Hi!

I recently got my hands on an AMC stereo console with phono. Naturally, it didn't work when I got it home. A quick inspection of the amp revealed a burnt up power filter resistor. Replacing the resistor got power flowing across the unit, but the volume was very low. Here's what I've done so far...

1. Replaced all of the tubes with brand new ones.
Rectifier tube (6ca4) (replaced with JJ EZ81)
Preamp tube (6eu7) (replaced with Sovtek unit)
Power tubes (6bq5) 2x (replaced with JJ EL84s)
2. Replaced the power filter capacitors
3. Replaced the preamp cathode bypass resistors and capacitors (I noticed the resistance on the bypass resistor was supposed to be 22k ohm, but in the circuit it only checked out at 2.1k ohm - is this normal?)
4. Replaced the power tube cathode bypass capacitor.
5. Replaced the high voltage power resistors to the preamp tube (both were showing much higher than the rated 220k ohm - one at 330k and one at 600k).
6. I have bypassed the tuner/equalizer completely by using an off the shelf phono preamp to rule out the pots for volume/balance/bass/treble.

So all of this and I still have the same problem. Low volume at the speakers. What kind of voltage drop should I see across the speaker transformers? I'm dying to get this thing working, but I am really stuck here. Any help would be greatly appreciated!!!

I'm getting 360VDC off the rectifier tube, but a max of 24V at the power tubes and preamp tube. I would expect a much higher voltage draw if it were amplifying properly, right?

I would guess you should be getting ~250vdc at the EL84 plates. If you're only getting 24vdc at those plates, you are drawing too much current somewhere. Did you replace the burned out filter resistor with the proper value? Lots of things could be wrong here.
Try unplugging your tubes one at a time and see if that helps bring up the voltage. You could also start at the furthest point of you B+ and try disconnecting the B+ feed line to each section and see what happens.

:I would guess you should be getting ~250vdc at the EL84 plates. If you're only getting 24vdc at those plates, you are drawing too much current somewhere. Did you replace the burned out filter resistor with the proper value? Lots of things could be wrong here.
:Try unplugging your tubes one at a time and see if that helps bring up the voltage. You could also start at the furthest point of you B+ and try disconnecting the B+ feed line to each section and see what happens.
:
Yes, I replaced it with another 150 ohm, 2 watt resistor.

Does it matter which tube I start with?

I should probably disconnect the B+ line that goes to the tuner in case something in the tuner is drawing the power. I haven't even opened the tuner yet.

Thanks for the suggestions!!!

Often when B+ is being loaded down excessively, the culprit is a shorted bypass capacitor. These are distributed at various points on the B+ bus and are used to route stray RF to ground to prevent it from getting onto the B+ bus and causing problems elsewhere in the set. When these caps short, they will take down a section of B+. Usually the B+ source doesn't dip much since there are series current-limiting resistors in place; but the "local B+" will take a voltage hit. This may be what is happening to your amp tubes if the plate voltage is only 24V. I recommend tracing the B+ voltage feed from the bad reading on the tube plate (24 volts) back to the point on the B+ bus where the voltage improves to near normal, which will be on the other side of a series current-limiting resistor. Go back to the "bad side of the bus" and start looking for caps hanging off it that go to ground (most likely there is only one per B+ branch), and remove them one by one until the voltage is restored. Then, replace the cap. It won't harm the set to operate it without the bypass cap, but leaving the cap out may cause it to be susceptible to "birdies" and other odd behavior.

This is probably why your "filter resistor" burned out in the first place. Like fuses, they never fail for no reason at all.

:Often when B+ is being loaded down excessively, the culprit is a shorted bypass capacitor. These are distributed at various points on the B+ bus and are used to route stray RF to ground to prevent it from getting onto the B+ bus and causing problems elsewhere in the set. When these caps short, they will take down a section of B+. Usually the B+ source doesn't dip much since there are series current-limiting resistors in place; but the "local B+" will take a voltage hit. This may be what is happening to your amp tubes if the plate voltage is only 24V. I recommend tracing the B+ voltage feed from the bad reading on the tube plate (24 volts) back to the point on the B+ bus where the voltage improves to near normal, which will be on the other side of a series current-limiting resistor. Go back to the "bad side of the bus" and start looking for caps hanging off it that go to ground (most likely there is only one per B+ branch), and remove them one by one until the voltage is restored. Then, replace the cap. It won't harm the set to operate it without the bypass cap, but leaving the cap out may cause it to be susceptible to "birdies" and other odd behavior.
:
:This is probably why your "filter resistor" burned out in the first place. Like fuses, they never fail for no reason at all.
:
Ok, there are 2 100nF caps going to ground from the preamp tube that I haven't yet replaced. I'll try clipping them first (I have replacements already) and then I'll lift the B+ going to the tuner. I'll try it at lunch and then I'll let you know how it goes!

