View Full Version : Still have no sound from RCA amp after mods


NCTubeLover
02-21-2012, 09:11 PM
Hi Everyone,

I've been working on my RCA RS-171 amp and still can not get any sound.
I've replaced the coup. caps under the chassis, the can caps and both audio transformers. At least the turntable works....

What's left to replace?
When it was first brought home all I had was a very loud hum. I knew the caps were bad.

I have the SAM schematics, I have redone 3 Magnavox amps with no problem.
This 59 RCA Orthophonic is driving me crazy. :tears:

Enclosed are some pics.

Any suggestions?

Greg

gadget73
02-21-2012, 09:34 PM
sounds like time to start checking voltages against the schematic. Since it does squat in both channels, methinks it will be a problem in the HV section.

cademan
02-21-2012, 09:44 PM
Where is your meter? You should have it tracked down if you see where the voltage stops. As Gadget has suggested, start with voltage measurements at the first can capacitor right after the rectifier tube.

Sir.Byrd
02-21-2012, 09:47 PM
Both channels hints at power supply, have you replaced all of the filter caps?

Blast
02-21-2012, 10:00 PM
Hi, Greg;

I feel like I got in the middle of something. I feel like part of the story is missing. Why would you have replaced both (audio) output transformers after a loud hum???

And, I'm with cademan, gadget and Byrd- let's get those voltage readings.

Brian

cubby01
02-21-2012, 10:33 PM
I feel like part of the story is missing. Why would you have replaced both (audio) output transformers after a loud hum???

That was my thought as well.

My money's on the PS cap wiring being knackered or something is touching ground that shouldn't be. Do the filaments glow?

NCTubeLover
02-22-2012, 12:33 AM
Hi Guys,

Thanks for the quick responses.

To answer your questions:
Yes, I replaced the audio transformers thinking they may be bad.:scratch2:
I replaced the caps including the can caps and I still had no sound; therefore I purchased new audio transformers thinking that's the problem.
Yes the filaments glow.

I tested the voltage, on the Can Cap side I have 324, 338, and 20 vdc.
Coming off the rectifier tube I have 328, 328 vac and 338, 338 vdc

I had low static coming thru the speakers when I touched my + lead the to the various lugs, as I tested the voltage.

When replacing the transformers, I connected the primary wires to the 2 P's since it is a SE amp.

If I was using it as PP I would have an additional wire to connect to the Center Tap.

The secondary connectors on the trans. have 6 lugs, I used 1-3 and connected them as the original RCA trans.

Replacement's are Stancor A3870, according to SAMs that is an equivalent part.

Off to bed...

cademan
02-22-2012, 12:46 AM
Single ended output transformers only have two wires on the primary side. One goes to pin 3, and the other goes to the positive of the can right after the rectifier.

Put back the original output transformers.

Blast
02-22-2012, 06:44 AM
Single ended output transformers only have two wires on the primary side. One goes to pin 3, and the other goes to the positive of the can right after the rectifier.

Put back the original output transformers.

Second that! :D

cubby01
02-22-2012, 07:52 AM
Single Ended? :scratch2: Pictured chassis looks possibly P-P to me. And since the noted alternative transformer is P-P, I'm thinking it probably is.

Can you post the schematic?

battradio
02-22-2012, 08:38 AM
the stancor is a univeral output transfor that can be for PP or a single tube .
http://www.bunkerofdoom.com/xfm/STANCOR_1961/Stancor_1961.pdf

dcgillespie
02-22-2012, 09:29 AM
This particular RCA chassis is a very unique animal. As I recall (I'll have to dig the schematic up again), it used a special pickup that inherently had the two channels out of phase. As a result, the amplifier would actually operate in push-pull when set for mono operation by the selector switch, or as two single ended amplifiers when set for stereo. The phasing issue when then dealt with by the speaker selector switch on the end of the amplifier if I recall correctly. All a bit hazy, but just realize that this is definitely not your typical run of the mill console amplifier.

