Hammond T 262 problem..

[Guest]

Hello, I just picked up this 1969 Tonewheel Organ and it's pretty sweet. Problem is after it warms up the sound cuts off completely, comes back to life intermittently and then becomes permanently garbled non musical, just gak for output. I've got it apart now, the leslie is working fine, all the keys work fine, likewise with the switches.... only good for a few minutes until things heat up.... the amplifier is solid state on this model, I have enough electronic experience to remove and replace bad caps or resistors or power transistors, but trouble shooting with a meter and schematic is just barely out of my league, though I can fake it a bit to obtain measurements but their meaning is mostly moot to me.....</p>

Thoughts?  carrying on with my methodical approach...</p>

Thanks in advance...</p>

Bobco </p>

[fredy2]

Re: Hammond T 262 problem..



Try some freeze mist (circuit cooler) to isolate bad components. Probably has a problem in preamp as usually power amps cook themselves permanently... but you can't be sure. It does sound like this is a thermal problem.</P>


Once you find a "sensitive area" use a hair drier and the cooler alternately to home in on the problem.</P>


Go slow, as you might shock the thing into a fix and then it will be back later.</P>


A cold or broken solder joint can also cause this.</P>

[Clarion]

Re: Hammond T 262 problem..



[quote user="fredy2"] Try some freeze mist (circuit cooler) to isolate bad components. Probably has a problem in preamp as usually power amps cook themselves permanently... but you can't be sure. It does sound like this is a thermal problem.[/quote]</P>


Sounds to me like a typicallyone or more defectiveoutput transistors. They heat up, junction separates, and you get that horribly distorted sound that comes from half an amplifier working. Feel the outputtransistors to see if one or more of them are running hot. On my old Classic Organ Works instrument, I used to buy output transistors by the dozen.</P>


This organ must be somewhat newer than 1969. Hammond was still using tube amplifiers at that time.</P>


</P>

[fredy2]

Re: Hammond T 262 problem..



If one of a push-pull ampis running hot, usually its mate is at fault...</P>


If one has a high failure rate of the transistors in a power stage, time for a redesign. Fixes include heat sink, higher rated transistors and a fan. Such fixes are usually relatively easy and safe to do if one has a bit of electronic knowledge.</P>


There are some quiet small fans. A rule of thumb is that ratings can be doubled with forced air cooling.</P>

[Clarion]

Re: Hammond T 262 problem..

[quote user="fredy2"] If one of a push-pull ampis running hot, usually its mate is at fault... [/quote]


'Suppose that makes sense, since the bad transistor is probably open and doing nothing.</P>


[quote user="fredy2"] If one has a high failure rate of the transistors in a power stage, time for a redesign. Fixes include heat sink, higher rated transistors and a fan. Such fixes are usually relatively easy and safe to do if one has a bit of electronic knowledge.</P>


There are some quiet small fans. A rule of thumb is that ratings can be doubled with forced air cooling. [/quote]</P>


In my situation, the amplifiers were being operated at only operated at a fraction of theirrating. Iam convinced that the problem resting with the ridgid mannerin which the TIP35C/TO-218s were mounted between the heat sinks and the circuit boards. There was just such a tremendous amount of expansion/contraction stress placed on these transistors caught between a rock and a hard place, that the junctions eventually separated after theygot hot. It has been suggested that the high failure rate ofotherwise problem-free amplifierswas associated with the speakers used with my organ.</P>


Anyway, that organ was 23 years old, and is nowgone. My new organ has Crown amplifiers, so I'm not overlyconcerned about their reliability. [;)]</P>

[Guest]

Re: Hammond T 262 problem..



Thanks for the responses, still hard at it, I've attached a pic of what I think to be the output transistors, I'm in the process of putting it all back together, and without a schematic as of yet it seems I don't have expression control, just full output, ... must have seft something out..... back to the garage..... any recommendations as to what to replace the transistors with? I'm thinking of using plain old transistors.... grilled cheese sandwich doesn't seem to work....ha</p>

Bobco</p>

 </p>

working on that jpeg attachment....hmmmm </p>

 </p>

http://viewmorepics.myspace.com/inde...ageID=41814555 </p>

[Frank Carvalho]

Re: Hammond T 262 problem..



Oh yes, the transistors behind the big aluminium sheets are the output transistors.</p>

Well I have a T-500 - almost same organ - and when I got it. the power amp had issues with the ground connections on the power amp PCB. I also had a few defect preamp transistors on that PCB causing the reverb to almost cut out. In general my power amp PCB seems to be of really poor quality. It is mounted using a number of screws at the edges, potentially putting it under tension Furthermore, the bottom side is fully exposed to the underside of the organ, which seems to me to be an invitation to even more trouble.</p>

If I were you I would take a look at the power amp as well, if the transistors are OK.
</p>

[fredy2]

Re: Hammond T 262 problem..



Changing transistors requires a little bit of engineering. You could cool them with a fan or replace with more capable ones.</P>


Find what is in there now and I can try to suggest a substitute.</P>


It sounds like your expression pedal has an issue. Does that one use the photo resistors? If so, burned out or disconnected lamp could be the problem.</P>