Have you got a schematic for this set? That would help a lot in identifying the parts where a B+ short might occur.

:Have you got a schematic for this set? That would help a lot in identifying the parts where a B+ short might occur.

It is in Sams Photofact 689-7. I checked my stacks and it is not one I have.

Richard

::Have you got a schematic for this set? That would help a lot in identifying the parts where a B+ short might occur.
:
:It is in Sams Photofact 689-7. I checked my stacks and it is not one I have.
:
:Richard
:
I do have the schematic in a hard copy. I can try to upload a picture of it.

:I do have the schematic in a hard copy. I can try to upload a picture of it.

Greg; if you prefer you can email me a copy of the schematic and I will post it for you. If you click on on my name (Next to the "Posted by") it should pop up your email with my address in it.

Richard

:Often when B+ is being loaded down excessively, the culprit is a shorted bypass capacitor. These are distributed at various points on the B+ bus and are used to route stray RF to ground to prevent it from getting onto the B+ bus and causing problems elsewhere in the set. When these caps short, they will take down a section of B+. Usually the B+ source doesn't dip much since there are series current-limiting resistors in place; but the "local B+" will take a voltage hit. This may be what is happening to your amp tubes if the plate voltage is only 24V. I recommend tracing the B+ voltage feed from the bad reading on the tube plate (24 volts) back to the point on the B+ bus where the voltage improves to near normal, which will be on the other side of a series current-limiting resistor. Go back to the "bad side of the bus" and start looking for caps hanging off it that go to ground (most likely there is only one per B+ branch), and remove them one by one until the voltage is restored. Then, replace the cap. It won't harm the set to operate it without the bypass cap, but leaving the cap out may cause it to be susceptible to "birdies" and other odd behavior.
:
:This is probably why your "filter resistor" burned out in the first place. Like fuses, they never fail for no reason at all.
:
Ugh...

Here's what I did...
1. Removed the Right channel 100nF cap to ground. No change.
2. Removed the Left channel 100nF cap to ground. No change.
3. Disconnected the high voltage to the tuner tubes from the power filter caps. No change.
4. Pulled the Left channel power tube. Voltage went up to 26V, but otherwise no change.
5. Ditto the Right channel power tube.
6. Swapped the preamp tube with an old one. No change.
7. Clipped the Left channel cathode bypass cap. No voltage change, but sound was getting worse.
8. Clipped the Right channel cathode bypass cap. Same as left.
9. Clipped the Left channel cathode bypass resistor and noticed the resistance across the mounting bracket was still 2.1k ohm without the resistor in place! There shouldn't be any connection across the bracket without the resistor unless it can go all the way through the preamp tube AND power tube. Is it possible that I'm getting something drawn back through the output transformers?

Rather than just swapping/replacing parts semi-randomly, it would be more effective if you started where the B+ is known to be bad, then trace it back to the point that it turns "good". Once you have the problem area localized to a single source node, you can concentrate on exactly what might be drawing down the voltage- usually it can only be a few parts.

This set may have multiple problems, but the very low B+ voltage is one that will prevent it from performing and is also a fault that is pretty easy to troubleshoot if you are methodical and can resist the urge to "shotgun" stuff on a hunch. The "divine inspiration/hunch" method is more fun, but the "methodical approach" is more effective.

:Rather than just swapping/replacing parts semi-randomly, it would be more effective if you started where the B+ is known to be bad, then trace it back to the point that it turns "good". Once you have the problem area localized to a single source node, you can concentrate on exactly what might be drawing down the voltage- usually it can only be a few parts.
:
:This set may have multiple problems, but the very low B+ voltage is one that will prevent it from performing and is also a fault that is pretty easy to troubleshoot if you are methodical and can resist the urge to "shotgun" stuff on a hunch. The "divine inspiration/hunch" method is more fun, but the "methodical approach" is more effective.
:
Understood. I was trying to do something to get the voltage to go up as you suggested. The voltage is 360V at the first power capacitor then it immediately drops at the second cap that feeds the two output transformers. The third cap feeds the output tubes and the last cap feeds the preamp tube and the tuner tubes. I'll have to work on posting the schematic, but I'm really beginning to think I've either got a transformer issue or my mounting brackets for the bypass caps have a problem.

