Hafler P500 help

[colebb]

Hello all!

This is my first post on this forum after a lot of lurking; please let me know if I should make any amendments to the post.

I recently came into a Hafler P500 amp which the seller stated had worked fine last he used it a few years ago. Before connecting it to any speakers I did notice a DC offset on the right channel varying between 10-500mv so I only tested the left channel with a signal. I've since also tested the right channel after the DC offset chilled out a bit (after 10-15 minutes of run time, DC offset remained roughly below 100mv, though I still only used cheap speakers to test). The P500 does appear to have a trim pot for DC offset but it's my inclination to avoid messing with that and instead to identify the cause of the DC offset.

Both channels input sound at full volume regardless of potentiometer position. I'm hoping that this might be solved by some contact cleaner inside of the pots but I have not yet found any information online regarding safe removal of the volume knobs on the P500.

It also appears that the amp was modified at some point to only allow the RCA inputs to work; several of the wires connecting the XLR and 1/4" inputs appear to be cut. Not sure if that info is really relevant but sharing just in case.

Does anyone have any advice or input as to where to start? It seems like the amp has two issues: whatever is causing the DC offset and the volume pots not doing anything.

I know there are kits available for refurbing these units and I am considering getting the barebones kit from fantasia but if it seems like my issues wouldn't be solved by that, I probably won't prioritize it (though likely will go through with it eventually).

Thanks in advance!

P.S. I haven't taken any photos yet but am happy to if it'd be at all helpful. based on other photos I've seen online, all major internal components do appear to be original.

 

[whoaru99]

I'd use the adjustment to adjusting it first. That's why it's there. Maybe some other KTB was in there diddling around with it.

If it cannot be brought into spec with the adjustment for that purpose, then consider digging deeper.

 

[colebb]

I'd use the adjustment to adjusting it first. That's why it's there. Maybe some other KTB was in there diddling around with it.

If it cannot be brought into spec with the adjustment for that purpose, then consider digging deeper.

amazing, thanks very much for your reply! I'll give the trim pot a try...

any suggestions as to the volume knob issue? for reference, the knobs do not seem to have any hex screws in the sides of them but they also seem very resistant to just being pulled off so I'm a little scared of breaking something. they do have what appear to be threaded holes in the sides of the knobs but they don't appear to have anything inside of them that a hex wrench could turn.

 

[whoaru99]

I have no idea how to remove the knobs and or pots.

However, given you say it appears to be modified, I'd guess the level controls are simply bypassed or wired/jumpered so they provide no adjustment/no attenuation.

 

[colebb]

I have no idea how to remove the knobs and or pots.

However, given you say it appears to be modified, I'd guess the level controls are simply bypassed or wired/jumpered so they provide no adjustment/no attenuation.

oh yeah good point. that seems like a pretty good guess.

thanks again! I'll post any interesting updates that arise.

 

[whoaru99]

Don't know how much experience in poking around inside amps you have.

But, if you haven't seen cautions and good intentions gone bad, be aware that one tiny slip with a test probe, screwdriver, etc., often results in a bad, if not catastrophic, outcome.

 

[avionic]

Welcome aboard Cole.

 

[Dave1027]

I suggest NOT purchasing a "refurb kit" and instead repair the unit first. Those kits may not have what that amp needs.

I do not think the problem with the GAIN knobs is something that can be cleaned out. Since you know the amp has been modified, it's likely somebody has bypassed those. Many amps do not have gain pots and always run wide open normally. You adjust the volume with the preamp and sources.

 

[colebb]

Don't know how much experience in poking around inside amps you have.

But, if you haven't seen cautions and good intentions gone bad, be aware that one tiny slip with a test probe, screwdriver, etc., often results in a bad, if not catastrophic, outcome.

good call! I don't have a ton of experiencing poking around amps but I do know enough to be very cautious with everything on the inside.

 

[colebb]

Welcome aboard Cole.

thanks! glad to be here

 

[colebb]

I suggest NOT purchasing a "refurb kit" and instead repair the unit first. Those kits may not have what that amp needs.

I do not think the problem with the GAIN knobs is something that can be cleaned out. Since you know the amp has been modified, it's likely somebody has bypassed those. Many amps do not have gain pots and always run wide open normally. You adjust the volume with the preamp and sources.

thank you for the advice! I think you're probably right about the volume knobs and I'll take your advice about those refurb kits too

 

[colebb]

I'd use the adjustment to adjusting it first. That's why it's there. Maybe some other KTB was in there diddling around with it.

If it cannot be brought into spec with the adjustment for that purpose, then consider digging deeper.

an update: I was able to adjust the DC offset to the point that the range of values at least roughly centered around 0 but still wildly fluctuates from -0.5v to 0.5v which still seems to indicate a problem somewhere. I'm hoping it's something as simple as a needing a few fresh capacitors and perhaps a resistor or two, but I'm not exactly sure where to begin with the diagnostic

 

[ivandezande]

Hello all!

This is my first post on this forum after a lot of lurking; please let me know if I should make any amendments to the post.

I recently came into a Hafler P500 amp which the seller stated had worked fine last he used it a few years ago. Before connecting it to any speakers I did notice a DC offset on the right channel varying between 10-500mv so I only tested the left channel with a signal. I've since also tested the right channel after the DC offset chilled out a bit (after 10-15 minutes of run time, DC offset remained roughly below 100mv, though I still only used cheap speakers to test). The P500 does appear to have a trim pot for DC offset but it's my inclination to avoid messing with that and instead to identify the cause of the DC offset.

Both channels input sound at full volume regardless of potentiometer position. I'm hoping that this might be solved by some contact cleaner inside of the pots but I have not yet found any information online regarding safe removal of the volume knobs on the P500.

It also appears that the amp was modified at some point to only allow the RCA inputs to work; several of the wires connecting the XLR and 1/4" inputs appear to be cut. Not sure if that info is really relevant but sharing just in case.

Does anyone have any advice or input as to where to start? It seems like the amp has two issues: whatever is causing the DC offset and the volume pots not doing anything.

I know there are kits available for refurbing these units and I am considering getting the barebones kit from fantasia but if it seems like my issues wouldn't be solved by that, I probably won't prioritize it (though likely will go through with it eventually).

Thanks in advance!

P.S. I haven't taken any photos yet but am happy to if it'd be at all helpful. based on other photos I've seen online, all major internal components do appear to be original.

The small signal transistors on all DH-500’s and 200’s go leaky, and cause bad DC offset. You need to replace them all. They are still made.

 

[whoaru99]

I'm hoping it's something as simple as a needing a few fresh capacitors and perhaps a resistor or two, but I'm not exactly sure where to begin with the diagnostic

I'd be inclined to consider any specific suggestions you get rather than generic/shotgun recapping. Recapping, while perhaps beneficial in the big picture, usually doesn't fix technical problems and if done before actually fixing the problem can result in more questions than answers.

 

[loudnoises]

The p500 did not come with rca jacks. You should post up pictures of your interior in case there are any other mods to the wiring.

Well documented amps. Don't just throw parts at em.

 

[ivandezande]

Yeah to be clear, it's not the caps. There's not many in there. Shoudl they be replaced, sure, but it's transistors causing your issue.

 

[colebb]

The p500 did not come with rca jacks. You should post up pictures of your interior in case there are any other mods to the wiring.

Well documented amps. Don't just throw parts at em.

oh yes you make a great point, thank you! here are some photos. happy to take more if that's helpful.

so far I've only removed the top and side panels and the right channel board isn't super easy to get an angle on but I included a well lit photo of the left channel board.