Fusible resistor blown? (long) - Charlie S in Maryland - Amp/Preamp Asylum

Amp/Preamp Asylum

Looking for a new Amp or Preamp? If you're after tubes, post over here.

Fusible resistor blown? (long)

69.72.27.240

In Reply to: Adcom GFA555-II oscillation (long & technical) - advise needed posted by tlenthe on March 27, 2005 at 08:12:48:

Hi Ted,
I'm not at all familiar with the GFA-555-II but am with their big brother, the GFA-565 monoblocks. FWIW, below are observations about the 565, on the assumption that the topology of the two amps MAY be similar. I believe they are both Nelson Pass designs of a similar vintage.
First, the 565 does have an internal Zobel network across the output, so I'm not surprised that the 555-II does also.
Second, the 565 has several small "fusible resistors" in the circuit. These are sacrificial protection devices intended to open circuit under certain fault conditions in order to save more expensive components.
Of interest, there are two 10-Ohm 1/4-watt fusible resistors and a 100-Ohm one that tie together the various circuit grounds (DC feedback servo reference, signal input ground, main driver/speaker output ground, and chassis/power plug 3rd pin safety ground). These fusible resistors are prone to blowing if there is a voltage potential difference between the AC power neutral/ground and input signal ground. I originally had this problem because of a stupid antenna grounding scheme in my system: Nearby lightning strikes blew the fusible resistors. Result was high-frequency buzz/hash, noise, and low output level.
The 565 also has a few other fusible resistors in the circuit, but they haven't given any trouble.
Try measuring DC resistance between combinations of the RCA signal input shell, the same channel's (low = black?) speaker terminal, and the power plug safety ground pin. An open circuit between any combination may indicate a problem.
Good luck!

This post is made possible by the generous support of people like you and our sponsors:

Follow Ups Full Thread

Topic - Adcom GFA555-II oscillation (long & technical) - advise needed - tlenthe 08:12:48 03/27/05 (10)

Solved!! (Adcom GFA555-II oscillation) - tlenthe 16:13:29 03/29/05 (1)

Hmmmm - chris_w 16:22:45 03/30/05 (0)

Fusible resistor blown? (long) - Charlie S in Maryland 16:00:00 03/29/05 (0)

More observations and another theory? - tlenthe 05:20:04 03/28/05 (0)

Why are you bothering? - Bare 18:16:29 03/27/05 (2)

:Why are you bothering... - Neil49 19:34:00 03/27/05 (1)

Re: :Why are you bothering...Theamp must be in the shop and have nothing better to do than putting down other inmate's - dimitri_P 23:53:06 03/27/05 (0)

My guess of what's going on - andy_c 12:13:28 03/27/05 (2)

Re: Description of circuit topology (Adcom GFA555-II) - tlenthe 16:48:15 03/27/05 (1)

Re: Description of circuit topology (Adcom GFA555-II) - andy_c 17:59:27 03/27/05 (0)

Follow Ups

Fusible resistor blown? (long) - Charlie S in Maryland 16:00:00 03/29/05 (0)

You can not post to an archived thread.