Author Topic: Console.. no, wait, portable repair and restoration  (Read 751 times)

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #15 on: March 16, 2017, 08:03:45 PM »
So when we last left the tube amp on the portable, the tubes would glow but not a sound would be heard from the speakers, even while turning all three pots to see if there might be a bad pot. I replaced the two .047uf electrolytic capacitors, and based on suggestions elsewhere, the rectifier and sand capacitor. The results: bumpkus. Same issue as before, the tubes glow, thats about it. Looking about the chassis, I dont see any caps or solders that jump out at me as bad. I suppose I can console myself that I likely didn't make it worse, however I admit that I am not sure what to do next.

Could the tubes be bad even though they glow? there is one 12AX7 and 2 50EH5 tubes; I suppose if the 12AX7 were bad, that would explain why there isn't a sound out either channel, no? All three appear to be the original Magnavox tubes! Conveniently the 12AX7 would be an easy tube to get a replacement for...  Any thoughts or suggestions? 

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #16 on: March 18, 2017, 07:27:36 PM »
Update.. I got my hands on a new but inexpensive 12AX7 and as expected no change. I also found my multimeter. It seems a bunch of the resistors off the 12AX7 are reading way off (and low) on the resistance gauge - I did check calibration against a good new resistor which measured that one accurately - so I will replace them.  I'm probably going to just replace all the capacitors anyway.  A multimeter test of the 50EH5 tubes and the old 12AX7 yielded the expected results - not that that eliminates bad 50EH5's, but at least there aren't any shorts in them. Ah well, I'll see what happens after I replace the resistors.

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #17 on: March 18, 2017, 08:42:52 PM »
Oh, one interesting note - I was doing checks of the resistance expected on the pins of the tubes (the schematic has them) (while the amp is unplugged, of course) - when checking pin 7 on the 50EH5's to just before the anode of the rectifier, I'd get a lil crackling in the speakers, likely from energy stored in one of the caps. Im careful about working around them. My amp is apparently wired like an 80-01-00 alternate wiring, which looking at the schematic in the downloads area is on the top right of the schematics page. Basically the resisitance I'm checking is between pin 7 and the rectifier, through the audio output transformer.  Clearly then, the multimeter is allowing a circuit to close and allow power to flow from the power filter caps thru the transformer.  I have a feeling this is a clue, but maybe I'm too tired to figure it out. Its the only sound I've gotten to come out of the speakers so far (hey, at least I know they work).  I'll sleep on it, see if it brings any ideas out.

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #18 on: March 18, 2017, 10:48:43 PM »
So, while pondering the meaning of the crackling, and one or two odd resistance readings, and after confirming which version of the power circuit this amp has - it's the 80-01-00 alternate version, to go along with the alternate 2-winding version of the output transformer - I decided to take another deep look at the replacement some previous owner did of the power-filter can capacitor. He cut the bottom off the can and mounted the three caps on it, taped together. looking under the myriad wires and such where it's installed, I found the problem - the 100 ohm resistor was not properly soldered where it attached to A, the 1000 ohm resistor, and the 80 mfd filter cap - in fact it wasn't attached at all. I soldered it back in place, and YES! Sound!! I feel a little silly missing it all this time - I picked up a magnifying visor today, to help my aging eyes, which no doubt helped me spot it finally (that and gently pushing aside the other things wired hither and yon above it).  I don't regret replacing any of the caps, and I think I will still replace those resistors whose readings were way off - R5 through 7, R13-15 - and eventually the remaining caps (they're cheap, and there isn't that many anyway).  I'm using the JJ 12AX7 tube I just got, but will probably swap the old one back in to see if there is a difference in sound. The Magnavox 50EH5 tubes seem to still work just fine. 55 years old and still going strong, amazing.. 

