B-52 AT-100, won't power on... please help!

[dreamconduit]

hi all, i just got a broken B-52 At-100 in a trade. it doesn't power on at all, totally dead. i replaced all the fuses, and while inside did a visual inspection- no visable swelled caps or fried resistor/ components. HOWEVER- the Surge Surpressor ( i believe is what it's supposed to be) item meant for spot RT-1 on the PS board is completely missing. i'm quite rusty w/ schematics, so does anybody have the specs.-or even better- a source for a replacement surge protector for rt1 on this amp?? from what i've read this part (rt1) is notorious for just outright falling out of the board from heat, like alot of the other parts in the AT-100. and could cause the no power, no light issue. so... i guess replacing that is where i start...

yes, i am confortable working safely w/ the voltages, have a DMM, and am totally willing to take it to a tech. i've already set that up. but my tech is backlogged a month or more, so the amp is gonna be sitting anyways. so, if i can get it up & running sooner, it'd be great.


any input help is greatly appreciated. thanks a ton!

[olddawg]

How about trouble shooting it instead of randomly replacing parts? You have a DVM? Measure the AC across the primary and the secondaries. You might have an open PT.

[dreamconduit]

i didnt think replacing what was clearly a missing component- that was leaving just 2 holes in the RT1 slot of the PS board was random. it seemed like a obvious problem. if not, sorry my rookie mistake. i came here for help w/ just that- troubleshooting. help me out by setting me straight. i can follow directions. what am i looking for on the primary and secondaries? thanks again.

[km6xz]

Many products can be assembled from the same pc boards, just with parts stuffed to the circuits of the different models so seeing some empty pads on a pc board means very little unless close inspection of the solder side show that something was once there and either unsoldered or cracked the joints and the part fell out. Do you have the appropriate DC resistance across the power transformer primary? Can you post a photo of the underside of the pc board?
The schematic shows a thermistor in series but it does not have to be on the PC board so tracing the primary side of the PS circuit should tell you if there is one missingat100 sch.PDF

[DrGonz78]

As it appears on the schematic the part RT-1 C60-11 is located right at Quadrants A3. It is right in series with the fuse on the hot side of the AC mains line. Look around the chassis to see if it did in fact fall out. Others here will know more than I about a replacement type part.

Looks like Stan updated his post to include the schematic.

[dreamconduit]

yes indeed. c60-11. after more research, it appears to be a thermistor. 10k ohm 5A . those are kinda thermal fuses. so, it does fit w/ the kid i got it from's story about it overheating then never working again, collecting dust for years. the thermistor could've fallen out anyware. i'll order one. i see TheTubeDoctor carries them. i wonder if there is a RadioSh&#k-ish version of these. anyone know?

again, thanks for everyones help/ ideas.

[km6xz]

It is a NTC Thermistor, with a cold resistance of 10 ohms and 5 amp rating. It should be common at any electronic supply house. As it heats up the resistance drops, so it limited cold in-rush current until it heats.

Edit: add link http://uk.farnell.com/ametherm/sl15-...ohm/dp/2113005 but all supply houses like Mouser, Newark or DigiKey will have it for less than $2

[Enzo]

Just for test: either tack solder a wire jumper in place of the thermistor then measure resistance across the power cord. You are then measuring the primary of the power transformer. If it measures continuity, then the thermistor may be all that is preventing power up. If there is still no continuity, then check the primary directly. Obviously once that is corrected the amp may have other problems.

Alternative is to leave it alone, aapply power, switch on, and then see if there is 120vAC across the thermistor pads. if there is, then the rest of the primary circuit is intact.

[g1]

Another alternate number for that part is CL-60, as used in several Fender models. It is also a 10ohm 5A NTC thermistor.
Some alternate tests:
in case the part has been bypassed, with unit unplugged, measure resistance between the 2 blades on the power plug, check with power switch in both positions. Any low resistance readings? If so, the part has been bypassed.

Similar to Enzo's test, with unit unplugged, you can put a jumper across the 2 blades of the power plug. Now measure resistance between the 2 pads where RT-1 should be. Try with power switch both ways. One way you should see a low resistance which will show the power transformer primary has continuity, and that the wiring and power switch are good. In this case all you should need is to install RT-1.

[olddawg]

If the thermistor is in series with the primary side, for test I would jumper it and power the amp up. The thermistor really doesn't do much and the amp will operate without it. Make sure you have the proper mains fuse. There may be other problems. You might want to make a lightbulb current limiter before you power it up with or without the thermistor.

[dreamconduit]

this community is amazing! thanks everyone for all the help/ input. i do have full current on the RT1 pads, so fingers crossed...but my little dog just got sick outta the blue after posting this thread last night, so i've been in nurse / vet mode all day. i'll get back into the workshop tonight and give a proper update.

again, thanks so much everyone! olddog- thx for the lightbulb trick reminder. i had long since forgotten about it, despite using it a ton in high school. guess im just gettin old. anyways-that'll be my approach since just a wire jumper worries me. i ordered the thermistor from a local supply house today. should have it by tuesday at the latest.

[km6xz]

The CL-60 designation is not the manufacturer's part number but an industry standard class that means 5 amp at a specified response time, and a cold temperature of 10 ohms. Each manufacturer has their own part number for it.
If you fire it up with the thermistor bypassed you could blow the mains fuse since the surge current is not limited by the missing 10 ohms. If you have a variac, the normal way of testing would be to start at a low mains voltage and ramp it up over a second or two and that will limit the current before the filters are charged.

[g1]

The CL-60 designation is not the manufacturer's part number but an industry standard class that means 5 amp at a specified response time, and a cold temperature of 10 ohms. Each manufacturer has their own part number for it.

May be true for "CL series" but not always for CL-60. Amphenol's actual part number for it is CL-60. It is usually marked as such by the manufacturer. The 60 is the only member of the CL series specified as 10 ohms, 5A.

[olddawg]

The CL-60 designation is not the manufacturer's part number but an industry standard class that means 5 amp at a specified response time, and a cold temperature of 10 ohms. Each manufacturer has their own part number for it.
If you fire it up with the thermistor bypassed you could blow the mains fuse since the surge current is not limited by the missing 10 ohms. If you have a variac, the normal way of testing would be to start at a low mains voltage and ramp it up over a second or two and that will limit the current before the filters are charged.

Yeah... You should always use a variac if you have one. I doubt it would blow a fuse unless you have a high mains voltage. Maybe because it has been setting for so long and the caps are drained. But... A light bulb limiter would limit the onrush too.

[dreamconduit]

thanks again olddawg- the lightbulb variac tip has once again saved me a ton of grief. so the amp is still in problem state. i've jumpered the RT1 via a tack soldered wire jumper- the result is the amp is still shorting/ blowing fuses (or would be, if i didn't have the variac in the power path). as is, it makes my 375W photo flood bulb light up bright once power is turned on. i've removed all 12 tubes. so, troubleshooting continues... i've got my DMM out, and am ready to start testing values.

so, if anybody could help me continue- i just need more guidance. i'm guessing the input / output transformers are next to check?

silver lining: at least my tech is gonna be able to fit me in earlier than expected. so, it'll go into the shop next week, regardless.
but i'd like to proceed (safely) with the diagnoses, if possible. i'm a fast learner, and can follow directions- and i have no problem admitting when i'm a dumbass. just give me the chance to learn more.

and thank you for all the help & info so far. it's been great. i can take pics if needed.
thanks again all