Scott 99C - No more sound

What about the resistance of the high voltage secondaries? Do they read 35 and 40 Ohms per the schematic? There could be a high resistance in the center tap transformer lead.
Yes
The resistence is 35.5 ohm and 40.5 ohm
Regards
 
yesterday
when I replaced the filter caps, I saw that the + and - legs of the C1B were very very close. There is a high possibility a short circuit there. I'm not sure about that.

If its has happened, may the rectifier tube have been damaged? Resistors? Capacitors?
Regards
 
yesterday
when I replaced the filter caps, I saw that the + and - legs of the C1B were very very close. There is a high possibility a short circuit there. I'm not sure about that.

If its has happened, may the rectifier tube have been damaged? Resistors? Capacitors?
Regards
Yes, if C1B was shorted, 410v/193 ohms=2.1Amps!! May have damaged the rectifier. Check all the wiring carefully for shorts to chassis. Move wires around away from the chassis and use a strong light to inspect it with. A slight abrasion in a wire insulation can cause a hidden short.
 
Thanks Triode
I will do. I will buy a new one NOS good rectifier tube too.
I will check the resistors from B+ too, and replace some wires.
In next weeks, i will post results here.
Regards
 
Hi guys
I replaced the rectifier tube
I checked the wires for shorts too, but i can't find none.
The problems stills.
The situation is now as follows. Power Tap and Output Tap are good (i did some tests like posts above).
All the filter caps are new
The tubes are Know good
All big resistors from Main B + have been tested, and when the amplifier is off, they have good readings. It should be OK.

When I turn it on with Dim Bulb Test (60W). The bulb is shinning weak, and the B + is just around 5 DCV.

I do not dare turn on without the Dim Bulb Test, because this amp has some hidden problem, like a short .... I am afraid to damage the TAPs.

Could someone help me find this hidden problem in this SCOTT 99C?

Regards from Brazil.
 
Is R54 a single power resistor with multiple taps? Its possible that the first leg could be open and not allowing any B+ to the circuit.

I would check R54-C, R46, and R47.

If the fuse is not blowing, you're not drawing excess current so I believe that there are no shorts. The power transformer is fine if you are getting the correct AC voltages coming out of it before the 5U4 rectifier tube. The output (speaker) transformer should be fine unless there is a critical short in one or both of the 6L6 tubes and in which case, the fuse should also blow preventing more damage depending on if the fuse is the correct rating for this amplifier.

Pics?
 
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Sorry guys
I've been very busy ... a lot of work. These vintage tube are a passion..a hobby for me..a fantastic hobby. Since June my scott 99c is standing on the bench.
So. Last night I decided restart work on it.
Remembering.
The problem Started like this.
I was listening to My Scott 99C, 70% of the total power.
At one point, I heard a small winch and the amplifier was muted.
The amplifier remained on, with the filaments lit.
I power off immediately. I felt a faint smell of burning ...

Thanks Century and Cademann for the latest posts.
The R54 is separated. There are 3 individual resistors. I checked all 3 (on circuit), and all are OK. It still replaces the R54C for testing. I tested R46too, and it is fine.
R46 and R47 should both be present? When i am looking for a diagram, there is something like a cut on circuit....
link to scheme here:
http://www.acousticpsychos.com/Files/H. H. Scott/Scott_99c.jpg

The situation now is this. I'm afraid to power on direct on the wall (117VAC). So, i removed all tubes, and connected it a Dim Bulb Test (60W).
Result. Without rectifier tube, VAC voltages from power transformer are OK.
After, i installed a rectifier tube, and stills using a Dim bulb test. The B + now is just 13VDC before the R54C (5U4 8 pin rectifier tube). After R54C the B + voltage is 12.0VDC
I did not try power on direct on the wall.
Can you help me to find where is the problem?
Regards, and thank you
 
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Welcome back. The way I use the dim bulb is as a go / no go indicator. I turn on the amp and observe the bulb; if it flashes then dims, I call it good and then plug in to full wall voltage and operate the amp including now-relevant voltage readings. If the DBT glows bight and does not dim then Houston, we still have a problem.

So - what did your bulb tell you?
 
