Hello, my 410 HD-130 started shocking me through the mic and then blew a fuse. Nearly all the filter caps failed ERS tests. So, I recapped all the electrolytic caps through out the whole amp, replaced the pair of 1KV 3A diodes (D9 & D10) on the rectifier board. The diode change because someone had been replaced one of them with two smaller diodes in series. I replaced the brown drops on the driver board as well. Lastly, I isolated the death cap from the circuit. Tubes all tested good but I swapped the EL34s with a known to be good matched quad set. It still shocks but doesn't pop the fuse.
The amp draws almost 3 amps on high power with standby switch closed (out of stand by). Through a current limiting 250 watt light bulb it glows like a 25 watt bulb. I'm trying to cure the low voltage and what feels like a voltage leak. FWIW the fuse doesn't blow anymore.
Test Points read as follows:
H = 568 VDC
G = 281 VDC
F = 266 VDC
A & B = +43VDC / -43VDC
Y = 0.460V max I can get (biggest amp draw at this setting)
at minimum setting it's cold and current on main AC drops to .5A. The 10 ohm 1 watt resistors on EL34 pin 8 spec out perfect. It's amazing how the bias controls the overall amp's current draw.
The PT failed a ring test with all secondary leads disconnected. With all the secondaries disconnected I could only get to 281VAC on the red lines, which explains the low test point H. So I replaced it with a NOS, original type. Mine is the early style with the stand by switch on the back panel and connected to the PT secondary side. So I didn't have to rewire the main power switch witrh the later PT styles. I don't play it on the low setting so I never cared about the LO setting tone problem. I still have the same issue of low voltage and heavy current draw when pin 8 is set near 460mV or more.
I tried jumpering the power switch but that didn't reveal a switch problem. The voltages are still too low but I'm not sure where to go next. Does anyone have any advice? Thanks
Static Test
Hi Rpbale, I'm assuming your amp is the early type with the tube (12AX7) phase Inverter. Have you tried "unloading" the power supply by removing all of the tubes and then re-measuring those test points (H, G, F, A & C). Also, it would be useful to verify those voltages change when you switch from Hi Mode to Lo mode. Feel free to record your measurements and let us know what you find. -mgriffin
Full voltage when unloaded
Thanks for your help. Yes mine is the early one with the tube driver.
The unloaded specs for Hi/Lo are:
A = +45/31v
B = -31.6/21.6v
C = -45/31v
F = 355/241v
G = 355/241v
H = 710/483v
Filament = 6.9/4.8VAC
Interesting that F & G are the same unloaded. With all these measurements well within spec I'd think it's something with components on the power sockets or the tubes or the OT. The 10 ohm 1wt check out but I haven't checked the 5KV protection diodes on the EL34 sockets. Should I put the tubes in one at a time and see what circuit kills the voltages?
Good to know.
I agree that something in the final stage is pulling too hard on the power supply. You're on the right track. Suggest you do a thorough visual and dmm component check around the tube sockets before putting the tubes back in. Fix any cooked resistors and diodes. Look for tracking in the sockets. If all the components and sockets look good, try putting in 2 tubes at a time so the push-pull on the OT stays balanced. The "shock on a mic" you mentioned is a concern. Maybe there is a loose ground to chassis connection or a wiggly solder joint. Clean and tighten every nut and bolt and jack. After you've done all that, hopefully Lars or mm210 will chime in with their years of experience. -mgriffin
410-HD130 short not fixed yet
So far, I've replaced the two .001uf 5KV at C39 & C40, and has two 50pf 1kv ceramic caps on pin 4 to ground. These 50 pf's are not on my early 12AX7 driver 2275-130 schematic but rather are on the later LM1458 driver schematic. I didn't see these on a service bulletin. I suspect these were added for noise or interference problems. I'd love to know the history on why these were added to the later circuit.
There's no change to the problem so far. Voltages are high +700v without the EL34's and still drop down to the sub 600v with the tubes in. I added the power tubes in as a pair and the voltage drops signficantly regardless of which pair of tubes are in place. Then I measured with all 4 back in place and the problem is still there.
