Leak Delta 30 Amplifier - One Channel Down - Page 4 - UK Vintage Radio Repair and Restoration Discussion Forum

dave cox · 3rd Oct 2010, 3:22 pm

Like AI, I think T1 is leaky/dead. It should not be conducting (not enough voltage on b-e) yet there is no voltage c-e. Also, It might be worth removing C11 & C17 for the time being and pop in new ones when everything else is sorted. This will eliminate any input leakage and isolate the a.c feedback loop.

BTW, I assume you are using a modern DVM. The input is operating at low current and one of those nasty AVO's would cause no end of trouble !! I'll now retire to a safe distance ...

dave

Wage Slave · 3rd Oct 2010, 3:23 pm

Quote:

Originally Posted by Station X

Quote:

Originally Posted by Alistair D

Hi Guys Looking at all those measurements and the different transistors I am now wondering if the schematic is not the correct one for this board. I am also now wondering is the board has DC coupled stages. Originally it seemed that there were 2 faults (T1/2 & T4/5). It could be that there is only 1. I am suspicious of R15 (470K), T1 being a little leaky or T2 drawing too much base current.

One way to determing that would be for Wage Slave to measure the transistor voltages on the other (working) board. If they're close to those on the diagram it's probably the correct one. If they're close to the readings on the faulty board we probably have the wrong diagram.

I did this with T5 earlier and the voltages were right. Every detail so far has been correct as far as components and connections go. I will measure the other transistors tomorrow to confirm they match the diagram.

Wage Slave · 3rd Oct 2010, 3:26 pm

Quote:

Originally Posted by dave cox

Like AI, I think T1 is leaky/dead. It should not be conducting (not enough voltage on b-e) yet there is no voltage c-e. Also, It might be worth removing C11 & C17 for the time being and pop in new ones when everything else is sorted. This will eliminate any input leakage and isolate the a.c feedback loop.

BTW, I assume you are using a modern DVM. The input is operating at low current and one of those nasty AVO's would cause no end of trouble !! I'll now retire to a safe distance ...

dave

It's not a Fluke but it is a reasonable DVM about 7 years old.

Remind me about the caps when it seems appropriate. I should have a set of caps arriving quite soon and a set of transistors in a week to week and a half.

Wage Slave · 3rd Oct 2010, 3:29 pm

Quote:

Originally Posted by Alistair D

Even comparative resistance measurements might help narrow down where the problem lies.
My earlier comment about T1 and R15 would only apply if that part of the board matched the cct.

Al

Happy to do that tomorrow as well. You'll need to be a bit specific though otherwise I will wind up doing the electronic equivalent of measuring the cat.

Wage Slave · 4th Oct 2010, 6:24 am

Quote:

Originally Posted by dave cox

Like AI, I think T1 is leaky/dead. It should not be conducting (not enough voltage on b-e) yet there is no voltage c-e. Also, It might be worth removing C11 & C17 for the time being and pop in new ones when everything else is sorted. This will eliminate any input leakage and isolate the a.c feedback loop.

BTW, I assume you are using a modern DVM. The input is operating at low current and one of those nasty AVO's would cause no end of trouble !! I'll now retire to a safe distance ...

dave

I removed T1. It tests as OK.

I also removed C11 and C17 as per advice.

I checked R15 (470k) and got as follows:

On 20M range.

Pos to R15 end connected to base T2 and collector T1 (T1 removed though). Negative connected to collector T3.

2.5 MegOhms

The other way round measured open circuit.

The circuit diagram continues to appear to be correct in every detail.

So, what next? To sum up, T5 and T1 are out and both seem to test ok. C11 and C17 are also out to facilitate further testing.

Wage Slave · 4th Oct 2010, 6:52 am

Quote:

Originally Posted by Station X

Quote:

Originally Posted by Alistair D

Hi Guys Looking at all those measurements and the different transistors I am now wondering if the schematic is not the correct one for this board. I am also now wondering is the board has DC coupled stages. Originally it seemed that there were 2 faults (T1/2 & T4/5). It could be that there is only 1. I am suspicious of R15 (470K), T1 being a little leaky or T2 drawing too much base current.

One way to determing that would be for Wage Slave to measure the transistor voltages on the other (working) board. If they're close to those on the diagram it's probably the correct one. If they're close to the readings on the faulty board we probably have the wrong diagram.

