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Looking for advice on repairing an 845 amp
Looking for advice on repairing an 845 amp
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Old 13th August 2021, 10:00 AM   #1
dch53 is offline dch53  Australia
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Default Looking for advice on repairing an 845 amp

Hi. Iíve been asked to fix a Voxativ 845 amp. The 845 is driven by a 300B via an interstage transformer.

Around the 845 bias potentiometer thereís a blown 22uF/450V capacitor and a discoloured 5W resistor. In the first picture below, Iíve indicated the blown capacitor. The discoloured resistor is under the red wire below and to the right of the capacitor.

I removed the capacitor and resistor. The capacitor is showing a resistance > 20M. The resistor is open circuit.

A check of the other matching monoblock indicates that the blown resistor is 5.1K. Its matching resistor is not discoloured at all.

Iíve included a picture of the whole amp to give an idea of the complexity of it.

I donít have the time or desire to try to reverse engineer a schematic for this amp and thereís no way the manufacturer is going to give me the schematic. Shipping it back to Germany from Australia for repair is not a great option.

Iíve researched similar amps and my best guess at the circuit around the pot is shown in the 3rd image. The 5.1K resistor R1 is the badly discoloured and open circuit one. The 22uF capacitor, C1 is the one with the bulging top that's leaked everywhere.

Weird pot value but thatís what I measured. Current should be around 3mA which should be no bother for the 5W resistor. In fact, it makes me wonder why thereís a 5W resistor in there. The pot is tiny (10mm in diameter) so it doesn't have a high power rating.

I could just assume the capacitor went short circuit and replace the resistor and capacitor but Iíd rather understand why the components failed in case there is some other problem.

Does the schematic look reasonable and are there any suggestions as to what might have caused the problem please?
Attached Images
File Type: jpg Dead cap.jpg (78.5 KB, 219 views)
File Type: jpg full amp.jpg (991.9 KB, 209 views)
File Type: png schematic.png (22.2 KB, 223 views)
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Old 13th August 2021, 12:00 PM   #2
Rod Coleman is offline Rod Coleman  United Kingdom
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Looking for advice on repairing an 845 amp
I do not believe a 450V cap blew up with -150V, just by itself. It's possible that the 845 tube failed with a grid-to-filament short. the short may have been transient (if the amp was knocked), or it may fail permanently. A short like this can mean very high voltage appearing on the grid and filament, since the bias vanishes with fil-to-grid shorts.

Fixing the cap in this case may just result in another blowup, and you also take risks with the OT. You can try testing the 845, or better, replace it.

Be sure that the bias voltage is a suitable value, once the R and C are replaced.The IT may have suffered damaged insulation in a blow-up, and should probably be tested too.
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Old 13th August 2021, 12:05 PM   #3
TMM is offline TMM  Australia
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I would guess grid shorted on the 845 or bias supply went positive and caused the cap to short which in turn took out the resistor. Possible that the cap just died randomly also.

If you have a variac it should be safe enough to replace the cap and resistor and turn things up slowly while monitoring the voltage of the bias supply and the voltage across the cap.

Otherwise best to trace out the rest of the circuit to get a better idea of any other culprits.
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Old 13th August 2021, 03:47 PM   #4
gideon1990 is offline gideon1990  Netherlands
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Rod maybe you know more about this; I have heard stories about 845 tubes getting damaged during shipping which may show up after using them for a while, I think it was also associated to some kind of "whispering" noise? Could that be related to this or is this most likely some physical knock/vibration like you already suggested?
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Old 14th August 2021, 02:43 AM   #5
dch53 is offline dch53  Australia
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Thanks all for your advice.

I measured the resistance from the pot to the grid of the 845 socket at 414Ω. If the pot feeds into the bottom of the interstage transformer maybe the transformer is OK.

I have a variac and spare 845. I'll order some of the resistors and capacitors and report back.

I'll test the original 845 separately.
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Old 14th August 2021, 04:31 AM   #6
daqvin_carter is offline daqvin_carter  United States
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Just a quick reminder that the 845 tube should be vertical when lit. Either upright or upside down. No horizontal use or anything in between.
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Old 14th August 2021, 04:55 AM   #7
dch53 is offline dch53  Australia
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Thanks very much for that! I was going to put the amp on it's side because the tube is taller than the transformers.

I hate working on amps on their side anyway.
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File Type: jpg IMG_1294.jpg (495.5 KB, 130 views)
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Old 14th August 2021, 05:01 AM   #8
Hearinspace is online now Hearinspace  Canada
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Looking for advice on repairing an 845 amp
Is that amp, this amp ?

If so could you ask Voxativ for schematic or at least relevant info.?

One might think that at that cost . . . . . . .
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Old 14th August 2021, 07:29 AM   #9
dch53 is offline dch53  Australia
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It is indeed! And no, the schematic is confidential. So much for right to repair!
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Old 14th August 2021, 09:13 AM   #10
Diabolical Artificer is offline Diabolical Artificer  United Kingdom
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It is indeed! And no, the schematic is confidential. So much for right to repair!
Annoying, but if you look at the underside of the amp, it looks pretty simple and should be easy to draw out. I would think the amp is generic and highly doubt they've done anything complex, it's not Audio Research complex. You'll probably laugh at how simple it is.

It'd be well worth drawing a schematic, it'll give you a far better insight as to how the amp works allowing you to do a better repair, it'll be a service to those techs who follow after you & it will also annoy the numpties at Voxitiv : )

One last thing, one would hope there are some protection circuits in this thing, those valves ain't cheap, not to mention the OPT's. Doesn't look like there is though.

Good luck, Andy.

Last edited by Diabolical Artificer; 14th August 2021 at 09:17 AM.
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