PL20 REV C board capacitors C3L & C3E went POOF

[62vauxhall]

Got the parts last weekend and during the week replaced the burnt resistors and blown out C3L & C3R. Returned to it this afternoon and installed Q1, Q2 and (shot) diode D80.

Did not order replacements at the time but I just now checked D3R & D20. D20 measured .546 but D3R was .356. Should not that be minimum .500?

If D3R has been damaged, can I use an IN4005 instead of an IN4004? Just asking because I have on hand a couple of the former.

 

[laatsch55]

.356 could be influences from other parts of the circuit. The only way to know for sure is to desolder one end...

 

[62vauxhall]

Done. But still the same reading.

1N4005 is a no go? If so, I can get 1N4004's tomorrow - the local shows them on hand.

 

[Gepetto]

Got the parts last weekend and during the week replaced the burnt resistors and blown out C3L & C3R. Returned to it this afternoon and installed Q1, Q2 and (shot) diode D80.

Did not order replacements at the time but I just now checked D3R & D20. D20 measured .546 but D3R was .356. Should not that be minimum .500?

If D3R has been damaged, can I use an IN4005 instead of an IN4004? Just asking because I have on hand a couple of the former.

Yes, 1N4005 is just a higher voltage rating version of a 1N4004

 

[laatsch55]

Gary. I don't know what the difference is between a 4 and a 5 , I suspect it's a voltage rating...

 

[Gepetto]

Gary. I don't know what the difference is between a 4 and a 5 , I suspect it's a voltage rating...

4005 = 600V
4004 = 400V

 

[laatsch55]

Thanks Sir Joe!!

 

[62vauxhall]

Yes, my thanks as well.

I went ahead with using a 1N4005 once Joe OK'd it and replaced both voltage regulators as well. So the new parts are:

R72R, R73L, R73R, R34L -because they were burnt
D3R - because of low reading
D80 - because it was bad
Q2 & Q1
VR1 & VR2
C3L & C3R - because they blew open

Once done, I brought the amp up to power with a variac and DBT and at full voltage, the DBT had a dim glow. Plugged in a discman, increased it's volume slowly and the amp played - for about a minute. Then DBT brightened so I shut 'er down.

After a 5 or so minute wait, I started increasing voltage on the variac. I thought it strange but the at little power, the discman, still connected, was being amplified enough to be clearly heard through the speakers. Once I had the AC power up to full, the DBT stayed dim.

Not being sure what to do next, I checked DC offset (without discman or speakers connected) which was about 12mv each side and bias current that was about 41mv. Have not yet adjusted that as I thought to learn what the hiccup was that that brightened the DBT.

The above took about 10 minutes and I left the amp powered up during the process. No change to DBT - just a dim glow at full power.

Anyone have any suggestions - voltage checks, shorts or....?

EDIT: Would incorrectly connecting the bias transistors cause that anomaly? I ask because I obtained them from Glen - a couple of "Lee's Specials" and I think Glen may may have connected them differently. I now have them connected as per the photograph provided by Joe.

 

[laatsch55]

Cold solder joint maybe??

 

[62vauxhall]

I don't think so. But your suggestion made me push on all the parts I replace and no change to the DBT.

However, I used the discman again, watching the DBT as it played. Please note that I have the discman plugged directly into the 400 and am using its volume control. So this might be a fundamental case of cause and effect but if the signal increases in amplitude, maybe it was a "dynamic" on the first CD I played or in this "just now" instance of increasing the volume, the DBT got brighter. Is this to be expected as the amp is under load?

 

[laatsch55]

Yes, as you load the amp the current will increase....a bunch...

 

[laatsch55]

WTF is a Lee's Special??

 

[62vauxhall]

Search me. When I picked up the board at Glen's place he gave me the two transistors and that's what he called them.

So I'm going to set the bias, button this amp up and when I have time, probably next weekend, I'll play it with a proper pre-amp for a few hours.

 

[62vauxhall]

Well, maybe the bias transistors are connected wrong as turning the pots does nothing. Readings are ~ 500mv regardless.

As mentioned, I connected them according to Joe's photograph. But I thought I did likewise at the get go when installing the board. Glen lent a hand when I goofed installing the output transistors and he altered the bias transistor leads, connecting them differently than how I had them.

When I removed the board for this parts replacement, I did make note of what wire went where but since it did not jive with that photo, I assumed I made a mistake on the diagram. Too late tonight but I think I'll put the wires in the order according to my diagram and see if that will then allow bias current to be adjusted.

Or is it possible those bias transistors got cooked by my electrical faux pas?

 

[orange]

WTF is a Lee's Special??

Cream cap.

 

[WOPL Sniffer]

Cream cap.

HUH????

 

[laatsch55]

Well, maybe the bias transistors are connected wrong as turning the pots does nothing. Readings are ~ 500mv regardless.

As mentioned, I connected them according to Joe's photograph. But I thought I did likewise at the get go when installing the board. Glen lent a hand when I goofed installing the output transistors and he altered the bias transistor leads, connecting them differently than how I had them.

When I removed the board for this parts replacement, I did make note of what wire went where but since it did not jive with that photo, I assumed I made a mistake on the diagram. Too late tonight but I think I'll put the wires in the order according to my diagram and see if that will then allow bias current to be adjusted.

Or is it possible those bias transistors got cooked by my electrical faux pas?

Yes, it's possible, but I would think they took a bigger hit when you lost low voltage regulation, but you did lose R1 on the backplane correct??

 

[62vauxhall]

Now that I don't know. My checking was confined to what's on the board. Did not check anything anywhere else. Won't be home until 9/10PM my time so will look then for R1 and measure it.

EDIT: Make that 10/11PM getting home. Just got an after shift job request and can't afford to turn it down. Yet another 17 hour day in this truck cab.

 

[laatsch55]

I'll be around Gary...

 

[62vauxhall]

Like I figured, home at 10PM. First thing I did was swap over the red and black bias transistor leads to correspond to my diagram and can now adjust bias which I set at 350mv. So I guess, problem solved - I hope.

And that would be a good thing too because I can't see a 1K ohm resistor anywhere on the back pane. I have an idea of the general area it should be located from the schematic but none with the color code brown - black - red are to be found. Unless I'm more tired than I feel.

To bagged to do anything further tonight, it was a long day and likely another tomorrow and another etc. until Saturday. Must hit the rack now because if I don't have my 4 1/2 hour's Z's, I get cranky.