Ashdown ABM 500 EVO Issue

[Billy Penn]

Looking good, Billy. Go for the FETs. I still advise bringing up cautiously - no speaker. Very carefully check there are no solder blobs or anything wired other than it should be before restoring power. Don't forget to remove the link you put in for this test.

Ok, I will order the FETs and install. Should I power up using a variac & light bulb limiter? I really appreciate the help. I feel 100% confident about doing work like this I just don't have much troubleshooting experience with SS amps. So again, thank you!!

[nickb]

I would definitely use the variac while monitoring the current on the V+/V- rails (most important) , the voltage on the V+/V- rails and the voltage on the output rail (i.e. before the relay).
You will need to measure the current on each rail as it is now (with the link) to get a baseline - write it down - call them I- and I+.

It's probably a good idea to set the bias trimmer RV1 to zero ohms before you start in case the new FETs run hotter. If you're able to get to the full +/-90V without any sudden current increases over I+ and I- then you are ready to bias it up. You want to set the bias RV1 trimmer for about an increase of 300mA. Double check the output rail is still close to zero then power off, plug in the speaker and test. The bias is a bit of a guess since they didn'y put it on the schematic. You can back it off if while using while a lowish volume if it sounds clean until you hear distortion and then increase a tad. Or check for crossover distortion with a scope if you have one. In any case don't go over 450mA.

[Billy Penn]

I would definitely use the variac while monitoring the current on the V+/V- rails (most important) , the voltage on the V+/V- rails and the voltage on the output rail (i.e. before the relay).
You will need to measure the current on each rail as it is now (with the link) to get a baseline - write it down - call them I- and I+.

It's probably a good idea to set the bias trimmer RV1 to zero ohms before you start in case the new FETs run hotter. If you're able to get to the full +/-90V without any sudden current increases over I+ and I- then you are ready to bias it up. You want to set the bias RV1 trimmer for about an increase of 300mA. Double check the output rail is still close to zero then power off, plug in the speaker and test. The bias is a bit of a guess since they didn'y put it on the schematic. You can back it off if while using while a lowish volume if it sounds clean until you hear distortion and then increase a tad. Or check for crossover distortion with a scope if you have one. In any case don't go over 450mA.

I am not 100% sure how to measure the current draw on the V+/V- rails. I would put one lead of my meter on the B+ from the rectifier and the other on a plate of a tube amp (shunt). Is it the same on this SS amp?

[52 Bill]

I am not 100% sure how to measure the current draw on the V+/V- rails. I would put one lead of my meter on the B+ from the rectifier and the other on a plate of a tube amp (shunt). Is it the same on this SS amp?

Are you kidding us here?

[nickb]

No, you cannot do anything like that. You are going to have to figure out how to break into the V+/- supply lines. Set your meter on a high range, probably 10A initially, just in case. If you haven't got two meters just do the V+ line.

[Billy Penn]

No, you cannot do anything like that. You are going to have to figure out how to break into the V+/- supply lines. Set your meter on a high range, probably 10A initially, just in case. If you haven't got two meters just do the V+ line.

Right. Obviously the meter has to be in series on the V+/- lines. I just need to figure where to break/insert. Thanks againyou have been very generous with your time. I appreciate it a lot!

[Billy Penn]

I installed the new FET's. AC cord from the amp to the light bulb limiter. Light bulb limiter into the variac. I brought up the voltage very slowly. Got to about 40% and had about 18vdc on the + rail. I noticed the large heat sink that the FET's mount to getting warm so I immediately shut it down. I'm not comfortable proceeding. Any thoughts on this? Am I not supposed to use both variac & limiter? Thanks!

[nickb]

Top marks for being cautious. The limiter is probably overkill. I would expect the FETS to dissipate around 30 watts by the time the voltage rails are all the way up so they will get warm. Are you monitoring the current on V+ and V- rails? It's essential to know that. What was the reading before you shut down? Did you turn the bias trimmer RV1 to zero resistance -also important?

[Billy Penn]

Top marks for being cautious. The limiter is probably overkill. I would expect the FETS to dissipate around 30 watts by the time the voltage rails are all the way up so they will get warm. Are you monitoring the current on V+ and V- rails? It's essential to know that. What was the reading before you shut down? Did you turn the bias trimmer RV1 to zero resistance -also important?

I did turn the bias trimmer down. I didn't measure the current because I cannot find a place to "break" either rail. The traces go directly from DR1 to the FET's. There is no break at all. I know this may sound ignorant of me but I do not know how or where to break the rail(s) to measure. I had the variac up to about 40% and the heat sink was getting warm and I thought there was a slight odor. I know sometimes new components when first powering up may have an odor but between the heat, slight odor and my lack of experience with SS amps I decided to bail and shut it down. I just didn't have a good gut feeling on this. Got any thoughts or feelings based on what I'm saying here?

[Jazz P Bass]

A warm heat sink at 40% power is Not good.

[nickb]

With the amp off I'd like you to measure the resistance between pin 1 of TR4 and pin 1 of TR1 it should be very close to zero. If not turn RV1 the other way and recheck the resistance.

You absolutely need to know the current on at least one rail. Do what you have to do to break the path. I hope you didn't skip a step where we wrote the current down before the FETS went in. We need to know that too.

[deveng]

Well, here's a trick I use sometimes. You can remove the output transistors and link the feedback path, check for <0.1V on the output and check the power rails, no speaker. This will tell you if the rest of the amp is OK for a DC point of view before you pop in the replacements. See attachment on how to do this.[ATTACH=CONFIG]21894[/ATTACH]

Can someone repost the attachment showing the feedback connection described above

[nickb]

Can someone repost the attachment showing the feedback connection described above

Here you go. Remember - no FETS and & no load for this to work. It just allows you to check out the low power side of things.

[deveng]

Nick,

Thanks for the quick response. I jumpered the feedback loop and with no transistors, no load and no input measured 1.4mV at the D3/Z1 junction. I then installed 2 known good MosFETs for TR1 and TR4. Attached 8 ohm dummy load and scoped the output. Looks like it works. Sine in Sine out clean. Don't have a variac so I may install pairs and monitor current.

Regards,
Jeff