OK, did this with the working board:

T5
Collector 37.6V (spec=35V)
Base 15.3V (spec=14V)
emitter 14.8V (spec=13.5V)

T4
Collector 15.3V (spec=14V)
Base 2.0 (spec=2V)
emitter 1.5V (spec=1.5V)

T3
collector 35.3V (spec=31V)
Base 22.6 (spec=19.5V)
emitter 22.0V (spec=19V)

T2
collector 22.6V (spec=19.5V)
Base 4.3V (spec=4.0V)
emitter 3.7V (spec=3.5V)

T1
collector 4.2V (spec= 4V)
Base 0.6V (spec= ?)
emitter 0.0V (spec=?)

Which all looks about right. Seems the circuit diagram is accurate.

Does this mean suspicion now falls on T2?

Station X · 4th Oct 2010, 8:02 am

Remove R15 (or just disconnect one end) and test it out of circuit. If it is open circuit replace it.

When a resistor is tested in circuit it should never read open circuit or more than its actual value.

If your meter has another range lower than 20M, but higher than 470K use that.

Station X · 4th Oct 2010, 8:05 am

Quote:

Negative connected to collector T3.

Don't overlook the fact that there could be a disconnection in the PCB track between T3 collector and R15.

Wage Slave · 4th Oct 2010, 8:58 am

Quote:

Originally Posted by Station X

Quote:

Negative connected to collector T3.

Don't overlook the fact that there could be a disconnection in the PCB track between T3 collector and R15.

Good morning and nice to hear from you. Track is OK.

Wage Slave · 4th Oct 2010, 9:04 am

Quote:

Originally Posted by Station X

Remove R15 (or just disconnect one end) and test it out of circuit. If it is open circuit replace it.

When a resistor is tested in circuit it should never read open circuit or more than its actual value.

If your meter has another range lower than 20M, but higher than 470K use that.

The range under i20M is 20k unfortunately. Will remove and measure.

"When a resistor is tested in circuit it should never read open circuit or more than its actual value."

That's what I thought. What about the one we tested earlier. Wasn't that open circuit in one direction? I'll go back and have another look.

Station X · 4th Oct 2010, 9:15 am

What I mean is that if a resistor is tested in circuit and it reads higher than its specified value or open circuit it is faulty. This is because if a resistor has something connected in parallel with it the resistance of the combination will always be lower than the original resistor's value, never higher and certainly not open circuit.

Station X · 4th Oct 2010, 9:25 am

Earlier you said that R32 might be open circuit. This is a 1K resistor so can be tested on the 20K range of your meter. If in doubt test it with one end disconnected or compare the reading with that for the same resistor on the other board.

The resistor has a capacitor in parallel with it. When you connect your meter this capacitor wil initially present a short circuit to the meter and give a low resistance reading. This should rapidly rise and then stabilise once the capacitor is fully charged.

Semiconductor junctions will also affect the resistance reading depending on whether they are forward or reverse biased by the meter. Take readings with the meter connected both ways. The higher or open circuit reading is more likely to be correct, but the only sure way of checking is to measure the resistor with one end disconnected.

Wage Slave · 4th Oct 2010, 9:29 am

Earlier we looked at R32 1k. If I measure it on the 2000 range it showed open circuit in both directions. On the 200k range it measured 41k one way and climbed to open circuit in the other.

After I took it out it measured 1k. I'm wondering if a joint was dry even though it looked fine?

Wage Slave · 4th Oct 2010, 9:34 am

Earlier we looked at R32 1k. If I measure it on the 2000 range it showed open circuit in both directions. On the 200k range it measured 41k one way and climbed to open circuit in the other.

After I took it out it measured 1k. I'm wondering if a joint was dry even though it looked fine?

Station X · 4th Oct 2010, 9:38 am

A dry joint is a possibilty. Refit the resistor, measure it both ways and compare the result with the good board. This will take account of any shunting of the resistor by other components.

When testing a resistor use the range next above the expected value, so in this case 20K. If the vaue is unknown start on the highest range and work down until a sensible reading is obtained.

Wage Slave · 4th Oct 2010, 9:47 am

OK. R32 after refitting reads 1k in both directions. I will check this against the good board a bit later but it looks good now.

In the meantime, R15 470k is out and measures open circuit. Could this be our fault?

Station X · 4th Oct 2010, 9:53 am

R15 supplies collector current to T1 and bias to the base of T2. It being open circuit won't help matters at all. Replace it.

Alistair D · 4th Oct 2010, 9:55 am

I think you mentioned that you had problems with this channel before it died.
R15 will have been rising in value before it died. This could explain the symptoms.

Station X · 4th Oct 2010, 10:00 am

Let's hope the replacement of R15 sorts out the problems with T1, T2 and T3. It won't help with the problems on T4 and T5 though as they have no DC connections to the earlier stages.

