HFMA1 amplifier strange behavior

[mauro]

Good morning to everyone here at Jukebox Addicts! I'm new to the group but have been in some other groups for many years.
I have been struggling with my VL200 amplifier for a very long time (on and off project).
Lastly I put it aside as I could not figure out why it had very little bass in the output, since amp is rebuilt, tubes are all good, cartridge good, crossover good, I thought perhaps the speakers. Anyway I decided to continue troubleshooting the other night and removed the amp cover and started inspecting the components around the bass and treble switches, I manipulated some resistors by hand and then decided to trace the signal with a scope. I used a scope I wasn't familiar with and when I touched the input from the cartridge I heard a loud static pop. I'm sure this is probably nothing but when I went to test the juke again I had a nice full sound? The strange thing is that the volume goes from zero to full with only a quarter turn of the control, the final 3/4 of the range just continues to play at full volume.
I get more than my fair share of strange ones!
What would cause the volume control to behave like this now?
Thanks,
Mauro

[Ron Rich]

Hi Mauro,
Good to see you here--
Yep--you sure do get some "strange ones"---
My GUESSES--and they are really "guesses", because I dono, and I do not have a schematic handy--1. on the "full sound" problem--a "cold solder joint" in the area of the tone controls, and/or a switch that was not making (good) contact, but now is-??
2. You "blew" something in the VC area--I would start by replacing the low end tubes, one at a time--
Letus know what you find--as I am sure you will-- Ron Rich

[mauro]

So I got into the amp a bit yesterday, I plugged in a remote volume control I had to see if it acted any differently and it did not disappoint.
Turning the remote volume control it acted a bit differently, the volume went up quickly again but not as quickly as the chassis mounted volume but at about the halfway point the volume actually decreased and as you continue turning the volume goes back up. I haven't found any other problem but didn't spend a great deal of time yet!
Just thought I'd post this in case anyone recognizes the symptom.
The tubes all tested strong with no shorts or gas.
Hopefully there will be some ideas,
I will post findings next time I'm able to get into this amp.
Thanks again!

[Ron Rich]

Hi Mauro,
My GUESS is that you have a couple of dirty/shorted volume controls ---- ? Got any Deoxit-5 ? Works wonders, if used as per can instructions-
Ron Rich

[robnyc]

"urning the remote volume control it acted a bit differently, the volume went up quickly again but not as quickly as the chassis mounted volume but at about the halfway point the volume actually decreased and as you continue turning the volume goes back up."

What is happening is the result of a problem in the loudness compensation connections or capacitors. The pot is tapped at approx 7k-ohms and 17k-ohms, though aged pots vary somewhat.....a lot.. What is happening is that the first tap or it's capacitor is either not connected well or is defective thus the volume goes up rapidly till around the midpoint where the second tap begins to mute the highs in relation to the bass as it should. From approx the 3:00 o'clock point upward neither tap will be in-effect and the bass control will have little effect and none with pot at max position.

One test is to turn the bass control to minimum. This takes the loudness capacitors out of the circuit. You should now see fairly normal level rise as you turn it up to full. If not, check the bass switch for high resistance connections and the resistors in that connection.

If the volume does rise normally, this can also point to poor switch connections and-or most often, bad capacitors (0.15) in the bass circuit. The dummy plug and RVC socket are common trouble spots and need to be cleaned periodically. Note that both the internal and RVC pots use the same internal parts associated with the bass control.

RobNYC