well enough alone, except for the FM indicator light. The 1k trim pot is not giving a reading despite having a Deoxit treatment - gonna try replacing it.  Q10, the 43P6 is a little weak; researched a replacement, an NTE 100 should work. Replacing the resistors next to it, and the original #1867 bulb and socket all test for continuity. I really wanna see that Stereo indicator light up! It would give me a great sense of accomplishment for the eight or ten bucks investment in parts.

Dave, will let you know if the Stereo indicator repair works.  And thank you so much, Dave, for pointing out Q13 & Q18 to me. Trying to find my small collection of heat sinks to put on those, or will fashion some.

About getting the AM section properly working again, not sure where to go with that just yet but I'm all ears.


TC Chris

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #12 on: July 14, 2019, 06:21:50 PM »
I can usually find the values of caps I need from Mouser. A few have been back-ordered, but at lower voltages lots are available.  Sometimes I'll settle for a radial where I need an axial or vice-versa.

Chris Campbell

Phillip Colwart

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #13 on: July 18, 2019, 12:30:01 PM »
Update - replaced those transistors  and all the resistors except one on the drive board, and thankfully, Channel 2 audio has returned.

Channel 2 audio is distorted and much lower in volume level than Channel 1.  Audio was from the FM section and also through the Tape In jacks, just to be sure. This was on the work bench, not reinstalled in the cabinet, so cannot blame the speaker selector switch.

Quit working on it pretty late last night, will go through and test transistors again, and also the Selector switch.  A radio station engineer/ham radio friend here mentioned if I have lost AM but have FM, it could be the selector switch.   May as well set up the oscilloscope again and try tracing the audio signal through Channel 2 to see what else I might find.

Any suggestions, hints, tips, caveats?


Motorola Minion

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Re: Astrosonic R214 Re-cap, then discovered Channel Two is dead...
« Reply #14 on: July 18, 2019, 12:51:15 PM »
The power supply is 36 volts, the maximum replacements should be no more than 100 volts, like you confirmed.

50 mf and 5 mf @ 25 or 50 volts is common in this chassis. Speaker coupling are a pair of 400 mf, but that is not a standard value. I see Mouser has a 390 mf 1t I would use 470 mf as its a standard value and may improve bass response. The 8 mf non-polarized caps that feed/protect the horns do not need to be any more than that but could be lower, say 6.8 mf which is available in 100 volt, non-polar.

For capacitors, JustRadios is who I deal with mainly because they cater to restorers, with all values on an excel spreadsheet to order. I have used Allied, Digi-key and RadioDaze also. Mouser has a very difficult website, too many capacitor choices and most are overkill and expensive.
« Last Edit: July 18, 2019, 01:24:56 PM by Motorola Minion »
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Dave