View Full Version : Scott 222B and phono imput


valvestart
09-01-2013, 06:54 PM
Hello to everyone,

I'm restoring a Scott 222B, it is working yet but the phono low and high imputs have too low gain, of course the low one works better. I'm using a MM Grado cartridge that works fine with another my Scott 233.

The ESR values I checked on the two 20/450 sections of the can capacitor (C204 and C205 on the attached schematic) were OK and I replaced the .002 and the .022 caps (C5 and C6) in its preamp stage with no improvements.

Could someone help me to find the reason why of that so low gain phono imput?


Thanks to all

Rob

dcgillespie
09-01-2013, 08:34 PM
You didn't mention it, but I am assuming that the problem is in both channels, and identically so. Therefore, if the tubes are good, then the problem will be something that is common to both channels, i.e., the power supply.

Check the heater voltages to the phono preamp tubes to make sure they are good, and the B+ voltage to the these tubes as well. Then check the voltages on the terminals of each tube versus the schematic.

Also of note, check the 82K resistor that runs between the cathodes of the two preamp stages in each channel. This resistor forms a positive feedback loop to increase the gain of the phono preamp section. If these are missing, or very high in value, then the gain of this section will be significantly reduced.

Other than these possibilities, it would be hard to imagine a separate malfunction in both phono preamps that would cause them both to have identically low gain.

Of course, this all assumes that the gain through the high level inputs is normal. You didn't indicate this either, but if this is not the case, then the phono preamps may not be your problem.

Let us know!

Dave

valvestart
09-02-2013, 03:20 PM
Hi,

Thank you so much for your informations Dave. Yes, the channels are identical with the same the problem. I can assure all tubes are well, to be sure I replaced the two 12ax7 with the other that worked well with the Scott 233 phono stage and all sounded the same. The amps works fine using a CD player, all other imputs work well.

I did some tests with the following results:

- Both of the 12ax7 of the phono stage have around 101V at Pin6 and 0.7 at pin1 (Schematic indicates 100V and 1V);

- There are 187V before the 47K (R204) and 252V after it (Schematic indicates 205V and 270V);

-Bias current checked with a probe is an average of about 30ma for each 6bq5;

-voltage at plate is 350V as per schematic.

I tested with the bias pot to read 38.5V across the 1.2K resistor (R201) as per specifications.

You can see above that the voltage is lower than it should be. I notice that the circuit on the schematic has some differences from the Scott I have (ie the R202 is 47K while I have a 10K) but I watched images of other 222B circuits and they were made like mine.

If it can be of any help I Disconnected the 56K R203 from the circuit and the voltages at the R204 became 202V and 270V then very close to the schematic indication.

I also checked the C202 4/250 and it has the right ESR value.

Can the C1 .001 mfd at the RCA connectors be relevant?

Thanks in advance,

Rob

Trio
09-02-2013, 04:51 PM
Have you traced with a scope at all? With what you have done so far, C1 can be very relevent here under some circumstances because it is between the chassis ground and circuit ground. Rarely will the two tie together directly without some issues developing. Even if it is a mica disc, I have seen those fail, though rare.

If it were mine, and the voltages were close, even if not as close as yours, I would apply signal and check to see that there was at least unity gain each step of the way. Chances are you will find the problem may still be corrosion- possibly on a tube socket connection or the selector switch contacts. Or one of the resistors in the signal path or equalization circuit is open or way out of value. Although looking at the circuit again, if switching between RIAA and tape has no effect, that may not be the problem.

I would check with the scope first, then check the voltage divider and make sure the resistors are in within spec for resistance, and check the connections themselves. A bad connection itself might be an issue because it is the only other thing that is common between both channels besides circuit ground, which might pay to check.

Patrice B
09-02-2013, 08:33 PM
I don't know if it applies to your case or not:

On my LK-72, I have to put the volume at half rotation at least to obtain a satisfying phono level (Rega P1).

On a Scott 222D, at half rotation, it's darn too loud for the JVC LA-21 TT...

valvestart
09-03-2013, 12:27 PM
Thanks for your hints, they are very much appreciated.
I don't have a scope then I have to try to find the problem without it. I also thought that older models have lower gain but the difference between the 222B and the 233 is too much.

I would say that the problem should be inside the phono preamp section since it works well with other sources. Though I found that the 56K R205 is between 280V and 105V in the schematic while I measured 312V and 40V. These 40V go to the pin3 of the 6BL8 but if this voltage is too low shouldn't it affect all the sources?

The phono preamp has only the low gain problem, the loudness knob must be all turned for a medium low volume. The sound quality is good but there is audible hum due to the the knob loudness at max position.

Patrice B
09-03-2013, 01:36 PM
Ok, I see. Not normal to put loudness at max, something is definitely wrong.

I would replace all the electrolytics to be sure...

Patrice

valvestart
09-05-2013, 07:10 PM
I think you are right I will change the electrolytics, they are all can capacitor. Would be better to buy other can capacitors (where?) or axials that are easier to find with a wide range of choices?

Rob

Patrice B
09-05-2013, 08:27 PM
I think you are right I will change the electrolytics, they are all can capacitor. Would be better to buy other can capacitors (where?) or axials that are easier to find with a wide range of choices?

Rob

It depends: some goes for the JJ cans available at the Tubestore, Tubedepot, Tubesandmore or other places (names??).

You could also re-stuff the original cans with single electrolytics (search here for "can re-stuffing").

