Heathkit W-7M pair blowing fuses

[markloft]

Bought a pair of Heathkit W-7M tube amps 3 months ago, in all-original condition with NOS Rubys installed about 5 years ago. They've sounded amazing up until 2-3 weeks ago when one of them started blowing a fuse, the 2-amp slo-blo. Two weeks later the other amp started doing the same thing. Tried new tubes, also NOS Rubys, tested well... same thing, don't think it's the tubes.

My local shop is stumped. They don't want to start replacing parts just to guess what it might be. They've tested at reduced load on a variac with dummy loads, as well as connected the usual way, and they too say they sound fantastic, until a fuse blows. They admit they don't have a clue why both amps (seems unusual that both are doing the same thing) blow fuses. They keep hoping something will fail, then they'll know what to replace, but until then...

Any ideas/guesses/suggestions?

 

[stanko]

Electrolytic caps in the power supply are leaking. You may be able to figure out which ones by feeling for heat since the fuse doesn't blow right away, but they are all bad. They should not be heating up. They all need to be changed. Checking for correct negative bias is a good thing as well. If the negative bias is defective, the outputs will increase idle current, causing an increased load on the power supply.

 

[ramjet]

Bought a pair of Heathkit W-7M tube amps 3 months ago, in all-original condition with NOS Rubys installed about 5 years ago. They've sounded amazing up until 2-3 weeks ago when one of them started blowing a fuse, the 2-amp slo-blo. Two weeks later the other amp started doing the same thing. Tried new tubes, also NOS Rubys, tested well... same thing, don't think it's the tubes.

My local shop is stumped. They don't want to start replacing parts just to guess what it might be. They've tested at reduced load on a variac with dummy loads, as well as connected the usual way, and they too say they sound fantastic, until a fuse blows. They admit they don't have a clue why both amps (seems unusual that both are doing the same thing) blow fuses. They keep hoping something will fail, then they'll know what to replace, but until then...

Any ideas/guesses/suggestions?

If indeed they are in "all original condition" then all of the power supply electrolytic and coupling capacitors are long overdue for replacement. Also, the bias supply has a selenium diode that is also getting long in the tooth. That your local shop is stumped "hoping something will fail" is interesting considering fuses don't blow unless something has.

 

[markloft]

Electrolytic caps in the power supply are leaking. You may be able to figure out which ones by feeling for heat since the fuse doesn't blow right away, but they are all bad. They should not be heating up. They all need to be changed. Checking for correct negative bias is a good thing as well. If the negative bias is defective, the outputs will increase idle current, causing an increased load on the power supply.

Thanks so much! This stuff is way over my head, but I do love the sound these things make.

 

[cademan]

My local shop is stumped. They don't want to start replacing parts just to guess what it might be. They've tested at reduced load on a variac with dummy loads, as well as connected the usual way, and they too say they sound fantastic, until a fuse blows. They admit they don't have a clue why both amps (seems unusual that both are doing the same thing) blow fuses. They keep hoping something will fail, then they'll know what to replace, but until

C'mon, your local shop doesn't know how to work on stuff if they gave you this kind of reply. Find someone else.

Could be a short in an output tube, short in a filter capacitor, short in the rectifier tube or bias supply. That's all it could be.

 

[markloft]

C'mon, your local shop doesn't know how to work on stuff if they gave you this kind of reply. Find someone else.

Could be a short in an output tube, short in a filter capacitor, short in the rectifier tube or bias supply. That's all it could be.

You could be right about the shop, they're not tube specialists, though they've done great work on some of my other gear. Thanks for the input about the shorts.

 

[cademan]

IMHO, tube stuff is easier to work on and to diagnose than solid state. They should be able to figure it out if they threw a scope and DVM on them.

 

[egellings]

Check bias supply, check coupling caps to output tubes to be sure they are not leaking current into their control grids, thereby making them more positive in voltage. If self-biased, check cathode resistor on outputs and any 'lytic that may bypass them.

 

[Mr Boochie]

C'mon, your local shop doesn't know how to work on stuff if they gave you this kind of reply. Find someone else.

Could be a short in an output tube, short in a filter capacitor, short in the rectifier tube or bias supply. That's all it could be.

It's so hard to find a good tube tech, these days. That old-school knowledge is slowly dying.

 

[markloft]

Update: got both amps back from my shop, that some of you criticized. My guy cleaned everything well, biased the amps for the new (to me) NOS Rubys we installed, said he found something called an "HK short" and a couple of places that he re-soldered. He says he's a minimalist when it comes to repairing older tube stuff... "if it ain't broke, don't fix it." He said the caps weren't leaking, everything checked out well on the 'scope. He played them for a week before telling me they were ready. $65 diagnostic charge, that was it. I've had them home and playing beautifully for a couple of days, they sound better than ever. Sure, a re-cap/major restoration is out there somewhere, but not needed right now. I'll keep giving this guy my business.

