Leak Delta 30 Amplifier - One Channel Down - Page 8 - UK Vintage Radio Repair and Restoration Discussion Forum

Alistair D · 16th Nov 2010, 2:10 pm

To investigate the fault or wait till you have re-capped it is is more a matter of personal taste. The symtoms you describe could well be caused by a faulty cap so it would be frustrating to find out that it was one of the components you were gong to change.

The crackle is too repeatable to be a connection problem so I would leave the connectors alone at the moment.

Should you want to investigate you will need a test amplifier to listen to various points. Do you have an old set of PC speakers?

Al

Wage Slave · 16th Nov 2010, 2:23 pm

Yes, I do happen to have just such a thing. A test amplifier eh? I like the sound of that very much. For the learning experience, regardless of whether it is a cap. how do I go about it?

Wage Slave · 17th Nov 2010, 4:54 am

Well how's that for timing? The crackle appeared last night and the new coupling caps arrived this morning. I couldn't resist fitting them straight away and yep the crackle has gone. It's also got a touch more volume and noticeably more and better bass. Nice when things work. Now to wait for the next fault to appear. We'll get there bit by bit.

Wage Slave · 20th Nov 2010, 11:04 am

Well I hope I am at the end of this job now. Son of crackle appeared again during testing while I was changing the crossover caps. Far more muted but the same sound and this time the sound in that channel remained weak and distorted. It seemed to be in or past the power amp board as it was still there with the preamp bypassed through the tape replay input. After a couple more power down/power up cycles there was a sort of thump in that speaker and the fault vanished again. How I hate intermittent faults.

Anyway my new ESR meter hasn't arrived yet so onwards with the scattergun approach. I changed the electros on the poweramp boards, got a bit physical with the edge connector sockets via some fine emery paper and checked the bias. All seems good now. I would really like this thing to just play nicely for a year or two which is why I decided to change all the electros - If they are going to start falling over one by one I would rather just get it over with. It does sound really quite good now so well worth all the effort and many thanks to all who have helped and guided me.

I have one question regarding the bias. Spec is 30 to 35 mA. On both channels when I inserted the meter and powered up the meter read 30 mA or so and then dropped like a stone to 7 or 8mA. I adjusted it back up to 30mA as my understanding that is what it should be at idle after stabilising not before. Have I misunderstood?

Silicon · 20th Nov 2010, 9:30 pm

Other items to check are:-
(a) The loudspeaker selection switches (you can bypass them by soldering wires across the contacts)
(b) The wiper on the pre-set variable resistors on the amplifier boards might be
making poor contact with the carbon strip.
(c) intermittently faulty transistors. Try tapping them gently with a plastic pen or use freezer spray.

The quiescent current might surge briefly as it charges up the output capacitors.

The amplifiers might not have been set to the full 30mA originally. It might have been adjusted for minimum distortion using a distortion meter.

Brendan

Wage Slave · 21st Nov 2010, 8:18 am

Well so far so good. It seems to be running very well now. Thanks for the advice. Tapping the transistors is well worth remembering as is shorting out the LS switch. The pots looked OK when I was fiddling with the bias.

You might be right about a distortion meter. However, the time it was made was not a golden era for quality and rigour. I have found a few pretty messy joints and also discovered C28 and C30 were transposed on the right hand power amp board. They are very close in value so it probably didn't matter very much but still....Just goes to show.

I've kept all the caps so it will be interesting to run them through the ESR meter when it arrives.It might show I have gone over the top on this but I would really like to have this thing working day in day out for a while without further fussing.

The situation is a bit odd here. For the smaller value caps the Japanese system of values seems to be completely different. So, I have to get them from the UK and getting a dozen is not much more expensive than getting one because the shipping charge is such a high proportion of the total cost.

Wage Slave · 26th Nov 2010, 1:34 pm

Well my ESR meter has arrived from Portugal of all places. Nice thing and very easy to use. I ran the caps I removed through it. The two big coupling caps were a bit high but not wildly so. I suppose that is about what I expected. Of the twenty or so caps from the boards, two were clearly faulty. The rest were below/at or very near to the guide ESRs.

A useful exercise. I expected worse results given this amp is about 35 years old. It certainly shows that automatically changing them all is not always a great idea even with an amp of this age. In this particular case, I think I still would have gone with mass replacement even had I known because I use the thing every day and for work. Therefore, I need reliability. If 10% have failed it is likely that others will do so in the reasonably short term. If however, I was only going to use it from time to time, and had easy access to parts, then it would make far more sense to only change the ones which were actually faulty.

Now to survey the capacitors in my Quad 33 and 303 before deciding which I want to change.

Alistair D · 26th Nov 2010, 2:40 pm

Has your crackle settled down?

Some of the caps you replaced would have been used to couple 1 transistor to the next and some to decouple emitter resistors. When these caps fail the result is low gain. the ESR meter comes into its own here.

You mentioned that your speaker coupling caps were a little bit high. Because these caps pass a couple of amps of speaker current this low ESR is more critical here.

Did you try thr phono plug dodge on the phono input?

Al

Wage Slave · 26th Nov 2010, 3:01 pm

Yes, the crackle seems to have settled down - fingers crossed. Seems to be working very well now.

I did try the phono plug dodge and it worked perfectly. As it happened my little local parts store had some green plugs in the bargain bin that cost me pennies.

Comment on the speaker coupling caps noted for future reference. Ta.

