Wrong voltages in Stromberg-Carlson ASR-433

[Raul]

In these weeks I am powering up many of my amps that haven't ben used in a few year.
One of these is the S-C ASR433, I powered it up slowly with a variac and then I let it play music for a few hours, I didn't notice any bad sound but before putting it again on the shelf I wanted to check some voltages and I found many of them wrong compared to the schematics.
For 6BQ5 while pin 7 is correct at 329-329 (schematics 330) pin 3 (9 instead of 15) and pin 9 (285 instead of 315) are low.
6U8 were rather good but in 12AX7 pin 1 (54 vs 115) and pin 6 (58 vs 110) are way lower.
So I checked the can caps and I got square (20uf) 285 (315), half-circle (20uf) 253 (265) triangle (10uf) 230 (130) and the fourth tab (10uf) 75 (280). So I thought that at least the two 10uf caps were bad and replaced them with two new 10uf 450V caps. I took voltages again but I got exactly the same results: 230 and 75V
A few years ago I replaced nearly all the resistors in this amp, I didn't checked them all now (but I can do if necessary) but I checked several of them (including all the few non replaced) and they are all within spec
Tubes were checked some time ago (not recently) with my simple B&K 667 tube tester and all tested good, maybe the tester missed some tube problem?
So now here I am, and also considering that my knowledge in electronics is very basic, puzzled and discouraged, any idea about what could be the problem and what to do/check now?
Thanks in advance

 

[gadget73]

Have you checked to see if the resistors are in spec, especially the ones on the power supply? With the voltages being that far off at the last tab, I'm thinking either some of those resistors are very far off, or you have some issue causing high current load somewhere around the phono stage end.

The 12ax7's are only used in the phono stage, so unless you are using the turntable input those do not matter. You can remove those and see if the other voltages come back to normal to see if its a problem with those two tubes.

 

[FlaCharlie]

Hi Raul - No reason to be discouraged, troubleshooting is a step by step process. The first step (replacing the old 10uf caps) didn't work so we just move to the next step.

Note to other posters: Raul and I have exchanged a few private emails on this previously. I'm responding here so others will know what's been tried already.

I believe he also checked the PS resistors previously. Correct, Raul?

To answer your previous (emailed) question, when you use the resistance table on a Sams Photofact you go through the whole thing and measure each pin on each tube. Unless it indicates otherwise, you measure to ground. As you can see, several of them are measured from the output of the last diode - so black probe on the diode and red on the tube pin instead of black on ground. Of course make sure you've drained all the voltage from the caps. So check those resistances and see if any are off by more than 15%.

On your 6BQ5 voltages, pins 7 (plate) is obviously good. Pin 9 (G2) is also within spec, not low. 315v +/- 15% ranges from 268v to 362v. Also, it's lower than the plate voltage, which it should be. Pin 3 (cathode) is low so you need to check the value of the cathode resistor and also replace the bypass cap, if you haven't already. You can use a 22uf instead of 25uf. See comment below about smaller caps and go up a step in voltage if possible.

Lets look at each cap and its tubes separately. The one that supplies the 7025/12AX7 should be 130v and it's way high at 230. If something is drawing more current, the voltage supplying it will drop and if it's drawing less current the voltage will rise. So we would expect that the 12AX7s are drawing less current than expected. At the same time, however, the voltages on the plates of the 12AX7s are too low. That is a bit confusing.

Before going further, try 12AX7s from another amp on the odd chance that one of them is bad, or test them if you have a tube tester. Also, try cleaning the sockets and make sure the pins are making a good connection with the socket.

Gadget's advice to pull the 12AX7s and see how the voltages are affected is a good idea too.

Assuming that you want to use the phono section and that the problem is still there . . .

So we need to check all the resistors on each triode section of the tubes. Here's the data sheet: http://tubedata.milbert.com/sheets/093/1/12AX7A.pdf

So check the plate resistors (pins 1 and 6), the cathode resistors (pins 3 and 8) and the grid leak resistors (which go from pins 2 and 7 to ground) and replace any that are out of spec. Also, there is a low voltage 50uf cathode bypass cap on pin 3 of each tube. Did you replace these earlier when you did the resistors?? In my experience these small electrolytic caps are almost always bad or out of spec. I never even bother to test them anymore, I just replace them. If you didn't replace them and don't have a replacement on hand, just disconnect one end of each cap for now and then recheck your voltages. You can replace them with 47uf caps. I usually go a step up in voltage since they are so tiny.

