Wurly 140 - germanium amp

[corgan4321]

Hello,

Working on a early 140 amp - learning lots about these old “totem pole” output schemes... but I’m very confused about the voltages they lost in their schematic.
I’ve swapped the old selenium bridge rectifier for four 4007s and am getting voltages now that are close to their’s: -20.2 and +19.2

However, I can’t understand how you could possibly get -20v at R13/R49 if the output of the rectifier is -20.2. By the time I get past the voltage drops of R47, R48, and R49 (which I’ve measured to be in tolerance) I’m at -13.1v

What gives?

https://cdn.shopify.com/s/files/1/0666/2821/files/140_Schematic_REBUILD_KIT_-_9JUL2019.jpg?131

Here’s the rest of my voltages at the transistors:

      E          B        C
Tr1  -6.45 / -6.3 / -12.86
Tr2 -0.25 / -1.32 / -3.32
Tr3  17 /  16.6 / 2 to 8V (trem LFO)
Tr4 -2.0 / -2.2 / -6.7
Tr5 -3.1 / -3.23 / -10.1
Tr7 -6.3 / -6.46 / -14.3

Tr8 -0.36 / -0.52 / -20.2
Tr9 19.2 / 19 / -0.3

 

[ruffrecords]

It is a bipolar supply with +ve and -ve rails.

Cheers

Ian

 

[corgan4321]

It is a bipolar supply with +ve and -ve rails.

Cheers

Ian

Yup, that’s correct. The positive and negative rails are about right out of the rectifier. I’m trying to figure out how they could get -20V after those dropping resistors I mentioned. I’ve only got ~-13v

Thanks

 

[hodad]

Have you already replaced all the "transilytic" electrolytic caps?  Those things tend to fail outright & could possibly be affecting voltages you see (I'm not that good with understanding what the voltages should be, but I have recapped a few of these.)  Also, do you have the service manual?  If not, let me know & I'll send you a PDF.

 

[corgan4321]

Have you already replaced all the "transilytic" electrolytic caps?  Those things tend to fail outright & could possibly be affecting voltages you see (I'm not that good with understanding what the voltages should be, but I have recapped a few of these.)  Also, do you have the service manual?  If not, let me know & I'll send you a PDF.

I replaced all the electrolytics and most of the other ones. I think I've only left 3 of the original caps which are all bypass caps on the inside of the chassis, C29, C30, C31. I've swapped out a handful of resistors too that were like 20% too high.

But I did have the same thought: maybe if a bypass cap was partially shorted it could cause this but I disconnected C29 to no effect. I didn't try disconnecting C-30 but that's not seeming to cause an issue with the base voltage at TR8. C31 is part of the global feedback loop for the output "totem pole" so i don't think that's causing an issue.

 

[corgan4321]

And no! I do not have that service manual. I would love to have any and all Wurly service manuals you might have. Thanks!

Have you already replaced all the "transilytic" electrolytic caps?  Those things tend to fail outright & could possibly be affecting voltages you see (I'm not that good with understanding what the voltages should be, but I have recapped a few of these.)  Also, do you have the service manual?  If not, let me know & I'll send you a PDF.

 

[Bo Deadly]

Because something is drawing too much current. The filter resistors are only 220, 220 and 120 so over 0.6V is 1.1mA. But at 7V that means that the whole thing is drawing 12.5mA.

This could be many things but clearly the electrolytics look like they're an issue. They are leaking current. So start by replacing those. But it could also be that something is not biased properly or, if the Germs are really early models, they could be leaking badly too.

 

[corgan4321]

The power transistors are NOS as are half of the smaller signal transistors. I suppose it might be worth swapping out the rest of the signal transistors but it looked as though someone had swapped those out sometime in the past 20 years so I held off on those.

Maybe I will get the parts for remainder caps to be replaced, I don’t think they are electrolytic though. They are the brown plastic fully encased style.

I’m not totally sure how to go about biasing this thing. I do know the four high wattage resistors are biasing it, but not sure of the relationship between them. It’s definitely not perfectly balanced right now because I’ve got some voltage at the speaker output and I’m looking for 0v there (explained here if you scroll down to “totem pole”):

https://sites.google.com/site/davidmorrinoldsite/home/trouble/trouble-amps/solid-state-history

Because something is drawing too much current. The filter resistors are only 220, 220 and 120 so over 0.6V is 1.1mA. But at 7V that means that the whole thing is drawing 12.5mA.

This could be many things but clearly the electrolytics look like they're an issue. They are leaking current. So start by replacing those. But it could also be that something is not biased properly or, if the Germs are really early models, they could be leaking badly too.

 

[Bo Deadly]

Unfortunately you can't just swap in whatever Germanium transistor so if someone swapped in the wrong thing, that might be difficult to overcome. Germanium transistors are extremely fickle. With the really old ones each particular transistor can have widely different leakage and the leakage affects bias. The not-so-old ones don't leak too badly and have a much better chance of working. Searching for GT2477 produces nothing even on the old transistor database site. But the part number doesn't strike me as the too-old variety. Post a high res pic.

Because the rebuild kit highlights the electrolytics and because of hodad's comment, I think you should still just start with replacing the electrolytics and then see where you're at.

 

[abbey road d enfer]

On the schemo, the voltage at the collector of TR7 is listed at +15.7V. Since the winding has small DCR, one can asume the voltage at R47-R48 is similar.
All the other voltages seem to indicate that the voltage at R49-R13 should be about 12-13V, which is what you measure.
Where did you get the notion that it should be about 20V?

 

[corgan4321]

Hi! Thanks very much for taking a look at the schematic. That's a good point - I'm totally willing to accept that I'm reading this schematic wrong. I see R13 as having +8V on one side and -20V on the other.

On the schemo, the voltage at the collector of TR7 is listed at +15.7V. Since the winding has small DCR, one can asume the voltage at R47-R48 is similar.
All the other voltages seem to indicate that the voltage at R49-R13 should be about 12-13V, which is what you measure.
Where did you get the notion that it should be about 20V?