Replacing a Solid State Rectifier

[Perkinsman]

Ok, I'll just replace both while I'm in there. Is a 1N4007 or 1N4008 okay to use as a sub? Besides this, can you also please answer my question regarding the power transistor?

[nevetslab]

C702, D702 both test good. Pulled and tested the rectifier, 3 transistors and most caps on the power supply and all tested good, however F2 was open again. I tested all 8 power transistors and they were good BUT one was different than the rest, apparently it had been replaced at some time. It was labeled MJ15025G BM1608 MEX and replaced one of the 2SB655's. Is this the correct substitution, and if not, could it be causing C719 (1000/16 cap) to overheat and F2 to blow? Also, when a power transistor blows, should they be replaced in matching pairs?

The 2SB655 and it's companion 2SD675 are 20MHz parts, 4 to 5 times faster (greater current gain-bandwidth product) than the MJ15025 (4MHz part). But, those xstrs (2SD675, 2SB655) are no longer in production. Both types have equivalent voltage and current ratings...mostly it's the bandwidth. MJ15024's and MJ15025's as complimentary pairs would suffice in the circuit, though being slower parts, could require altering the compensation. Someone else following this thread might have a faster TO-3 pair to suggest. Yes, in this mismatch, I'd either try and find another 2SB655, or similar replacement pairs.

As you found the original 15A Bridge had failed and replaced it, this together with the pair of 15,000uF/63V supply caps sourcing the power amp circuits, and the output xstrs are what need that current, I have to wonder what caused the bridge to fail. I doubt if the mismatch in speed on the one substitute xstr would cause the bridge to fail, though if the insulating heat sink washer is damaged....such as a non-greased mica insulator now breaking down and leaking, that WILL cause a high current short. Greaseless insulators being re-used isn't recommended, as the insulation system can be damaged after peeling the xstr up when replacing the xstr. Check for shorts between heat sink and the case of the TO-3 parts (collector).

[Enzo]

FAir enough, but he has had caps exploding, and TO3 shorting to chassis ought not do that.

[nevetslab]

I know, and that cap (C719) exploding is pretty bizarre, as that circuit path isn't part of the power amp circuit. That circuit seems to only be involved with the display lamps & Muting circuit. We DON'T know the status of those 15,000uF/63V buss caps thus far.

[Perkinsman]

Can I use a 1N4007 or 1N4008 to replace D702?

I found a pair of sb655 nos. Do I need to replace both on that amp or just the mismatched one?

Also, when I tested the 15A in circuit, it tested bad & that’s what I reported here but once I removed it to be replaced, it tested good. So what I have is F2 fuse blowing & C719 getting hot. Once I replace c702 & d702, I’ll update.

[nevetslab]

On the SB655's, I'd replaced both, just to be safe. What kind of insulators are being used? Greased Mica, or greaseless silicone-rubberized style insulators? Are you getting ANY resistance between Heat Sink (Ground) and the metal cases of any of the output xstrs?

The 15A bridge rectifier is testing bad when installed, while tests good when out of circuit? Are you measuring a short, or low resistance reading between Ground and either 15,000uF/63V filter buss caps? Does it change when unplugging the 15A Bridge's (+) & (-) output terminals? Your comments with regards to the 15A bridge suggests there is a substantial short on either/both supply connections on the two 15,000uF buss caps. Do the power amp circuits ahead of the eight output xtrs unplug from the system? Perhaps there's still a short on the circuit board(s) of the power amp to account for the bridge reading bad.

There is a way to test the 15,000uF buss caps in-circuit, if you have a scope and a square wave signal (Function Generator). If you do, I'll add an article on how to set up a test to measure ESR of caps, which also measure the capacitance, while injecting 200mA to 400mA of current into the caps. I never invested in an ESR meter, having that function on my impedance bridges, as well as being able to cobble this ESR circuit with the scope & square wave generator.