You might try removing the B+ loads one at time from the second filter cap node until the voltage jumps up to normal (probably 250+ volts or so). That will tell you which output transformer is sucking all of the power. (Both transformers are unlikely to have failed in the same way.) Then check the offending transformer for a primary winding short to the core (ground) and that the bypass cap on the output tube plate side of the transformer is shorted to ground.

I don't think that an output tube fault could suck down that much voltage without the tube plate glowing bright red and (shortly after) self-destructing. Problem almost has to be in the output transformer side of things.

Usually when there is a plate-side bypass cap short, the transformer gets smoked (burned open) because it can't handle the DC current overload. But maybe your set has more robust transformers than others.

:You might try removing the B+ loads one at time from the second filter cap node until the voltage jumps up to normal (probably 250+ volts or so). That will tell you which output transformer is sucking all of the power. (Both transformers are unlikely to have failed in the same way.) Then check the offending transformer for a primary winding short to the core (ground) and that the bypass cap on the output tube plate side of the transformer is shorted to ground.
:
:I don't think that an output tube fault could suck down that much voltage without the tube plate glowing bright red and (shortly after) self-destructing. Problem almost has to be in the output transformer side of things.
:
:Usually when there is a plate-side bypass cap short, the transformer gets smoked (burned open) because it can't handle the DC current overload. But maybe your set has more robust transformers than others.
:
I doubt this old AMC is the most robust system, but I like the way it looks and I'm really excited to get it working. I've always wanted to be able to listen to a record and dance without having my dad yell at me for making the needle skip!

I'll try lifting the resistor (R73) then look at the transformers one by one.

Here is the schematic for the AMC F636 Amp:

I would suggest lifting the leg of R73 between R73 and R72 and see if the 300V source goes up.

If it does not go up then the issue is on the 300v source.

If it goes up to 300V (or thereabouts) then your draw is on the 280 or 250/220v source. Resolder it, lift the leg between R73 and R72.

If your 300V source goes up to around 300v then your issue is on the 250/220v source.

Richard

:Sir Greg . . . . . .
:
:
:
:I don't think that you mentioned any hummmmmmmmmmm, but an open C68A filter section can be the reason
:
: of the cause your initial problem of R74 charring /opening up.
:
:
:Also the "suspicioned " 2.1-K across the negative feedback loop of that R/C pair used, is a " red herring" to you,
:
: if you look more closely at the circuitry, you will see that the cathode resistor of the 6EU7 is also shunting
:
: ACROSS the pair.The same circuitry procedure is duped on the OTHER channel.
:
:
:Lastly, how good is your color code reading expertise at reading resistors, is there a chance that you misread
:
: and actually have a decade or centary higher value installed in place of a 150 ohm value . . .e.g. 1500 or 15000 ohms.
:
:I just now realized that I have used up all my sick days, so I'm calling in dead.
:
::
::I would suggest lifting the leg of R73 between R73 and R72 and see if the 300V source goes up.
::
::If it does not go up then the issue is on the 300v source.
::
::If it goes up to 300V (or thereabouts) then your draw is on the 280 or 250/220v source. Resolder it, lift the leg between R73 and R72.
::
::If your 300V source goes up to around 300v then your issue is on the 250/220v source.
::
::Richard
::
:
:
Thanks for posting the schematic, Richard! I like your idea of lifting R73 before I lift the transformers from C68B. I will try that first...after I put all the other caps and resistors back on!

Sir Edd,
I hope you're right about the red herring! I just really didn't want the problem to be coming from the transformers. I've already replaced C68A (and B, C, & D) and thought for sure that would fix my problem. I admit that my color code reading skills are weak, but my ability to read a number on a package should inspire confidence. :) (plus I checked it with my multimeter before I installed it.)

