700B blew one Right channel 5A supply fuse

[62vauxhall]

Standing by.

If I'm to check transistors on this PL20B board, the printing in the diagram downloaded from HiFi Engine is not clear enough to read the Q numbers. The PL14 diagram was more legible.

Also not sure of what transistors they are and would need to know so I can look up what pins are E, B and C.

 

[laatsch55]

The transistors are the same, I'll post a copy of the Pl 20 layout..and schematic..

 

[laatsch55]

In the meantime, power the amp up to just over 40 volts on the rails and see if the bias will adjust on the left channel...

 

[laatsch55]

In the realm of service manuals, you get exactly what you pay for......and anything you get for free is worth what you paid for it...

 

[62vauxhall]

In the meantime, power the amp up to just over 40 volts on the rails and see if the bias will adjust on the left channel...

Rails is bus bar ground to storage cap positive?

With 42 volts showing (variac at 60 volts) bias is 18.2mv which is bias pot at lowest point. Turning bias pot full up only increases voltage to 18.5mv.

EDIT: BTW fuses are still out.

 

[62vauxhall]

In the realm of service manuals, you get exactly what you pay for......and anything you get for free is worth what you paid for it...

Agreed. I had a bad experience downloading a manual from a pay as you go website that put my computer out of commission, took three days to fix and cost $120.

That may have been an isolated occurrence and maybe that website was dodgy. If you recommended a website and it would help, I'd give it another go.

 

[laatsch55]

Phoenix member---rick-----is rick@stereomanuals.com He has manuals for most anything and is a hell of a nice guy to boot...

 

[62vauxhall]

Phoenix member---rick-----is rick@stereomanuals.com He has manuals for most anything and is a hell of a nice guy to boot...

Thanks, I can make out the numbers on what you sent and will check the transistors now. I'll look at Rick's website when I'm done.

 

[laatsch55]

The rails have to have the fuses in if we are checking bias in output devices. By leaving the rail fuses out we are able to check power supply specific problems without endangering output devices in doing so....

 

[62vauxhall]

I put the fuses back in and measured voltage between the bus ground and capacitor positive and this does not make sense to me.

With fuses in and the variac at 60 volts, I only get about 6 volts but with fuses out, get 42 volts.

Be that as it may, with fuses out left channel bias voltage is 185mv and again, unaffected by rotating the bias pot.

Have not check diodes yet, just the transistors. Did not want you waiting as long as it would take to check the diodes too. And if I am to check diodes, you say I can do than in circuit?

These are the reading I got from the transistors on the board the odd ones more than once:

Q1 Left
B to E: .634
B to C: .641
B to E: OL
B to C: OL

Q1 Right
B to E: .638
B to C: .644
B to E: OL
B to E: OL

Q2 Left
B to E: .632
B to C: .638
B to E: OL
B to C: OL

Q2 Right
B to E: .639
B to C: .644
B to E: OL
B to C: OL

Q3 Left
B to E: OL
B to E: OL
B to E: .659
B to E: .665

Q3 Right
B to E: OL
B to C: OL
B to E: .657
B to C: .663

Q4 Left
B to E: OL
B to C: OL
B to E: .612
B to C: .636

Q4 Right
B to E: OL
B to C: OL
B to E: .588
B to C: .631

Q5 Left
B to E: .536
B to C: .534
B to E: .835
B to C: OL

Q5 Right
B to E: .548
B to C: .527
B to E: .832
B to C: OL

Q6 Left
B to E: .379
B to C: .613
B to E: .379
B to C: OL

Q6 Right
B to E: OL
B to C: OL
B to E: .649
B to C: .646

Q7 Left
B to E: OL
B to C: OL
B to E: .597
B to C: .591

Q7 Right
B to E: OL
B to C: OL
B to E: .594
B to C: .592

Q8 Left
B to E: .052
B to C: .100
B to E: .169
B to C: OL

Q8 Right
B to E: .053
B to C: .153
B to E: .054
B to C: OL

Q9 Left
B to E: .489
B to C: OL
B to E: .169
B to E: .166

Q9 Right
B to E: .501
B to C: OL
B to E: .175
B to C: .171

Q10 Left
B to E: .587
B to C: .571
B to E: OL
B to C: OL

Q10 Right
B to E: .594
B to C: .573
B to E: OL
B to C: OL

 

[laatsch55]

What do you get on the negative terminal of the PS caps?? With the fuses in?? The 6 volts tell you you have a bad short somewhere on the backwall or chassis. I don't believe at this time it's on the board. Is the bulb bright with the fuses in??