Wage Slave · 4th Oct 2010, 10:09 am

Quote:

Originally Posted by Station X

Let's hope the replacement of R15 sorts out the problems with T1, T2 and T3. It won't help with the problems on T4 and T5 though as they have no DC connections to the earlier stages.

Could the problems in T4 and T5 be explained by R32 not being connected properly and hence open circuit?

Anyway, many many thanks guys. It is very satisfying to have come this far. Now I have to go searching for a 470k resistor. That is not going to be easy round here!

3rd Oct 2010, 3:22 pm	#61
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,061	Re: Leak Delta 30 Amplifier - One Channel Down Like AI, I think T1 is leaky/dead. It should not be conducting (not enough voltage on b-e) yet there is no voltage c-e. Also, It might be worth removing C11 & C17 for the time being and pop in new ones when everything else is sorted. This will eliminate any input leakage and isolate the a.c feedback loop. BTW, I assume you are using a modern DVM. The input is operating at low current and one of those nasty AVO's would cause no end of trouble !! I'll now retire to a safe distance ... dave

4th Oct 2010, 8:02 am	#67
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: Leak Delta 30 Amplifier - One Channel Down Remove R15 (or just disconnect one end) and test it out of circuit. If it is open circuit replace it. When a resistor is tested in circuit it should never read open circuit or more than its actual value. If your meter has another range lower than 20M, but higher than 470K use that. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

4th Oct 2010, 9:15 am	#71
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: Leak Delta 30 Amplifier - One Channel Down What I mean is that if a resistor is tested in circuit and it reads higher than its specified value or open circuit it is faulty. This is because if a resistor has something connected in parallel with it the resistance of the combination will always be lower than the original resistor's value, never higher and certainly not open circuit. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

4th Oct 2010, 9:25 am	#72
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: Leak Delta 30 Amplifier - One Channel Down Earlier you said that R32 might be open circuit. This is a 1K resistor so can be tested on the 20K range of your meter. If in doubt test it with one end disconnected or compare the reading with that for the same resistor on the other board. The resistor has a capacitor in parallel with it. When you connect your meter this capacitor wil initially present a short circuit to the meter and give a low resistance reading. This should rapidly rise and then stabilise once the capacitor is fully charged. Semiconductor junctions will also affect the resistance reading depending on whether they are forward or reverse biased by the meter. Take readings with the meter connected both ways. The higher or open circuit reading is more likely to be correct, but the only sure way of checking is to measure the resistor with one end disconnected. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

4th Oct 2010, 9:29 am	#73
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Leak Delta 30 Amplifier - One Channel Down Earlier we looked at R32 1k. If I measure it on the 2000 range it showed open circuit in both directions. On the 200k range it measured 41k one way and climbed to open circuit in the other. After I took it out it measured 1k. I'm wondering if a joint was dry even though it looked fine?

4th Oct 2010, 9:34 am	#74
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Leak Delta 30 Amplifier - One Channel Down Earlier we looked at R32 1k. If I measure it on the 2000 range it showed open circuit in both directions. On the 200k range it measured 41k one way and climbed to open circuit in the other. After I took it out it measured 1k. I'm wondering if a joint was dry even though it looked fine?

4th Oct 2010, 9:38 am	#75
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: Leak Delta 30 Amplifier - One Channel Down A dry joint is a possibilty. Refit the resistor, measure it both ways and compare the result with the good board. This will take account of any shunting of the resistor by other components. When testing a resistor use the range next above the expected value, so in this case 20K. If the vaue is unknown start on the highest range and work down until a sensible reading is obtained. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

4th Oct 2010, 9:47 am	#76
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Leak Delta 30 Amplifier - One Channel Down OK. R32 after refitting reads 1k in both directions. I will check this against the good board a bit later but it looks good now. In the meantime, R15 470k is out and measures open circuit. Could this be our fault?

4th Oct 2010, 9:53 am	#77
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: Leak Delta 30 Amplifier - One Channel Down R15 supplies collector current to T1 and bias to the base of T2. It being open circuit won't help matters at all. Replace it. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

4th Oct 2010, 9:55 am	#78
Alistair D Nonode Join Date: Jan 2010 Location: Edinburgh, UK. Posts: 2,008	Re: Leak Delta 30 Amplifier - One Channel Down I think you mentioned that you had problems with this channel before it died. R15 will have been rising in value before it died. This could explain the symptoms. __________________ I won't tell you how I discovered that.

4th Oct 2010, 10:00 am	#79
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: Leak Delta 30 Amplifier - One Channel Down Let's hope the replacement of R15 sorts out the problems with T1, T2 and T3. It won't help with the problems on T4 and T5 though as they have no DC connections to the earlier stages. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.