CRACK OTL - Issues After First POWER ON [resolved]

[mipheres]

Hi,

Just finished building my CRACK OTL for HD650.
All "Resistance check" went OK
But almost all "Voltage Check" FAILED.
6080/6AS7 tube filaments glow weakly.
All B terminals are checked with 0V, same with As

I suspect the POWER TRANSFORMER.
Also how can the tube glow if all B inputs measure 0V?
maybe the volt-meter  is off?

Please assist.

Best Regards
YM.

 

[fullheadofnothing]

Are you measuring AC or DC?

Is your meter auto-ranging? If not, what range are you using?

 

[Paul Birkeland]

Where you are measuring "0V" are the terminals that make the tubes glow, so this is very likely a measuring issue. 

Try measuring the DC voltage of a 9V battery with your meter, just to get a feel for it.

-PB

 

[mipheres]

Hi,

I set voltometer to VDC 20 and measure 1.6v from a new SIZE AA BATTERY---> meter works ok.
meter set to VDC 600 - power transformer terminal 6,7,9,10 -measure 074
meter set to VDC 20- power transformer terminal 4,5 -measure 0.00

meter set to VAC 600 - power transformer terminal 6,7,9,10 -measure 160
meter set to VAC 200- power transformer terminal 5 -measure 5.00
meter set to VAC 200- power transformer terminal 4 -measure 0.00
meter set to VAC 200- power transformer terminal 5 -measure 5.00
meter set to VAC 600- power transformer terminal 2 -measure 223
meter set to VAC 600- power transformer terminal 1 -measure 0

 

[Paul Birkeland]

These measurements show that your power transformer is working perfectly.

Set to DC volts/200, can you let us know what voltages you have in your amplifier that don't match those in the manual?

 

[mipheres]

Voltage Check
meter on 200VDC
terminal,recommended,measurement
1 ,75-90  ,-00.5
2 ,170      ,-00.5
3 ,0          , 0
4 ,170      ,-00.5
5 ,75-90  ,-00.5
6 ,0          ,0
7 ,100      ,0
8 ,0          ,0
9 ,100      ,0
10, 0        ,0
11, 0        ,0
12, 0        ,0
13, 170    ,-00.5
14, 0        ,0
15, 185    ,-00.5
20, 0        ,0
21, 206    ,218
A1, 90      ,-00.5
A2, 0      ,0
A4, 0      ,0
A5, 0      ,0
A6, 90    ,-00.5
A7, 0      ,0
A9, 0      ,0
B1, 90    ,-00.5
B2, 170  ,-00.5
B3, 100  ,0
B4, 90    ,-00.5
B5, 170  ,-00.5
B6, 100  ,0
B7, 0      ,0
B8, 0      ,0

 

[fullheadofnothing]

The capacitor that is supposed to be on 14 and 15 is on 15 and 16.

 

[Paul Birkeland]

15, 185    ,-00.5
21, 206    ,218

There's a 270 Ohm resistor that goes from 21 to 15.  Yours is not properly connected.

 

[mipheres]

@fullheadofnothing
I moved the CAP to 14-15, see picture
@Caucasian Blackplate
the resistor seems connected ok , 15-21

the result is the same.
Can the 270Ohm capacitor be damaged?

Thanks guys...

 

[Paul Birkeland]

The capacitor that you had improperly mounted may well be quite damaged.

You can remove it from the amplifier, then retest your voltages.  If the cap is the only issue, the amplifier will function properly but be too noisy to use (just replace the 220uf/250V cap with a new one if this is the case).

-PB

 

[mipheres]

@Caucasian Blackplate
removed the suspected capacitor, same result.

measured the 270ohm resistor (15-21) and it looks off (see picture attached)
measuring the other 270ohm provided good result, also the 3k capacitor works ok.
I can remove the suspected resistor and measure it without other parts attached , if suggested.

Thanks.

 

[Paul Birkeland]

Yeah, that resistor has opened up, most likely due to the placement of that 220uF capacitor.  Both the capacitor and that resistor need to be replaced before performing any additional tests. 

You will also want to carefully use a piece of insulated wire to short the leads on each side of that defective resistor for a while to bleed off any residual high voltage.

-PB

 

[mipheres]

thanks, Caucasian Blackplate.

I guess the capacitor is an easy one to get but what about 270ohm resistor, Should I ask Bottlehead for those extra parts?

 

[Paul Birkeland]

A 250 Ohm 10 Watt resistor should be easy to find, inexpensive, and will work just fine.  A 250 Ohm 5 Watt resistor will also work, but finding one with long enough leads may be difficult. 

-PB

 

[mipheres]

Hi.

Got and installed new 250ohm resistor with 220uf capacitor.
started the voltage test and got OFF reading on first check.

Terminal_1 is reading 140V instead of
Also 3k-Ohm resistor is running very hot , so I switched off.
Did not continue testing the rest.

what do you suggest to do next ?

...

 

[Paul Birkeland]

Check your solder joint that connects A4 and A5.

 

[mipheres]

Hi,

As you can see A4-A5 are ok.

the reading from Terminals:
1-140v
2-160v
3-0
4-160v
5-80v

Also, the second led does not glow.

 

[Paul Birkeland]

Inside the 12AU7 are two vertical plate structures.  Can you see little glowing dots and the top and/or bottom of each of these structures while the amp is running?

 

[mipheres]

no , I dont see the small tube glow.
The bigger tube is working.

 

[Deluk]

Make sure the unlit led is soldered to the central pin. It looks a bit suspect there. Small tube glow varies. Some hardly glow at all and only show when its dark.