16th Nov 2010, 2:10 pm	#141
Alistair D Nonode Join Date: Jan 2010 Location: Edinburgh, UK. Posts: 2,008	Re: Leak Delta 30 Amplifier - One Channel Down To investigate the fault or wait till you have re-capped it is is more a matter of personal taste. The symtoms you describe could well be caused by a faulty cap so it would be frustrating to find out that it was one of the components you were gong to change. The crackle is too repeatable to be a connection problem so I would leave the connectors alone at the moment. Should you want to investigate you will need a test amplifier to listen to various points. Do you have an old set of PC speakers? Al __________________ I won't tell you how I discovered that.

16th Nov 2010, 2:23 pm	#142
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Leak Delta 30 Amplifier - One Channel Down Yes, I do happen to have just such a thing. A test amplifier eh? I like the sound of that very much. For the learning experience, regardless of whether it is a cap. how do I go about it?

17th Nov 2010, 4:54 am	#143
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Leak Delta 30 Amplifier - One Channel Down Well how's that for timing? The crackle appeared last night and the new coupling caps arrived this morning. I couldn't resist fitting them straight away and yep the crackle has gone. It's also got a touch more volume and noticeably more and better bass. Nice when things work. Now to wait for the next fault to appear. We'll get there bit by bit.

20th Nov 2010, 11:04 am	#144
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Leak Delta 30 Amplifier - One Channel Down Well I hope I am at the end of this job now. Son of crackle appeared again during testing while I was changing the crossover caps. Far more muted but the same sound and this time the sound in that channel remained weak and distorted. It seemed to be in or past the power amp board as it was still there with the preamp bypassed through the tape replay input. After a couple more power down/power up cycles there was a sort of thump in that speaker and the fault vanished again. How I hate intermittent faults. Anyway my new ESR meter hasn't arrived yet so onwards with the scattergun approach. I changed the electros on the poweramp boards, got a bit physical with the edge connector sockets via some fine emery paper and checked the bias. All seems good now. I would really like this thing to just play nicely for a year or two which is why I decided to change all the electros - If they are going to start falling over one by one I would rather just get it over with. It does sound really quite good now so well worth all the effort and many thanks to all who have helped and guided me. I have one question regarding the bias. Spec is 30 to 35 mA. On both channels when I inserted the meter and powered up the meter read 30 mA or so and then dropped like a stone to 7 or 8mA. I adjusted it back up to 30mA as my understanding that is what it should be at idle after stabilising not before. Have I misunderstood?

20th Nov 2010, 9:30 pm	#145
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,163	Re: Leak Delta 30 Amplifier - One Channel Down Other items to check are:- (a) The loudspeaker selection switches (you can bypass them by soldering wires across the contacts) (b) The wiper on the pre-set variable resistors on the amplifier boards might be making poor contact with the carbon strip. (c) intermittently faulty transistors. Try tapping them gently with a plastic pen or use freezer spray. The quiescent current might surge briefly as it charges up the output capacitors. The amplifiers might not have been set to the full 30mA originally. It might have been adjusted for minimum distortion using a distortion meter. Brendan Last edited by Silicon; 20th Nov 2010 at 9:45 pm.

21st Nov 2010, 8:18 am	#146
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Leak Delta 30 Amplifier - One Channel Down Well so far so good. It seems to be running very well now. Thanks for the advice. Tapping the transistors is well worth remembering as is shorting out the LS switch. The pots looked OK when I was fiddling with the bias. You might be right about a distortion meter. However, the time it was made was not a golden era for quality and rigour. I have found a few pretty messy joints and also discovered C28 and C30 were transposed on the right hand power amp board. They are very close in value so it probably didn't matter very much but still....Just goes to show. I've kept all the caps so it will be interesting to run them through the ESR meter when it arrives.It might show I have gone over the top on this but I would really like to have this thing working day in day out for a while without further fussing. The situation is a bit odd here. For the smaller value caps the Japanese system of values seems to be completely different. So, I have to get them from the UK and getting a dozen is not much more expensive than getting one because the shipping charge is such a high proportion of the total cost.

26th Nov 2010, 1:34 pm	#147
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Leak Delta 30 Amplifier - One Channel Down Well my ESR meter has arrived from Portugal of all places. Nice thing and very easy to use. I ran the caps I removed through it. The two big coupling caps were a bit high but not wildly so. I suppose that is about what I expected. Of the twenty or so caps from the boards, two were clearly faulty. The rest were below/at or very near to the guide ESRs. A useful exercise. I expected worse results given this amp is about 35 years old. It certainly shows that automatically changing them all is not always a great idea even with an amp of this age. In this particular case, I think I still would have gone with mass replacement even had I known because I use the thing every day and for work. Therefore, I need reliability. If 10% have failed it is likely that others will do so in the reasonably short term. If however, I was only going to use it from time to time, and had easy access to parts, then it would make far more sense to only change the ones which were actually faulty. Now to survey the capacitors in my Quad 33 and 303 before deciding which I want to change.

26th Nov 2010, 2:40 pm	#148
Alistair D Nonode Join Date: Jan 2010 Location: Edinburgh, UK. Posts: 2,008	Re: Leak Delta 30 Amplifier - One Channel Down Has your crackle settled down? Some of the caps you replaced would have been used to couple 1 transistor to the next and some to decouple emitter resistors. When these caps fail the result is low gain. the ESR meter comes into its own here. You mentioned that your speaker coupling caps were a little bit high. Because these caps pass a couple of amps of speaker current this low ESR is more critical here. Did you try thr phono plug dodge on the phono input? Al __________________ I won't tell you how I discovered that.

26th Nov 2010, 3:01 pm	#149
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Leak Delta 30 Amplifier - One Channel Down Yes, the crackle seems to have settled down - fingers crossed. Seems to be working very well now. I did try the phono plug dodge and it worked perfectly. As it happened my little local parts store had some green plugs in the bargain bin that cost me pennies. Comment on the speaker coupling caps noted for future reference. Ta.