Trouble shooting help please [resolved]

[JamieMcC]

Season’s greetings guys

I am having a bit of trouble tracking down an issue which has recently developed with my Eros. I noticed the last time I used it one channel started to have a slightly lower volume and when next used there was no sound at all from that channel.

Trouble shooting wise so far I have done no more than look for loose connections and tried a different set of EF86 and 6922 tubes.

An observation made is that on the offending (red) channel  C4S /servo board the two HLMP-6000A LED's (the ones closest to the front RCA and ground post sockets) initially glow brightly when power is turned on but then quickly dim and look to go out completely, all other HLMP’s glow brightly.

The Eros so far has been working perfectly for around 300 or so hours.

I do have another phono I can use in the meantime and understand it is the holiday season so if you can suggest the next step to help narrow down the problem when you all get back to work that would be very much appreciated.

Cheers
Jamie

 

[BNAL]

Have you done any voltage checks? That would help isolate the problem. It sounds like a bad solder joint.

 

[JamieMcC]

I hope with the info given rather than just measure everything the BH team will be able to narrow down some likely causes and be a bit more specific in what to measure and the areas to investigate further.

 

[Doc B]

It really helps up to figure out what is happening if you post the voltage measurements.

 

[JamieMcC]

It really helps up to figure out what is happening if you post the voltage measurements.

These are my voltage readings first figure is the channel with the problem followed by the reading for the working channel all DC readings I have 240V transformer version of the Eros

KReg  2V      1.5V 
BReg  55V    97V
OB    56V    95V
ba      0V        0V
Oa    223V    166V
Ia    226V    225V

 

[Paul Birkeland]

Yeah, that tells us that the regulated power supply back by the 12BH7 is doing its job, and confirms that there's a miswire or potentially a swapped pair of resistors on the C4S board above the offending channel.

This could be something as simple as the STP going to pins 4 and 5 on the EF86 socket of that channel coming loose and not allowing the tube to heat, or it could be a broken or poorly soldered jumper on the C4S board.

Did this Eros pass its voltages checks perfectly initially?

 

[JamieMcC]

Hi Paul thanks 

I am pretty certain the initial votage checks were all good and both boards had only the odd 1 or 2 volts difference between them. The amp has been working faultlessly and sounding superb for the last 3 months and has been getting a lot of use 

I have reflowed the STP cable at pins 4 and 5 and the jumper wires. The resistor values were also all correct but I had to pull a couple that I could not see the codes for (I normally ohm every one out to check before installing) and when doing so I busted the 301 ohm resistor (R1 B side) I heated one side as you do and it came out no problem but when I flowed the other leg gave a gentle pull and the end of the resistor fell off  : 

I've pulled a few resistor now on different projects and this was the first one thats busted I dont know if it might have been damaged prior to my removal at all but it fell apart pretty easy and I will need to find a replacment before I can check my voltages again.

I do notice my tubes pins in the middle C socket are not a very tight fit when turning the amp over to do the voltage checks, loose enough that the tube is at risk of falling out if I'm not careful.

Cheers
Jamie

 

[JamieMcC]

Sorry forgot to update here but back up and running, voltage checks on both boards checked out all ok

Even though I replaced a resistor that was damaged when pulled for checking and reflowed some joints I have a sneaky suspicion the problem might have been much simpler and down to user error and not fully pushing home the 6922 tube into the socket. 

Thanks for the help over the holiday period, much appreciated.
Cheers
Jamie