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #19 on: March 19, 2017, 10:43:28 AM »
So, this morning I put the original Magnavox 12AX7 tube back in, and as expected, it works just fine. There is still some hum in the system, even with no source hooked to the amp, but not nearly as bad as the transistor-amp console was when I got it (before I replaced the can capacitor on that unit). I figure that the updates, etc that I will be making will address most of that. It still sounds good to me, the hum is only a problem on really quiet passages of music.  I've temporarily marked on the plug which prong should always go in the narrow slot. Upcoming work on this unit will include:
1) Replacing the power cord with one that has a polarized plug.
1a) Install a isolation transformer. Figure out how to do it without the half-wave problem. I looked over the recommended page and it didn't include that, but I think it would be a good idea. This "portable" is 25 pounds already, whats another pound or two.  Add a fuse to the circuit as well.
1b) Investigate modifications per that page for where the power switch is in the circuit. I'm thinking of adding a power switch and phono-jack inputs that would let me use the amp for external things like my iPod without having the phonograph on to turn the Collaro unit on. 
1c) Speaking of phono-jack inputs, there's a pair on the outside of the rear of the cabinet that look original and are no longer connected to anything. I suspect this was done to reduce shock risks, but I need to find a manual for the unit to see what they were for - external speakers? External inputs? I suspect the former, as the internal speaker connections have a second, unoccupied tab on them, in addition to the occupied ones for the built-in speakers.
2) Replace R5-7 and R13-15 (see previous comments).
3) Replace remaining caps with mylar poly film caps.
4) Figure out what the function of the circuit in the schematic off the "Loudness" (volume) pots is that doesn't exist on my unit, and whether I should add it.  It consists of two caps, C3 & C4, and a 220k resistor, between the source phono jacks' outer conductor, one of the volume pot's prongs, and the floating ground. Probably irrelevant as I suspect my version of the amp is short the fourth connector on the volume pots. If it's particularly helpful, I might replace the volume pots.. potses.. potentiometers.  ;D
5) I think I will add a 1 ohm resistor in series to the 22ohm R21 and 33ohm R22, which are right off the AC power of the house (soon off the secondary taps of the isolation transformer). The unit was originally designed for 117V, though the Sams manuals say 110-120VAC power supply, I think it would be a good idea to add just a bit more resistance into the circuit to account for the 3 extra volts, and give it a bit more headroom for power spikes.
6) Clean out and repair the Collaro changer. Re-wire its cap to the changer chassis, like the one that came with the unit (now in the console) had it. 
7) Other general cleanup of the cabinet, including replacing the feet, some of which are broken. The condition of the cabinet is solid, but the external leather, etc is pretty beat up, and at some point the changer spindle damaged the wonderful "Magnificent Magnavox" logo on the fabric interior of the cover. This unit isn't so special, however, that I'm afraid of either learning on it, or modifying it.   For example, something I might do someday is replace the front panel with one that has a window showing the tubes glowing away inside. Back in 1962, the tubes weren't a big deal, maybe even old fashioned and something to hide, but nowadays, why not show them off?

Also, someday, when I have enough knowledge, I may look into designing a mod of the amp to substitute more easily obtainable power tubes for the 50EH5's.  For example, the EL84, another power pentode. I'd have to design a way to cope with the reducing the volts going into the filament circuit to 6.3V rather than 50V, and allll the other differences in various input and output specs. No doubt on a daughterboard.  It would be an interesting project, but it will be a long while before I feel capable of pulling it off!   

As for the coffee table console and its transistor amp, I have the caps I need to replace the remaining originals on that unit, but to be honest, it sounds pretty good now, and getting that amp out of the unit takes a bit of work. So, I think I will concentrate on the portable for now.

All your previous suggestions have given me the confidence to keep working on these units, and that with care I can get them working without getting myself killed in the process. It's very much appreciated, as are any other suggestions.

VinylAddict

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Re: Console.. no, wait, portable repair and restoration
« Reply #20 on: March 28, 2017, 09:18:35 AM »
Really cool thus far Mike. This sure is an interesting hobby.
I'm in MA and currently restoring a RCA console. It's a small world. (My thread is right below yours)

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #21 on: April 01, 2017, 07:13:59 PM »
Hi, VA! Yeah, its a lot of fun. I'm holding off on getting another idler wheel for the Micromatic in hopes of getting my hands on another Micromatic the old fashioned way - luck like I had coming across the first two a week apart.  ;D  In the meantime, I'll concentrate on the firestone and its V-M changer.