The case of a Yamaha CA-600 and the black legs of death - UK Vintage Radio Repair and Restoration Discussion Forum

MeerKat · 19th Oct 2023, 12:02 am

A friend ask me to look at one of these, this one in just about mint condition and hadn’t been got at before.

These are well respected units and go for good money. I like them, I’m of an age that likes valves and transistors, I will deal with ICs but not my preferred animal and as for surface mount boards not my cup of tea at all and as I only do repairs for friends I can pick and chose the stuff that interests me.

It had no pilot light and made strange hissing noises when the low filter was engaged, noise on one phono channel and after about half hour of running the volume slowly decreased, advancing the volume caused the lower frequencies to ‘break through’.

The lamp was an easy fix as I had a suitable miniature bulb.

It was easy to check the main power rail and that was OK.

A close look at the circuit boards revealed all the 2SC1345D transistors had black legs, this immediately rang the alarm bells as I had had dealings recently with an Akai GX635D that are known to suffer with this problem, apparently over the years corrosion forms on the transistor legs and eventually creeps up into the transistor contaminating the actual transistor die causing all sorts of problems.

Voltage checks revealed the current through TR501 on the Filter board slowly increased as the amp warmed up turning TR503 harder and harder on until it saturated, I measured 41.4v base 42.1v emitter and 42.05v collector, so hard on! Similar was happening in the other channel.

2SC1345D are hard to get hold of and I was quoted silly money on one site, now as the rails on these run at 50v that ruled out the usual low noise transistors. I eventually settled on KSC1845FTA from Mouser, these are low noise and have a VCEO/VCBO of 120v.

I replaced all the suspects, 12 in all IIRC. Not a bad amp to work on, reasonable access to the boards. I checked the 2SC1345Ds I had removed on my Peak DCA55 and they all tested OK but that was no surprise.

On test all was restored to normal and one pleased friend.

Now to look at his dead Linn Kairn…….and I think there is more to come, he’s a bit of a collector….

stevehertz · 19th Oct 2023, 7:20 am

Good job, well done. I wonder if removing the black 'corrosion' from these transistors would fix them as they test good until warmed up? Maybe once this is removed the process is halted. Just a thought.

Ted Kendall · 19th Oct 2023, 8:26 am

I've dealt with this in the past by cleaning transistor legs by scraping and solvent. Hasn't bounced yet. If it took decades to occur in the first place, it might take decades to return. Perhaps some conformal coating on the exposed legs would help prevent recurrence.

19th Oct 2023, 12:02 am	#1
MeerKat Hexode Join Date: Jan 2016 Location: Ramsgate, Kent, UK. Posts: 303	The case of a Yamaha CA-600 and the black legs of death A friend ask me to look at one of these, this one in just about mint condition and hadn’t been got at before. These are well respected units and go for good money. I like them, I’m of an age that likes valves and transistors, I will deal with ICs but not my preferred animal and as for surface mount boards not my cup of tea at all and as I only do repairs for friends I can pick and chose the stuff that interests me. It had no pilot light and made strange hissing noises when the low filter was engaged, noise on one phono channel and after about half hour of running the volume slowly decreased, advancing the volume caused the lower frequencies to ‘break through’. The lamp was an easy fix as I had a suitable miniature bulb. It was easy to check the main power rail and that was OK. A close look at the circuit boards revealed all the 2SC1345D transistors had black legs, this immediately rang the alarm bells as I had had dealings recently with an Akai GX635D that are known to suffer with this problem, apparently over the years corrosion forms on the transistor legs and eventually creeps up into the transistor contaminating the actual transistor die causing all sorts of problems. Voltage checks revealed the current through TR501 on the Filter board slowly increased as the amp warmed up turning TR503 harder and harder on until it saturated, I measured 41.4v base 42.1v emitter and 42.05v collector, so hard on! Similar was happening in the other channel. 2SC1345D are hard to get hold of and I was quoted silly money on one site, now as the rails on these run at 50v that ruled out the usual low noise transistors. I eventually settled on KSC1845FTA from Mouser, these are low noise and have a VCEO/VCBO of 120v. I replaced all the suspects, 12 in all IIRC. Not a bad amp to work on, reasonable access to the boards. I checked the 2SC1345Ds I had removed on my Peak DCA55 and they all tested OK but that was no surprise. On test all was restored to normal and one pleased friend. Now to look at his dead Linn Kairn…….and I think there is more to come, he’s a bit of a collector…. __________________ Andrew Illegitimi non carborundum

19th Oct 2023, 7:20 am	#2
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 9,009	Re: The case of a Yamaha CA-600 and the black legs of death Good job, well done. I wonder if removing the black 'corrosion' from these transistors would fix them as they test good until warmed up? Maybe once this is removed the process is halted. Just a thought. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

19th Oct 2023, 8:26 am	#3
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,778	Re: The case of a Yamaha CA-600 and the black legs of death I've dealt with this in the past by cleaning transistor legs by scraping and solvent. Hasn't bounced yet. If it took decades to occur in the first place, it might take decades to return. Perhaps some conformal coating on the exposed legs would help prevent recurrence.