"New" Magnavox Astro-Sonic, needs repair!

Hi guys, Im new to the site...actually surprised to see that there are groups of people/websites dedicated to old stereos and what not, Very Cool! Anyways, I just got a Magnavox Astro-Sonic Model # 1ST637 for my 30th birthday from my parents the other day. This has got to be the coolest present ever, I think my dad wanted to keep it for himself, I'm gonna have to find him one for fathers day now, lol. Its in excellent condition, with only a few light scratches in the woodwork. The only problem is that there is no output from the right speakers. I switched the connections at the chassis and the problem went to the left side. So, I know that all speakers do work, and the wiring between the chassis to the speakers is good. There is a slight hum in the speakers and when I adjust the balance pot the speakers crackle a bit, so there is some sort of signal being transmitted. I assume the problem is in amp/tuner unit itself? It's solid state. Not sure on the year. I did a search here and have heard about re-caps? Is that replacing capacitors? Also, I cannot find a schematic to aid in this. http://www.alltelmypics.net/photos/photo04/6f/10/ed17899b49bf_cc473b92-61aa-41d8-a4d2-c5a9d1d46ade.jpg
http://www.alltelmypics.net/photos/photo04/78/63/e6898e8eb193_b119c92e-c035-4937-9dfc-96dc2519e36a.jpg

I forgot to mention - The two numbers I got off the chassis
R-204-01-30
425372

Age and neglect take its toll on these things, but nothing's impossible to repair.
The chassis should be pulled and gone over, but it's not an easy job for a novice to take on.
I use a special tool that Magnavox made to removal, and am familier with these beasts.
If the output transistors are defective on the bad channel, special matched pairs must be used as replacements for reliability.

Okay, I've replaced a ton of caps and resistors and I've now got some sound in the right channel but its not very loud and kind of static-y. It seems that the balance pot may be a problem. Is there any way to bypass or eliminate it? It has three wires - one for left, one common and one right. Also, I've tested the transistors and they all seem to have the same failure. But I can't find exact replacements. I checked with an online database to cross reference the part number, but it wasn't listed. What they are - Bendix 39P1K, and Ive got four of them. Any idea where I can for sure find replacements? Ive been searching a lot of those vintage parts and repair places and all seem to carry parts for everyones stereo except mine, lol.

Also, here is how I tested the transistors.

* The base-emitter (BE) junction should behave like a diode and conduct one way only.
* The base-collector (BC) junction should behave like a diode and conduct one way only.
* The collector-emitter (CE) should not conduct either way.

And the CE did conduct one way on all four. And one of those conducted both ways.
Is this the proper way to test them?

OneEyed wrote:Okay, I've replaced a ton of caps and resistors and I've now got some sound in the right channel but its not very loud and kind of static-y. It seems that the balance pot may be a problem. Is there any way to bypass or eliminate it? It has three wires - one for left, one common and one right. Also, I've tested the transistors and they all seem to have the same failure. But I can't find exact replacements. I checked with an online database to cross reference the part number, but it wasn't listed. What they are - Bendix 39P1K, and Ive got four of them. Any idea where I can for sure find replacements? Ive been searching a lot of those vintage parts and repair places and all seem to carry parts for everyones stereo except mine, lol.

Also, here is how I tested the transistors.

* The base-emitter (BE) junction should behave like a diode and conduct one way only.
* The base-collector (BC) junction should behave like a diode and conduct one way only.
* The collector-emitter (CE) should not conduct either way.

And the CE did conduct one way on all four. And one of those conducted both ways.
Is this the proper way to test them?

Seems you're way off.

Are you checking these out-of-circuit?
If not, are you considering the loading of the power supply and/or driver circuits?

I checked them all out of circuit. I read on a few different websites and they both gave the same instructions on how to test them. If I'm testing them wrong, please correct me. Thanks.

Sounds like the driver board is in need of service.
All 4 outputs rarely fail at once.
These amps are not easy to troubleshoot, particularly over the internet, my suggestion is to let an experienced restoration tech who's dealt with Maganvox handle it.

Do you know of any good techs in Iowa? I've been searching and found a few that are about three hours away, but have no idea about their reputation. Not sure if I want to risk shipping this unit, but I guess If it comes down to it, I might have to.

Hey OneEyed,
I looked in one of my trusted RCA SK Series X-ref guide. A single replacement for the 39P1 is Sk2009, a matched pair is SK3013. You need a matched pair if only one is bad. Another trusted X-ref lists GE-3, Elmira is ELM121MP (MATCHED PAIR) and Motorola 230 or 232. I'm not looking for more, most likely you would get a GE product regardless which one you ordered. And if replacing all four power amps (some recommend regardless if only one is bad) you need two matched pairs. I can't imagine what a tech would charge to work over your work. I think you have probably massacred this Astro-Sonic. You should look for one that is working and does not require anything other than phono needle and record changer lube job. They all do not require recap.
Bill

shedradios wrote:Hey OneEyed,
I looked in one of my trusted RCA SK Series X-ref guide. A single replacement for the 39P1 is Sk2009, a matched pair is SK3013. You need a matched pair if only one is bad. Another trusted X-ref lists GE-3, Elmira is ELM121MP (MATCHED PAIR) and Motorola 230 or 232. I'm not looking for more, most likely you would get a GE product regardless which one you ordered. And if replacing all four power amps (some recommend regardless if only one is bad) you need two matched pairs. I can't imagine what a tech would charge to work over your work. I think you have probably massacred this Astro-Sonic. You should look for one that is working and does not require anything other than phono needle and record changer lube job. They all do not require recap.
Bill

Indeed not all need caps, but in some cases the reason for bad output transistors stems from leaky speaker-coupling caps, in other cases the driver transistors and associated components are the reason.
Merely replacing the (expensive) output transistors is foolish unless the cause is determined and rectified first.
Bias resistors could be bad as well.
This all takes a professional technician to troubleshoot properly.