Still have no sound from RCA amp after mods

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

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.

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.

Both channels hints at power supply, have you replaced all of the filter caps?

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

Blast said:

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

Click to expand...

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?

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...

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.

cademan said:

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.

Click to expand...

Second that!

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?

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

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

Here's the schematic.

I will look into the amp later today.

Greg

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

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

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

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

NCTubeLover said:

This 59 RCA Orthophonic is driving me crazy. :tears:

Enclosed are some pics.

Any suggestions?

Greg

Click to expand...

Got a big hammer?

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

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