AMEK TAC 850series PSU

[johnnyscotch]

Hello,

Hoping someone here can help with this problem. I have a TAC Magnum console I'm trying to get up and running. I have an 850 series TAC PSU that I had refurbished by GRS. They also refurbished the master section of the console and two channel strips.

So, to make a long story short, GRS is the worst company I have ever dealt with and my issues with them are beyond the scope of this thread, so I'll just leave it at after a YEAR of dealing with them and trying to get my equipment back I am finally at a point of trying to get this all working and now I have a problem with the PSU that they will not deal with because apparently everyone at the company is out of town.

I had a tech friend of mine who is pretty good take a look at the PSU. At first, I thought I might have blown the +17 and -17v rails, but it was just the fuses and my tech buddy thinks it may be one or more of the transistors located on the heatsinks that need to be replaced. (I've attached pictures).

My problem is that my tech manual for the console has a schematic that is for the 750 PSU, so it's a little different. The other problem is that only one of the transistors in question is marked. It's MJ2955, which is an easy part to source, but how can I figure out if that is same part for all 4 locations??

Does anyone here have a schematic for the 850series PSU? I got my manual from Colin at Audio Maintenance and I reached out to him, but haven't heard back.

Thanks for your help!

Eric

 

[johnnyscotch]

2

 

[johnnyscotch]

3

 

[johnnyscotch]

4

 

[jwhmca]

Kinda all hard to read, but the schematic looks like an PNP-NPN type thing, so they wouldn't all be the same part number.

 

[jwhmca]

Are you sure they are (all) shorted?

 

[johnnyscotch]

To be honest we're not sure of anything, that's why I'm trying to sort the schematic so we can see what's up.

The 32a transistor is in a different location.
We're only looking at the 4 transistors in the "can" packaging on the heat sinks.

I don't think we can go by this schematic rigidly.

 

[jensenmann]

If your buddy is guessing instead of measuring what´s going on then he might be the wrong person to trust in. No offence intended. The PSU is the life insurance of your desk. Make sure that it´s taken care of very well by someone very competent.

From looking at the picture I´d say that 750 and 850 PSUs have different circuits, at least partially.

One source of trouble in these PSUs are the power regulator and transistor sockets. Make sure their contacts are still ok. Otherwise solder directly....

 

[johnnyscotch]

No offense taken.
He took some measurements and said the problem was probably the transistors. Next step, find out what they are and replace them, then more measurements.

It's a $2 part to replace the transistors, figured it's a reasonable place to start. Just need to figure out what they are.

It's a shame GRS is so useless as a company, they could tell me the part # in an email, but their communication and practices are so terrible, even if someone does get back to me, it will be months.

If your buddy is guessing instead of measuring what´s going on then he might be the wrong person to trust in. No offence intended. The PSU is the life insurance of your desk. Make sure that it´s taken care of very well by someone very competent.

From looking at the picture I´d say that 750 and 850 PSUs have different circuits, at least partially.

One source of trouble in these PSUs are the power regulator and transistor sockets. Make sure their contacts are still ok. Otherwise solder directly....

 

[jensenmann]

Usually fuses blow for some reason. One of them could be the dead transistors, the other one could be a fault in the desk (which in fact could have caused the dead transistor/regulator). If your buddy has a good bench supply with current limiting, then you could try to power your desk from that one to see if the current consumption is too high. If yes, then the desk is faulty and killed your PSU (no fault at the GRS side then), if no then the problem was in your 850 PSU.

 

[johnnyscotch]

GRS definitely replaced the caps, and at least one of the transistors, the others look original and old.
Even if they did the right  job mechanically, that's not the issue. Taking 6 months to rebuild it, losing other parts for months and zero communication is the bigger problem, when you're trying to run a business.

We are thinking the desk probably has a short somewhere and could have caused the problem. The fact is that now without the desk plugged in, as soon as the PSU is powered up, the fuses blow after a minute or so.
It definitely has a problem.

We're going to test the desk as well. Unloading the frame and loading a channel at a time is no quick process!!

 

[moamps]

My problem is that my tech manual for the console has a schematic that is for the 750 PSU, so it's a little different. The other problem is that only one of the transistors in question is marked. It's MJ2955, which is an easy part to source, but how can I figure out if that is same part for all 4 locations??

It's very strange that markings on other cans are missing. Are they erased intentionally or are disappeared  because of overheating? ? IMO, they are regulators LM338, and according to your picture this power supply gives 4 voltages. Could you please post the picture of the front side where control LEDs are located? As I know, your console needs +/-17V and 48V. 

If fuses blow without console plugged in power supply, you should check rectifiers (sometimes only one diode in the bridge is shorted), capacitors and regulators.

 

[mitsos]

The TO3 parts are two LM338 regulators and two 2955 transistors. You can tell which is which if you look at the traces, or lift the heatsink to see the silkscreen underneath.

If this has quick connects like the 750 psu, you could try powering each rail individually to see which one causes the fuse to blow. 

 

[moamps]

The TO3 parts are two LM338 regulators and two 2955 transistors.

If you look more carefully to the picture of the power supply in post 1, you will see that there is only one high power regulator (LM338 + 2n2955). There  is obviously also +5V  reg, so I'm not sure is it 750/850 power supply at all. 

 

[johnnyscotch]

thanks for the replies,

I got a hold of a schem for the 850.
The 3 other cans are all LM338k

I ordered parts for all the transistors and regulators

 

[damien]

Hi, I want to replace the fan on my AMEK/TAC 350 Series PSU which is really loud.
The current fan is a 115V DC 80x80mm tapped directly from the transfo main input.

I was wondering if I could just install a DC 24V PWM "noiseless" fan, which would be temp-controlled with a small external board and powered together from the +18V rail ?
Is the fan going to introduce some noise in the +18V line going to the console?

Thanks and happy new year 2025!

 

[jensenmann]

Your current fan is most likely AC, not DC. In the format you mentioned are loads of very silent fans available. Better to use these. I wouldn´t risk loading the audio rails with a fan (which then would be underpowered) and introduce more noise in the system.

 

[Matt Syson]

The 750 and 850 supplies are very different and there are a couple of variants of the 750 as well. The 850 started with DC powered fans and I think eventually AC powered. The 750 has 3 power rails, plus and minus 17 and 48 and was built with 2 'power ratings' where some had a larger transformer and an AC powered fan. If you really have a Magnum desk then it should have a 750 supply as the negative rail of an 850 may not have sufficient current capability, depending on the actual size of the Magnum.
Note that Silent fans are usually useless because the 'silence' is possible because it is not moving sufficient air to cool things properly.
The 850 does indeed have 4 power rails plus and minus 17 Volts, 5 and 48 Volts but the negative rail is only good for about 4 Amps.
If in doubt read the manual. ALL AMEK/TAC desks were delivered with manuals relevant to the desk supplied.

 

[Matt Syson]

Hi, I want to replace the fan on my AMEK/TAC 350 Series PSU which is really loud.
The current fan is a 115V DC 80x80mm tapped directly from the transfo main input.

NO it will be 115 Volt AC fan and probably is a bit loud BUT any replacemet MUST shift as much air as the original so will probably not be much quieter. Using a DC fan will cause the unit to get hotter and if temp controlled it will probably run flat out anyway.