700B blew one Right channel 5A supply fuse

[62vauxhall]

I should have fitted the new bias transistors in their brackets immediately after reading Gepetto's post like I said I would. Instead, I left it till last and wired them incorrectly. Fortunately I had enough sense to at least re-read that post again, realized I done wrong and corrected it.

The board is now done and I am wondering what you would have me do next?

 

[laatsch55]

Check the 1N4004 diodes on the back wall, and if those are good, solder the board back in...

 

[62vauxhall]

Done.

Diodes were .369 ~ .383 in one direction and "OL" in the other.

Board is now soldered back on.

 

[laatsch55]

Changed the outputs yet? - When you do just put in the RCA-410's and bring it up on the dim bulb. If everything is fine , put in another row all the way accross, then dim bulb it again, then another row accross, etc. Get the bias close on the first row, but you'll have to adjust it as you go. Check the service manual as to where you check for bias, if you have any questions on that let me know...

 

[laatsch55]

Waiting with bated breath.....

 

[62vauxhall]

I had some questions about the process of changing output transistors but before I get there, I do believe I have a problem.

I thought to check those bias readings with the old transistors in place. Plugged in the amp, the panel lamps came one but within a minute or so, a small click and the lamps went out.
Unplugged the amp, checked the fuses but they were all OK. Plugged the amp in again. Once more, the lamps lit for a minute, small click and lamps out. That click is either from one of those round devices on the back panel or from the front panel board. I ran a finger along the left channel transistor rows - warm to the touch but not excessively. The right channel rows were a lot warmer. That was only within about two minutes of being powered up.

Would this be a fault within the old output transistors or a screwup I made to the board?

For what it's worth, the power switch was left in the on position and I was just plugging and unplugging the power cord. While plugged in, a substantial spark was created at the AC outlet. I thought it had done this before, maybe not as profound, but I attributed it to a large current draw from the amplifier. When I had the amp hooked up to a system, I remember the room lights lights would dim when it was turned on.

 

[laatsch55]

Those small round devices are thermoswitches. They open when things get hot. Checking the bias first is a must. If you have to high a bias it gets hot. Please go with the new outputs. We stll don't know if the old RCA-410's were any good. If you can have your meter on the bias test points you can get a good idea where it stands before it gets hot enough to shut down. Did you install the DC protect board?..Also what we do to tell the general health of the amp right after bias is in the ball park is DC offset. Meter on the speaker posts, + to +, - to- Should read less than 20 millivilts DC.

 

[laatsch55]

I had some questions about the process of changing output transistors but before I get there, I do believe I have a problem.

I thought to check those bias readings with the old transistors in place. Plugged in the amp, the panel lamps came one but within a minute or so, a small click and the lamps went out.
Unplugged the amp, checked the fuses but they were all OK. Plugged the amp in again. Once more, the lamps lit for a minute, small click and lamps out. That click is either from one of those round devices on the back panel or from the front panel board. I ran a finger along the left channel transistor rows - warm to the touch but not excessively. The right channel rows were a lot warmer. That was only within about two minutes of being powered up.

Would this be a fault within the old output transistors or a screwup I made to the board?

For what it's worth, the power switch was left in the on position and I was just plugging and unplugging the power cord. While plugged in, a substantial spark was created at the AC outlet. I thought it had done this before, maybe not as profound, but I attributed it to a large current draw from the amplifier. When I had the amp hooked up to a system, I remember the room lights lights would dim when it was turned on.

The spark at plug in is normal along with light dimming. How is the dim bulb tester acting?

 

[62vauxhall]

100 watt bulb is near full intensity.

Sorry, didn't realize you posted twice. Didn't see the first one.

I'm going to give this a rest for the night, it's been a long day and will have at it again tomorrow..

 

[laatsch55]

Too much current draw resulting in heating of the output stage. Possibly a short in the outputs. Have you dismounted and remounted the outputs? (old ones)

 

[62vauxhall]

Haven't touched them. Should I do as you said earlier - start with the 410's or replace them all. If just the 410's to start, pull all transistors first or go row by row?

 

[laatsch55]

OK, tomorrow, dismount all ouput devices and test each one individually and we'll go from there. I'll be wotkin tomorrow but i'll chck in with my phone occasionally. From now on , if the bulb gets bright and stays that way turn the amp off immediately. Bad things can happen all over the plaace if a high current condition is allowed to remain...we'll get er man...

 

[laatsch55]

Go row by row, that way if something is wrong there's not that many devices to go through, or associated components to check.

 

[laatsch55]

And by rows I mean accross, not up and down..

 

[62vauxhall]

At the moment, all output devices are pulled. Checked each one and all seem OK.

I have not yet put in those new 16,000MF capacitors. Should I do that, put back the old or new transistors and try again with the dim bulb?

Or, is it back to the board for me?

 

[laatsch55]

Put in the new big cap add. Then just the RCA 410's then dim bulb it.

 

[62vauxhall]

While in the process of removing the filter caps I was abruptly called away for a time and am now unsure of how they tied together. It looks to me like the schematic says the copper strap would go from POS of one to NEG of the other. For some reason I envisioned the copper strap going NEG to NEG.

The original old filter cap left in the amp, the lower one, I can position according to adhesive residue therefore the copper strap would go on it's POS terminal. Am I right to interpret the schematic whereby the strap gets connected to the NEG terminal of the upper capacitor?

With both capacitors removed, I now see where the bridge rectifier goes. With the capacitors out of the way, is this the opportune time put in the new rectifier or should I wait and deal with the output transistors first?

When I start putting those transistors back in, I shall use the new ones you sent me.

 

[laatsch55]

The cap bus bar goes negative to positive , top to bottom.
There is a chance the BR is hurt so now is a good time to change it. Yes , use the new outputs I sent .

 

[62vauxhall]

Got it.

I have the bridge off and see that it had heat transfer compound applied. Should I do likewise with the new one?

I notice the symbols ~ and - at two of the new bridges's terminals. Must I connect it in a certain way or is it irrelevant?

 

[Gepetto]

Got it.

I have the bridge off and see that it had heat transfer compound applied. Should I do likewise with the new one?

I notice the symbols ~ and - at two of the new bridges's terminals. Must I connect it in a certain way or is it irrelevant?

The ~ is the 2 AC inputs FROM the transformer secondary. The + and - on the bridge go to the B+ and B- bulk capacitors respectively. The ground is the center tap wires coming out of the transformer secondary which go to the copper bus bar.