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79 Ampeg SVT Volume Drop

#82
No 4 ohm load on an SVT?

IF you know what you are doing, unloaded presents no problem for testing and troubleshooting. That said, 99% of folks probably shouldn’t be inside their amp anyway.
 
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#87
Before you reinstall the output tubes, retension the pins in the tube sockets.

A few years ago I had an amp that was fine on the bench. When I put it on a speaker, it started blowing fuses intermittently. The second time it blew a fuse there was a bright flash and a loud noise like a cannon shot. Tightened up the pins in the tube sockets with a dental pick, and it has been fine since.

Make sure and discharge the capacitors before attempting the procedure.
 
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#89
Finally had a day to spend with the SVT again. Blown resistors replaced and any others that were out of spec. I did discover a completely fried Hum Balance pot - replaced with a 100ohm and R53, the little 10ohm resistor to ground was fried as well - don't have a replacement handy.

Also cleaned up all wiring - much easier to see where things are going.

I also revisited the cap wiring per suggestions. The only difference between what you all posted is the Pin 2 wire from Pre-Amp we discussed. That is tied to C9A based on the previous cap job - still not a 100% on that one. Everything else follows diagrams.

I also noticed ground continuity to exterior of Cap Cans. I've not installed those before, is that expected? I do have the plastic insulators in place. Bolts are isolated.

Hum balance center connection is to C9 Ground. Left and Right runs to Heaters of V8.

Current limiter is ready to go. Hopefully picking up a Variac before I try and power it up.

IMG_0630.jpg

IMG_0621.jpg
 
#90
Yes the case of the twistlock caps are grounded. They are mounted on insulators so there is only one path to ground.

The ground flows from J1 on the back panel. A line goes from J1 to the Power PC card and also to the Molex

See added red line:
upload_2022-9-24_18-31-59.png


upload_2022-9-24_18-41-6.png


The case of the capacitors should be connected to the end of the red trace on the lower right corner.
 
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#91
Man, that soldering work needs some serious help.

I would also double-check that there are no more lifted pads and PCB damage. What's there is borderline hack job, especially around D5 and D6, R52, D18-20 and around D12. If there is an issue with PCB damage, that could also cause problems.
 
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#92
numberfourthree said:
I also revisited the cap wiring per suggestions. The only difference between what you all posted is the Pin 2 wire from Pre-Amp we discussed. That is tied to C9A based on the previous cap job - still not a 100% on that one. Everything else follows diagrams.
Click to expand...
I think you are okay with this. Check where it goes to the preamp.
Fliptops sells this part.
100 Ohm Hum Balance Pot, NOS OEM - Product Details
 
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#93
Cleaned up solder as best as I could as a number of those areas were a mess from previous 'repairs'. The traces are so delicate and there are a few repairs and a few spots of glueing them back down.

Used a Variac and the Limiter this time - no tubes - and slowly warmed up. No fuses blown but still seeing very high voltage on B - it's coming that way straight from Pre-Amp reads 549Vdc. Also reads the same on pre-amp terminal strip, so confusing. Perhaps R47 (8.2K 5W) there is bad? Reads ok cold.
 
#94
numberfourthree said:
Cleaned up solder as best as I could as a number of those areas were a mess from previous 'repairs'. The traces are so delicate and there are a few repairs and a few spots of glueing them back down.

Used a Variac and the Limiter this time - no tubes - and slowly warmed up. No fuses blown but still seeing very high voltage on B - it's coming that way straight from Pre-Amp reads 549Vdc. Also reads the same on pre-amp terminal strip, so confusing. Perhaps R47 (8.2K 5W) there is bad? Reads ok cold.
Click to expand...

Voltages will read higher because the power supply is not under load. With the tubes in, the readings will be lower.

Resistors can read differently if there’s an issue, hot vs cold.

Do not turn the amp on without the speaker cab or a dummy load connected.
 
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#96
numberfourthree said:
Thank you. Yes I’ll continue tests with tubes but the readings for 5 have been high all along, even with tubes.

As stated earlier always keeping it hooked up to 4 ohm load.
Click to expand...

The voltage references on the schematic are with a wall voltage of 120VAC. Use an AC voltmeter to see what your wall voltage is. If your mains is higher, the power supply voltage readings will be higher. The important thing is, are the DC readings that you take plus some headroom, within the voltage ratings of your power supply capacitors.
 