The other cap supplies the 6U8s. Here's the data sheet: http://tubedata.milbert.com/sheets/093/6/6U8A.pdf

You say the voltage on that cap is way low - only 75v instead of 280v but you said earlier (private email) that you were getting 171v on pin 1 (plate, which should be 195v so it's in spec) of the 6U8 triode which is supplied by that cap. That would be impossible if the supply was only 75v and passing through a resistor too. So measure that again.

If it turns out that it is out of spec, check all the resistors on the plates, cathodes and grids, including G2 on the pentode section. There is also a 4uf cap between pin 3 and pin 7 so that cap is suspect too and should be replaced. I'm not sure if they make 4uf caps anymore, so just parallel two 2.2uf in each position. Again, go a step up in voltage if possible.

 

[FlaCharlie]

On your 6BQ5 voltages, pins 7 (plate) is obviously good. Pin 9 (G2) is also within spec, not low. 315v +/- 15% ranges from 268v to 362v. Also, it's lower than the plate voltage, which it should be. Pin 3 (cathode) is low so you need to check the value of the cathode resistor and also replace the bypass cap, if you haven't already. You can use a 22uf instead of 25uf. See comment below about smaller caps and go up a step in voltage if possible.

Re: the 6BQ5 cathode voltage. I just looked at the schematic. The lower voltage on the cathode is actually better. The stock amp ran those 6BQ5s really hard. Dropping 15v over a shared 150 ohm cathode reisistor means that each tube is dissipating 15.75w and, according to the data sheet, the max plate dissipation should be 12w. That's not uncommon. Many manufacturers ran the piss out these tubes. With 9v on the cathode, assuming your shared resistor is actually 150 ohms, the dissipation is only 9.45w. That includes the screen dissipation so it's pretty conservative now. I'm guessing you have well used tubes in it and putting new ones in may affect the voltage.

I would still suggest a new bypass cap even if you leave the resistor alone.

 

[Raul]

Have you checked to see if the resistors are in spec, especially the ones on the power supply? With the voltages being that far off at the last tab, I'm thinking either some of those resistors are very far off, or you have some issue causing high current load somewhere around the phono stage end.

The 12ax7's are only used in the phono stage, so unless you are using the turntable input those do not matter. You can remove those and see if the other voltages come back to normal to see if its a problem with those two tubes.

I removed the two 12AX7 (I don't use the phono inputs) and without those tubes the voltage that was at 70 jumped to 260 (280) so it becomes in spec, while if I remove only one is stays at 70, if I remove the other one instead it goes to 145. But the voltage at the "triangle" of course is still high at 230 (130)
The voltage of 6BQ5 stays the same: 9v at pin 3 (15), 330 (330) at plate and 290 (315) at pin 9

You say the voltage on that cap is way low - only 75v instead of 280v but you said earlier (private email) that you were getting 171v on pin 1 (plate, which should be 195v so it's in spec) of the 6U8 triode which is supplied by that cap. That would be impossible if the supply was only 75v and passing through a resistor too. So measure that again.

Yes that's very strange, I measured it again 6U8 is about 170V at pin 1, it's the same value with the cap voltage at 70 or at 250 (without 12ax7 tubes), and I don't understand why removing 12ax7 brings the voltage of the 4th cap (that goes to 6U8) to normal while the voltage of the "triangle" cap that feed the 12ax7 remains the same too high at 230 instead of 130.

 

[gadget73]

The 130v supply feeds only the phono stage, so that one will be high with the 12ax7 tubes removed.


Can't say I understand why pulling the 12ax7 would alter the 6U8 voltages either. I wasn't looking at the schematic earlier, now that I am I don't see how those sections really interact very much. What voltages are you getting on the 6U8 and where? The voltage at pin 6 will be very much determined by the voltage at pin 3, and that is controlled by the condition of the tube and the 1M resistor being good. Also it will want the 330K resistor from pin 6 to be good as well.

Voltages on the 6U8 triode section will be controlled by tube condition, and the set of resistors around the tube. This one needs the two 39K and the 2.2K to be reasonably on value. The 470K isn't very critical but it needs to not be open.

Also keep in mind there is a tolerance on all of those voltage measurements. The Sams schematic allows +/- 15% so don't get too concerned if its not exactly matching.

 

[FlaCharlie]

It's demons! Demons, I tell ya!