I lifted the 50pf just to see what would happen but knowing this is DC I didn't expect a change and there wasn't. And the unit is drawing 1.75 amps on the mains with all 4 tubes in and point Y is maxed at 0.38 volts. Still way under the 0.5v target spec. That's the highest I can set the bias. I don't know if this indicates anything, but if I lower the 6CA7 bias trim pot so that point Y is closer to 0.1v then the current on the mains drops to about a 1/2 amp. Obviously way too cold of a bias. But does that indicate where the problem is?
The remaining components to replace are on order, even though they meter correctly. They are the two MR250-5 5KV diodes, the two 10 ohm 1W resistors (R56 & R61) and the two 1.5K 1/2 W resistors R57 & R62. I suppose the EL34 sockets too. But they physically look fine. I'll leave those until the end. The .047uf 600V cap C41 has been replaced that goes between the OT secondary and ground. Should I try the circuit with the MR250-5 diodes lifted just for a test? The are only OT flyback protection diodes.
The one thing I haven't done is to check it with the driver tube out. I didn't do that because the driver tube is new. I'll check that today.
Thanks for the input!
Rick
Yes pull the PI tube
Hi Rick, We're still quite puzzled and looking for clues. For Data's sake, go ahead and pull the PI tube and measure/report the Voltages with the Power Tubes installed. Also check the Bias supply components real good. The voltage at point B should be a little negative of the voltage at point Y. Are the 2 16V Zeners in the bias supply working properly? The Ref Designators are too blurry to read. Are the 2 150uF Caps leaky? And yes, the 50pf caps are for noise reduction. Hang in there. -mgriffin
Bleeder resistors
Hi Rick,
have you checked the bleeder resistors 150k/2W on the rectifier board? Does the amp draw any significant mains current with all tubes out? Is your bias voltage at pin 5 on each of the EL34 sockets the same as point B on the circuit board?
281VAC on the red lines on the PT sounds about right if you ask me. What does your replacement unit show?
Cheers,
Lars Verholt
This is good input. I'll
This is good input. I'll measure pin 5 vs B and B vs Y out tonight. The mains current draw too. My recollection was it was about ~250mA or less without tubes and 1500-1750mA with tubes I'm pretty confident the rectifier board is clean as I have replaced it's caps and diodes. Not resistors though because they spec'd out okay. The replacement Power Trans was slightly higher voltages with the original with the PT in and out. Even the bleeders spec'd out okay. Again thanks for the help.
Updated voltages
Imv, the 150k/2wt bleeders are both exactly 170k. A little out of spec. At 110% they should be no more than 165k. Doesn't seem like that would cause the volt drop with the tubes in though. The unit does draw about 450mA on the mains w/o the driver and power tubes and 1750mA with the tubes. Point B is -30.5v and pin 5 is -30.5v. The 281v was on the point G with the tubes. Spec is 360v.
mgriffin155, the 150uf caps in the bias area are new and the bias v is -/+43v without tubes and -/+32v with tubes. With tubes pin 5 is -22.7v, w/ tube -30.5v. The bias adjusts but not all the way up to spec at 0.5v. Highest is about .419v and that makes it draw the full 1750mA on the mains. If I drop the bias back to .3v then the mains drop to about 500mA. But that not the right spec. Without the inverter tube the voltage on the power tube pin 3 and 4 are still low. No change with or without the inverter in.
I think I may have the zeners in stock and can try another pair. Point B is -31.9v with tubes. Point Y is pin 8 and is .419v with tubes, zero v w/o of course. So B is quite neg compared to Y. Is that what you meant?
Thanks to both of you for all the help. I feel like the problem is so close to being found.
Ground and Neutral
Hi Rick, We've been through the obvious stuff. Time to think outside the box. You mentioned earlier about getting a small shock with a mic plugged in. For interest's sake, would you mind checking to see if there is any voltage between Power Supply DC Gnd and Chassis Gnd? Also, same request for Neutral and Gnd at the Power Cord. We're looking to see if voltage happens with respect to current drain going up. -mgriffin
Will do
I think check that at the very beginning. But let me double check it to be sure.
