Harrison 4032 PSU - MCR649AP-2 SCR replacement?

[Mikasonic]

Hi

I have a Harrison 4032 PSU which has a blown MCR649AP-2 (Thryristor) that needs replacing on the +18v rail. The MCR649AP-2 is obsolete so i am desperately trying to find a suitable replacement so i can replace all 4pc that are in the PSU. Does anybody know of a suitable replacement? As Harrison and MCI are very similar, the MCI JH 500/600 PSU may have also used the MCR649AP?

Any help appreciated!

Mike

 

[Walrus]

https://www.ebay.co.uk/itm/192139555263

 

[Mikasonic]

Thanks, i have seen a bunch of NOS available online but as this thing is on 24/7 and is powering a 40ch console not the usual 24ch or 32ch then i would really prefer to change them out for modern replacements rather than replacing them with more 40 year old components.

For anybody else reading this thread, i am looking for any info on possible modern replacements for the MCR649AP-2

 

[Walrus]

I ASSume it is being used as crowbar protection for the 18 V rail (and similar for the other rails).
Therefore any Thyristor with similar Gate turn on voltage and currents, and maximum current handling
will probably work. I've never seen Thyristors in T03 package before, so you may have to go for a different style.

 

[Mikasonic]

Thanks for that!! Yes, it is acting as a type of crowbar protection on the +/- rail outputs. As i understand it is an SCR (Silicon Controlled Rectifier) type. I was hoping someone may know a direct replacement but i suppose i will just have to find an SCR with the same specs but it will be a different package as when i took a quick look at Mouser there were no SRC or Thyristors in a TO-3 package.

 

[Mikasonic]

After looking at Mouser for a suitable replacement the 2N6504G should work with a bit of 'Mac Gyvering' it to the cooling block.

I have attached the datasheets for the MCR649AP-2 and the 2N6504G if anyone is kind enough to compare them and give me a second opinion on if the 2N6504G will work as a replacement.

 

[Walrus]

Yes, i think that should work.
Do you know why the original failed?

 

[JohnRoberts]

as long as you cover the voltage/current specifications you should be OK... (should be).

I still have scars on my as_ from thyristors that misbehaved over the decades.... I've had some inexplicable problems with triacs used in power switch applications. In one Peavey SKU, I punted and replaced the triac with a mechanical switch, after the triac manufacturer could not explain the fault mode. I have also used plenty that worked as advertised so YMMV.

JR

 

[Mikasonic]

Thanks for the input and for taking the time to compare the datasheets!! More than appreciated!

Probably 40 years of serious neglect with no TLC is highly likely the cause... Everything measures OK and it puts out +/-18v to the console with no fault lights but there is a ton of crap and noise on the Master bus when maxed. Also when i measured the current draw into the console the +/-18v is current limiting @ 6.5A. I am not 100% certain as to what a 40ch Harrison should draw on the +/-18v rails but i read somewhere that each channel draws approx. 200mA so multiply that by 40ch and that is 8A + the Master section channels on top, so should be around 9A? The max current output is 10A on the +/-18v rails so to be 'Belt n' Braces' safe i'm changing out all of the flintstones 2N3055 voltage regulators to 2N5885 which can handle way more current without getting as hot. The Harrison 32 series PSU is exactly the same PSU for 24ch/32ch/40ch consoles and has no fan just huge heat sinks.

The console is now fully restored so i know that the console is fine. As the console was put in to storage in 1990 until last year, it would not surprise me if this problem has been on the PSU since before it was put in storage...

I will post an update here when i have installed the 2N6504G's just to confirm they are working and share this info for any other lost souls in the future with the same issue.

 

[JohnRoberts]

If it is an overvoltage crowbar it won't be easy to test...

When restoring an old large console be careful about the temptation to swap in modern low noise op amps that draw more current.

JR