Traynor YBA-1 open power rail resistor

[Diverted]

Hi all, I was given a Traynor amp to look at that was DOA. Owner had bought it (history unknown) and played it for about an hour or two. Loved it ... then it went silent. Nothing.
Opened it up, checked transformers etc, no obvious issues with caps or resistors I could see. So I powered it up at about 60vdc just to put some juice through it. Discovered straight off that V1 had no plate voltage, but first few nodes did. So I started checking resistors in the B+ line and discovered the 10K (R29) between nodes C and D open. I replaced the resistor and the amp is playing.

My question is about finding the cause of the resistor opening. It showed scorch marks underneath after I pulled it, so it saw some excessive current. Am I right to assume that there is a fault downstream of that resistor causing the high current draw that fried the resistor?

The amp's all original, never been touched. Original filter caps etc etc etc.

Thanks!

[acheld]

I'm not real familiar with Traynor as a manufacturer, but wouldn't it be possible that the resistor was underspec'd?

[sluckey]

Am I right to assume that there is a fault downstream of that resistor causing the high current draw that fried the resistor?

Most likely suspect is C15.

[Diverted]

I'm not real familiar with Traynor as a manufacturer, but wouldn't it be possible that the resistor was underspec'd?

It looked to be a 1/2 or 1w resistor. Definitely small.

[sluckey]

Some schematics show 1 watt and that should be fine for the small load. I would replace with a 10K/3W MO resistor.

    https://el34world.com/charts/Schematics/files/Traynor/Traynor_yba_1_yba_4_bass_master.pdf

[tubeswell]

^what Steve said in Reply#2^


(a shorted C15 will make the whole B+ drop across the supply resistor causing excessive smokiness)

[2deaf]

Can you post the whole schematic?

[Diverted]

Sure, here's the full schematic. Not the best resolution but good enough.

[thetragichero]

there were a few variations on the same theme, but you should get the general idea: https://www.thetubestore.com/lib/thetubestore/schematics/Traynor/Traynor-Bassmaster-YBA1-Schematic.pdf
look familiar? certainly not too complexi

[2deaf]

Sure, here's the full schematic. Not the best resolution but good enough.

Thank you for your prompt attention.

Can you post the rest of the schematic that you posted a piece of in your original post?  Also, I would love to see some gut shots of this amp.  I've never seen that particular variation in the flesh.

[Diverted]

Here you go 2deaf. Photo size limitations on this site so I posted high-res shots here. Click on them to enlarge.

https://imgur.com/a/tTh7HSM

[dwinstonwood]

A question: should those little tan rectifier diodes be replaced, too? I'm just curious whether diodes go bad over time.

[sluckey]

Those diodes should be fine. I've seen a lot of those in the military and FAA. Silicon diodes don't just fade from good to bad. It's like a switch... one moment they're good, next moment they're bad. No half-bad weak inbetweeners. Same goes for silicon transistors.

[2deaf]

Here you go 2deaf.

Thank you very much.

[2deaf]

I drew up a schematic looking at your pictures.  If you see any discrepancies, I would appreciate your pointing them out.