Author Topic: 100w Suitcase with permanent fuzz in one channel  (Read 1356 times)

Offline ppeterso2

  • Fiesta Red
  • **
  • Posts: 36
    • View Profile
100w Suitcase with permanent fuzz in one channel
« on: July 11, 2015, 02:01:50 PM »
I have the Rhodes Suitcase that originally had the sliders and 100w amp.  The amp has been rather reliable until now. Over the years I've secured and resoldered the connectors and gone that extra mile with it.  One channel  is now distorting like a fuzz pedal that I can't turn off.  I've replaced the big 11000uF caps and checked the headphone muting jack (jumped and bypassed it to make sure) I've switched the speaker jacks to make sure all 4 speakers/wires are fine. 

Lastly I swapped the 2 pcbs (for channel 1 and 2) part 012835 in the manual and the problem moved to the other channel! So I've isolated the problem to one of the 2 pcb's 012835.  I've replaced the 1.0uf cap--what should I do next?  I see transistors Q6 and Q7 (a 29b and 30b) in there but don't know if there are other known failure points?

Any advice would lift my spirits lol

Offline pianotuner steveo

  • MIDI Mark V
  • *****
  • Posts: 3341
  • A keyboard player in love with vintage guitars!!!
    • View Profile
Re: 100w Suitcase with permanent fuzz in one channel
« Reply #1 on: July 11, 2015, 07:09:45 PM »
That sounds like output transistor(s) to me.

1960 Wurlitzer model 700 EP
1968 Gibson G101 Combo organ
1975 Rhodes Piano Bass
1979 Wurlitzer 206A EP
1980 Wurlitzer 200A EP
1980 Wurlitzer 270 Butterfly Grand
2000 Yamaha acoustic piano
2004 Hammond XK3
2009 73A Rhodes Mark 7
2009 Korg SV-1 73
....and a few guitars...

Offline ppeterso2

  • Fiesta Red
  • **
  • Posts: 36
    • View Profile
Re: 100w Suitcase with permanent fuzz in one channel
« Reply #2 on: July 11, 2015, 08:37:02 PM »
That sounds like output transistor(s) to me.

Thanks for the response! Are you talking about those large transistors fixed-mounted behind the heat sink?  I agree that's what it sounds like, but when i swapped the 2 cards (that have a few smaller transistors on it) the problem moved to the other channel.  It seems like the problem would have stayed if the one of the fixed mounted transistors where bad?

Since I posted this, I tested all the diodes (one is irritating because you have to disconnect it to test it) and I noticed something strange:  The Base and Emitter of one of the smaller transistors (the RCA 30b) on the good sounding card had continuity (MM buzzer sounded) and the bad card had no continuity between the base and emitter.  This thing is getting weird!

Offline pianotuner steveo

  • MIDI Mark V
  • *****
  • Posts: 3341
  • A keyboard player in love with vintage guitars!!!
    • View Profile
Re: 100w Suitcase with permanent fuzz in one channel
« Reply #3 on: July 11, 2015, 08:40:53 PM »
I'm speaking in general terms. I am not familiar with this exact amp.

The bad part(s) will be on the card that has the the bad audio. Not anywhere else.

1960 Wurlitzer model 700 EP
1968 Gibson G101 Combo organ
1975 Rhodes Piano Bass
1979 Wurlitzer 206A EP
1980 Wurlitzer 200A EP
1980 Wurlitzer 270 Butterfly Grand
2000 Yamaha acoustic piano
2004 Hammond XK3
2009 73A Rhodes Mark 7
2009 Korg SV-1 73
....and a few guitars...

Offline pnoboy

  • Mark I
  • ****
  • Posts: 395
    • View Profile
Re: 100w Suitcase with permanent fuzz in one channel
« Reply #4 on: August 16, 2015, 05:06:58 PM »
There's no true shortcut to finding a problem like this.  The best thing to do is to use an oscilloscope to look at the signal as it goes from stage to stage.  At some point, you'll see a normal signal, and then at the next stage it will be bad.  Then you'll know where to look for the problem.  Also, make sure you have a schematic.

Offline vanceinatlance

  • Mark I
  • ****
  • Posts: 223
    • View Profile
Re: 100w Suitcase with permanent fuzz in one channel
« Reply #5 on: August 17, 2015, 09:40:18 PM »
Might not hurt to look at the 5uf np cap (c2) if im reading the schematic right.