700B blew one Right channel 5A supply fuse

[laatsch55]

No, they look ok Gary. Check the resistors and call it good. They need to be over 5 ihms or weird shit tappens......instability...

 

[laatsch55]

Those on the posts are .022 mylars, they'll do.....

 

[62vauxhall]

Resistors are 10.2 Ohms to 10.5 Ohms.

So capacitors are fine?

Edit: Good, they're soldered back in place as are those diodes.

 

[laatsch55]

Good deal.

 

[62vauxhall]

Things changed for me time wise today and I was not able to get new co-ax cable until now - just got home. I was asked at the last minute by some friends to help with a move and couldn't bring myself to say no. In appreciation, they offered me dinner tonight so I'll be leaving for there again shortly. When you live alone, you do not turn down a free meal. When I get back, I'll start replacing the co-ax cable but before I do, please advise what you think about the stuff I got.

I should have checked out the suggested cable .... You are looking for shielded with a CM designation. 2 conductor #22, or #20. Either one...

Two stores I figured, were most likely to have it but neither knew anything about a CM designation. I described what I wished to use it for, explained there was a heat issued and that the original cable had foil in addition to braided shielding.

The first store showed me two quite thick cables (18GA & 20GA) about twice the diameter of what's in the amp. The second store first showed me solid core co-ax and then the cable I wound up buying. I was told it was used in guitar amplifiers and can withstand 85 degrees Celsius or 185 degrees Fahrenheit. The outer sheath is rubber, it has foil and braid for shielding, some textile threads but two centre conductors not one. If I can't twist those both together to make connections, I can use one or the other. Plus it's approximately the same diameter as the original.

Please let me know if you think this stuff is OK. It was only $0.58 per foot so no biggie if I have to find something else. Does that CM designation translate into an RG number?



EDIT: Scratch that comment about it being about the same diameter, it's way thicker. I'll cut off a small sample and go back to that shop in the morning and if no luck, will try the other two stores I did not have time to visit today.

 

[laatsch55]

Gary just use one conductor, cut the other flush with your stripback. Thats way better cable rhan CM, ya done good...

 

[laatsch55]

Thats way good interconnect cable also.

 

[62vauxhall]

As usual, a longer job that I thought. I'm in the process of replacing all the shielded lead and ran into some difficulty around the input jacks. You may recall, this one has 1/4" jacks not RCA's. To make room for soldering the leads, I had to remove them and in doing so, found one of the isolators disintegrated and in pieces, So I'm off the the parts store again to find some.


That copper plate behind those jacks had so much solder on it that it flowed where it shouldn't have. It created a layer preventing one of the jacks from seating properly. Meaning there is not enough thread sticking through the hole to catch the nut. I'll try solder wick or scraping to get enough if it off.

 

[laatsch55]

It's always something isn't it?

 

[62vauxhall]

I could not get an isolator for 1/4" jacks but bought instead a pair of surface mount RCA jacks that include a two piece plastic isolator.

Looking at where 1/4" jacks are mounted, are the jacks really isolated from the chassis?

There is that piece of copper behind the chassis holes that the jacks pass through which to me looks like it could easily make contact with the jacks. Does that piece of copper have to be there?

If I were to replace the 1/4" with RCA's, could I not remove that piece of copper, mount the RCA's to the chassis and attach the ground bus plus the shield portion of the leads (which are all soldered onto the piece of copper) to the star ground point?

Hope this makes sense.

 

[laatsch55]

Yeah, but you'll make a mess of the jacks when ya go to put the shielded and the direct/normal switch wiring to them....

 

[62vauxhall]

Understood, more work and no room to do it in. Those jacks though, they do have to be isolated, right?

I test fitted the RCA's and one was isolated, the other not then put the 1/4" back where I had the non isolated RCA and it wasn't isolated either.

 

[laatsch55]

Yes they should be isolated to keep ground loops out...

 

[62vauxhall]

Well I do have a problem then. With that copper plate to contend with, I am not able to isolate those jacks. Not exactly sure how they could have been isolated in the first place since the jack is metal on metal with the copper plate and the plate is grounded.

This cross section diagram of jack mounting is as I see it and I cannot think of a way to isolate them so I'm open to suggestions or ideas.


Now that I have them, I wouldn't mind using the RCA jacks. The two piece isolator they came with made me think that without that copper plate in the way, and mounting through chassis holes only, they would be isolated.



EDIT: I thought I'd mention that I replaced all 24 input jacks on a Hafler pre-amp with jacks similar to these. The originals were surface mounted to fibreboard strips and the strips snapped. I had stainless steel strips custom fabricated and successfully mounted the jacks. The were all isolated because they only had to pass through a single thickness of metal.

 

[laatsch55]

Put the copper plate on the jacks first. The jacks are not isolated from the copper plate, the copper plate is isolated from the chassis.After putting the jacks through the the copper plate, then the isolating bushing, then put that assembly through the chassis, then the other bushing on the outside, then the nut...

 

[laatsch55]

Nice diagram BTW Gary. how is the fit in the chassis?

 

[62vauxhall]

Through the hole chassis fit is good if that's what you're referring to. The RCA jacks have exactly the same outside diameter as the 1/4" jacks.

So mounting the jack, putting it in in contact with the copper plate which is grounded, how can I use the meter to check if it's isolated? I have one meter lead to the star ground and the other to the outside of a jack and it beeps.

Since contacting the copper plate connects the jack's outside to ground, how can it be isolated? Meter leads to chassis and jack beeps also.

Thanks about the diagram I liked drafting in high school so do try to make good ones. Primitive now - high school was some time ago.

 

[62vauxhall]

I removed the jacks and suspended the copper plate so it itself is not in contact with the chassis. The only connection is that large bare, I assume, rail wire soldered to it. If that copper plate is supposed to be isolated from the chassis, it isn't because connecting meter to chassis and to plate causes the meter to beep.



I had that plate completely off earlier when I mounted the RCA's. When I put it back in, I bent that large wire in such a way to facilitate soldering the lead shields.

 

[Gepetto]

I removed the jacks and suspended the copper plate so it itself is not in contact with the chassis. The only connection is that large bare, I assume, rail wire soldered to it. If that copper plate is supposed to be isolated from the chassis, it isn't because connecting meter to chassis and to plate causes the meter to beep.

View attachment 14826

I had that plate completely off earlier when I mounted the RCA's. When I put it back in, I bent that large wire in such a way to facilitate soldering the lead shields.

There is a schematic with the optimal ground scheme shown in the WOPL archives.

 

[62vauxhall]

There is a schematic with the optimal ground scheme shown in the WOPL archives.

Thanks. I read through the posts and looked at the schematics oldest to newest and unless I am grossly mistaken, the (as it's so descriptively called) floating ground plate should not have a ground wire soldered to it.


There most certainly is in mine and I might add, an outrageous amount of solder was used to make the connection. It is therefore not a floating ground plate in this case.



The other end goes to a chassis grounding tab at the Normal/Direct switch.



But it is isolated mechanically from the chassis by these spacers.



If I clipped that wire off, it would certainly isolate the input jacks. Should I do that?

If that ground shouldn't be there yet it is, was it added after manufacture?


Also there are no 220 Ohm resistors from input pin to ground. Does that apply strictly to WOPL conversions or all 700B's?

If I examined your schematics more closely, I may find other differences.