Yes, good strategy. If the 300V node is getting sucked down, it could either be something sitting directly on that node (which is just the audio output transformers) OR something "downstream" pulling down the 280 or 250 volt nodes which in turn is dragging down the "upstream" (source) node. Opening each resistor in the B+ stream will let you home in on the offending node.

Did you install the electrolytic caps correctly? If installed "backwards" (+ to ground and - to B+), they will quickly overheat and short out internally when reverse-biased. Usually accompanied by interesting sound effects and projectile-vomited electrolyte.

Looking at the schematic, there are no plate bypass caps to worry about, so if the fault is on the 300V node, one of the audio output transformers is shorted. Not much else to point the finger at.

The 100nf caps that you snipped out had no possibility of drawing down the B+ bus since they are fed from B+ via a 220K ohm resistor.

Whatever the fault is, it is something that is sinking a heck of a lot of current. I'll bet that the resistor that you replaced is smokin' hot.

:Yes, good strategy. If the 300V node is getting sucked down, it could either be something sitting directly on that node (which is just the audio output transformers) OR something "downstream" pulling down the 280 or 250 volt nodes which in turn is dragging down the "upstream" (source) node. Opening each resistor in the B+ stream will let you home in on the offending node.
:
:Did you install the electrolytic caps correctly? If installed "backwards" (+ to ground and - to B+), they will quickly overheat and short out internally when reverse-biased. Usually accompanied by interesting sound effects and projectile-vomited electrolyte.
:
:Looking at the schematic, there are no plate bypass caps to worry about, so if the fault is on the 300V node, one of the audio output transformers is shorted. Not much else to point the finger at.
:
:The 100nf caps that you snipped out had no possibility of drawing down the B+ bus since they are fed from B+ via a 220K ohm resistor.
:
:Whatever the fault is, it is something that is sinking a heck of a lot of current. I'll bet that the resistor that you replaced is smokin' hot.
:
Alright, here's what I did last night...

I replaced all of the caps and put the system back to normal.

I lifted R73 as Richard suggested and got C68B to 250V.
I replaced R73 and lifted R72
C68B = 250V
C68C = 86V
I cut the wire to the preamp tube and got
C68B = 250V
C68C = 140V
I replaced the wire to the preamp tube and cut the feed to the tuner
C68B = 250V
C68C = 96V
This suggests there is a problem in the 280V leg, but I replaced R72 just to see what things were like in the 250/220V leg.
C68A = 367V
C68B = 77V
C68C = 12.6V
C68D = 12.5V
I cut the 280V feed to the preamp tube and got
C68A = 367V
C68B = 70V
C68C = 12.9V
C68D = 12.7V
This suggested that I have an even bigger problem in the 250/220V leg going to the power tubes. So I started cutting wires to the individual tubes. I cut the Left tube first and got
A = 367V
B = 90V
C = 17.8V
D = 17.6V
Replaced the Left and cut the Right tube
A = 367V
B = 86V
C = 16.6V
D = 16.6V
Cut both tubes
A = 370V
B = 280V
C = 85V
D = 85V

Wait a minute...I may have switched the wiring to the C and D capacitors. So, if I'm feeding the preamp from C when it should be D, could that be the problem?

Have you tested the 6CA4 rectifier tube? Dropping the kind of voltage that your set is apparently doing across the filter resistors would almost certainly result in at least one resistor in the string smoking and incinerating itself.

UNLESS the 6CA4 tube is worn out and can't supply enough B+ current to meet demand. In which case it will be loaded down to a very low voltage on its output (cathode pin).

The easiest way to check this is to take voltage readings across the first resistor in the string (R73)
with all of the loads connected and the set operating.
You can then calculate the current by dividing the measured differential voltage by the resistor value (measure the resistor, too, with the set off- resistors can ve mismarked). The tube is capable of outputting 100mA current; if the calculated value is much less than this, I would suspect that you have a weak rectifier tube.

:Have you tested the 6CA4 rectifier tube? Dropping the kind of voltage that your set is apparently doing across the filter resistors would almost certainly result in at least one resistor in the string smoking and incinerating itself.
:
:UNLESS the 6CA4 tube is worn out and can't supply enough B+ current to meet demand. In which case it will be loaded down to a very low voltage on its output (cathode pin).
:
:The easiest way to check this is to take voltage readings across the first resistor in the string (R73)
:with all of the loads connected and the set operating.
:You can then calculate the current by dividing the measured differential voltage by the resistor value (measure the resistor, too, with the set off- resistors can ve mismarked). The tube is capable of outputting 100mA current; if the calculated value is much less than this, I would suspect that you have a weak rectifier tube.
:
The 6ca4 tube is putting out 360-370V with everything in place. I will have all of my tubes (new and old) checked today. The first resistor in the string is R74 rated at 150 ohm. When I was only getting 24V at C68B the voltage across R74 was 340V. 340/150 = 2.2 A. Yikes. Is that even possible for this transformer/rectifier tube combo?