In my case, I sometimes use JJ cans, Richey Gold cans (found at Justradios.com) or simply replace the caps with singles under the hood when space is available...

I hope it helps.

Patrice

NOSValves
09-06-2013, 11:20 AM
Hi,

Thank you so much for your informations Dave. Yes, the channels are identical with the same the problem. I can assure all tubes are well, to be sure I replaced the two 12ax7 with the other that worked well with the Scott 233 phono stage and all sounded the same. The amps works fine using a CD player, all other imputs work well.

I did some tests with the following results:

- Both of the 12ax7 of the phono stage have around 101V at Pin6 and 0.7 at pin1 (Schematic indicates 100V and 1V); Pin 6 and pin 1 are the plates and the voltages should be similar.

- There are 187V before the 47K (R204) and 252V after it (Schematic indicates 205V and 270V);

-Bias current checked with a probe is an average of about 30ma for each 6bq5;

-voltage at plate is 350V as per schematic.

I tested with the bias pot to read 38.5V across the 1.2K resistor (R201) as per specifications.

You can see above that the voltage is lower than it should be. I notice that the circuit on the schematic has some differences from the Scott I have (ie the R202 is 47K while I have a 10K) but I watched images of other 222B circuits and they were made like mine.

If it can be of any help I Disconnected the 56K R203 from the circuit and the voltages at the R204 became 202V and 270V then very close to the schematic indication.

I also checked the C202 4/250 and it has the right ESR value.

Can the C1 .001 mfd at the RCA connectors be relevant?

Thanks in advance,

Rob

First thing that caught my eye in bold above....

was the pin 1 comment a typo? The voltages that are important here are pin 1 and 3 and 6 and 8. 1 & 6 are the plates and 3 & 8 are the cathodes. Please reply with those voltages.

valvestart
09-06-2013, 03:54 PM
Hi,


Thanks for your reply, you are perfectly right, it was my mistake. I changed some caps and today I got these values:

Pin1: 103,1 and 103,1 V
Pin6: 103,5 and 103 V
Pin3: 0.84 and 0.77 V
Pin8: 0.79 and 0.76 V

Do you think that 40V to the pin3 of the 6BL8 I mentioned before is really too low and may affect the phono imput while the other imputs work well?

Patrice thanks for your suggestions about the electrolytics

NOSValves
09-06-2013, 04:57 PM
I missed your comment on the 6BL8 voltages, the 6BL8's are driver/phase inverter that are always in use regardless of the source chosen. So that has nothing to do with this phono problem. 6BL8 voltages are often all over the map depending on the tube inserted.

My HH Scott 222B shows 100V (1 & 6) on the plates and 1V on the cathodes (3 & 8) of the phono section so you have a little problem with the current draw.

The Sam's photofact shows 120V on the plates and .9V on the cathodes of the phono tubes. By the way it also shows 40V on pin 3 of the 6BL8's.

If you want a copy of the Sams email me and I'll send it back. My email is on my website.

Have you taken a close look at the back of the input RCA jacks to make sure the amp was not modified for both inputs to be high or higher level inputs? The easy way is to check this is measure the resistance from the center signal connection at the back of the RCA jack to the 10K input resistors on pin 7 it should read 10K ohms from the low input and 110K from the high input.

I'd be checking the value of all the phono section resistors... a picture posted of the phono section area of the amp would be helpful.

NOSValves
09-06-2013, 05:00 PM
Oh and Patrice is absolutely correct you should replace all the filter cans in these amps. But if the phono section is not humming like a beast most likely those will not solve the low gain problem.

valvestart
09-07-2013, 10:22 AM
Hi,

Now the phono is working fine, I changed the two can caps sections connected to the 47K of 20/450 C204 and 205 and the 10/450 C206. I also replaced the .02 C5 but I think the actual problem where the 20/450 above mentioned.
Though I still get too low plate voltage at about 100V while I'm suppose to get 120V as per the schematic I kindly received from NOSvalves.

I should be able to post a photo of the circuit tomorrow.


Thanks to all

Patrice B
09-07-2013, 01:23 PM
Hi,

Now the phono is working fine, I changed the two can caps sections connected to the 47K of 20/450 C204 and 205 and the 10/450 C206. I also replaced the .02 C5 but I think the actual problem where the 20/450 above mentioned.
Though I still get too low plate voltage at about 100V while I'm suppose to get 120V as per the schematic I kindly received from NOSvalves.

I should be able to post a photo of the circuit tomorrow.


Thanks to all

Good to know that it is working fine...

valvestart
09-08-2013, 12:08 PM
Hi,

I attached the images of my 222B circuit.

The Sam's photofact shows 120V on the plates and .9V on the cathodes of the phono tubes. By the way it also shows 40V on pin 3 of the 6BL8's

Your Sams shows the actual circuit I have then I should get the 120V on the plates instead of the values I already posted.

measure the resistance from the center signal connection at the back of the RCA jack to the 10K input resistors on pin 7 it should read 10K ohms from the low input and 110K from the high input.

I measured the resistance with the following values:

Low imput: 10.15K and 9.87K
High imput: 112.6K and 119.9K (one 100K at the RCA jack measured 110K)

There are some little differences of the resistor values on the phono stage e.g the 280K at pin1 measured 303K and 286K. Also I got 5.5K for the 82K between pin3 and 8 but the measured should be affected by other parts like caps since few days ago I got the right value when I tested disconnecting it.

As you can see I inserted some temporay caps I alredy have till I'm going to buy new caps.