[Perkinsman]

There is a way to test the 15,000uF buss caps in-circuit, if you have a scope and a square wave signal (Function Generator). If you do, I'll add an article on how to set up a test to measure ESR of caps, which also measure the capacitance, while injecting 200mA to 400mA of current into the caps. I never invested in an ESR meter, having that function on my impedance bridges, as well as being able to cobble this ESR circuit with the scope & square wave generator.

On the SB655's, I'd replaced both, just to be safe. What kind of insulators are being used? Greased Mica, or greaseless silicone-rubberized style insulators? Are you getting ANY resistance between Heat Sink (Ground) and the metal cases of any of the output xstrs?

I replaced both SB655's. They use the mica type insulators that someone had gobbed on paste. I removed them all, cleaned & reapplied new paste to all of them. I powered up and C719 still gets hot and F2 blew again.

The 15A bridge rectifier is testing bad when installed, while tests good when out of circuit? Are you measuring a short, or low resistance reading between Ground and either 15,000uF/63V filter buss caps? Does it change when unplugging the 15A Bridge's (+) & (-) output terminals? Your comments with regards to the 15A bridge suggests there is a substantial short on either/both supply connections on the two 15,000uF buss caps. Do the power amp circuits ahead of the eight output xtrs unplug from the system? Perhaps there's still a short on the circuit board(s) of the power amp to account for the bridge reading bad.

I don't know how to answer this because i don't really understand how to do some of these things but I'm willing to learn. What I do know is that the current rectifier is good BUT I may have wired it incorrectly when I replaced it. I'll post a pic of the existing rectifier and my wiring.

[Enzo]

Blows fuse again? Um... weren't we using a bulb limiter whwile we serviced this thing?

Without going back through it all, You pulled the AC wires from the transformer off the rectifier and it doesn't blow fuses, yes? Did you connect the transformer wires back to the rectifier and disconnect the red and white wires to the caps? Now blows or not?

I just looked, but I don't see a schematic posted.

You have film caps across each diode in the bridge - that's fine - but could one be shorted? Just check with an ohm meter right where they sit. Takes 20 seconds.

[Jazz P Bass]

From Post #18: https://music-electronics-forum.com/...etch?id=912197

[Jazz P Bass]

Here is a page with some nice pics of the internals of the amp: http://www.tapeheads.net/showthread.php?t=27955
They may help in navigating.

[Enzo]

Thanks Jazz.

I see C719 doesn't seem to be related to the big bridge and its supplies. If C719 is blowing up, I don't doubt F2 blows. It looks to me like that power supply board uses individual plug-on wire connectors. ANy chance you have a wrong wire plugged onto post #7.

Try this test. Remove fuse F2, power on, and measure AC vvoltage on the empty fuse clip. One should have the voltage from the transformer, the other should show about zero due to the missing fuse. C719 is only a 16v cap, soif there is more than about 10vAC on that incoming wire, it will burn up the caps.

[52 Bill]

Check cap C702 with your ohmeter. If it is shorted it would cause the diode to test bad in circuit but okay out of circuit and it would send ac voltage directly to cap C719, causing it to heat up. If that ends up being the problem replace C719 as it has probably been damaged.

[Perkinsman]

Here is a page with some nice pics of the internals of the amp: http://www.tapeheads.net/showthread.php?t=27955
They may help in navigating.

Thanks so much for that link to those pics. Without F2 in, both sides were showing high vac. I zoomed in on the rectifier & the plugs to the powers supply board that I had pulled off & put back on several times during this troubleshooting thread. Attached is the pic of my power supply board. Notice the difference? Once I corrected the green & white wires I powered up (on the dbt Enzo, lol) & voila, lights, sound & C719 didn’t get hot! One side of the fuse was 6.4vac, the other was low mv. Thanks for your patience & helpful advice.

[g1]

The cap that was getting hot should be replaced for reliability. It may be damaged and has at least been stressed beyond it's limits.

[Perkinsman]

The cap that was getting hot should be replaced for reliability. It may be damaged and has at least been stressed beyond it's limits.

I remember you had said that earlier in the thread so I put in a new Nichicon. Thanks for all your ongoing help.