[HAM] Vibrato Problem

[David Anderson]

Mark,

That's your problem. The vibrato line should be at ground potential or within about one millivolt. If I were you, I would disconnect the brown vibrato drive wire from the preamp. If the positive DC voltage goes away, it's coming from the preamp, and unless something is shorted to the 1uF capacitor lead between the capacitor the terminal, you have a leaky 1uF capacitor. Replace it, and the problem should go away.

If there is a DC voltage on the vibrato line, the line acts as a voltage divider, alternately charging and discharging the scanner, which, after all, is an air capacitor. That's what you're hearing. 

David

-----Original Message-----
>From: Mark Laesch <bluetop at wwnet.net>
>Sent: Apr 14, 2008 1:01 AM
>To: The Hammond Forum <hammond at zeni.net>
>Subject: Re: [HAM] Vibrato Problem
>
>David,
>
>Voltages at the line box at the caps is at .55 volts DC.  This is with the 
>expression pedal at full open.   I did check the brown wire coming off of 
>the preamp to the line box. It measured 1.4 vdc with the expression pedal 
>wide open and with the vibrato switch in any of the C positions. When the 
>vibrato switch is in any of the V positions, the voltage on the brown wire 
>measures .0001 vdc with the expression pedal wide open. The resistors 
>replaced R7,R8, R19, R20. R25, R29 and R36.  The only other wierd voltage 
>that I thought might be too high is at the 12AX7...Pin1 and 6.  According to 
>the print the voltage should be 145 and 140 vdc respectively.  I have 190 
>VDC at pin 1 and 6. R29 and R33 were changed, and upstream of the .33 meg, 
>the voltage is 292 vdc.  I dont have an O scope {:>(
>
>Regards,
>
>Mark
>
>
>----- Original Message ----- 
>From: "David Anderson" <thermionic27609 at earthlink.net>
>To: "The Hammond Forum" <hammond at zeni.net>
>Sent: Monday, April 14, 2008 12:08 AM
>Subject: Re: [HAM] Vibrato Problem
>
>
>> Mark,
>>
>> For comparison, I checked the voltages at the line box capacitors on
>> mine.
>>
>> I got varying DC voltages, but no more than in the +/- 1 millivolt
>> range in either C or V positions. That would be accounted for by
>> small shifts in the preamp's power supply as a result of varying line
>> voltage.
>>
>> If you are finding +/- 0.5 VOLTS, not millivolts, something is wrong.
>> Is there a DC voltage at the vibrato line drive wire (brown) preamp
>> terminal?
>>
>> If yes, there might be something wrong with the preamp's vibrato
>> input circuit that is causing the circuit to oscillate when presented
>> with the higher load of the vibrato line in Chorus settings.
>>
>> I recall you said screen and plate resistors were replaced. Is it
>> possible that a mistake was made, creating a  low frequency
>> oscillation at the cathode of the 12BH7?
>>
>> It would be interesting to put an oscilloscope on the vibrato line
>> drive wire and/or the 12BH7 cathode and work backwards from there.
>>
>> David
>>
>> On Apr 13, 2008, at 11:10 PM, Mark Laesch wrote:
>>
>>> David,
>>>
>>> I have not checked this. My check indicated that when in any of the
>>> the C
>>> positions,  varying DC voltages in the negative range .554 to -.595
>>> volts on
>>> the top of the capacitors. When in the V position,  zero volts DC.
>>> Does
>>> this indicate C13 is bad?
>>>
>>> Regards,
>>>
>>> Mark
>>
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