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Old 20th February 2011, 06:43 AM   #1
Zoodle is offline Zoodle  Australia
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Default Advice needed re repairing Audio Research VT100

I've been asked to repair an Audio Research VT100 mk3, which has at least one blown resistor (please see 2x photos). I believe the blown resistor is a 100ohm, 3w, 5%, wire wound. Am I right?

As for the other one: AR documents suggest it's a 1ohm, 2w, 5%, 'blue line', but my read of the codes gives a different answer.

I intend replacing all the 100ohm resistors (8); is it worth replacing the "1ohm" ones as well (if that's what they are)

I'd be grateful if you could positively identify both parts, and for any other advice, as I want to do a top quality job (BTW, the owner doesn't want to wait for AR to deliver parts, so I'll be ordering from RS)

The amp has previously had overheating issues (a placement problem, now resolved), so I'm wondering if pre-emptive replacement of other parts might be a good idea. Again, I would be grateful for advice.

Lastly, am I right in thinking that the PCB traces are silver? If so, I should use silver solder(?)

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Old 20th February 2011, 07:18 PM   #2
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Quote:
Originally Posted by Zoodle View Post
I've been asked to repair an Audio Research VT100 mk3, which has at least one blown resistor (please see 2x photos). I believe the blown resistor is a 100ohm, 3w, 5%, wire wound. Am I right?
Yes, that is what the resistor is.

Quote:
As for the other one: AR documents suggest it's a 1ohm, 2w, 5%, 'blue line', but my read of the codes gives a different answer.
Looks about right to me. Brown, black, gold, gold. The gold multiplier moves the decimal point one position to the left. So it's one ohm 5%. And 1 ohm isn't critical at all. Could be 2 or 3 ohms even.

Quote:
I intend replacing all the 100ohm resistors
Unnecessary. If the resistors are in good condition with no signs of overheating, there's no need to replace them unless it makes you feel better. That goes for the 1 ohms as well. The most important thing is to establish what made it (them) burn. They don't fry like that unless there's a good reason. And a defective tube is the most definitive answer. I would certainly replace that tube, and carefully test all the others. Test them hot if possible. The G1 bias is fixed at about -50 volts, so look for that on pin #5. Be sure to clean the board under the old resistor and try to mount the new one slightly elevated off the board if room permits.

Quote:
The amp has previously had overheating issues (a placement problem, now resolved), so I'm wondering if pre-emptive replacement of other parts might be a good idea.
Visually inspect all resistors for signs of overheating and discoloration and replace any that look stressed. Measure them if you're unsure.

Quote:
Lastly, am I right in thinking that the PCB traces are silver? If so, I should use silver solder(?)
I've never heard of AR using silver traces on their boards. It's probably just tinned copper. I'd use regular solder myself unless I knew for sure there was silver in the traces.
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Old 21st February 2011, 01:55 AM   #3
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I agree with Hollow State. Use RA flux 63/37 Sn/Pb solder. Either Kester or MultiCore is FINE.
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Old 23rd February 2011, 07:33 AM   #4
Zoodle is offline Zoodle  Australia
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Update: I replaced the blown resistor, powered up, and for a few seconds all seemed good, before a very faint 'crack' was heard... a real crack, in the class of tube 12...

Sigh....
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Old 23rd February 2011, 10:08 AM   #5
sajti is offline sajti  Hungary
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Quote:
Originally Posted by Zoodle View Post
Update: I replaced the blown resistor, powered up, and for a few seconds all seemed good, before a very faint 'crack' was heard... a real crack, in the class of tube 12...

Sigh....
I think that the tube failed, and it takes away the resistor, when heating up.
The 1ohm resistor is for the bias testing, so keep the 1ohm value, to measure same bias as on the other output tubes.

Sajti

Last edited by sajti; 23rd February 2011 at 10:19 AM.
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