I remembered that I had somehow removed (intact!) the factory schematic that was pasted onto the inside of the bottom cover on my 433. After much digging I found it - of course it was not in my file folder of S-C schematics. Some of the PS voltages are slightly different but the PS circuit is the same. So we can rule out the possibility that there is some kind of major mistake on the Sams. It happens.

Something is truly wacky here. Pin 1 of the 6U8 (plate of the triode section) has a 39k plate load resistor which connects directly to the voltage source. The Sams says this source should be 280v. My factory schematic says it's 260v. Regardless . . . How could Pin 1 possibly read the same voltage whether the measured source voltage is 70v or 250v???

And, as Gadget says, pulling the 12AX7s should not affect voltages on the cap that supplies the 6U8s. There are three resistors between those two caps.

I think you're measuring something from the wrong place. Or maybe something is not wired correctly. Let's see . . .

Start at pin 1 of the 6U8 and find the 39k resistor that connects to it. It should be the only part between pin 1 and one of the 10uf caps. Measure the voltage on the cap side and then on the tube side. Do this right at the resistor. That way you're not measuring the wrong cap by mistake. Try it with and without the 12AX7s installed.

That cap, by the way, should have a 1/2 watt 10k resistor between it and the 20uf cap that supplies voltage to pin 9 (G2) of the 6BQ5.

The other 10uf cap supplies voltage to the 12AX7s. The only parts between it and Pins 1 and 6 of the 12AX7s is a 100k resistor - one resistor on each pin 1 and one on each pin 6 of both 12AX7s. Again, with the 12AX7s installed, measure on each side of the resistors, right at the resistor, not at the cap.

It might help if you make a chart and systematically measure, and write down, the voltages at each pin of each tube. Remember, if you measure the heater pins, they will read in AC not DC. In your emails to me you mentioned some tube pin voltages but it was not complete.

There is a 100k 1/2 watt resistor between the 10uf cap that supplies the 12AX7s and the 20uf cap that supplies the 6AV6s. The cap that supplies the 6AV6s is connected to the 20uf cap that supplies voltage to pin 9 (G2) of the 6BQ5s. The resistor between those two 20uf caps is a 10k 1w part, not a 1/2 watt. Double check to see if you've got it wired correctly and also measure the resistors between the caps in the PS to confirm that they are in spec.

Hopefully we can get this figured out. If not, you might have to call the Pope and request an exorcism!

 

[Raul]

I checked carefully the area around the can caps and I found a strange thing, the two 10uf caps are inverted, i.e. the wires from the "triangle" cap that should go to the 12AX7s actually go to the 6U8s and the wires from the "no sign" cap go to the 12AXTs instead of 6U8s. They were connected this way before I replaced the two 10uf caps, it's not my mistake.
This finally expains the impossible increase of voltage from 70 on the cap to 170 at pin 1 of 6U8, because I was measuring the wrong cap as I followed the schematics and I didn't check where the wires actually go.
I know that they are both 10uf caps but this exchange compromise something or doesn't change anything?
Here is a pic ("triangle" is on the right, "no sign" is below)

I notice also that the two capacitors connected to the diodes are 100uf and not 50uf, these were replaced by me in the past and I don't remember if they were already 100uf before (but I don't think I increased them by myself)
Regading your questions:

Start at pin 1 of the 6U8 and find the 39k resistor that connects to it. It should be the only part between pin 1 and one of the 10uf caps. Measure the voltage on the cap side and then on the tube side. Do this right at the resistor. That way you're not measuring the wrong cap by mistake. Try it with and without the 12AX7s installed.

with or without 12AX7 installed (of course) voltages on pin 1 across the resistor are the same, 170V toward the tube and 233 toward the cap.
Anyway voltages of 6U8 are within spec exept pin 3 a bit low: 1 172 (195), 3 30 (40), 6 61-67 (75), 8 65 (70) 9 -3.1-3.2 (-3.5)

That cap, by the way, should have a 1/2 watt 10k resistor between it and the 20uf cap that supplies voltage to pin 9 (G2) of the 6BQ5.

correct

The other 10uf cap supplies voltage to the 12AX7s. The only parts between it and Pins 1 and 6 of the 12AX7s is a 100k resistor - one resistor on each pin 1 and one on each pin 6 of both 12AX7s. Again, with the 12AX7s installed, measure on each side of the resistors, right at the resistor, not at the cap.

pin 1 and 6 with 12AX7 70V before and 50/55 after the 100K resistor, without 12AX7 250V (before and after resistor): I tried also two different 12AX7 but nothing changed so the problem seems to be why the 10uf cap with tubes gives 70V instead of 130V? I don't think it's a problem of the can cap as I replaced it with the same result

There is a 100k 1/2 watt resistor between the 10uf cap that supplies the 12AX7s and the 20uf cap that supplies the 6AV6s. The cap that supplies the 6AV6s is connected to the 20uf cap that supplies voltage to pin 9 (G2) of the 6BQ5s. The resistor between those two 20uf caps is a 10k 1w part, not a 1/2 watt. Double check to see if you've got it wired correctly and also measure the resistors between the caps in the PS to confirm that they are in spec.

