Phonic MAX 860 problems...

[anton1974]

Hi guys,
I have searched for the Max 860 repair and found this thread. I thought I would try to ask for advise here. Both channels of my Max 860 do not work. The amp was probably overheated since the fan is mechanically seized and does not turn; the fan voltage is OK though. Something probably went wrong due to the overheating. It seems that the protective circuit disengages the speaker relays quite quickly. The speaker output seems to have a short circuit to the ground. I measured the voltage on the emitter of Q204 as it was suggested by Jazz P Bass, it is not negative. What else could I check? Any info is appreciated.
Anton.

[Jazz P Bass]

The 'protect' circuit is monitoring the output for unwanted Vdc.

If there is dc volts on the output then you most probably have a blown amplifier.

Check the output transistors for a shorted condition (which they should not be in)

[anton1974]

Thank you for reply, Jazz P Bass.
An update after some measurements and testing: the speaker outpus are not shorted. The right channel's relay is energized and the channel works: I applied the signal and played some music. The left channel's relay does not energize. However, when the contacts of this relay are shorted, the channel works! So, it seems that the protective circuit of the left channel has a malfunction and the output transistors are probably OK. For some reason, the yellow "PROTECTION" LEDS of both channels are on. I measured the DC offset on both channels, on right channel it is 20 mV while at left channel it is 34 mV. It is not that high, right? On a juncion of R526 and C508 of the left channel I measured 0.44V while at the similar junction of the right channel (R426 and C408 it was about 10V. What else could be tested?

[g1]

On a juncion of R526 and C508 of the left channel I measured 0.44V while at the similar junction of the right channel (R426 and C408 it was about 10V.

The schematic shows R426 and C408 for the left channel, not the right. Please confirm which is which.
Which channel has the 10V at that junction?

[anton1974]

The schematic shows R426 and C408 for the left channel, not the right. Please confirm which is which.
Which channel has the 10V at that junction?

Page 10 of the manual, "Power amp output (R) schematic" shows R426 and C408.
Page 9, "Power amp outpu (L) schematic" shows R526 and C508/

To summarize:

Right channel: R426 and C408 junction, V = 10V. Relay energized, the channel works, but the yellow LED is ON. DC offset 20 mV.

Left channel: R526 and C508, V = 0.44V. Relay is not energized, yellow LED is ON. DC offset 34 mV. The channel works when the contacts of relay are shorted.

[Chrille]

Page 10 of the manual, "Power amp output (R) schematic" shows R426 and C408.
Page 9, "Power amp outpu (L) schematic" shows R526 and C508/

To summarize:

Right channel: R426 and C408 junction, V = 10V. Relay energized, the channel works, but the yellow LED is ON. DC offset 20 mV.

Left channel: R526 and C508, V = 0.44V. Relay is not energized, yellow LED is ON. DC offset 34 mV. The channel works when the contacts of relay are shorted.

Hi,
Did you manage to find and fix the problem?
I have the same problem on one of my amplifiers.
/Chrille

[Larry Stover]

I realize this is from quite a while back and you may likely already have the solution, but I was working on an 860 with the same symptoms and came across your site while searching for answers. I discovered the problem to be the ribbon cables, one at the front especially but also the one connecting the input PCB. They are intermittent and the +/- 15VDC supply to all the preamp stage is unstable causing no or distorted audio and the green LED's may light solid. In my case the +15VDC went completely away. I found replacement cables and plug headers on Ali Express and replaced them both to solve the issue.

[The Dude]

Welcome to the place and nice of you to join just to try helping others with your solution!