(The 6ca4 tube is putting out 360-370V with everything in place. I will have all of my tubes (new and old) checked today. The first resistor in the string is R74 rated at 150 ohm. When I was only getting 24V at C68B the voltage across R74 was 340V. 340/150 = 2.2 A. Yikes. Is that even possible for this transformer/rectifier tube combo?)

No, and it's physically impossible for the first resistor to dissipate that much power: P=VI, so 340*2.2=748 watts. Absurd. As suggested earlier by someone in the thread, you probably have a resistor that is actually an order of magnitude or two larger that what you want (1,500 or 15,000 ohms, maybe even 150K). That would also account for the voltage drop under load, and the fact that the resistor apparently hasn't vaporized itself yet.

:(The 6ca4 tube is putting out 360-370V with everything in place. I will have all of my tubes (new and old) checked today. The first resistor in the string is R74 rated at 150 ohm. When I was only getting 24V at C68B the voltage across R74 was 340V. 340/150 = 2.2 A. Yikes. Is that even possible for this transformer/rectifier tube combo?)
:
:No, and it's physically impossible for the first resistor to dissipate that much power: P=VI, so 340*2.2=748 watts. Absurd. As suggested earlier by someone in the thread, you probably have a resistor that is actually an order of magnitude or two larger that what you want (1,500 or 15,000 ohms, maybe even 150K). That would also account for the voltage drop under load, and the fact that the resistor apparently hasn't vaporized itself yet.
:
Right, I didn't think that was possible. So the resistances across R74 and R73 are reading much higher (and fluctuating) than they should be. Readings start out in the Mohm range then drop to kOhm. That's new. I would blame my novice soldering skills, but should the meter read through the 150 ohm resistor instead of across a bad solder?

(...should the meter read through the 150 ohm resistor instead of across a bad solder?)

Not sure what you mean. You should be able to ohm from lead to lead on the resistor (i.e., at points on the leads just outside the body of the part) and get a reliable reading. (Make the readings with the set power off, of course.) If you suspect the solder joint at the lead ends, you can ohm from the lead to the solder surface where it attaches; if you get something other than zero ohms you have a bad joint.

:(...should the meter read through the 150 ohm resistor instead of across a bad solder?)
:
:Not sure what you mean. You should be able to ohm from lead to lead on the resistor (i.e., at points on the leads just outside the body of the part) and get a reliable reading. (Make the readings with the set power off, of course.) If you suspect the solder joint at the lead ends, you can ohm from the lead to the solder surface where it attaches; if you get something other than zero ohms you have a bad joint.
:
:
For whatever reason, when I try to check resistance across R74 and R73 my meter goes haywire showing a varying resistance in the Mega to kiloohm range. When I check R72 I get a nice 1.5k Ohm reading. When I switch my meter's leads on R74 and R73 I get O.F. The soldered connections check out ok.

I decided to remove all of the loads from the power filter capacitors except for the output transformers on C68B. When I do this voltages on all lines go up to where they are supposed to be.
A = 365V,
B = 280V,
C = 255V,
and D = 255V.

When I connect the preamp tube to D the voltage drops to 95V and everything else stays the same for A, B, and C although I do get a nasty hum through the speakers (both sides).

When I disconnect the preamp and connect either power tube the voltages drop all over.
B = 77V
C, and D = 17V

I intend to replace all of my power reistors when the new 470 Ohm 5 watt shows up, but I'm not sure where to go from there. Here's what I'm thinking...

Connect only the preamp to D. Lift C78 then C75 to see if the voltage goes up at D. If the voltage does go up then I have to figure out the issue at the power tubes. Does this seem logical?

Thanks for all of your help!

You gentlemen were right all along. The 150 ohm resistor I put in was actually 150k!!! On the bright side, I sure learned a LOT about tube amps along the way.

Thanks for your input!