Here there is a difference, as you can see in the pic the resistor between the 10 and 20 cap is 220K (red/red/yellow) not 100K, while the other resistor 10K 1W is correct. But on the schematics glued under the bottom of the chassis this is 220K (so mine is correct) and the 2W resistor coming from the diodes is 2200 (in my amp and chassis schematics) instead of 1000 in Sams. So I think this should be correct anyway.
So what to do now?

 

[gadget73]

I believe the doubler caps are 100uf stock. 100 or 220uf is fine as a replacement.

 

[FlaCharlie]

I checked carefully the area around the can caps and I found a strange thing, the two 10uf caps are inverted, i.e. the wires from the "triangle" cap that should go to the 12AX7s actually go to the 6U8s and the wires from the "no sign" cap go to the 12AXTs instead of 6U8s. They were connected this way before I replaced the two 10uf caps, it's not my mistake.
This finally expains the impossible increase of voltage from 70 on the cap to 170 at pin 1 of 6U8, because I was measuring the wrong cap as I followed the schematics and I didn't check where the wires actually go. I know that they are both 10uf caps but this exchange compromise something or doesn't change anything?

Just as I suspected, it was wired incorrectly . . . so no need for an exorcist. It should be switched to the correct way because the two sections supply significantly different voltages - the cap that feeds the 6U8s supplies twice the voltage.

If you were able to fit 100uf doubler caps in there that's great. They must be skinny caps to fit in that shallow chassis. In series they are effectively the same as a 50uf cap. I opened mine up and I used a pair of 82uf caps that are thicker and barely fit. Since this is not a tube rectified PS, you can go higher and, within reason, it should be an improvement.

with or without 12AX7 installed (of course) voltages on pin 1 across the resistor are the same, 170V toward the tube and 233 toward the cap. Anyway voltages of 6U8 are within spec exept pin 3 a bit low: 1 172 (195), 3 30 (40), 6 61-67 (75), 8 65 (70) 9 -3.1-3.2 (-3.5)

Don't worry about the 6U8 voltages for now. The two sections are very interactive and, if they are anything like the similar 7199, tubes vary a lot. We'll come back to them later to get them dialed in.

pin 1 and 6 with 12AX7 70V before and 50/55 after the 100K resistor, without 12AX7 250V (before and after resistor): I tried also two different 12AX7 but nothing changed so the problem seems to be why the 10uf cap with tubes gives 70V instead of 130V? I don't think it's a problem of the can cap as I replaced it with the same result

When you remove the 12AX7s there's no voltage drop across the resistor because there is no current being drawn. With them installed the voltage is too low on the tube side because there is too much current being drawn.

Edit: Errr, not too much current. The Sams and the factory schematics differ on the 12AX7 voltages but a quick look suggests the issue is with the supply voltage. So . . .

Again, first step is to connect the correct cap to the 12AX7s. It's the cap with the 100k resistor between it and the previous 20uf cap. After you've got it connected correctly, measure the values of the plate resistors and the cathode resistors on the 12AX7s and make sure they're in spec. Four plate resistors and four cathode resistors.

Here there is a difference, as you can see in the pic the resistor between the 10 and 20 cap is 220K (red/red/yellow) not 100K, while the other resistor 10K 1W is correct. But on the schematics glued under the bottom of the chassis this is 220K (so mine is correct) and the 2W resistor coming from the diodes is 2200 (in my amp and chassis schematics) instead of 1000 in Sams. So I think this should be correct anyway.

On my factory schematic the values of the PS resistors is the same as on the Sams. Since the Sams was likely not published immediately after the amp was introduced, and because my factory schematic agrees with the Sams, I'm guessing that your amp is an early production model and that they changed the schematic afterwards. Since all your supply voltages are a bit low I would suggest changing the 220k to a 100k and change the 2.2k 2w to a 1k 2w (or 5w if you don't have a 2w resistor) too. That would raise the PS voltages of everything that follows just a bit too.

