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'67 Frank Head with background distortion, intermittent tremolo, and noisy reverb. [message #20003]

Sat, 11 January 2014 21:43

ARKustom
Messages: 37
Registered: January 2014
Location: United States

Member

Hi, I need some help with a '67 head. I notice a fuzzy distortion behind the note when playing. I'm pretty sure I'm not overdriving the amp, it does it at low volume levels and with the volume knob on the instrument rolled back. Also, it seems like which direction the power switch is flipped makes difference in the sound of the channel and the ratio of normal signal to the background distorted signal.

Now, on to the tremolo. Sometimes it works and sometimes it doesn't , but the intensity knob will change the tone of a sustained note.

The reverb works, but adds a lot of noise.

Any help trying to fix these issues would be greatly appreciated.

Thanks

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Re: '67 Frank Head with background distortion, intermittent tremolo, and noisy reverb. [message #20004 is a reply to message #20003]

Sun, 12 January 2014 01:47

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Welcome to the place. There is a schematic for your amp in the technical section of the site. Are you schematic friendly? What sorts of tools and skills do you have?

The fizzy distortion sounds like a bias problem in the power amp. The bias is set by the resistors connected to the output transistors.

The vibrato is a true pitch shift vibrato and not a normal tremolo. It actually is a phase shift circuit that is optically coupled with a bulb and photoresistor. If the low frequency oscillator does not run, turning up the vibrato control will cause a change in the signal tone. Bad caps in the oscillator circuit are the most common problem.

The noisy reverb could be a noisy transistor in the reverb circuit or perhaps a noisy resistor.

I'd start with the distortion problem and then fix the FX.
The amp also has atleast two large case caps that should also get changed out.
Both of these are 500 mfd caps, one on the reverb board and one on the output driver board mounted on the rear wall of the amp.
You can replace all of these caps I posted about with ones of a higher voltage rating and reap the benifit of longer lived parts.

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Re: '67 Frank Head with background distortion, intermittent tremolo, and noisy reverb. [message #20005 is a reply to message #20004]

Sun, 12 January 2014 10:33

ARKustom
Messages: 37
Registered: January 2014
Location: United States

Member

Thanks, I can solder pretty decently. I'll have a look at the schematic and see I can figure anything out.

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Re: '67 Frank Head with background distortion, intermittent tremolo, and noisy reverb. [message #20006 is a reply to message #20003]

Sun, 12 January 2014 13:14

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

There are many electrolytic type caps in this amp on all the boards, your Frank like mine of the same vintage likly uses all white Mallory brand caps and I can state that you likly have many bad ones and the rest have drifted in value by a bunch.

I would start your repair work by first changing out all of
these caps so that you have a known good base line to start from in terms of furthur trouble shooting in the case that the replacement of these does not fix all of the amps problems, which it may just do!

Also note that a light distortion heard behind a note could well be a speaker issue so I would try the amp thru different speaker(s) if you can.

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Re: '67 Frank Head with background distortion, intermittent tremolo, and noisy reverb. [message #20007 is a reply to message #20006]

Sun, 12 January 2014 14:56

ARKustom
Messages: 37
Registered: January 2014
Location: United States

Member

Well, I fixed the distortion issue, it was indeed a speaker issue. I had recently put in a pair of EV Force 15's (Replacing some generic Eminence speakers, the originals were gone before I got it.) I suppose I didn't have time to listen to them closely right after the installation. It stopped when I reinstalled the Eminences.

Only the vibrato and reverb to worry about, now. I suppose that maybe after thinking about it, that I would describe the reverb issue as more of a white noise/hiss/hum increase as the reverb level is increased. The reverb does work.

[Updated on: Sun, 12 January 2014 15:00]

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Re: '67 Frank Head with background distortion, intermittent tremolo, and noisy reverb. [message #20008 is a reply to message #20007]

Sun, 12 January 2014 21:01

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

The reverb control is a balance control between the straight signal and the reverb signal, so as you turn up the control you will hear more reverb signal and less straight signal.

If I remember correctly, there are three transistors in the reverb return circuit. There is no test that I know of to find noisy transistors other than substitution. You could ground out the reverb signal at the output of each of the 3 transistor stages to see if you can tell where the most noise is being generated.

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Re: '67 Frank Head with background distortion, intermittent tremolo, and noisy reverb. [message #20010 is a reply to message #20003]

Mon, 13 January 2014 08:24

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

You can at least test the send, or input side of the reverb pan for the noise problem with out a great deal of pain.
To do this you will need to make up or buy a adapter to let you plug the RCA jack from the pan into another amp.
If you than find the the noise to signal level is out of wack then you know that the noise issue is from the transistors on the send amp side of the pan, and not the return side.
Note that these early amps do add hum into the amps output sound as the reverb is turned up even when all is functioning normal, excess hiss is not normal however!
Note also as Bill posted that due to the design of the reverb circuit if the hiss part of the problem gets worse while turning up the reverb level than the noise / hiss issue may be transistors on the return / output side of the reverb pan.

[Updated on: Mon, 13 January 2014 08:25]

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Re: '67 Frank Head with background distortion, intermittent tremolo, and noisy reverb. [message #20012 is a reply to message #20003]

Mon, 13 January 2014 12:45

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Generally speaking, as there is no direct signal path from the drive transistor to the reverb output signal any hiss that the drive transistor makes will not be heard in the output.

Looking at the schematic, the RV board is PC301. This board is the one that is located on the end of the chassis, opposite from the power supply. There are 3 transistors on the return side of the circuit, Q302, Q303 and Q304. Q304 is the last transistor in the signal path before the reverb control. I would start there.

See if you can figure out the parts on the schematic that I have mentioned, and see if you can figure out where they are on the actual pc board.

There are two things that will add hum to the reverb signal. One is the 500uF filter cap that is on the RV board and the second thing is the ground loop caused by the type of tank that was used in these heads.

The 500uF cap (marked C306 on the schematic) that is almost always bad is a black plastic cased axial cap often with white or red ends. I always test these whenever I see them in any of these amps. These caps are also found on the preamp and driver boards

The original reverb tanks came with grounded input and output jacks. And many of the amps also have a separate ground wire that is soldered to the metal tank itself. This allows for three separate ground circuit paths which will cause added hum to the reverb signal. On my own amps, I cut the grounded shield from the RCA cable that carries the drive signal at the circuit board end. This will help reduce the reverb ground hum.

[Updated on: Mon, 13 January 2014 12:47]

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