Bought with blown right channel, all 4 outputs blown. Pulled blown outputs, bypassed with 1k resistors to track voltage. Wouldn't leave standby. Replaced some bad components, still standby. One transistor on the driver board and the two caps in the protection circuit, out of protection, 15mv offset on right. 50v on left. Shut it down immediately. Outputs still okay on diode check, I don't have another set, so being very careful.
Where do you start on something that far out of wack? Going to bypass the outputs and start checking the circuit again. Seeking a bit of advice.
Where do you start on something that far out of wack? Going to bypass the outputs and start checking the circuit again. Seeking a bit of advice.
There are 2 ways to approach it.
1/ Route driver stage back into LTP with output transistors removed. Check voltages.
2/ Dive in and check all components on the pcb with component checker.
1/ Route driver stage back into LTP with output transistors removed. Check voltages.
2/ Dive in and check all components on the pcb with component checker.
1st question: is the picture in #2 related to the op's question?
In the protection circuit, check for C13 & C14 as these have swapped polarities (wrongly drawn or designed if placed).
Check for D411 (MV5W) as this could be a suicide diode (refer to the sony's ta4650/ta5650), better replace it with 2 x 1n4148 (or eqv.) in series.
If the protection kicks in, the current source Q411 is switched off, leaving the diff pair to run on lower current. This will change many settings in the amp.
If you're afraid blowing up the power transistors, replace them (one pair is sufficient) with low power bjt's, like bc546/bc556 (rated 65V, or eqv.) with a 1k0 resistor at the emitter. If they fry, it won't harm.
In the protection circuit, check for C13 & C14 as these have swapped polarities (wrongly drawn or designed if placed).
Check for D411 (MV5W) as this could be a suicide diode (refer to the sony's ta4650/ta5650), better replace it with 2 x 1n4148 (or eqv.) in series.
If the protection kicks in, the current source Q411 is switched off, leaving the diff pair to run on lower current. This will change many settings in the amp.
If you're afraid blowing up the power transistors, replace them (one pair is sufficient) with low power bjt's, like bc546/bc556 (rated 65V, or eqv.) with a 1k0 resistor at the emitter. If they fry, it won't harm.
You were indeed correct on the cap polarity in the protection circuit. Too new at this to doubt the circuit board.
D411/412 both read good. The only suspect diodes are D407, 408, 409, 410 which are hidden under the metal shroud over the base of the filter caps. Unfortunately, requiring the removal of the bridge rectifiers which is a chore. One is showing reverse voltage. No idea which.
I've got the 546/556s installed, only brought the variac up a few volts before I had over a volt on the outputs. Time for coffee. I appreciate the tips,
D411/412 both read good. The only suspect diodes are D407, 408, 409, 410 which are hidden under the metal shroud over the base of the filter caps. Unfortunately, requiring the removal of the bridge rectifiers which is a chore. One is showing reverse voltage. No idea which.
I've got the 546/556s installed, only brought the variac up a few volts before I had over a volt on the outputs. Time for coffee. I appreciate the tips,
These are flyback diodes, if one is broken, a rail voltage is on the output indeed. Cut them out all, replace them with fat body 1N5401 series or better. (Beware of the smaller body types, less performance, less SOA).
I advice: replace D411/412 no matter what: timebombs maybe - just plain 1N4148 or eqv's proove rock solid.
The 546/556 can be just under par for this amp, eleaborate at more appropiate types better. Suggestions / contributions here are very welcome!
Been pulling my hair out on this one. Never dawned on me that they were hidden until this morning. We will see what happens.
The556/546 combo seems to be doing the job. They just have to create an artificial environment.
Thanks, and will keep you posted. I may or may not have the 5401s on hand. I've stocked up fairly well, but there are always holes.
The556/546 combo seems to be doing the job. They just have to create an artificial environment.
Thanks, and will keep you posted. I may or may not have the 5401s on hand. I've stocked up fairly well, but there are always holes.
Holy crap. Under the heatsinks, Under the bias resistors. I've got the whole back end removed. Replacing everything that's inaccessible when assembled.
Started reassembly, but the weekend is going to be nuts. Will keep you updated
Skeleton key of the amp. Where is that cap used for? Looks like a HV tube supply type... curiuos.
I'm trying to get up to speed.
"I've got the 546/556s installed,---"
What's being mentioned here? I couldn't spot them.
Thanks.
Does this mean removing Q438,Q440,Q442,Q444 and installing resistors base to emitter? Sounds like a good plan and should allow frontend to be debugged.
"I've got the 546/556s installed,---"
What's being mentioned here? I couldn't spot them.
Thanks.
It's one of the main filter caps. Odd design, their is another set only slightly smaller dedicated to the front end. Does kind of look like an old multi-section, but two are just mounting tabs.
BSST, Citizen had made a couple of suggestions to be able to track down problems using T0-92s in place of the unobtanium output devices.
Looks like it's all coming back apart, was monitoring DC out on the bum channel while slowly running the variac up. Things were looking pretty good until they weren't. Smoked one of the "safety" T0-92s, took out one of the buried diodes.
Going to set this one aside for a day and finish repairing a couple of pads on a Citation 12 board, repopulate it, and claim the easy victory. Have to replace a couple of failed outputs on that one, the Russian pair installed (I would guess) in the 90's, went south. One is dead and the other is not healthy. Then I will get back into the trenches with this one.
I'm missing something really stupid in this process. Clear the head and start from square one.
Looks like it's all coming back apart, was monitoring DC out on the bum channel while slowly running the variac up. Things were looking pretty good until they weren't. Smoked one of the "safety" T0-92s, took out one of the buried diodes.
Going to set this one aside for a day and finish repairing a couple of pads on a Citation 12 board, repopulate it, and claim the easy victory. Have to replace a couple of failed outputs on that one, the Russian pair installed (I would guess) in the 90's, went south. One is dead and the other is not healthy. Then I will get back into the trenches with this one.
I'm missing something really stupid in this process. Clear the head and start from square one.
Unavoidable...
Can you post pictures of the top side of the pcb? Maybe the forum spots a issue.
With this amp, there isn't much to see. Most of the action is on a sub board which complicates testing. Will put some pics up. Have stripped all of the T0-92s, the two blown diodes, and checked every resistor. Will repopulate over the next couple of days. Too much other stuff going on.
D405/D... is another of such a suicide diode (near C409/C410 but removed): if they break, the biasvoltage is open and will maximize (and so the bias current). Replace with 2 x 1N4148 (or eqv). Check for continuity there.
D411/412 are right under the alu part with the blue mark.
D411/412 are right under the alu part with the blue mark.