As others have suggested, I would reinstall the original output transformers, and then further, systematically go through the unit and convert it to a permanent 2 channel single ended stereo unit, removing all of the switching nonsense necessary to support the two different modes of operation.

Good luck with it!

Dave

NCTubeLover
02-22-2012, 12:29 PM
Here's the schematic.

I will look into the amp later today.

Greg

edlipman
02-22-2012, 01:24 PM
It looks like you have the output transformers wired incorrectly.
As someone before said put the original transformers back in.
The output transformers should be wired , for the primary , using the two wires at the ends of the primary winding and the center tap wire should be taped up and not used. This is a single ended amp.
The tuner does not even have a multiplex stage for stereo. Just an audio amp stage to invert the audio for the right channel ? ?
Hope this helps

dcgillespie
02-22-2012, 02:51 PM
Yep -- that's the one I mentioned earlier in post #12. Notice:

1. On the top output transformer, the blue wire goes to the plate, while on the bottom transformer, the RED one goes to the plate. This is not a famous Sams mis que, but the way it was originally wired. This is done so that the amplifier can operate in true push-pull mode when mono is selected and a balanced mono signal is presented.

2. The tuner is mono, and so requires a phase inverter in one channel to allow the push-pull to operate properly. Without it, a common mode mono signal would simply cancel in the push-pull output for no sound.

3. The stylus IS a special out of phase stereo unit (only three terminals), allowing mono records to be amplified in push-pull, while stereo signals are still correct as well: The stylus is out of phase between the channels -- but then so are the output transformers (see #1 above), so the ultimate phase produced between the channels is still correct then when stereo records are played.

For this amplifier to operate properly in mono (push-pull), it MUST have a balanced mono signal presented between the two channels. A common mode mono signal presented to both channels will produce no output.

For this amplifier to operate properly in stereo (single ended), it MUST have the stereo signals presented out of phase with each other to prevent acoustic bass cancellation between the two channels during playback.

While the whole design idea was a rather novel concept, it is woefully out of date in this day and age. For proper operation today, the unit MUST be modified into a normal in-phase stereo amplifier. Then, you can enjoy it in the same vein as so many other single ended stereo console amplifiers that were produced back in the day. To do that, simply copy the top channel (left) circuit in every detail into the bottom (right) channel, and enjoy.

If you were operating this unit in stock form in mono mode, and supplying a common mode mono signal to both channels for test purposes, then there will be NO output from the amplifier.

I hope this helps!

Dave

NCTubeLover
02-22-2012, 04:56 PM
Hi,

I have been looking over the comments.

I do not really want to modify the wiring on the amp. Keep it original.
If I find a tuner for it, I want it to work with it.

Also, it is only stereo if you have the additional speaker cabinet connected, which I do not own. A switch on the amp lets you go from mono to stereo.

A few of you suggested I reinstall the original audio transformers.
When I removed the old RCA ones, I unsoldered the wires and resoldered them to the new transformers. New trans. did not have wire leads.
I did not disconnect the leads from the amp chassis.
I had to cut the wrap around the trans. to access the wires. You can look at the pics I have included.

There is a pic of how I connected the Primary wires to the trans.

Also I included the instructions for the Stancor universal audio transformers.

Thanks again,

Greg

dcgillespie
02-22-2012, 06:34 PM
Greg -- If you plan to keep the unit all original and play modern stereo sources through it using the stereo extension speaker, you will be required to reverse the phase to the speaker(s) of one channel. Otherwise, there will be an acoustic hole in the middle at bass frequencies. Of course, if you use the original turntable pickup and/or tuner, you will need to reverse the phase back again for those sources.

In mono mode, the unit is converted to push-pull operation. This makes the installation of any replacement output transformers very tricky to make sure the phasing of the replacement units mimics that of the original units. If they are phased wrong, you will get NO sound at all out of the amplifier in mono mode. If only for this reason alone, it would be a good idea to reinstall the original output transformers if at all possible. Also, for this unit to produce ANY traditional modern mono source, the source must have a balance (to ground) signal applied to the input of both channels. Otherwise, it will produce no sound at all, regardless of how the speakers are connected. While balanced signals are typically available from PA and sound support panels, they rarely if ever exist with traditional home hifi/stereo equipment.