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#97
IMHO, Turn the variac down!

Part of the reason you use the variac is so you can feed the amp a low voltage (only enough to troubleshoot). By keeping the voltage low you reduce the risk of damage. If damage does occur, it's likely to be less extreme.

It's also important to keep the voltage low enough so you don't push any of the power supply capacitors over their voltage rating. AFAIK R47 and R48 are the only sources of shunt resistance in the positive power supply rails. So there should be no current flow at Node B and C. If no current is flowing between R49 and R50, the voltages at Node A, B, C should all be the same. This could mean you are cooking C12B with 562V. If this is not the case, then identify where the unexpected current is flowing...but turn the variac down first.

Without current flowing to the tubes, the taps will not have any significant sag. If you feed the amp 120V with no tubes, the voltage at Node A should rise. AFAIK 720VDC or higher would be fairly normal.

Getting correct operating voltages depends on the components being the proper value and the tubes pulling the correct amount of current. I previously mentioned that it seemed odd that your plate voltage was near the actual schematic value, while the voltage at node B was really high.

Also you indicated that the voltage at the junction between C10 and C12A was not centered. Is this still the case? If yes, either a component is bad or something is not connected properly. It's common to replace the balance resistors (R47 and R48) when you do a cap job.

If your volt meter is only rated to 600V a direct reading at node A is suspect. To get this reading with a 600V meter, measure from ground to C12A. Then move the ground lead to C12A and measure to C10. The two readings should be almost identical, and the total is the voltage at node A.
 
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#99
More testing and exploring on the endless haunted SVT project...

1st issue is V9 that flashed a few weeks ago is getting very hot, while v8-V4 stay cool. I have a spare Svetlana 6550 which solves the issue but then that tube stays cool while the rest of the 6550s get warm. Likely a short in the GE 6550, sadly. Will test tubes later this week.

Using Variac and Lightbulb Limiter, with all tubes installed, 150w bulb is bright at 120vac - though not full brightness. Removing power tubes dims the bulb somewhat but not fully. Adjusting bias increases brightness so bias circuit seems to be working ok. Putting amp in standby - bulb goes out - which seems to point to filter caps or tranny. Though I can see nothing amiss in filter cap section and have rechecked wiring.

Tested OT. Primaries show Blue to Blue / White = 56.5ohms, Red / White to Blue 28.5ohms looks normal aside from the low values. Anyone have resistance measurements for a 320825-1 transformer?
Secondaries run either .2 or .4ohm btwn Orange, Red, Brown or btwn Blue, Green, Yellow. No resistance (open) btwn Primary and Secondary and open to chassis ground.

Ran 6.3vac with the variac through the desoldered OT. Primary Blue + Red White to Blue White = 4.32vac, Blue + Blue White to Red White = .570vac
Secondaries: 6.60vac on Red, 6.58 Orange, 11.30 on Green, 13.76 on Blue
I'm not as familiar with testing OT voltages so if anyone a smoking gun pls. let me know.

I have noticed a grounding issue. The CE Can Caps ground lugs are connected to its case so using those lugs as ground bus creates continuity btwn the can exterior - possible ground loop to entire chassis and exterior of trannies. Perhaps this is the short the Limiter is seeing? Otherwise I've checked all wiring, traces, etc. Replaced the little blown 10ohm resistor from J2 to ground lug. I can't find anything odd on wiring aside from those can caps.

Ran a test signal through the amp (everything connected - all tubes and speaker). Initial signal seemed stronger and then got weaker over time. Got a nice full range of tone at startup and thought perhaps the amp was ok but plugged in a bass and not at all - anemic and weak. This is still with the 150w limiter in place, no bias, etc. Probing amp with signal found nothing, tubes sockets all quiet. Instruments and cords are all good. Speaker cab seems fine.

Really pulling my hair out on this one. I'll test all tubes again and if need be order a new sextet.

* Correction (6550 pins 2, 7 and 8 heaters and cathode all connect to ground as they should it seems, not pin 4).
 
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#100
If it was the filter caps with the symptoms that you describe, the filter cap(s) would be HOT to the touch after a few minutes of operation (prior to bursting).

The resistance values on the OT look high to me.
 
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