Linear Power 952

clayton

Member
2004-04-05 1:56 am
LA.
I am in need of schematics for this amp. The thermal protection has been disconectedand don't know where it goes It is still mounted to the bottom of the board. When power is applied I get 12v on the power supply transistors center leg, but only 5v on the output transistors center legs. I think that there is poss something wrong with the turn on section. Which might have something to do with the thermal protection not being connected. All power & output transistors test good.
 
I have some temp switches that you can simply add to the 12 volt turn on line to solve this issue LP is out of business and the people that have any info are cashing in on that fact. They are simple amps to work on been in them plenty over the years. Plus I have a few locals out here running LP is SPL competitions so I can check them for you if time is not a issue..:)
 

clayton

Member
2004-04-05 1:56 am
LA.
The temp switch is still on the board just the wires have been disconnected. What is the best way to check if its still good. The way I have always checked these is to ohm it out and apply heat to it. If the ohms increase it is still good. Would this be the correct way?
 

Perry Babin

Member
Paid Member
2003-11-20 11:01 am
Louisiana
www.bcae1.com
In their old amps (maybe all of their amps) Linear uses a switch (thermostat), not a thermistor. It should show either open or closed. You can heat it with your iron to make it toggle. I know some of the thermostats were normally closed but I don't know if all were.

If the switch is still on the board, you should be able to see where they go. There should be open solder pads just at the end of the wires.
 

clayton

Member
2004-04-05 1:56 am
LA.
Yes it is a themostat like you said Perry. This one has continuity till heat is applied. The wires have been cut to about 3" and I can not see any spots where it looks like they would go. Some of the turn on transistors look like they have been changed. They also test good.
 
If its a closed temp switch until heated, just run the 12 volt turn on lead through the switch. When the amp overheats it will open and the amp will shutdown.
The turn on lead on has about 1 milliamp of current on it so the switch contacts should last a lifetime.
If your worried about switch noise just apply a ceramic cap 0.1 ufd or there about across the switch contacts.
I have done this several times before to Orion and PPI amp many years ago for some of my more troublesome clients to warranty.

One of the local shops here liked to move and isolate temp sensors trying to defeat the temp controls on amps to give their clients a amp that would not temp cycle. The end results was a blown amp on my bench :whazzat: :whazzat:

If you can wait a bit I will contact one of my 4 local LP clients and take a look see about the temp switch connection. I am sure its all the same for each LP model as these were just scaled amps of a single design....:)
 
Some of the amps will not power up without the thermal in the circuit. For example, on the 2502 the thermal connects the base to the emitter of a transistor to keep it switched off. When the thermal opens (or is out of the circuit), the transistor will turn on pulling the collector low which will shut the amp off.

His amp is of an older design which varies significantly. The 2502 has several ICs in the supply. His is a self-oscillating design, if I'm not mistaken. I'm not sure how the thermal is used in his amp.
 

clayton

Member
2004-04-05 1:56 am
LA.
Ok, did a little more checking. The voltage on the center legs of the outputs were not 5v. I read it wrong it is .5v. I think Perry is dead on. The 1st turn on trans has 12volts on all 3 legs, but the others only have it on 1 leg. Justonemore I don't have anything to loose by waiting. This amp was given to my son.
 
Hi Perry, Re-reading his first post I take it the amp is in working condition, hence my work around for his temp switch issue.

Now If I took his post incorrectly and the amp is non functional then I would worry about the correct connection, But as I read his post I take it that some super tech has Bypassed his temp switch on the board and he is just trying to restore some semblance of normal and reliable operation to the amp.
So my temp wiring work around will work properly and restore the correct temp cycle for the amp without issues.

At least until I can look at a few of my local clients amps to see the correct connection that LP did over the years. LP's year to year variations were really not that significant, as there amps were really never that complicated. There convoluted lay-out was more of a challenge then there design ever was.

I have had several requests lately to design a replacement for there Black module they use in most of their higher models, but I am hesitant because I just don't see a market for that much work.

The Chief design Engineer for these amps now works here in SoCal for TRU technologies so there is a source for a correct answer without paying the $150.00+ repair fee that is listed on the old LP web-site that is operated by some local group left over from the LP operation.


If I am wrong please correct me:) :) :)
 
Many of these ol amps have defective electrolytics. Many times, these can prevent the amp from powering up. If the amp doesn't power up after you get the thermal back in the circuit, you may want to change all of the small electrolytics in the supply. I don't remember this amp but many of the older amps have a row of components along the edge of the board. Any small electrolytics in that row should be replaced.

Can you post a few good photos of the power supply section of the amp (both top and bottom)? If you have a good camera and can take high resolution photos, post them on a server (post the links to them here) or send them to me and I'll drop them on my server. Maybe the photos will jog a few memories. If you email multiple photos to me, zip them into one file before sending them.

babin_perry@yahoo.com
 

clayton

Member
2004-04-05 1:56 am
LA.
Perry Babin said:
Many of these ol amps have defective electrolytics. Many times, these can prevent the amp from powering up. If the amp doesn't power up after you get the thermal back in the circuit, you may want to change all of the small electrolytics in the supply. I don't remember this amp but many of the older amps have a row of components along the edge of the board. Any small electrolytics in that row should be replaced.

Can you post a few good photos of the power supply section of the amp (both top and bottom)? If you have a good camera and can take high resolution photos, post them on a server (post the links to them here) or send them to me and I'll drop them on my server. Maybe the photos will jog a few memories. If you email multiple photos to me, zip them into one file before sending them.

babin_perry@yahoo.com

You are talking about the caps in the power supply right? There is a total of 6. 2 of them are extremly small 16v 33uf, 4 are 35v 470uf, and the other 1 is 25v 1000uf. Should I change all of them in the power supply. I emailed you some pics of the amp. Have you had a chance to look at them yet?
 

ca90ss

Member
2007-09-20 4:31 am
I don't have a 952 but I checked my 452, 652 and 1502 and they're all in the same general area on the board. I circled where you should look, hope it helps
LinearFront.jpg