Wurlitzer EP200, transistor issue

[AJStrummer]

Hello

I was asked to look at an EP200. When I received it, the driver and output transistors were fried and filter caps leaking. I have sourced replacement transistors and installed them as well as re-capping. I'm getting a clean signal up to Tr7, and nothing beyond it. I have removed and tested it with an Atlas DCA55 and it tests OK. From the schematic the voltages I should see are B-1.85V C-15-27V E-1.25V. What I'm reading is B-1.42V C-0.20V E-0.11V. The supply voltage is 42.7V. All resistors have been tested and are OK. Any ideas as to how to get the TR7 to conduct and pass a signal?

TIA
AJS

[The Dude]

If the transistor is not shorted, you could have an open resistor. You are missing collector voltage. Follow from your supply through R73, through R16, through R24. See where voltage drops and how much. What is the voltage at the junction of R73 & R16? What is the voltage at the junction of R16 & R24?

[g1]

Also, I think there are various 200 schematics, even of the EP, so make sure you have the right one for the serial # of your unit.

[AJStrummer]

Thanks for the replies gents. I have remeasured at those junctions. The power rail is 42.5V, junction of R73/16 is 42.2V, junction of R16/24 is 39.9V. R24 was where I lost the collector voltage and I removed the original resistor. I have made R24 out of a 2.7K & 1K in series. After the 2.7K there is 10.7V, and 0.2V on the collector.

[g1]

What are TR7 E and B voltages now?

[The Dude]

If you are dropping that much voltage across R24, TR7 must be shorted. I would check again, or replace it. It's possible it might check fine on a DVM, but fail under load. .

Also, in post #1 you show a voltage drop of about 1.3V B-E. This indicates a bad transistor unless I'm misinterpreting your readings. Polarity matters and it's hard to tell for sure if your "-" signs are minus signs or dashes. It's why I use "=" signs as separators instead of "-" . Far less confusing.

Edit: Other possibilities are that the wrong transistor is installed or the transistor is installed incorrectly as it relates to basing diagram. If you replaced the transistor with a substitute, maybe it had a different basing diagram than the original?

[AJStrummer]

Hey The Dude.

Thanks for all your input. See if I can find a replacement transistor and pop it into TR7.

G1, I had replaced 24 before taking all the measurements so the base and emitter voltages are the same

[AJStrummer]

Morning gents.

I replaced TR7, no change. The original was a 2N3859A. I put in a 2SC2634. Going from All Transistors it should be a suitable replacement. Looking at schematic I see R23 is selectable in resistance. I tried a few different values there. With a 33k in R23 I now have the following voltages at TR7

B +0.85
C +0.17
E +0.12

That should bias TR7 better?

Cheers

[The Dude]

The 2N3859A is a 500mA transistor. The 2SC2634 is only 200mA. I wouldn't use that as a substitute. The common 2N3904 would be a better one. That said, you still have no collector voltage. The original part (11-0774) I believe was E-B-C basing diagram. Both of the transistors in your post are E-C-B. I am wondering if someone put in a substitute transistor and did not correct for the basing diagram. At this point, I would double check the transistor basing diagram with the circuit. In other words, look up the datasheet for whichever transistor you use and check it with the circuit. Make sure, using the transistor basing diagram from the datasheet, that emitter goes to R15, collector goes to R24, and base goes to C20. It may be as simple as rotating the transistor.

[g1]

The manual I have lists the 11-0774 as 2N3859A, which is ECB basing.
If the boards match, it is oriented as in this picture.

[AJStrummer]

Make sure, using the transistor basing diagram from the datasheet, that emitter goes to R15, collector goes to R24, and base goes to C20. It may be as simple as rotating the transistor.

Hey The Dude, I've checked and TR7 is positioned correctly. B to R23/C20, C to R24, E to R15

[AJStrummer]

The manual I have lists the 11-0774 as 2N3859A, which is ECB basing.
If the boards match, it is oriented as in this picture.

Hi g1, yes, Tr7 is oriented like in your pic.

Cheers

[g1]

Looking at schematic I see R23 is selectable in resistance. I tried a few different values there. With a 33k in R23 I now have the following voltages at TR7

B +0.85
C +0.17
E +0.12

That should bias TR7 better?

Cheers

I think you may want to go the other way with R23. Just lift one end of it and recheck those voltages.

[g1]

And if that gets things moving in the right direction, R23 should be selected so that TR9 emitter voltage is half of what is at it's collector.

[Helmholtz]

I think TR 7 is biased into saturation, so high collector current and no signal gain.
The emitter voltage is way too low.
It is essential that the emitter voltage is lifted by the current through R14, thereby lowering the base-to-emitter bias.
This in turn requires the voltage at the emitter of TR 9 to be around 23V.
It is possible that the amp needs a load to operate properly because the bias current through D4 needs a DC path to ground.