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Messages - gbono

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Amplifier Discussion / Re: Altec 770 Biamplifier
« on: July 09, 2020, 04:00:09 PM »
Opps I had switched out the HF/LF boards with another amp and the bias pots were turned up. THX Enzo. BTW the bias is supposed to be set using voltage across the EB of the output transistors (Q1/3?)- the spec setting is 17mV?

Amplifier Discussion / Altec 770 Biamplifier
« on: July 07, 2020, 07:39:10 PM »
I have several Altec 770/771 biamplifiers that have been rebuilt but have an issue with excessive heating at idle. The schematics are for the 770 version but the design is the same for the 771.

The problem is that the heat ink will get up to 125F/50C when the amps are idling with no input and 8 ohm loads attached. I have tested many of these amps and they appear to operate in class AB and usually don't have this issue. The amps that I have now all meet their output specifications and play program very well but get hot. I replaced the zeners on the EB of the output transistors, cleaned and reapplied new heatsink compound to the mica insulators and checked the "stabistors" (CR1/2).

Can the amp run in class A?

Amplifier Discussion / Re: Sunn Dymos
« on: June 11, 2020, 12:16:29 AM »
Yup the bridge was intermittent - replaced it and power supply works. next it's will this 50 year old pass a signal.

Amplifier Discussion / Re: Sunn Dymos
« on: June 10, 2020, 06:28:27 PM »
I'm having a problem with the supply voltages. If I measure across the bridge (A-B) I get +2VDC. How do I verify supply voltages?

Okay just saw your post - still only get 2VDC from B to chassis ground. 40VRMS into bridge. Bad bridge?

Amplifier Discussion / Re: Sunn Dymos
« on: June 09, 2020, 02:45:57 AM »
So if I should see +15V across C105 and -15V across C104? Supply voltages not referenced to chassis ground.

Amplifier Discussion / Re: Sunn Dymos
« on: June 07, 2020, 04:22:22 PM »
Yes the image shows the hardware mounted but that's not what I have on the bench it's an image off the web. What happens if points A and B are connected to chassis ground -i.e.  the collectors are not isolated?

Amplifier Discussion / Sunn Dymos
« on: June 06, 2020, 07:02:30 PM »
I have a Sunn Dymos with all the output/driver/reverb transistors removed. Looking at the schematic, it looks like these transistor's collectors (they are all TO-3 2N3055) should be isolated from the chassis - is that correct? Point A and B on the schematic would be grounded without isolation but i don't have any mounting hardware with this chassis to verify how the collectors should be connected.

The jpeg is off the net for reference only.

Amplifier Discussion / Re: Backline 600 no output
« on: May 27, 2020, 04:41:32 PM »
Thanks Enzo ... There was a 13V trigger coming out of the mute circuit to R8 but zero volts across R4. As you suggested, R33 was open (R34 is good). Once I get a few 10 ohm fusible resistors from Mouser I will see what happens next.

Amplifier Discussion / Backline 600 no output
« on: May 25, 2020, 05:52:49 PM »
G&K backline 600 which initially required new output transistors (drivers also changed). Amp powers up, supply voltages look good +-50V, +-14V, etc., and has strong signal into power amp board (base of Q7a) but no signal at output or at D6 anode.

I believe the issue is the mute/fault circuit. Voltage across R4 is 0V. Checked all transistors in fault/mute circuits but nothing is damaged. Amp powers up and preamp stage works so power on and protect is working but no output voltage on mute node?? How does the mute/fault work on this design?

I vote for battery powered - heck get it to work of some solar modules too  ;)

Amplifier Discussion / Re: Peavey CS400
« on: March 20, 2020, 02:22:14 AM »
Back on this since it's solitary confinement here...

The inductor/coil is not open and all solder joints are good. No shorted capacitors and the diode across the output good too. DC on the output terminals is .9V channel A and 5V on channel B.

Amplifier Discussion / Re: Peavey CS400
« on: March 11, 2020, 06:55:19 PM »
So the inductor looks good. The leads are soldered to the pcb without issue. I noticed that the connector that ties the inductor, ground and +- 59V is also blackened (got hot). Could the issue be that C4 is shorted?

Amplifier Discussion / Peavey CS400
« on: March 09, 2020, 09:27:41 PM »,%207-79)%20Schematic.pdf

CS400 with damaged (but functional) R16/17 - part of a zobel network? The amp is causing the light bulb limiter to glow brightly but the amp did not blow fuse. Is this an issue with high frequency oscillations on the output?

Amplifier Discussion / Re: Gallien Kruger GT200
« on: January 03, 2020, 03:00:26 PM »
Thanks g1....

So Q3, D1 and R9/10/11 form a VBE multiplier?

Amplifier Discussion / Re: Gallien Kruger GT200
« on: December 30, 2019, 04:01:12 PM »
VBE of the output transistors sits at 1.1 V.
So how are output transistors biased in this design? Point "A" is 12V and R8/9 provide bias to Q7? How is Q9 biased?

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