 

[62vauxhall]

Scratch that comment. Fuses were in when I checked bias the second time.

 

[laatsch55]

Ok, Jer just suggested, leave the rail fuses out on the left channel, let's get it isolated to one channel or the other, or both..

 

[62vauxhall]

What do you get on the negative terminal of the PS caps?? With the fuses in?? The 6 volts tell you you have a bad short somewhere on the backwall or chassis. I don't believe at this time it's on the board. Is the bulb bright with the fuses in??

Voltage at PS cap terminals is 6 volts on one and minus 6 volts at the other

With fuses in and vaiac at 60 volts, the bulb glows orangeish. Tried the 100 watt bulb alone straight into the variac at 60 volts and intensity is the same.

So if the problem is on the back wall and since I did some unsoldering/resoldering to check resistors and diodes but can't see an obvious short, should I be thinking about mass component removal and replacement?

Any chance of a faulty transformer?

Those PS caps do have the copper bus joining Negative of one to Positive of the other?

 

[62vauxhall]

Ok, Jer just suggested, leave the rail fuses out on the left channel, let's get it isolated to one channel or the other, or both..

Right fuses in - left fuses out produce the same plus/minus 6 volts at the capacitor terminals - DBT glows.

Left fuses in - Right fuses out, voltage is plus/minus 38 volts - DBT does not glow.

 

[jbeckva]

Right fuses in - left fuses out produce the same plus/minus 6 volts at the capacitor terminals - DBT glows.

Left fuses in - Right fuses out, voltage is plus/minus 38 volts - DBT does not glow.

There ya go!

That means your issue is with the right channel. Now.... again with the right fuses out and it powered OFF, measure the *resistance* (not diode check.. straight up "ohms" or the omega symbol) on each "rail" on the right channel, to the buss bar on the caps. The best way is to follow the red and black wires (assuming they are colored that), from the fuseholders for the right channel, that go to the sockets.

Tell us what the readings are, for both positive (red) and negative (black), right channel rails to the buss bar common.

 

[jbeckva]

Also... when you do each measurement, reverse the leads... for the positive you want the red lead on the fuseholder wire/black to buss bar, and for negative the opposite.. black lead to fuseholder/red to buss bar.

 

[62vauxhall]

There ya go!

That means your issue is with the right channel. Now.... again with the right fuses out and it powered OFF, measure the *resistance* (not diode check.. straight up "ohms" or the omega symbol) on each "rail" on the right channel, to the buss bar on the caps. The best way is to follow the red and black wires (assuming they are colored that), from the fuseholders for the right channel, that go to the sockets.

Tell us what the readings are, for both positive (red) and negative (black), right channel rails to the buss bar common.

One white wire from a right channel fuse holders attaches to a lug on a three terminal strip, another white wire from the other right channel fuse holder attaches to a rail. At that rail with meter at Ohms, the reading is OL.

I checked the other three rails in the right channel and they are currently at 15 M Ohms but have been declining slowly and steadily.

 

[jbeckva]

One white wire from a right channel fuse holders attaches to a lug on a three terminal strip, another white wire from the other right channel fuse holder attaches to a rail. At that rail with meter at Ohms, the reading is OL.

I checked the other three rails in the right channel and they are currently at 15 M Ohms but have been declining slowly and steadily.

See pic... measure at these two points to ground, and between these two and the right channel positive output terminal. And again - with the right channel fuses pulled.

 

[62vauxhall]

Also... when you do each measurement, reverse the leads... for the positive you want the red lead on the fuseholder wire/black to buss bar, and for negative the opposite.. black lead to fuseholder/red to buss bar.

With red to fuse holder black to bus bar - 0 OL
With black to fuse holder red to bus bar - an unsteady 51 K Ohms.