Leak Delta 30 Amplifier - One Channel Down

[Wage Slave]

Originally Posted by SteveCG

I am 99.9% certain that they are one and the same. However the Stereo 30 is a very different beast from a Stereo 30 Plus.

Yes, as far as I can see, with the exceptions that the transistor numbers are different and there are three extra fuses in the Delta circuit that weren't in the plus circuit.

[dave cox]

Hmmmm, the linked schematic voltages around T1 don't look right. It's collector is annotated at 4V with it's base tapped into about 1/5th of 3.5V. In my book it would be permanently cut off I think T1 should have its operating point set by the feedback loop via T2, R18 and R19 but I can't make sense of that schematic. Maybe that beer I drank last night had more effect that i thought

Is R19 *actually* a 470R ? I'm thinking it must be a higher value ...
What part number is T1 marked with ?

BTW, the voltages you measured are crazy ! T4 is likely dead, the other maybe OK but I can't figure the crazy input circuit.

dave

[dave cox]

I just noticed that your measurements of T1 collector and T2 base are inconsistent, they should, according to the schematic, be at the same potential.


dave

EDIT
Re-flow T2's base solder joint and check it's connected to junction of R15 / T1 collector.

[Wage Slave]

Originally Posted by dave cox

I just noticed that your measurements of T1 collector and T2 base are inconsistent, they should, according to the schematic, be at the same potential.


dave

EDIT
Re-flow T2's base solder joint and check it's connected to junction of R15 / T1 collector.

You are right. I rechecked and got.

T2
collector 30.7V (spec=19.5V)
Base 0.2V (spec=4.0V)
emitter 0.0V (spec=3.5V)

T1
collector 0.2V (spec= 4V)
Base 0.0V (spec= ?)
emitter 0.0V (spec=?)

My mistake obviously. Reflowed it and rechecked. Seems fine.

[Wage Slave]

Originally Posted by dave cox

Hmmmm, the linked schematic voltages around T1 don't look right. It's collector is annotated at 4V with it's base tapped into about 1/5th of 3.5V. In my book it would be permanently cut off I think T1 should have its operating point set by the feedback loop via T2, R18 and R19 but I can't make sense of that schematic. Maybe that beer I drank last night had more effect that i thought

Is R19 *actually* a 470R ? I'm thinking it must be a higher value ...
What part number is T1 marked with ?

BTW, the voltages you measured are crazy ! T4 is likely dead, the other maybe OK but I can't figure the crazy input circuit.

dave

R19 appears to be 470 ohm. It is marked yellow, purple,brown,silver.

I pulled T4 and tested it according to the sticky here and it seems OK unfortunately. I have one on order so will change it anyway just in case when the parts arrive.

T1 is marked as a BC149, T2 to T4 are BC184L and T5 is BC142.

Anyhows this seems to be a tougher nut to crack than first we thought . Thank you so much for your help. (and patience!) Maybe, buying a second unit for spares is the best way out of this.

[Station X]

T4
Collector 30.6V (spec=14V)
Base 31.8V (spec=2V)
emitter 1.6V (spec=1.5V)

T4 base is fed by a simple potential divider consisting of R29 and R30. As you've removed T4 I suggest that you measure the voltage from the negative line to the junction of these resistors. You should get something like 2V. If not measure the values of the two resistors.

EDIT. If the resistors measure OK and the voltage is wrong with T4 removed, remove C22 and check again.

[Alistair D]

Regarding T1/T2 measure R19 as well. You dont need to remove T1. If the reading is much less than 470 ohms C12 may be leaky. It is OK to power up the amp with C12 removed to see if the voltages return to normal.

We are a long way from replacing the board yet. It just takes a little bit longer to find the fault with thousands of miles between us. Think of the learning experience.

Al

[Wage Slave]

Originally Posted by Alistair D

Regarding T1/T2 measure R19 as well. You dont need to remove T1. If the reading is much less than 470 ohms C12 may be leaky. It is OK to power up the amp with C12 removed to see if the voltages return to normal.

We are a long way from replacing the board yet. It just takes a little bit longer to find the fault with thousands of miles between us. Think of the learning experience.

Al

It measures 479 ohms. Is it still worth pulling C12 do you think? I couldn't agree more about the learning experience - Very nice of you guys to act as the guides. Onwards!

[Wage Slave]

Originally Posted by Station X

T4 base is fed by a simple potential divider consisting of R29 and R30. As you've removed T4 I suggest that you measure the voltage from the negative line to the junction of these resistors. You should get something like 2V. If not measure the values of the two resistors.

EDIT. If the resistors measure OK and the voltage is wrong with T4 removed, remove C22 and check again.