Therefore, you will ultimately be best served to service and use the unit in stereo mode only, with the speaker phase reversed in one channel. That will allow the unit to operate properly with all modern stereo sources. All mono functions then should simply be disregarded, since they won't work anyway.

I am hardly trying to scare you off of this project, but as I said in my original post, this amplifier is a very unique animal, and requires a thorough understanding of its design and operating conditions, to be able to service it, and have it work properly in the modern world.

I hope this helps. Good luck with it!

Dave

rpampt
02-22-2012, 07:19 PM
This 59 RCA Orthophonic is driving me crazy. :tears:

Enclosed are some pics.

Any suggestions?

Greg


Got a big hammer?:D

Blast
02-22-2012, 09:19 PM
Wow.

After finding a two year older mono console (the SHF-4) last summer I did a lot of research on it before I began bringing it back to life. During that time I also discovered and read about this unique stereo amp. I wondered if I would ever see, or hear about one in the future.

I've been reading this thread with great interest and, Dave, I just wanted to commend you for a brilliant overview of the theory of this amp's (and the whole console's) operation. Your advise to the OP about what it takes to make it work today in today's world was, in my opinion, absolutely top notch. You went to a lot of time and trouble to inform the OP and address his specific situation and problems.

Whether, or not, the OP wishes to take your advise is his business. But, based on your expertise and well written advise, if it were me in the OP's shoes I'd be converting it right now. :yes:

Brian

dcgillespie
02-22-2012, 11:25 PM
Thanks for the very kind words, Brian. To the casual observer, this amp looks to be yet just another run of the mill single ended stereo amplifier used in countless consoles from the late 50s and early 60s. The vast majority of them are rather easy to clean up, recap, and re-tube, to restore dependable, original performance. As such, they make for great entry level projects for so many who are new to this hobby, and can create a lot of satisfaction for a job well done when operation is restored, and music is heard.

But this unit is quite different, so it's easy to get into trouble with it real quick without a good understanding of its basis of operation. Based on that, I was simply trying to give the OP (or anyone interested) what that basis is, and let him go from there. Hopefully, he has enough info now to proceed based on his own comfort level to effect repair. I'm glad you found the information helpful too!

Thanks again.

Dave

Blast
02-23-2012, 06:51 AM
I've been studying the schematic and find this whole console and thread very interesting. So, let's see if I understand... This console was sold during a time of transition between mono and stereo. Stereo tapes had already been around, stereo records were just becoming available, and FM stereo was still a few years away. This console was designed to not only be compatible with both mono and stereo sources of the era, but also to utilize it's full power output capability whether, or not, only the internal speaker was playing by itself (mono) or the external speaker was attached, as well. Does that about sum it up?

Brian

dcgillespie
02-23-2012, 08:14 AM
Brian -- you have absolutely nailed it.

One further tidbit of interest about the design concerns the tape recording facilities included. It uses a connector identical to that used to connect the signals from the unique phono pickup and tuner used with this amplifier. That suggests that a unique companion tape deck was also available for it as well.

You rightly mentioned that stereo tapes preceded stereo records which, like stereo records, used a standardized recording format. For our purposes here, that standardization includes both channels being recorded in phase. For the tape desk in this unit to properly interact with commercially prerecorded tapes available at the time then, it would have to have had one of its channels reversed in polarity at the output of the deck -- just as the unique phono pickup and the phase inverter tube on the tuner chassis provided for. And like the tuner, this was likely accomplished with a phase inverter tube on-board the tape deck.

However, in record mode, to have any resulting tapes made be able to properly interact with other machines of the day, the deck would also have had to insert the phase inverter circuit in one channel of the recording signal as well. Otherwise, it would have recorded any signals from the phono or tuner with the channels out of phase with each other on the tape, which would have been incompatible with other machines of the day.

