Eico ST-84 channel help

jaguar ride

Active Member
Howdy everyone,
I'm having trouble with channel 1(which I guess is supposed to be right) on an St-84 preamp. Trouble as in, it ain't working. Based on an extremely basic troubleshooting process(swapping cables), it looks like the problem is something with the input. Signal still comes out of channel 1 output. I tried all the inputs on channel one -mic, phono a, phono b, tape head- and none of them work.

I have zero knowledge of electronics, so this could be something simple, or something not worth the trouble, I really don't know. I figure now is a good time to start learning.


Oh yeah, all odds are really against this thing. I'll tell you what it's been through because that very well may have a hand in this thing being busted.

I found it on the side of the street while I was skateboarding to catch the train. I had no idea what it was other than it said "stereo" on it. I assumed it didn't work, but I still really wanted it, but the train was coming and I'd be skateboarding around the city all day, so I hid it in some bushes on the sidewalk to pick up later in the day. Well, I forgot about it because I'm a knucklehead. Until 2 days ago. It had been ONE MONTH since I hid it. It was still there(amazing since it was on a well traveled street in San Francisco), upside down, barely hidden, and the power cord had been cut. Oh yeah, it had also rained four or five times since I hid it. Perfect. So I opened it up and replaced the power cord, plugged it in, and somehow, it turned on. Amazing. When I tested it with a turntable, it sounded bad with crazy humming, feedback, and scratches. Cleaned the pots, it sounds great. Amazing that the only issue I'm having is with channel one(or maybe it's not amazing, things were built to last back then)


So there you have it. I think that's all of the info. Would really like to get this thing fully functioning. If anyone can PLEASE offer some advice or tips or insight, that would be much appreciated. Thanks!
 
Thanks! Now I feel a lot better about those 'senior moments' that happen from time to time.

If it hasn't had at least a couple of weeks in a warm & dry environment, then be patient. Make sure there's absolutely no residual moisture inside the transformers before operating it.

Okay; a question or two... Do you have a decent voltmeter, and know how to use it without injuring yourself? How about the Eico ST-84 manual, or at least a schematic? Have you tried the most basic troubleshooting trick in tube stereo gear, i.e. swapping tubes between good and bad channels?
 
Thanks for the reply. It's been a couple weeks since it rained, but I do live in a pretty fog heavy area. It looks dry. Been powering it on for the past two days with no problems other than channel issues. I'll give it some more time though.

I did print out the schematic thinking I might be able to follow the signal flow and catch anything visibly busted. More difficult than expected.

I don't have a voltmeter, but it looks like I'll be buying one today or tomorrow. As I said, I have no background in electronics or tube equipment, so everything I'm doing right now is a first for me. I'll try the tube swap and see what happens. Hopefully I don't mess it up even more.

Looks like a good time to pick up some tube and electronics reading material!
 
Switches switches and more switches. Give the switches a good back and forth rinse and repeat scenario to clean the contacts so to speak. I had a finicky Harman Kardon Citation I that the switches would cut the signal out. Drove me crazy until I replaced them. That's a good start. Testing the tubes would be a good second.
 
As I said, I have no background in electronics or tube equipment, so everything I'm doing right now is a first for me.

Better learn how to be safe when working on tube gear. The filter caps can hold a charge even when the unit is unplugged if they have not been discharged. Hot tubes can drain them but you will only know if the unit is safe to touch if you measure the voltage and, if found, discharge the caps.

If this does not make sense, you need to do some research before opening the unit.
 
Ok, update after a long vacation outta town:

The channel still doesn't work. Discharged the caps, picked up a multimeter and some tube/electronics books, swapped the tubes using a tube that I know works from a guitar amp, cleaned the switches. Nothing. Running out of basic troubleshooting ideas.

Anyone have any ideas of what it could be/other areas to check? I'm determined to get this thing working even if it takes forever.

Thanks.
 
Jag -- It will help to know if the sound is missing on all inputs or just some in channel 1.

One thing that you can do to help identify where the cause may lie is to feed a high level signal into the tape input jacks, and turn the tape monitor switch on. Doing this eliminates (obviously) the phono preamps, but more importantly, all of the wiring associated with the selector and mode switches as well. If the sound is still missing in channel 1 in this scenario, the problem must be associated with the balance or level controls, the wiring associated with V3, or the wiring associated with V5A (all Eico designations).

If the sound is present in the above scenario, then the problem will be associated with the selector or mode switches, or possibly the phono preamp (V1) if the problem is only present when phono is selected.

Keep us posted on what you find.

Dave
 
Thanks for the input fellas.

None of the inputs on channel 1 work(although a VERY low signal comes through when I crank the level up).

Bypassing the preamp works, not only straight from tape in to tape out, but also from any channel 1 input to tape out. Which makes me think the problem is after the selector and somewhere closer to the output. I could be completely wrong though, as I have no experience and am treating this as a learning experience.

Will post photos if I can borrow my roommate's digital camera.

I appreciate the help. Thanks.
 
The next logical step would be to measure anode voltages at V3, V4, and V5. That will tell us pretty quickly whether DC bias conditions are at the root of your problem.

FYI: Channel 1 is normally the LEFT channel. Think of the channels being numbered from left to right...
 
Channel 1 is indeed left. Whoever tossed this thing to the curb was kind enough to label this channel.

The next logical step would be to measure anode voltages at V3, V4, and V5. That will tell us pretty quickly whether DC bias conditions are at the root of your problem.

I'm sorry, but I have absolutely no idea what this means. Is it possible to explain over the internet, or maybe have a link? I'm assuming by looking at the schematic these are the tubes, correct? Not sure about the measuring though...
 
Do you have a meter? You don't have to be a rocket scientist to know how to take basic voltage readings of the good working channel, and compare them to the bad unworking channel.

