Ampex tube reel-to-reel stereo playback, right channel not working?

[AardvarkBry]

When I switch it to mono mode, I get the same signal out of both the left and right output, when it’s only supposed to come out of the left. When I switch to stereo, I get a normal signal out of left, but the faintest signal out of the right.

I’ve checked every voltage in the entire machine, and everything is pretty much spot on, until I get to the cathode follower output. On the cathode side of C6 and C11, I’m getting 34volts on both, but between the capacitor/resistor and the switch, I get 1volt output on the left channel, but only 100mv on the right channel, whether or not it’s switch into mono or stereo. If I check the output jack, in mono, I get 1volt on both, but in stereo, I get 1volt on the left, and 100mv on the right.

Is this a problem in this old 1959 switch, or is the problem in the c11 r25 combo? The capacitor is an old poly bumblebee, and the resistor is an old carbon comp.

 

[radardoug]

You need to check levels through both repro chains to make sure you are getting the right levels at every point. You dont indicate whether you are getting ac or dc volts. Its an important difference. We can only guess, you are the person sitting in front of the unit. You need at least a scope to trace with.

 

[AardvarkBry]

You need to check levels through both repro chains to make sure you are getting the right levels at every point. You dont indicate whether you are getting ac or dc volts. Its an important difference. We can only guess, you are the person sitting in front of the unit. You need at least a scope to trace with.

The measurements are DC. I couldn’t get a good AC reading with my multimeter. I don’t have a scope. I have an old Heathkit RMS VTVM IMW-38 somewhere. I’ll dig it out and measure levels.

When you play a tape in mono, it comes out perfectly strong out of the right channel, and equal to the left side mono. When you switch it to stereo, the signal out of the right is very faint, and you end up hearing more of the machine operating, than you do signal. I checked every DC voltage in the whole machine, every plate, grid, cathode, and everything is pretty close to right on the money. I replaced every old electrolytic cap in the whole machine, and add a filter choke, hoping that might solve the issue. It greatly improved the performance in mono, but did nothing for the stereo. I’ve swapped out every tube multiple times. One thing I found curious, was that the right channel output never had a ground connection. You could tell nothing was ever soldered to the jack. I connected a ground, but that seemed to add a little more buzz, and that was about all.

 

[radardoug]

Well, to test things out you need test equipment, or give it to a pro.

 

[Ike Zimbel]

When I switch it to mono mode, I get the same signal out of both the left and right output, when it’s only supposed to come out of the left. When I switch to stereo, I get a normal signal out of left, but the faintest signal out of the right.

I’ve checked every voltage in the entire machine, and everything is pretty much spot on, until I get to the cathode follower output. On the cathode side of C6 and C11, I’m getting 34volts on both, but between the capacitor/resistor and the switch, I get 1volt output on the left channel, but only 100mv on the right channel, whether or not it’s switch into mono or stereo. If I check the output jack, in mono, I get 1volt on both, but in stereo, I get 1volt on the left, and 100mv on the right.

Is this a problem in this old 1959 switch, or is the problem in the c11 r25 combo? The capacitor is an old poly bumblebee, and the resistor is an old carbon comp.

I would meter the switch and see what the resistance is through each set of contacts. It should be low, like around +/- 1 Ohm or less. If you get really high readings, it's a problem with the switch.

 

[HG-Tacos]

The switch seems to be behaving properly to me. The mono switch on that kind of equipment just sums all inputs to all outputs. It was never designed to only come out the left output when mono was engaged.

This says to me that your output section is OK. You need to focus on your input section - and you have two signal paths there.

You've already stated that you get very low level when playing back tape. Is it a tape that came with this recorder? Is it a tape you recorded on this recorder?

Play a known-good stereo tape and see what happens. Then check the Monitor circuit.

You should have the ability to monitor whatever is on the right input and send it straight out the right output by setting the far left knob to "Monitor."

You should see signal present on the meters and hear whatever you're feeding into the input. Try this for left & right. If it works on one side and not the other, swap your cables and see what happens.

You could have an issue with the input in general, which is your Monitor section. This will cause you to record low-levels or no level at all on the affected channel. This will be evident on any tape you record on this machine and try to playback -and will also stay consistent if you play the tape on another machine.

Or you could have an issue in the repro circuitry, which is what plays back the signal from the tape. If this is the case, a known-good tape will still exhibit the symptoms you already described.

Pro tape gear has a lot of tweakable stuff for the input level, bias, and repro levels. This probably has all fixed components with something blown out.

 

[AardvarkBry]

The switch seems to be behaving properly to me. The mono switch on that kind of equipment just sums all inputs to all outputs. It was never designed to only come out the left output when mono was engaged.

This says to me that your output section is OK. You need to focus on your input section - and you have two signal paths there.

You've already stated that you get very low level when playing back tape. Is it a tape that came with this recorder? Is it a tape you recorded on this recorder?

Play a known-good stereo tape and see what happens. Then check the Monitor circuit.

You should have the ability to monitor whatever is on the right input and send it straight out the right output by setting the far left knob to "Monitor."

You should see signal present on the meters and hear whatever you're feeding into the input. Try this for left & right. If it works on one side and not the other, swap your cables and see what happens.

You could have an issue with the input in general, which is your Monitor section. This will cause you to record low-levels or no level at all on the affected channel. This will be evident on any tape you record on this machine and try to playback -and will also stay consistent if you play the tape on another machine.

Or you could have an issue in the repro circuitry, which is what plays back the signal from the tape. If this is the case, a known-good tape will still exhibit the symptoms you already described.

Pro tape gear has a lot of tweakable stuff for the input level, bias, and repro levels. This probably has all fixed components with something blown out.

Thank you for all the input. Initially I thought it might be a problem with the record head/input section, so I recorded a tape stereo and played it on another machine, and it was fine. I then played a known stereo tape, and it was the same issue. I adjusted the oscillator bias controls a lot, and they didn’t make a difference. There are two weird adjustable switches I’ve never seen before, that say “see note 9” which says they’re factory set and calibrated. That seems like a good candidate. They look like very delicate components.