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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Magnavox Model 1ST525A

I just got this phonograph/record player from my parents and wondering about it's quality and would it be worth refurbishing, plus adding an auxiliary to it for an mp3 player. Its a Magnavox Stereophonic and sounds ok through the radio but I need a new needle for the record player. I assume update the caps, the tubes are all running well and what else would need to be cleaned (ie tube sockets etc...)
 

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The turntable/cartridge is a piezoelectric POS, which can destroy valuable records. :( It should go!

It looks like the amp uses PP 6V6 O/P tubes and an EZ81/6CA4 rectifier.

It's blurred and I can't be sure, but you may have an AUX position on the source selector. That's where an external, line level, source (like a CDP or MP3 player) would attach.

Are you going to use the console's speakers? As is frequently the case with console amps, the O/P "iron" is not "meaty". Circuit modifications, including triode wired "finals", would allow for maximum fidelity in reproduction. The price of doing that is an approx. 50% reduction in O/P power and the need to use mid to upper 90s dB. sensitive speakers.
 
If you go to a moving magnet cartridge tracking @ 1.5 gram or less, to preserve rare vinyl records, you'll probably need a different turntable. You might be able to fit it in the old good looking console. I abandoned my Mother's ceramic cartridge RCA record player in 1969 because I could hear new records deteriorating at the second play. The best bargain in the US is a used BIC turntable, which looks very cheap (so they went bankrupt fast) but performs admirably. I bought mine in 1979, sold off the AR turntable that skipped too much when the floor bounced, and have been using my BIC 940 ever since. I have a 1979 Shure M97 era IV magnetic cartridge, still in production I see from Amazon.
Moving magnet cartridges typically put out 25 mv instead of the volt of a ceramic cartridge, so you'll need a preamp stage to play records. A transistor or op-amp preamp, or a retired disco mixer, would do the job. I'm using a herald-electronics RA88a disco mixer, purchased for $15, modified with an external power supply (wall transformer) to remove the hum of the practically unused mixer. I also put $3 of op amps and bypass capacitors in it to remove the hiss. A competently designed but road weary disco mixer would probably need some potentiometers and maybe power supply electrolytic cap replacement. The premium disco mixer is the Peavey, but they didn't sell very many. Behringer sold a bunch, they are not easily repairable IMHO. Regular mixers don't have an RIAA curve on the inputs to handle LP's or 45's.
Used LP's is a cheap hobby, I bought 19 of them last week at the charity resale store for $.50 each. Maybe 10% to 50% will be too damaged to play twice, but the damage seems to be concentrated on the popular artists played on poor (heavy armed) equipment by rowdy teenagers.
I wouldn't worry so much about modifying the amp right away. After new e-caps, the biggest bang for the buck is replacing high value (plate) caps with metal film instead of carbon comp. Cuts hiss audibly. Magnavox output transformers are unlikely to be full frequency, anyway. I would think about upgrading those before I worried too much about triode vs pentode connection. and changing speaker design with some wood and baffling to a ported reflex design could possibly really improve bass without spending much money, just a lot of time.
 
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The graphic Bruce uploaded provides us with lots of info. Unfortunately, corners were cut in this design. The 6CA4/EZ81 is rated for 150 mA. and it's being asked to provide a good deal more. :( Please observe that the tuner/preamp gets its B+ from the same supply as the power amp.

The editorial error on the graphic of saying 5U4 instead of 6CA4 is a history lesson. A previous version of the console amp used a 5U4, which is adequately rated at 250 mA. The manufacturer saved money by simplifying the power trafo (no 5 VAC winding), using a less costly tube, and a less expensive socket. The phenomenon of "shafting" the customer for a bit more profit is not new. Grrr!!

IMO, the easiest solution to the overworked B+ rectifier is to switch to SS diodes. That will raise the rail voltage, which is something that can be taken advantage of. The operating point of the 6V6s will be more favorable. :D

6EU7s are scarce and priced accordingly. Fortunately, the triodes in the bottle are the same as those in a 12AX7. Rewire the sockets and install Sovtek 12AX7LPS tubes. The 'LPS contains a spiral wound, hum bucking, heater, like the 6EU7 contains. :)
 
Ok I have put in a 3 prong power chord and replaced some caps. I also installed a volume knob onto the amp portion. I plugged in my mp3 to the input on the amp and get about 1/10th the power I had when I ran it through the tuner. I assume the tuner had a pretty loud preamp in it. Can I reattach the tuner just to the power supply on the amp and then run an mp3 player through the tuner to get the increased sound level back or do I need to reattach all of the cables attaching the tuner to the amp? does the tuner adjust bass, treble and volume from within itself before it goes to the amp input?
 

rparsh

Member
2009-09-21 2:22 am
Hello Bruce, I live in Lilburn, Ga and also have one of these Magnavox amp units from a console. It has four 6V6 and two 6EU7 with a 6CA4 rectifier. It is stamped 88-02-00 on top and 414223 on the back of the chassis. This forum is the first info I have found on these units. Thanks forany info, rparsh