Help! Problem with my Technics Amp cutting out .

[Drewpicker]

I seem now to have lost the 39v at C701 and C702 ... the amp is still working and cuts out when i increase the volume ... i tested the ac outs of the transformer and all are ok .... this week-end when 39v were present the amp stayed on longer at higher volume ... maybe the fault is in this line ...and this will help narrow down the possibilities... ive tried to understand the power lines in the schematic but its beyond me ... maybe ive done something to cause this but i'm pretty sure this isnt the case .... thanks for any help with this ...

[PJL]

This is a very complex system. The power relay is controlled by IC801 (System Control) but that has a datalink to the ST-CH730 unit where there is a microcomputer. The ADI connection to the ST-CH730 serves multiple purposes by using different sink resistance values and includes the buttons and the fan/DC offset fault signal.

Lifting one end of R391 (47K) will tell you if the error is coming from the fan/DC offset fault signal, otherwise the fault must be detected in the ST-CH730 unit.

There is also a link you can short (TP701) that will force the power to remain on to help with fault finding.

I suggest you check all power supply voltages marked up on the circuit, particularly the output of the regulators, -15V and 14.8V.

[Drewpicker]

Thanks PJL ... ill try to look at all of that ... i don't know if you read earlier in the thread (post 10) where i mentioned a fault in the st-ch730 on the display board ... the photos show that it appears that something has burnt out on this board ... wiping the components left dark traces on my fingers ... ive looked and looked at this board and the only thing that looks to have maybe burned out is a strange bulb in the display itself ... could this be imbalancing things and trigerring the cut out ?

[vidjoman]

Originally Posted by Drewpicker

the only thing that looks to have maybe burned out is a strange bulb in the display itself

If you are referring to the pip on the end of the glass then that is where they seal the device - it is not a bulb.

[PJL]

It is not dependent on the channel being overloaded and that suggests it is not the power amplifiers.

It still trips when there are no speakers connected and that suggests it is not the power supply.

Lift R391 and try again.
Check the regulated power rails.
I can't see anywhere where the output level is being fed back to the ST unit to enable it to trip on high volumes other than the fan/DC offset fault line. I have not looked at the ST circuit.

[PJL]

The ST manual does list F61 as a DC offset fault with the CH unit although the circuit clearly shows the fan can trigger this too. If lifting R391 does not get rid of it then the ST unit must be incorrectly reporting the error.

[Drewpicker]

Hi PJL .... I lifted R391 yesterday and tested the amp but I had to go to band practice so had no time to check anything.... the amp doesn't cut out anymore... yesterday it would cut out when I pressed the v-bass or eq buttons but today they're working fine ... maybe they were a little dirty as i haven't been able to use this amp for a while ... at a certain volume level on 'source direct' the fan is not operating and upon pressing EQ the fan starts up ... I haven't been able to check the power rails as I'm not sure how to do that .... I figured I should put negative on the chassis and at least test the pins of IC501 and IC551 ... but I'm afraid that I'd blow one of them up ... better to be cautious me thinks ... as it is the amp seems to be working perfectly today ... I haven't tried the cd player and cassette unit yet ... have you any idea why the display is "partial" ... are they just led's that don't work anymore... or a capacitor or something needs changing ... I'm not particularly bothered about this but as I've got everything open ... if it's an easy obvious fix I might as well do it ... if not I'll put it all back together and try the cd player and tape deck with it ... I presume that I can leave it as it is with R391 lifted as the fan still works as normal... thanks a million PJL for your help with this .. and a huge thanks to everyone who contributed their help on this ...
Andrew

[duncanlowe]

The display is a type called vacuum fluorescent (VF), not LED. The 'bulb' that someone explained is where the air was sucked out to make it a vacuum. I'm not experienced in faults with VF displays, but I believe they have an inverter to increase the voltage to drive the display making it glow, and if that's right then maybe the circuit isn't making enough volts for it to glow properly.

[PJL]

So it's most likely the fan stall detection circuitry but less likely the DC offset detection. It needs to be fixed or the amp could damage your speakers.

[Drewpicker]

If it's the fan stall detection then as the fan is working how will it damage the speakers .... does R391 affect other parts of the amplification system or just the fan stall detection?

[PJL]

Remember the fault signal we have disconnected also reports a DC offset on the speaker outputs. A DC offset will kill a speaker.

The way the fan stall works is the -15V that powers the fan is fed through R371 and charges the capacitor C371. If it continued to charge it would pull the emitter of Q354 negative and Q354 would conduct and send the fault signal to the ST module. The ST module then cuts the power. However, there is a resistor R358 in series with the fan and as the fan rotates it produces pulses across R358 which pass through C383 and momentarily turn on Q371 that discharges C371 before it can trigger the fault.

Either there is a faulty component in this circuitry, or the current pulses produced by the fan are insufficient to trigger Q371. You would really need an oscilloscope to see what's going on.

One option is to lift one end of R371 (68K) and refit R391 which would disable the fan stall check but leave the DC offset detection working.

[Drewpicker]

Hi PJL ... i refit R391 and figured from what you said that I might as well try replacing C371 and C363 (I'd spent some time scouring the board for C383 then looked at the parts list to find it didn't exist ... so I followed your explanation with the schematic and realised you meant C363) ... and lo and behold it appears to have solved the problem... I haven't had time to give it a long test but 5 mins on a reasonably high volume and no tripping out ... I remember reading a thread by someone along the lines of "it's always a capacitor" .... hope this is solved now ... I'm going to have a look at the VF display problem (thanks Duncan) ...in the interest of thoroughness and to maybe learn something.. in the meantime I'll give the amp a better work out .... I have an old late 70s 20w guitar amp that was given to me.... an ibz20 it turns on but doesn't work... i cant find a schematic for it anywhere.... that'll be my next project/thread... if you fancy helping me on that one youd be most welcome.... no computer chips or data connections or auto cut-off on that one thankfully.... thanks for your time, patience and incredible knowledge... without your help I'd still be nowhere on this one .... I'll report back on my progress with this .... all the best .

[PJL]

Sorry about the mistake, small writing and poor eyesight! Pleased to hear it is fixed. I know nothing about VF displays so will leave the display to others...

[Drewpicker]

Confirmation that the amp has been working for over a week now with zero cut outs ... i didn't bother with trying to fix the vf display in the end .. another day maybe ... ....enormous thanks to PJL I'm currently listening to my vinyls thanks to you ... and big thanks to everyone else who contributed their ideas to the problem... all the best ..... Andrew