Ampeg SVT-II no pro no go

[nevetslab]

The SVT II I have here has 220 ohm screen resistors, and no diodes that used to be there with the 22 ohm value. Usually we see screen resistors in the range of 470 ohm to 2.2k on power tubes. Orange uses 1K on their 6550's in the AD-200 bass amp, for instance. And the maintenance note I included only stated it was to extend the life of the power tubes (by reducing the screen current).

We could compare the open circuit secondary voltage on that 5-leaded winding that sources both the bias supply and the screen supply. Your plate voltage is in agreement with the stated schematic voltage for the plate supply (660VDC), so all that fits.

Did you measure the screen supply voltage at the power tugbes or at the filter cap (which also feeds the driver tubes plate voltage?

Where do you source parts over there? Farnell? Nichicon, United Chemicon, Cornell-Dublier are typical sources, all being 10mm L/S snap-in low-proile caps. I probably have the data sheets in my database for the Nichicon & United Chemicon parts, having replaced some in the past.

I just looked at the schematic you're using. Does that match your amp? I see a voltage doubler circuit in use (C8 & D13), followed by D11 & C9 to form a half-wave rectifier circuit for the screen supply. I haven't lifted out the motherboard in my chassis, so I don't know if this one here is the same as yours. So, you have one additional cap C8 that could be suspect.

[Enzo]

We see screen resistors of 470-2200 ohms, yes, but they are on screens that sit at essentially the plate voltage. These 220 ohm resistors are for screens that sit at half the plate voltage or less.

[nevetslab]

We see screen resistors of 470-2200 ohms, yes, but they are on screens that sit at essentially the plate voltage. These 220 ohm resistors are for screens that sit at half the plate voltage or less.

The Orange AD200 has 375VDC screen supply along with the 660V plate supply, using the 1k/5W screen resistors

[Enzo]

I am not the theory guy, but I don't recall the SVTs burning up 220 ohm resistors. What are the odds someone putting the Orange together chose those values based on tradition?

Though I didn't phrase it as such, my post #47 was more of a question than a statement.

[nevetslab]

Nor am I the theory guy. And of course you're absolutely correct, where that range of screen resistors are from the plate potential and not from a lower supply as in the SVT & the Orange. I've seen a number of screen resistors in SVT's burn up, both when they were 22 ohms as well as 220 ohms, even though the protection circuit acts pretty quickly to turn off the HT pwr xfmr. Sorry about the previous reply...no offense intended.

Any thoughts on if the screen supply being much lower in shzmm's SVT II than the working amp I have in the shop? 380VDC vs 308VDC. I've never been down this road, so mostly, like the rest of us, looking for clues.

[Enzo]

No offense perceived, no problem, I was just thinking out loud so to speak.

Yes, any shorted tube can burn up whatever resistor is there.

I guess if my screens node was way low, I'd track it down through the power supply. My prints have voltages everywhere, so we can determine if we started out with too low to start with, or if a later node was being loaded. And certainly look for ripple.

[christarak]

[QUOTE=Chuck H;479968I hate telling a customer that something is above my skill set. I'm more likely to call them and tell them I need more time if they're ok with that. Then I charge for the repair (or finishing task) based on what time it likely would have taken someone with specific experience to handle. Seems fair and I'm never sorry for the experience.[/QUOTE]
I am having a similar hair-pulling frustration with a JVM410 at the moment. I have spent way more time on it than I should, at the cost of other repairs. When do you throw in the towel?

[Chuck H]

I am having a similar hair-pulling frustration with a JVM410 at the moment. I have spent way more time on it than I should, at the cost of other repairs. When do you throw in the towel?

Maybe after lifting the coupling caps at the PI end and retesting without improvement

[Enzo]

I don't tell them it is above my skill set. Frankly it probably isn't. But the answer eludes me this time. I tell them I am unable to solve the puzzle. I also usually continue by telling them that doesn't mean it is unrepairable, just that I can't seem to do it. I tell them another tech might spot the problem quickly.

We have all managed to miss simple solutions to problems, just as we all have/will find answers to very difficult problems in a short time. No one bats 100%, do not be ashamed when you miss.

In certain circles, I am a guy techs turn to when they get stuck. And it often is the case that what I find is simple, an oversight, a mistake, a faulty assumption. I am sure someone who followed up on ones I couldn't solve many times found simple solutions I somehow could not see.

[christarak]

Maybe after lifting the coupling caps at the PI end and retesting without improvement

Ha! Well said Chuck. It'll my first check when I get back home on Thursday. Thanks mate ... and Jazz P ... and Enzo