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Hi all,
I acquired an amp similar to the one I already have (see thread: http://www.audioasylum.com/cgi/vt.mpl?f=set&m=74593). Whereas the one I have was well made and needed just some minor tune-up, this amp needs a lot more work. Almost all capacitors are leaking, and some of the metal-film resistors have disintegrated (see attached image). I also think the diodes are no longer working. Before starting the repair, I hope you can help me answer these questions:
— What event could have caused this massive fail inside the amp? I know the owner was using a 12BZ7 instead of the spec'ed 12AX7 tube, but not sure if that could have caused such a disaster.
— Resistors: Should I replace all low-watt metal film resistors, or just the ones that are visibly damaged? Also in the image, you can also see a red resistor. This and ones like it are high-watt resistors. What type are they? Do you think these need replacing too? (They look/measure OK, and that would add a lot of work).
— Capacitors: Due to the circuit layout, I am mostly restricted to using axial capacitors, which leaves me with Nichicon VX for the aluminum ones. Are these sufficiently good? Do you have other suggestions for the aluminum caps?
— Diodes: The circuit sheet specifies ERC16-12 diodes. I found the spec sheet (attached). It’s in Japanese, but the important specs are clear. Would these Vishay avalanche diodes be a good replacement? I am concerned about the difference in forward current vs forward voltage between the two, but I don’t know enough to tell if this is an issue. Are there better diodes to use?
Note that the circuit schematic is in the thread referenced above.
Thanks!
Follow Ups:
to engage in these shoddy build practices.I just posted one a few days ago but the Tango iron is iron to be envious of.I'm assuming this is a SET amp and like Paul and the guys said below,you need to change all the components out because those are questionable.
The fortunate thing is,you look like you may have some decent real estate in the amp to install better components.Diodes,I would use the Schottky-Cree diodes.Filter caps,being a low powered amp if its single ended,I would definitely use film caps in the main B+ supply such as motorruns or regular poly film caps such as Electronic concepts,CDE,or Panasonic,or Wima.Here is a picture of a 100uf@600v EC poly standing next to a JJ dual 100uf@500v electrolytic..You can see they made EC poly compact which is a plus.It a 1/4 shorter than the JJ but it is bigger around.
Clean up that poor workmanship first and foremost.
Honest amplification is better than excessive 2nd order distortion anytime.
I have to agree with Chip647, at least based on the pictures and description.
The blasted resistor is part of a series string clumsily soldered to another resistor, hence almost certainly a replacement for the original. Since this is a 211 amp, is is quite probable that the resistor was not rated for the actual working voltage.
The nearby electrolytic capacitor has what appears to be a burn from a soldering iron - indicating poor technician skills. This assessment is supported by the many leads oddly bent and what appears to be a cold solder joint on a bus wire - hard to tell for sure.
These things suggest to me that many parts were replaced by someone with inadequate technician skills and inadequate engineering knowledge. You really MUST have a schematic to follow to determine what the correct parts values are and what the requirements are. Things that look original - neatly soldered with straight leads and measure the correct value - might still be OK. Anything with oddly twisted leads is probably a replacement and likely to be the wrong value or the wrong rating. One must assume that leaking oil caps are badly made or stressed beyond their specification or both.
The diode sheet you attached indicates a silicon diode of 1200v/2.0A rating. They are the main power supply diodes.
One thought - if this was wired for Japanese 100v power and plugged into American 120v or European 230v mains, that would take out a lot of components before the fuse blew. That could explain the amateur repair attempt... I'd actually do what you can to test the transformers before proceeding too far; if they are toast it will be mighty expensive and difficult to complete a restoration! You can check for correct DC resistances of windings and lack of shorts between windings. A Hi-Pot test would be advisable but might be hard to arrange, since you'd have to find the original specs to know what voltage to test to.
Thanks Paul! Indeed the work on the amp is pretty bad. The only section that's neat is the circuitry around the small signal tubes. I wish I can just tear out all the wires and re-do the whole thing, but that will be a very big job. I think if I replace the metal film resistors, the capacitors and the diodes, it will a good start as the amp will at least be in working condition. It seems that the high-watt resistors are OK (at least the ones that give a stable measurement ... once I disconnect the old caps, I will check all of them).
How do I test the DC resistance on the output transformers? I verified there are no shorts between the primary and secondary windings.
The power transformer is a Tango MS-UVD which is 100V. I checked the fuses. They are rated 250V, and were arced but not broken. Is this normal?
I took more pictures, but they are painful to look at :)
Someone had their way with this poor amp, painful to look at.
Try to match the build of the amp you show in the referenced post of the "good" amp.
I would replace every part you show with the types and values from the schematic.
LOL ... a butcher is funny. And this is not the worst set of caps. While inspecting it, I wondered why the oil caps connected to the diodes where glistening, and after touching them, I realized it's the oil from the cap! The diodes are these canister diodes that I've never seen before and they don't have continuity either way. The worst however are the 500V 100uF caps. They're all corroded on the bottom.
The good amp has diodes that look like avalanche diodes (teardrop shape), but I cannot tell what the specs on them are. I cannot get a hold of the Fujii diodes listed on the schematic either. I'm just wondering what is an equivalent (or higher rated) replacement.
The caps and resistors are easier to replace.
batriq, hi. Check your e-mail.
Sorry it's a long post, but I would appreciate any help, especially about the diodes.
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