As I said, it was a novel idea to have two single ended amplifiers operate in stereo when the optional stereo extension speaker was connected, and stereo sources were available -- but then have the unit function as a fully push-pull mono amplifier with the internal speakers only, when the extension speaker was not used, and only mono sources were available. As you stated, this allowed full power output to be available regardless of the speaker setup used. It was the epitome of putting the consumer first by allowing for full compatibility with whatever mono or stereo source you may have had at the time -- it worked properly with either format!

This is a concept that is lost on today's engineers, who simply abandon a format when it no longer serves their commercial needs, and then force the consumer to buy all new again to stay current. I am still impressed that I can turn on any number of my old mono FM tuners, and still hear the full program material, even though that medium now broadcasts in stereo -- because the engineers championed compatibility. Or that black and white TVs worked right up until the end of analog TV broadcasting even though the medium went to color broadcasting decades ago -- again, because compatibility was championed. Those are values that are all but gone today. But enough of my rant.

Finally, it is interesting to note, that for all the compatibility efforts included in this console series, FM stereo was clearly not on the horizon yet, as no provisions were made in the tuner section for adding a future multiplex unit for that purpose. Besides that omission though, this unit was just about as much all things to all (console) people as you could get back in the day. What a design!

Dave

NCTubeLover
02-23-2012, 12:25 PM
Dave, Brian and others,

Thank You for all the assistance in this unique RCA amp! :yes::yes:

I do want to repair it and really wish I could find a repair shop near me.

Larry of RCAVictorland in Little Rock, Ar. does not work on this type of amp, bummer.


I'm going to sulk for awhile, had a crappy job interview today.

Anyone know of a great repair shop??

Thanks again,

Greg

drtechno
02-23-2012, 08:12 PM
drag a o-scope out and see where you loose the signal

NCTubeLover
02-23-2012, 08:28 PM
I have a scope, a Tektronix T22.

I've been testing the resistors with a dig. meter and some are way over spec also a few actually test at 0 ohms. Looks like I need some replacements.

Greg

gadget73
02-23-2012, 09:00 PM
resistors almost never drop in value with age. Pull one end out of the circuit and re-test. I always test in circuit, and if it reads high, I know its bad. If it reads low, I may pull it loose and re-verify.

dcgillespie
02-23-2012, 09:46 PM
Greg -- If you don't mind shipping, I will be happy to get you back to "square one" so to speak. As I understand it, you've replaced most of the usual suspects, so all that is left is potentially improper connections or a defective component or two causing your problems. If the repairs are routine, I'll be happy to make them. If they are extensive, I'll determine and document which components need to be replaced, and then let you have at it. All you will need to cover is the shipping each way.

Good luck with your job hunt.

Dave

NCTubeLover
03-10-2012, 12:29 AM
Hey Everyone,

I finally got that stupid RCA amp to work.:banana::banana::banana:
Also I replaced all the resistors too:yes::yes:

Let's see: new can caps, coupling caps, output transformers and resistors.
Spent about $300 in parts.

Gee, that is what eBayers and Craig lister's are asking for "as is" units!

This amp is my most difficult project to date.

I now have 3 fully working tube consoles!!:music:

I have been listening to a little Elvis, Chubby Checker, and Dave Cortez "The Happy Organ.

It doesn't have that much bass.
Guess, I am used the the 15" woofers in my Maggie.

Thanks for the words of encouragement and support.:thmbsp::thmbsp:

Greg

gadget73
03-10-2012, 01:00 AM
So what did it finally end up being ?

NCTubeLover
03-10-2012, 01:31 AM
I must have a had a few bad resistors.

I also looked over the schematic very very carefully and checked to make sure I did not connect something up wrong. Not sure what the magic bullet was....

RWood
03-10-2012, 10:26 AM
It doesn't have that much bass.


Could be the phasing of the signal, either at the input (cartridge to preamp) or output (speakers).

As Dave wrote, this is a complex unit in that regard. If you get a bass boost when listening in mono versus stereo, I would suspect a phasing problem.

RWood