Thank you for the suggestion. It is 2V.

[Wage Slave]

Originally Posted by Station X

T4 base is fed by a simple potential divider consisting of R29 and R30. As you've removed T4 I suggest that you measure the voltage from the negative line to the junction of these resistors. You should get something like 2V. If not measure the values of the two resistors.

EDIT. If the resistors measure OK and the voltage is wrong with T4 removed, remove C22 and check again.

I am really sorry. I have just looked at my notes and seen that I measured it as 1.9 first time. A transcription error on my part. I'm checking all the others.

[Station X]

Never mind, we all make mistakes. If the base was at 1.9V and the collector really was at 30.6V, then T4 isn't pulling any current through R31. You've tested T4 out of circuit and it's OK. Check R31, it's possibly being shorted by a fault in T5, in which case remove T5 and check again. Also check R32. If it reads low remove C25 and check again.

EDIT. You say that the emitter of T4 measures 1.6V. Assuming that R32 is OK Ohm's law tells us that 1.6mA is flowing through R32. That current must be coming from T4 and most of it should be flowing through R31 causing a volt drop in it and bringing the collector down to 14V. Something is obviously wrong here. Connect the leads of your meter to the ends of R31 and check what voltage you have across it.

[Wage Slave]

Originally Posted by Station X

Never mind, we all make mistakes. If the base was at 1.9V and the collector really was at 30.6V, then T4 isn't pulling any current through R31. You've tested T4 out of circuit and it's OK. Check R31, it's possibly being shorted by a fault in T5, in which case remove T5 and check again. Also check R32. If it reads low remove C25 and check again.

Thank you for your patience! R31 reads 15.15k with negative to the base of T5 and 4.63k with positive to the base of T5. That looks OK? I did pull T5 earlier and tested it with the diode tester on my meter. It seemed OK.
I think R2 and C35 should be OK because they are off the board and the good board works in either slot. Working on checking them though.

[Station X]

Now I'm confusing you! I meant R32, not R2 and C25, not C35. I've edited my posts to show R32 and C25.

[Wage Slave]

Originally Posted by Station X

Now I'm confusing you! I meant R32, not R2 and C25, not C35. I've edited my posts to show R32 and C25.

That's OK. All good practice. I found them and checked the colour code to make sure I had the right ones. Now for R32 and C25. Sorry the wrong smilies crept in there somehow! I was smiling I promise! Right, now this is more interesting. R32 seems to be pretty much open circuit unless I try to measure it at the very top range on my meter ie 20 Meg when it does some odd stuff slowly increasing in resistance way above 1k which I suppose is to do with C25 not it?

[Station X]

R31 is OK. You're getting two different readings because depending on which way you connect the meter you forward or reverse the junction of T5 which is in parallel with it.

If R32 is OK, and I think you'll find it is, I cannot explain why the collector of T4 is at near 30V rather than 14V. Time to put back T4, which has been tested and check the voltages again. If the collector of T4 is still high measure the voltage across R31.

[Alistair D]

It is probably not worth pulling C12. I will let station X help you out with the T4 problem first rather than have you going in all directions.

One thing to remember when doing resistance measurements is that you have a perfectly good board for comparison but please dont remove any components from it and risk inducing different faults.

Al

[Wage Slave]

Originally Posted by Station X

R31 is OK. You're getting two different readings because depending on which way you connect the meter you forward or reverse the junction of T5 which is in parallel with it.

If R32 is OK, and I think you'll find it is, I cannot explain why the collector of T4 is at near 30V rather than 14V. Time to put back T4, which has been tested and check the voltages again. If the collector of T4 is still high measure the voltage across R31.

Right T4 back in and still has 31.3V on the collector. Across R31 I got 3.3V. I also got quite an impressive little arc to my probe at one point but all seems well.

[Station X]

What voltage do you have on the base and emitter?

[Alistair D]

Originally Posted by Wage Slave

R19 appears to be 470 ohm. It is marked yellow, purple,brown,silver.

I pulled T4 and tested it according to the sticky here and it seems OK unfortunately. I have one on order so will change it anyway just in case when the parts arrive.

T1 is marked as a BC149, T2 to T4 are BC184L and T5 is BC142.

Anyhows this seems to be a tougher nut to crack than first we thought . Thank you so much for your help. (and patience!) Maybe, buying a second unit for spares is the best way out of this.

T2 toT4 BC184L. These transistors have a different pinout to the ones quoted. I think they may have been incorrectly replaced in the past.

[Station X]

Good point. According to Towers the leadout is L21.

With the flat side of the BC184L transistor towards you and the leads pointing upwards , from left to right the leads are B, C and E.