Also, a quick check on the output transformers would be good place to start. Remove the output tubes, and do a quick check of both transformers using the ohms scale on the meter.

Both should be within a few ohms of each other. If one is radically different in ohms compared to the other, then it is probably open (shot). This may not be your problem, but check everything that you can do easily.
 
Jag -- The "anode" that Mike refers to (also called the "plate") is the high voltage element within each operating section of the tubes (the 12AX7 tubes used in an ST-84 each have two sections, and therefore, two plates).

For these tubes, V3 serves strictly channel #1, V4 serves strictly channel #2, while V5 serves both channels (Eico designations)

With the chassis turned upside down, look at the tube sockets for these particular tubes. Notice that each tube socket has 9 pins with a gap in the circle of pins at one point. The pins are numbered 1-9 with pin #1 being the first pin going in a clockwise rotation from the gap. The plate pins are numbers 1 & 6 for these tubes.

If you have no way of specifically identifying which tubes these are, for lack of anything better, they will be the three 9 pin tubes that are located farthest away from the selector switch. There is also a 7 pin tube used in this preamp that is not at issue here, so do not consider it in these discussions.

Set your meter so as to be able to measure about 200 volts DC. The black (or negative) lead should be firmly connected to the chassis, the red positive lead is then (carefully!) touched to pins #1 & 6 of V3, V4, & V5. Use the readings you take on V4 and V5B (identified as pin #1 on that tube) as your "Good" readings since they operate in channel #2. Your defective channel #1 will be the readings taken on V3 and V5A (identified as pin #6 on that tube).

Specifically, Eico says you should see 140 volts DC on both pins #1 & 6 of V5, and on pin #6 of V3 and V4. You should see 180 volts DC on pin #1 of V3 and V4.

This is not a complicated procedure to do, but it may seem so at first. It is also a good way to start learning about how vacuum tube electronics works -- but be careful. There are high voltages within the chassis that under the right conditions can be lethal. Wear shoes and socks and keep one hand behind your back when you take such readings -- that way no current can flow across or through your body should you accidentally slip with your hand. This is not meant to scare you off at all, but is simply good basic safety guidelines to follow whenever you are working around high voltage.

Good luck and let us know what you find!

Dave
 
dcgillespie -

Thanks a lot for the help. However(and I'm probably going to sound like a knucklehead here), I gotta ask before I do something stupid/dangerous.... does the unit have to be plugged in/turned on while testing the anodes? I'm getting no reading as of right now, so I'm either doing something wrong, or I gotta plug it in. I've got the negative lead attached to the chassis and touching the positive, and nothing.

I would normally not think of plugging it in while poking around, but since I've drained the filter caps and you've still mentioned high voltages I'm beginning to question myself...

Thanks again
-Seth
 
Ok, here's what I ended up with:

V3 #1- 167v V3 #6- 125v

V4 #1- 164v V4 #6- 115v

V5 #1- 174v V5 #6- 174v


Off from what Eico says, but fairly similar to each other.
 
Yes they are off, but today's line voltages will distort the readings somewhat (cause them to read higher), so that is not a real concern. Next: (1) Great job getting the readings! and (2) It proves that all the necessary tubes are drawing appropriate current to amplify as they should. Therefore, the problem is likely going to come down to things like dirty RCA jacks, lose connections, a signal short somewhere, etc.

Here is an easy test you can do to help trace the signal through the unit:

1. Connect a high level signal into the tape input jack of Channel #1, and turn the tape monitor switch on.

2. Connect a short RCA to RCA mono (or one side of a stereo) connector cable into the tape input of Channel #2. Leave the other end alone.

Since channel #2 produces sound, you can use it to trace the signal through channel #1 and see where it stops:

1. Connect the preamp to your power amp as you normally would, and turn the units on. Observe the precautions mentioned earlier. Center the balance control and turn the volume control to ~ 11-12 o'clock position. Turn your signal source on. You will not hear any sound. However, if you touch the tip of the open end of the connector cord, you should hear a buzz. No danger of shock here.

2. Now touch this tip then (and only the tip -- the grounding sleeve connects to nothing) to the following locations:

A. Both connections on the balance control. By following the general wiring you can determine if channel #1 is the front or rear section of these controls. Once you determine which section of the control goes to which channel, it remains consistent between the balance, level, bass, and treble controls. Remember V3 is in the defective channel, so whichever control section that pin #7 of this tube connects to will determine which section of all controls serves the defective channel. Once it's determined, always use the same section of the various controls through out the test.

B. The top lug of the volume control. This will be the outside lug with a wire connected to the balance control.

C. The center lug of the volume control.

D. Pin #7 of V3.

E. Pin #2 of V3.

F. Both terminals of the rumble switch. Be sure to determine which side is used for Channel #1. It will be the side fed from the circuitry associated with V3.

G. All terminals of the Bass control.

H. All terminals of the Treble control.

I. Pin # 7 of V5.

J. The center pin of Channel #1's output jack.

You should likely pick up sound in step A above. If not, then it has already stopped at the tape monitor switch or its wiring. Assuming you do pick up the sound in step A above, you should lose it at some point as you move through the steps. When you determine where it stops, you will have determined very closely where the problem is. The sound will normally be louder or softer at different points through out this test. You are looking for where it basically disappears.

Again, good luck with it, and report back!

Dave
 
Not getting sound from step A. There is a very low signal coming through constantly, but it is VERY low. Maybe I'm doing something wrong? I'm going out through Preamp out, which is what I would normally be using. Should I be in Tape out instead?

Again, I really appreciate the help here. Hope I'm not overstaying my welcome and asking too many stupid questions. Just trying to learn, and the books only teach so much.
 
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