GAP Pre73 - won't power up

[frazzman]

Hi all,

I have a GAP Pre73 (Chinese neve 1073 mic ore clone) that decided to bight the dust moments before a session today. Powers on with a very very faint LED and has that distinct burning smell. These units are apparently pretty well known for having a lousy PSU section. It's powered by an external 24vAC adaptor.

I popped the lid off and took some measurements at the output of the PSU at the 24V and 48v connectors - both measured fine. I noticed that there was no burning smell with the mic pre PCB disconnected.

I then reconnected the mic pre PCB to the PSU board and took measurements at R8 (+24v test point) - this is only measuring 2.4V. Similar story on the 48v measurement.

I noticed that there is a 270R 5W resistor on the PSU board that gets extremely hot. Users have mentioned this as a culprit in the past but it tests ok here at least on the DMM

Could I be right in that there is possibly a short on the mic pre PCB or the PSU is crapping out under load?

Any ideas ?

Cheers

 

[pucho812]

times like this you can start with the senses test. visually look for any burnt parts and traces, look for blown caps, look for burnt out resistors, look for anything out of the ordinary. Then smell it, sniff for burnt parts.  I would suspect you had a transistor/transistors go belly up or maybe a tant cap  go boom.  but anything is possible at this point.

 

[frazzman]

times like this you can start with the senses test. visually look for any burnt parts and traces, look for blown caps, look for burnt out resistors, look for anything out of the ordinary. Then smell it, sniff for burnt parts.  I would suspect you had a transistor/transistors go belly up or maybe a tant cap  go boom.  but anything is possible at this point.

Thanks Pucho!
The TIP122 transistors on the PSU smell pretty fishy. It's just strange that the PSU is working in isolation but I am almost certain that's where the problem is. I was surprised that the +24 and +48 voltages tested ok.

 

[pucho812]

Well your testing without a real load. 

 

[frazzman]

Well your testing without a real load.

True true, what I meant to say was that the PSU voltages are ok on their own ... When the pre PCB is disconnected... And also no smell.

The 270R 5W resistor in the PSu board is insanely hot! You could easily fry an egg on it within 5 seconds of power up

 

[pucho812]

right

 

[mitsos]

Load the PSU with a 200R, 5W resistor between the 24VDCand ground (without the preamp connected of course).  If it gives you 24V DC then it's (probably) OK.    While it's on, check the 270R for less heat than before.

Then test the preamp with a known working 24VDC power source. 

 

[frazzman]

Load the PSU with a 200R, 5W resistor between the 24VDCand ground (without the preamp connected of course).  If it gives you 24V DC then it's (probably) OK.    While it's on, check the 270R for less heat than before.

Then test the preamp with a known working 24VDC power source.

Hi Mitsos,

I tried your suggestion with the 200r resistor in place I was still able to measure 24vDC. I guess the PSU circuit isn't where I should be focusing. I also replaced the 270R 5W resistor in the PSU section with a 10W replacement. It still seems to get really hot, with or without the mic pre board connected.

I reheated all the joints on the mic pre board to try and rule out a dry joint or the like. I suppose it's time to try and track down a schematic from the manufacturer.

Appreciate your help

 

[mitsos]

Your PSU is probably OK. 

If you can't get a schematic from the mfg, grab the EZ1290 or the original neve, yours should be the same.

good luck.

 

[frazzman]

Trying to work out the circuit by virtue of reverse engineering the ez1290 schematic ... It's translating some what but there are a lot of different values.

Anyway, it's clear to me there is something on the preamp board that is drawing too much current. With the preamp board disconnected, I get 24v output from the PSU board.

With preamp board connected, I only get 2.4v on the 24v output from PSU board

 

[templemark]

Does the Power Supply still read 24V will some 200 ohm or other dummy load present?

 

[frazzman]

Does the Power Supply still read 24V will some 200 ohm or other dummy load present?

Hi Templemark,

Yes with 200ohm Dummy load I am reading 24V from the PSU.

However as soon as mic pre board is connected the 24v reads as 2.4v and the 270R resistor in the PSU starts heating up big time and starts smelling pretty awful

Cheers
James

 

[pucho812]

obviously the fault lies in the mic pre board, side of the circuit and the PSU is fine.  Test your transistors on the mic pre side  I suspect you have one that has failed.

 

[mitsos]

This might help a little, it's the schematic from the TNC ACMP 73, from the group buy years ago.  I think they had the same values as the original neves but I don't remember.  I do remember modifying the ACMP 84s I got, but mostly caps in the EQ section and tantalums where they had electrolytics.

 

[frazzman]

obviously the fault lies in the mic pre board, side of the circuit and the PSU is fine.  Test your transistors on the mic pre side  I suspect you have one that has failed.

Thanks Pucho.

I've tested all the BC184 transistors. Every collector and emitter leg measures the expected 600ish ohms.

Once I did this I then went on to connect the negative lead of my DMM to the base and the positive lead to either the collector or emitter pins.

As I understand it - in this circumstance - I should expect both junctions to read infinity... This is true of all my transistors expect for 3 transistors where I do not get such a reading - with this test condition.

I am not sure if that indicates a defect with these 3 transistors but it struck me as odd that 3 transistors would exhibit the same defect.

Mitsos - thanks for that schematic it looks much closer to what I am looking at.

 

[frazzman]

So I made some progress...

In an attempt to isolate the problem to a particular part of the circuit - I lifted the 1w/12R resistor that feeds 24v to the output stage. Turned the pre on and now have proper voltage and the power LED is no longer dim.

I had a feeling that it was either the 2n3055, TL072 or LM339 in the output/meter section that was the culprit - however that was not the case

There are 3 other transistors in the output stage - BC184s - they test ok in circuit though

 

[templemark]

I would think that it is probably a failed transistor that is burning up.

When you recreate the problem do any of the transistors get extremely hot within seconds? (....down stream from the 5W resistor)

 

[frazzman]

Finally got this sorted!
After replacing all the transistors and still having the same Issue - I decided to work my way through the other  output stage components as I knew the problem was in there.

Turned out the culprit was a 2200uf/25v electrolytic (6c5 on the schematic mitosos provided earlier).

After replacing that cap all is well.

I'd like to say there was a measured & concise diagnostic approach taken to arrive at this conclusion, but in reality the 2200uf cap was the largest cap in the output stage - so having ruled out the transistors I thought I'd start focusing on the caps.

Call it intuition but probably more like luck.

Thanks everyone for your help.