700B blew one Right channel 5A supply fuse

[62vauxhall]

No worries, totally understand. I'll monitor the forum for any forthcoming suggestions.

 

[laatsch55]

I don't understand why we had very little DC offset in the left channel and now we have 37 volts. Was the DBT bright on that last test?

 

[62vauxhall]

I don't understand why we had very little DC offset in the left channel and now we have 37 volts. Was the DBT bright on that last test?

Bright, no but was glowing orange.

I had to go back through the posts and the last one I can see where I reported offset voltage was this. It was when the non insulated screws were noticed holding the rows of output transistor sockets .

At same 28 volts on variac, Left channel bias is 91.4Mmv bias and offset is minus 3.68 volts.

Neither bias voltage or intensity of DBT is affected by adjusting bias pot.

Preceded by:

Not surw what I should do here. The RCA's are back in and as I was increasing voltage on the variac, the DBT started to glow at 29 volts whereas before replacing the bias transistor and .33 Ohm resistors, this level of glow took 50 volts. Variac is now backed off to zero.

Should I proceed with measuring bias and offset voltage and if so at what voltage on the variac?

EDIT: With variac at 29 volts bias is 1.84 volts and unaffected by adjusting bias pot as is intensity of DBT. Offset is minus 1.58 volts.

 

[laatsch55]

The logical answer would be something got F&^%$# up between those posts....but how ....I'm lost....

 

[jbeckva]

If the screws were installed at the time of the readings, they would effectively (and very much so) hold down any voltage that made it across the collectors on both ends, and since the collectors on the negative side are wired right to the output..

How at that point would you see ANY voltage ... at all... now there's a mystery too!

I have to take a break on this too Gary.. coming off of 20 hours "overtime" in the last 3 or 4 days while getting over a cold.. gotta rest the bones, and take it easy the rest of the evenin..

 

[62vauxhall]

The logical answer would be something got F&^%$# up between those posts....but how ....I'm lost....

Not sure if this would explain it or not, but recall that I put those screws in at the opposite ends from the RCA 410's? And the reason was to keep the rows of transistor sockets stable?

Earlier on, I was flipping the amp frequently and the weight of the faceplate was tugging at the wire bundle, therefore the transistor sockets, pulling at them, bending the rails.

I just spotted this broken solder joint at rail to left channel Q18. Since there's no transistor in the socket, probably not not the issue, right?




EDIT: corrected to left channel Q18.

 

[laatsch55]

Nope, the outputs are paralleled....that would have just taken that output off line is all...

 

[62vauxhall]

Too bad.

It's about now that I'm inclined to photograph everything, document all connections, remove all components and replace them with new ones.

EDIT: On the back wall that is.

 

[laatsch55]

Yep, the intermittent nature of things leas me to believe there are some things floating around under the sockets. it doesn't take much to F&^% your world up under there....

 

[NavLinear]

Too bad.

It's about now that I'm inclined to photograph everything, document all connections, remove all components and replace them with new ones.

EDIT: On the back wall that is.

Gary,

Have you considered upgrading to the White Oak backplane and controller board? A little more out of pocket money but the end result will be a better amp. Thinking out loud here.

Nav

 

[62vauxhall]

Yep, the intermittent nature of things leas me to believe there are some things floating around under the sockets. it doesn't take much to F&^% your world up under there....

I just noticed a notation on the schematic you sent a few days ago about the bias transistors which says E = red - B = black - C = Brn.

I compared that to a diagram I made several weeks ago of a 2N5088 and hate to say, but I do not have them wired that way - E and C leads are reversed.

Since this problem seems to revolve around bias voltage, is switching those leads the big fix? I can get more 2N5088's tomorrow because they don't seem to care for disconnection re-connection considering where they are.

Once I change them (again) I'll check bias and offset voltage once more and post tomorrow.

 

[62vauxhall]

Gary,

Have you considered upgrading to the White Oak backplane and controller board? A little more out of pocket money but the end result will be a better amp. Thinking out loud here.

Nav

Yes, this how I ended my first ever post:

I understand there is now a loaded PC board for these amplifiers. Could something on the existing board have the blown that fuse and would a new loaded board be beneficial? Or is it more likely power transistors again and/or the associated 1W carbon composite resistors?

 

[laatsch55]

I just noticed a notation on the schematic you sent a few days ago about the bias transistors which says E = red - B = black - C = Brn.

I compared that to a diagram I made several weeks ago of a 2N5088 and hate to say, but I do not have them wired that way - E and C leads are reversed.

Since this problem seems to revolve around bias voltage, is switching those leads the big fix? I can get more 2N5088's tomorrow because they don't seem to care for disconnection re-connection considering where they are.

Once I change them (again) I'll check bias and offset voltage once more and post tomorrow.

No. Those hand written notes on the schematic are not correct.

 

[62vauxhall]

No. Those hand written notes on the schematic are not correct.

Damn!

In that case, I will go shopping for replacement resistors, diodes and 3 lug terminal strips for the back wall. Resistors will be 2 watt versions of the originals and the same IN4004 diodes specified on the schematic. The terminal terminal strips are to have on hand in case the originals don't make it through the dismantling process.

I intend to do one side at a time and will only disconnect wires necessary to gain access to the rows of transistor sockets and rails, the connections of which I shall re-heat to ensure that joints are solid.

Hopefully this process will reveal/allow removal of any debris accumulated during my attempted repair and any that may have collected during past work plus create tidier solder connections in general.

This is going to take me a day or two (I hope) and I will post once it's back together.

 

[laatsch55]

Ok.

 

[62vauxhall]

I got everything except .27 ohm resistors so re-building the back wall will take longer than planned. I guess they're an oddball size or there's too little demand for a store to stock them. All I could get were two and I need ten.

Must now buy them online and wait for them to be sent.

 

[62vauxhall]

None of the online vendors I checked had any .27 ohm resistors in stock but some place local will special order 10 of them for $10.

ETA is 2 weeks so I guess I won't be posting again until after I have them. Hope y'all don't forget about me.

 

[laatsch55]

Mouser doesn't have any?

 

[62vauxhall]

Looking again at Mouser's site, they do have them. When I first checked, I did not see a .27 Ohm designation but now see they refer to that value as 270 mOhms. At the time, I thought mOhm meant meg ohms.

I understand now that Mohm = megohms and mOhms = milliohms.

There are metal oxide 2W 270 mOhm available for $ .16 each but shipping is $20 and I've already paid the local shop $10 for a special order. Mouser would be quicker but I'm reluctant to order those too because the total investment would be over $30.

After an extended period of unemployment, I was to start a new job on July 7th but was told yesterday that the position was no longer available. So with uncertain income potential (again), I must watch my pennies.

Will need to ask the museum I borrowed the variac from if it's OK to keep it a few weeks longer.

 

[laatsch55]

Gary.......let's think this over for a moment.......if you can get it to me, I'll get it back ....are you going to keep the amp or just fix it to sell it?