25V AC on Neutral
I'm showing 25VAC on the neutral line of the power cord to chassis. Zero volts on power supply ground to chassis. I took the PS ground from the ground side of the filter caps. Is the the right place? Incidentally, the death cap has been lifted since the beginning.
Additional info
The 25VAC on the neutral was via an isolated power supply. Measured again while connected directly to mains showed about 500mVAC on the neutral to ground.
Bias Supply ?
Hi Rick, As you have verified, bias point B is negative relative to point Y as it should be. You had me worried with 25VAC on the Neutral. 500mVAC is much more realistic and 0V between DC Gnd and Chassis Gnd is good. That being said, I'm as stumped as you. You mentioned that the bias supply lines A and C (+/-46V) fall to +/-32V with the power tubes installed. That seems like an excessive voltage drop to me but perhaps Lars and/or Mike Kaus have a better knowledge and experience with the old school MM amps. -mgriffin
The Mystery Continues
I'll hone in on the bias supply and see if I can find anything in their. It's drawing a full amp on the mains in standby. That does not seem right and might indicate the problem is not in the HV section. Tubes out and it drops to 0.5 amps. Seems like it's on the heater circuit? Would the 5 heaters draw a full amp? Have to look that up.
I rechecked the VAC from Chassis Ground to AC Neutral and I definitely have 25VAC there, but only when on the isolated power supply. Why? Is it because the ground is isolated from earth ground. It's 0VAC when connected directly to mains. I can't imagine it's because lifted the C-49 death cap.
I also recheck the driver tube's current draw.
Heater current and GFI test
Assuming "warmed up tubes", EL34/6CA7 heater current is around 1.5A @ 6.3V or about 9.5 watts per tube. Thirsty little buggers. 4 tubes x 9.5 watts is 38 watts. 12AX7 heater current is 300mA @ 6.3V for about 2 watts. That adds up to about 40 watts total for the 5 heaters and would end up being about .333 amps / (PT conversion efficiency) at the Mains. Long story short, the 5 tubes should consume about .5A from the Mains. While on the quest to find phantom current paths, has the amp ever been plugged into a GFI outlet? If not, give it a try. If there is a current mismatch between hot and neutral, it should trip the GFI breaker. -mgriffin
Think load limiter was confusing things - RESOLVED
The differences between the measurements when on the isolated power supply vs. the earth ground mains have been eating at me too. One thing I might not have been clear about is the whole time it's been on my B&K isolated supply plus I've had in series on the hot lead a 250 bulb watt load limiter. So, rather than just remeasure the neutral to chassis on the direct mains I remeasured all the test points.
Of course I didn't think of doing this until after I replaced some more parts that arrived yesterday. I switched out the following:
On the main board: both zener 16w 1w and it's 620 2w resistors (it's 150uf 50v lytics were already done earlier)
At the EL34 sockets: four 1.5k 1/2w and two 10 1w resistors, and the 5KV flyback diodes (the .001 2KV caps were done earlier)
The only thing that stood out while replacing these last night was one of the 5KV flyback diodes fell apart without any particular pressure. I thought that was very interesting. It could have been leaking at higher voltage and been the cause of not being able to maintain the 700+ volts under load. In hindsight, I could have checked the voltages with it just lifted for test purposes.
This morning, while on the direct earth grounded mains, the loaded test points at 130 watt setting, measured as follows:
test (factory spec)
A = +45v (+46v)
B = not remeasured but (Y is well in spec now)
C = -45v (-46v)
F = 321v (340v)
G = 347v (360v)
H = 701v (725v)
Y = 0.5v (0.5v) I'm easily able to get a 1/2 volt with adjustment room to spare
I'm happy with all these results and the amp sounds good. The amp has updated lytics throughout. While the flyback diode might have contributed significantly, I think the load limiter was confusing things. I'll only use the load limiter initially when dealing with an unknown amp and stop using it once I know there's no lethal shorts. This amp originally started out with blowing the fuse which lead me to using the load limiter. I'm clearly a bit of a nube on all this.
I'd be curious if any of you feel like I missed something or have a different conclusion.
Thank you again for all the help.
Sweet
Hi Rick, Congratulations. Sometimes, the end justifies the means. Tenacity pays off and fhe voltages look great. Rock on. -mgriffin