 

[zackthedog]

Update: got both amps back from my shop, that some of you criticized. My guy cleaned everything well, biased the amps for the new (to me) NOS Rubys we installed, said he found something called an "HK short" and a couple of places that he re-soldered. He says he's a minimalist when it comes to repairing older tube stuff... "if it ain't broke, don't fix it." He said the caps weren't leaking, everything checked out well on the 'scope. He played them for a week before telling me they were ready. $65 diagnostic charge, that was it. I've had them home and playing beautifully for a couple of days, they sound better than ever. Sure, a re-cap/major restoration is out there somewhere, but not needed right now. I'll keep giving this guy my business.

"HK" probably means heater-to-cathode. Glad you got the running again for such a bargain.

 

[markloft]

Glad here too, mostly to have them back running so well, but certainly glad it didn't cost me much. Got a non-working "parts only" Heathkit SP-2 preamp last week to go with these. It powers on (with a Variac), no smoke, no excessive heat, and no sound. The tubes glow faintly when running at full voltage. Left it with my shop yesterday, hoping/guessing it's something minor. I got it for next-to-nothing, so even if I have to put a few bucks into, I think I'm going to enjoy it.

 

[tuberadios69]

Sure, a re-cap/major restoration is out there somewhere, but not needed right now

I'm not trying to be a jerk but if all the electrolytic caps in both amps are original, you are playing Russian Roulette right now. The W-7M's were last made in 1962 so your internal caps and resistors will be at least 60 years old this year if they are original! That is way beyond their expected life. One of them will fail; just when is anyone's guess. Are you really willing to risk destroying a power transformer or output transformer just to save some dough by avoiding way over due maintenance? Your tech says none of the caps are leaking but I'll bet they are weak and ready to pop at the most inopportune time. I'd at the very least replace any large multi-section caps and, the selenium rectifiers. Check to see (using your hand) how warm the external caps between the transformers are next time you run the amp. (don't grab the leads underneath, just the outside of the caps). They should barely be warm. If they are hot, they are going bad and will fail sooner than later.

 

[Palustris]

[QUOTE="markloft, post: 15542072, member: 208821" Sure, a re-cap/major restoration is out there somewhere, but not needed right now. [/QUOTE]

Macbeth:
If it were done when ’tis done, then ’twere well
it were done quickly.

 

[markloft]

I'm not trying to be a jerk but if all the electrolytic caps in both amps are original, you are playing Russian Roulette right now. The W-7M's were last made in 1962 so your internal caps and resistors will be at least 60 years old this year if they are original! That is way beyond their expected life. One of them will fail; just when is anyone's guess. Are you really willing to risk destroying a power transformer or output transformer just to save some dough by avoiding way over due maintenance? Your tech says none of the caps are leaking but I'll bet they are weak and ready to pop at the most inopportune time. I'd at the very least replace any large multi-section caps and, the selenium rectifiers. Check to see (using your hand) how warm the external caps between the transformers are next time you run the amp. (don't grab the leads underneath, just the outside of the caps). They should barely be warm. If they are hot, they are going bad and will fail sooner than later.

Seems like a valid point. I'm a complete newbie when it comes to maintaining/repairing tube gear. Lots to learn, I know. But I'm so new that I don't know enough to know which is the more valid argument here. Don't fix anything that's not broken vs. go ahead and do the work now before it costs even more and the right parts may have become even scarcer. I can appreciate a repair shop not wanting to spend my money needlessly. I guess that's the question, is the re-cap work needed right now, even if only to avoid a more expensive repair bill in the future. I'll see how warm the external caps are in the next day or so.

 

[ramjet]

It is not about fixing what's not broken or doing work now that will cost more later, it's about replacing parts that are well past their expected service life. Certain parts, when they fail (and they will) will (possibly/probably) destroy parts that cannot be replaced/repaired. You came here looking for "Any ideas/guesses/suggestions" and have been told what needs to be done to ensure that they will continue to work in the future.

 

[knockbill]

Yep, PS and coupling caps aren't "fix when broken" parts... They have a time limit on them,,, Kinda like leaving original oil in an old engine,,, When they fail it can be too late to change em...
I tack new caps in any amp I find, to make sure it will last thru bench testing... Cheap insurance, and easy enough to remove, if I'm not going to rebuild the amp...