Low Pin B6 (OA) Voltage

[Quiet_Storm]

I just finished assembling my first Paramount, and I'm having some voltage issues. It looks like I'm getting a reasonable voltage to the C4S input (IA on the PCB or Terminal 16). 486 V looks close enough to 468 V. But something seems to be going wrong with the B6 (OA on the PCB) voltage. Instead of 300 V, I'm getting around 100 V. Also, the B side LEDs aren't lighting up.

I'm getting low values in several other places, but I'm thinking it stems from this issue. Any ideas what might be wrong? The right parts are on the board, the A side LEDS are on, and the solder joints look fine. Would a bad transistor do this?

On a semi-related note, what is the role of the two transistors on the A side of the board? The circuit diagram makes it look like they're lowering the voltage and current. Are these values just stepped down across the two transistors? Does one lower the voltage and one lower the current? Sorry I don't remember much from school about how transistors work.

 

[Paul Joppa]

Do you have an original or a version 1.1 Paramount? I'll comment assuming it's v1.1:

If the B side LEDs are not lit up, then there is no current running through them. From the diagram, you can see that the 100v you have goes through the LEDs, then 149K R2(B), then through 149K R2(B), then 2.49K R3(B) to ground. One of those connections is broken, presumably - or else the LEDs are in backwards.

It's the voltage dropped across the 2.49K resistor R3(B) that sets the shunt regulator voltage, so there's no surprise that voltage is not correct. It's possible (for example) that an error on this chain could damage the B-side 431 and short it, which would lead to about 100v on the shunt reg triode.

You can check the voltages on the varios resistors mentioned above, and on the 5670 pins 6, 7, and 8. This should give us more clues.

 

[Quiet_Storm]

Thanks for the fast response. I have a version 1.1.

So I jumped the gun and swapped out PCBs. I'll troubleshoot the one with issues when I finish up the right channel later this week. The new one is lighting up properly and most of the voltages are right now. Can you take a look at the values I got to make sure I'm ok?

Terminal .... Expected VDC 2A3 ... Actual VDC

Terminal strips
1 ... 464 ... 458
4 ... N/C ... N/C
5 ... 217 ... 198
6 ... 0 ... 198
9 ... A2/3 ... 151
10 ... 233 ... 0
11 ... 164 ... 233
12 ... 0 ... 164
13 ... 94 ... 0
15 ... 26 ... 26
16 ... around 464 ... 474
17 ... 0 ... 0
18 ... 0 ... 0
19 ... A2/3 ... 151

Four pin socket
A1 ... * ... 200
A2 ... around 464 ... 458
A3 ... 217 ... 151
A4 ... * ... 198

Nine pin socket
B1 ... 3.1VAC ... 3.1VAC
B2 ... 4.1 ... 3.2
B3 ... close to 0 ... 0
B4 ... A2/3 ... 151
B5 ... 0 ... 0
B6 ... 300 ... 302
B7 ... 0 ... 0
B8 ... 6.3 ... 7.8
B9 ... 3.1 VAC ... 3.1VAC

I think the expected zener diode string voltages are wrong in the instruction manual. The way I have it set up appears to be working properly, and that explains the inconsistencies on 10-15. My main concerns are terminal 6 and A3. Also B2 and B8 are a little off. Should any of this concern me? Thanks!

 

[Paul Joppa]

Your voltages are fine.

I don't remember whether there is any sticky post with corrections, but the manual voltages are wrong. Only T10 and the T11-T15 voltages are off for the 300B, but for the 2A3, the following corrections should be made:

T5 should be about 200 (not 217)

T10 should be 0 (not 233) It's grounded!

The Zener string is inactive once the regulator kicks in, so the voltages at T11, T12, T14, and T15 are meaningless. Only if the regulator is not functioning will you see reliable voltages, roughly 68v at T15, 136v at T14, 204v at T12, and 272v at T11. These voltages apply to the 300B as well.

A1 and A4 should be about 200v

A3 should be about 155v, actually the A2 voltage divided by 3

B2 is the bias of the driver; it's fine as long as it's over 2.5v.

B8 is the shunt reg bias; it's probably between 5 and 10 volts, but as long as the plate B6 is 300volts then the tube is regulating and all is well.

 

[Quiet_Storm]

Great news on the voltages. I gave the left channel a brief listen, and I'm starting to get very excited.

That victory motivated me to stay up late and finish the right channel. I'm getting similar voltages to my original situation. So it looks like it's definitely something on the shunt regulated active load pc board. The LEDs are installed correctly, so I took the voltages you requested.

With one lead attached to ground, I get the following readings:

149k (R2) 83.1 V on one side and 42 V on the other side
149k (R4) 42 V on one side and 0.65 V on the other side
2.49k (R3) 0.65 V on one side and 0 V on the other side

B6 85.1 V
B7 0 V
B8 0.35 V

The B6 starts at around 350 V and then drops pretty fast. Judging by these numbers, the resistors appear to be properly connected in series, but the current is low. Any thoughts on what would cause this?

 

[Quiet_Storm]

Thanks for the help so far Paul. Any thoughts on what might be causing my problem? I'm at quite the loss over here.

 

[Paul Birkeland]

149k (R2) 83.1 V on one side and 42 V on the other side
149k (R4) 42 V on one side and 0.65 V on the other side
2.49k (R3) 0.65 V on one side and 0 V on the other side

The B6 starts at around 350 V and then drops pretty fast. Judging by these numbers, the resistors appear to be properly connected in series, but the current is low. Any thoughts on what would cause this?

I would recommend replacing the 75v zener diodes and the TL431 regulator, assuming you are 100% sure that everything else is in its proper place. It may be easier to call Eileen and ask about the possibility of starting with a fresh board.  Do be aware that you can't just look at a zener diode and tell that it's blown, and if one or more have failed and you replace the green board, you will end up having to start over again.

 

[Doc B]

My hunch is that the 431 may be shot, as PB suggests. The new Zeners are pretty rugged.

 

[Quiet_Storm]

Thanks everyone! I'm pretty sure the zener diodes are ok since my left channel had the same issue until I swapped out boards. I called up Eileen, and I have a new TL431 on the way. Talk about great customer service. Hopefully I'll be up and running soon!

 

[Quiet_Storm]

I'm up and running. Once I swapped out the LM431, the voltages read almost exactly the same on both channels. Thanks everyone!

 

[Doc B]

Damn, I'm good. OK, PB is good too.