After you've got the PS sorted out and the resistors on the 12AX7s in spec, what are your voltages?

 

[FlaCharlie]

I believe the doubler caps are 100uf stock. 100 or 220uf is fine as a replacement.

Actually, they are a pair of 50uf caps. Here's the PS section of the Sams which has the same parts as my factory schematic, although they are drawn a bit differently. These amps were definitely a bit low on PS capacitance.

 

[Raul]

Again, first step is to connect the correct cap to the 12AX7s. It's the cap with the 100k resistor between it and the previous 20uf cap...
Since the Sams was likely not published immediately after the amp was introduced, and because my factory schematic agrees with the Sams, I'm guessing that your amp is an early production model and that they changed the schematic afterwards. Since all your supply voltages are a bit lowI would suggest changing the 220k to a 100k and change the 2.2k 2w to a 1k 2w (or 5w if you don't have a 2w resistor) too. That would raise the PS voltages of everything that follows just a bit too.

SOLVED!!!
The cap was already connected correctly to the 12AX7, even though the wiring was inverted (so that "triangle" cap was actually "no sign" and "no sign" is "triangle" as both are 10uf) the cap going to the 12AX7 was already the correct one with the 100K resistor (mine as I told you was 220K instead) between it and the 20uf cap so there were no wiring to change. At this poin the only change I could still make was replacing the 220k resistor with the 100K and the 2200 2W with the 1K 5W as per Sams and your schematics, and voilà the magic happened, all voltages became instantly correct.
Now I have "square" 310 (315), "half-circle" 260 (265), new-triangle 132 (130) and no-sign (old triangle) 255 (280). This brought also pin 1 and 6 of 12AX7 from 50-55 to 100/105 (110-115), it gave to 6U8 those 10-15V more to become closer to specs (pin 1 from 172 to 186, schematics 195) and reduced the difference on 6BQ5 from 40 to 20V ( pin 7 and 9 325 and 305, before they were 325 285, schematics 330 - 315).
So it seems that everything is in spec now. So replace those two 10uf can caps wasn't necessary but it surely won't hurt, but I think that I leave the other two (square and half-circle) alone as they seem to filter very well and very close to spec. Thank you very much for now and let's move to another amp...

 

[FlaCharlie]

SOLVED!!!
The cap was already connected correctly to the 12AX7, even though the wiring was inverted (so that "triangle" cap was actually "no sign" and "no sign" is "triangle" as both are 10uf) the cap going to the 12AX7 was already the correct one with the 100K resistor (mine as I told you was 220K instead) between it and the 20uf cap so there were no wiring to change. At this poin the only change I could still make was replacing the 220k resistor with the 100K and the 2200 2W with the 1K 5W as per Sams and your schematics, and voilà the magic happened, all voltages became instantly correct.
Now I have "square" 310 (315), "half-circle" 260 (265), new-triangle 132 (130) and no-sign (old triangle) 255 (280). This brought also pin 1 and 6 of 12AX7 from 50-55 to 100/105 (110-115), it gave to 6U8 those 10-15V more to become closer to specs (pin 1 from 172 to 186, schematics 195) and reduced the difference on 6BQ5 from 40 to 20V ( pin 7 and 9 325 and 305, before they were 325 285, schematics 330 - 315).
So it seems that everything is in spec now. So replace those two 10uf can caps wasn't necessary but it surely won't hurt, but I think that I leave the other two (square and half-circle) alone as they seem to filter very well and very close to spec. Thank you very much for now and let's move to another amp...

That's great news. As I said, you just gotta take it step by step.

I would still be tempted to replace the remaining original electrolytics since you're in there already and there are only two caps left. Although, I admit that I don't automatically do that either.

I have an old Eico cap checker that reads lytics better than my DVM and a cheap ESR meter that I use to make these decisions. I measure the ESR of the old cap and compare the reading to that of a new replacement. Sometimes I actually find that the old cap measures better (lower ESR reading) than a new cap. If there is no significant difference in ESR I then test the cap on the Eico to measure the cap in terms of value and leakage. If all is OK, I'll leave it in.

Did you replace the two 50uf 6v cathode bypass caps on the 12AX7s? And the two 4uf 150v caps on the 6U8s? Definitely replace those.

 

[larryderouin]

Aw RATS I wanted to see it try to spew pea soup all over the room.