If R 54 is indeed open, you will not have any sound. I suspect that the cause of failure is a shorted 20uf capacitor c1 ( The can cap directly after the open resistor) You can order new can caps from a variety of suppliers online, including Hayseed hamfest, and CE. You also could restuff the can or mount a new electrolytic under the chassis. These are sweet little. Amps, I own 2 for stereo. Good luck
 
Thanks RWood
The Dim Bulb Test (60Watts) shines when i power on, and after some seconds it decrease the bright until the filaments are lit, a little strong.
If I use a 100 watt bulb, the filament would quickly light red. As it is a 60W bulb, it shines a little stronger.
I did a similar test on a revised Harman Kardon Trend 2, and the bulb's bright similar. So I do not think it shows a short signal.
But.. i am afraid. I felt a faint smell of smoke in the last time i power on the wall.....
Regards
 
thanks trainbufftony
The R54 is OK... but i just measured then when power off. Maybe the heat can change the resistance value?
All electrolitc caps was replaced by new ones, mounted under the chassis. I tested then now (connected on circuit) with a capacimeter, and all tested good. So, i think, the problem is not here...
I love this scott.
Regards
 
If it produces the expected voltages with the rectifier out, but then something is pulling down the B+ when the rectifier is inserted, something is still wrong with the power supply. Sounds like you have already checked cap polarity, so I would be looking closely at each HT connection at the tube socket. Since the plate connections, pins 1 and 6 of the 12AX7s have at least two leads coming off them (a plate load resistor and a coupling cap), it is possible that something way down the line is touching another component or socket pin. Maybe not enough to show an obvious short on the DBT.

Here's what I would do. With the amp powered down and caps drained, I would clip an ohmmeter lead to pin 8 of the rectifier socket and work your way along the high voltage supply line all the way to the plates of the phono tube. You should be reading the cumulative resistance of everything in between the points you test. Sams manuals usually show the resistance readings but I didn't see the chart on the schematic you posted. But you can add it up on your own. Also see if you get any reading between there and ground. Should be many megohms of resistance.

One last thing for now, regarding the burning smell. If you are like me, panic sets in and we are doing well just to get the unit turned off when we smell something burning. But if you have the presence of mind to then get your nose in there and find out WHAT was burning, you'll make troubleshooting a lot easier. Remember, before fixing the 'what' you need to know the 'where'.
 
Thanks for reply Rwood
here is the resistance readings, from Sams scheme.
I wrote in red reads from my 99C. M means Mega.
I hope this helps
I can try to turn on again, to track down the smell of burning.
Regards
 

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Have you tested your tubes yet? It's possible that one could have a heater to cathode short.

Don't be afraid to plug it in to the wall. It looks like you have no power transformer, output transformer, or filter capacitor shorts as long as its not blowing the fuse.
 
Thanks RWood and Cademan
I'm testing it only with the 5U4 rectifier tube. I replaced the rectifier with a known one, and the problem persisted.
Well, I took the risk of plugging it direct at wall (117VAC), to identify the source of the smoke smell.
The B + goes to 200VDC. I can listen a loud humming on the loudspeaker. The resistor R54B (160 ohm) overheated (the brown resistor in photo), and it came out smoke and smelled burn (from this resistor).
Then I unplugged it quickly. So, i checked the fuse, and the fuse in it is 3A. In the schematic indicates 2A ... I hope this did not cause any problems ...
The R54B measured with ohmmeter indicates 150 ohm, when power off
Which should be my next step.
Regards
 

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You should have a 2 A fuse. Loud hum is usually indicative of an open power filter capacitor or one that might not be grounded properly.
 
Thanks Cademan
I will replace the fuse.
I'll check all the big electrolytic capacitors.
Any suggestion on what can i do about the overheated resistor?
Can something be causing it to overheat?
Thank you again.
 
Thanks Cademan
I'll check all the big electrolytic capacitors. Any suggestion on what can i do about the overheated resistor? Can something be causing it to overheat?

Yes, a short in the big electrolytic capacitor(s) and fuse not the proper rating.

Just measure that resistor. Those things are damn near bullet proof and if it still measures within spec, it's fine but you had better find the culprit as not to keep putting stress on that resistor and/or other parts in the circuit.
 
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