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Faulty Micro-Tech 1200LX


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#1 Wrighty

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Posted 26 August 2005 - 09:38 AM

Hi,

I've been given 3 old Crowns to sort out by a colleague, with various faults. I'm currently working on a Micro-Tech 1200LX (March 1987 vintage) which was reported as having the ODEP indicators doing odd things, although the amp seemed fine in operation.

After a good clean out and complete resolder of the front panel PCB to eliminate dry joints, it's still faulty. Ch 2 is normal - ODEP light comes on quickly after power up. Ch 1 ODEP light slowly fades up after a few seconds.

I've done the first few tests in the service manual, up as far as setting up the ODEP voltages. At this point, Ch 2 is fine, but R121 has very little effect and R132 has no effect on the voltages measured on U100 and U103 respectively. U100-6 measures about -12.3 to -13.2V (as you adjust R121) and U103-6 measures about +11.4V with no difference as R132 is adjusted.

Some more checks around the PCB revealed that the +VCC line is at about +7.7V and -VCC is at -78V on Ch 1. Ch 2 is approx +/-43V, as it should be. Measurements taken with respect to chassis ground.

If I unplug the main board from both output modules, Ch 2's rails are still stable, but on Ch 1 they gradually float down to about +27V and -60V.

I've been round the output module quickly with the meter and all the output and driver transistors appear to be fine, as do all the diodes. I can't see much else on the schematic which could be pulling the reference point up. I even went as far as changing C302 and C303 to make sure I didn't have a couple of leaky caps, to no avail.

Any suggestions?

The main module PCB is D6304B2 and the output modules are P10115-7.

Cheers,

Richard Wright

#2 Wrighty

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Posted 09 September 2005 - 10:38 AM

Hi,

Anybody got any ideas on this one?!

Cheers,

Richard

#3 KWhitehead

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Posted 10 September 2005 - 07:12 PM

Richard,

I'm currently in London for a trade show so I don't have any schematics in front of me. There's two thing to check, 1) you didn't state you checked output bias voltages. Measure across R02 and R21 on the output module for 320mv. If you have that the go to the ODEP circuit on the main board. There's two resistors in that circuit that go from VCC to the dual transistor pack. If one of those opens that can cause high ODEP voltages and it can be unadjustable.

Thanks,

Kip Whitehead
Crown Technical Support

#4 Wrighty

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Posted 11 September 2005 - 07:56 PM

Hi Kip,

Thanks for your reply.

Ah, PLASA, I take it...? I was hoping to go this year but I've got too much work on and London is a 3-4 hour train journey from where I am!

I've checked the output bias voltages - they're done before the ODEP adjustment in the check-out procedures. They set up to 300-320mV no probs.

I'll have a look for and check those 2 resistors in the ODEP circuit - thanks for that info. I've got a couple of other projects on this week so I may not get a chance to have a proper look until next week but I'll let you know how I get on.

Cheers,

Richard

#5 Wrighty

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Posted 12 July 2006 - 05:01 AM

Well, that faulty Micro-Tech 1200LX went on the shelf for a few months and finally came out again this week to continue the repair...

The biases all adjusted fine but ODEP seemed to be causing a problem. Kip - you were spot on with the open resistor - R134 was open circuit. Replacing that allowed me to adjust the ODEP voltages correctly. Thought I was on to a winner, but then I discovered the supply was still wildly unbalanced - I was measuring about +12/-73V on Ch.1 and adjustment of R132 caused the ODEP light to go from off to full on sad.gif

I did my favourite trick of flipping the main module round and using Ch.2 input circuitry on Ch.1 output module, which told me immediately that the problem was on the main module and definitely not the output module. Even knowing that, it still took ages to find the cause of the fault.

I removed Q100 and Q103 (as suggested by Kip in another post about a faulty Macro-Tech) to remove ODEP from the signal path. Still got a problem. Checked all the resistors around Q115 and Q116 and even swapped Q115 and Q116 with Q215 and Q216. Still the same. Eventually, after a lot of head scratching and checking voltages at various points, I discovered that there was a huge difference in voltage between J600-3 and J800-3, with the main module unplugged from both output modules. I started looking at other things connected to that pin, which I hadn't already checked (purely following tracks on the PCB, as the pin number marking for J500/600/700/800 is a bit sparse on the Micro-Tech 1200 schematic J0275-8), which led me to resistor network N102. Swapped it with N202 and BINGO! There's the faulty component!

I think that when R134 (18k 0.5W resistor) went open circuit, the current flowed through N102 instead. The 33k resistor connected between pins 5 and 7 (7 being connected to J600-3 [-VCC]) is actually measuring more like 330k now, so it's definitely suffered.

Time to get on the phone to Harman Pro UK and order some bits...

I'm learning more and more each time I fix grounded bridge Crowns - I've done small things on a couple of CSLs, a major repair and refurb on another CSL and this will be the 3rd Micro/Macro with multiple odd faults which I've done now. I like the serviceability of these amps, the commonality in design between so many models and the fact that all the schematics and service manuals are freely available for download - and this forum can be a godsend too! Keep it up, Crown!

Cheers,

Richard