Can't get much help on other forums

[rockpassion]

Doc, I am wondering if I can get some help and questions answered on this forum regarding two old IPC AM-1026 theater amps I am trying to get up and running.  I have tried a number of other forums but I am not getting any response that are really helpful.  I have all the original schematics for this and I feel I can learn a lot by trying to get these things up and running. 

I know it is not a Bottlehead product but I do connect into a BeePre preamp.  So there is a little connection.

Thanks

Richard

 

[Doc B]

We will do what we can. Basically you do it like you troubleshoot a kit. Measure any voltages at points where they might be listed on the schematic, be careful to watch for parts overheating,etc., and shut the thing down immediately if you think it might be shorting somewhere. If you have a variac to bring the voltage up slowly so much the better. If any voltages are off let us know where it is in the schematic and we can take a guess at what might be going on. In general if the amp has electrolytic caps they are the number one suspect with the other caps being number two. But there can also be open resistors, open inductors, etc.

Pictures of the underside will probably be helpful too.

 

[rockpassion]

Thanks Doc.  I do have a Variac and will bring it up very slowly.  I has been about 15 years since it was last turned on, I have or will have replaced almost all the capacitors including the one electrolytic per amp.  I hope to begin the turn on process tomorrow.  I will start my questions then.

I love this forum!!!

Thanks,

Richard

 

[Doc B]

I can't say why, but my experience with old gear has been that you need to start watching closely when the variac hits about 90VAC. That seems to be about the minimum to get the heaters working and the amp will start to come to life. Holding the V there for a while before you go higher might be helpful.

 

[rockpassion]

Thanks Doc.  I got it to 70 vac and the OD3 regulators flashed.  I have a small 8 ohm speaker hooked up to it and it begin to buzz. I also have my old FPII hooked up to it so  I am not sure but could it be a ground loop problem. 

One issue I had when hooking everything up was, do I use the System Ground or the Channel Ground to hookup to the speaker.  The System Ground appeared to be coming off the 0 terminal on the Output Transformer. 

I have turned the Variac up to 80 vac and my OD3 are glowing along with all the other tubes.  The buzz has quieted down a bit but is still there.  I am increasing the Voltage by 10 volts every hour.  When I hit 120 volts I will keep it there for about and hour.  Will that be enough.

Other than the buzz so far so good. 

Richard

 

[Grainger49]

When working on older equipment I take an old computer IEC power cable and cut off the equipment end.  First you need to determine the minimum voltage to ground impressed by the transformer.  I wire nut the hot and neutral, measure from neutral to chassis, swap hot and neutral and do the same measurement.  This lowers the ground current. 

Then ground the booger and work safely.

 

[rockpassion]

I turned on the amp using a Variac.  I hit about 60 - 70 VAC and got a really, really loud hum.  I first used the System Ground as my negative lead into my speaker.  I changed the lead for the negative to the Chassis Ground and the really loud hum became a small high frequency squeal when I hit about 80 VAC.  I traced the wiring for these two grounds and found the Chassis Ground is coming off the grounding on the 6SL7 socket hardware.  The System ground is coming from the 0 terminal on the output transformer that then goes to the two large can capacitors (4uf 600v).  I did not replace these large capacitors.

I did read that the filter capacitors maybe bad.  How do I tell which are the filter capacitors.  Are they part of circuit associated with the power transformer.  I did not replace the 3 big 4uf 600v can capacitors and the 8uf 250v capacitors. 

Here is a URL to see the schematics I have, http://audiorv.wordpress.com.

Thanks

Richard

 

[Paul Birkeland]

I'd post pictures of what you have, we can draw little circles around the suspect components.

It looks like you have some 20uF and 4uF power supply caps in there, as well as a couple 8uF bias supply caps.  Just out of curiosity, what voltage rating do all of these carry?  The high voltage power supply appears capable of delivering 800-900V, which I suppose isn't out of the ordinary for 807's running as pentodes.

I would change out CR1 and CR2, just replace them both with a single 1N4007 (or UF4007 if you're feeling extravagant), as they could be old selenium diodes.  I would also change out those 8uF bias supply caps for some good 220uF lytics.  This is the one power supply that you can't allow to fail.

-PB

 

[rockpassion]

Paul,

The 20uf is 3 value can, 2 x 20uf @450V, and 1 20uf at 25 V.  I replaced this with an exact value replacement.  However, I have not changed the 8uf capacitors.  They are rated at 250v and the 4uf are 600V.  I did not attempt to measure them while the amp was running.  Should I?

I think i have some UF4007 diodes already.  On the 8uf capacitors you say 220 uf but what voltage.  Should I stay with the 250 v value. 

Richard

 

[Paul Birkeland]

Yeah, 250V isn't a bad idea.

Are the 4uF 600V caps electrolytic or film?  In either case, I'd be tempted to toss some 4.7uF Solens in there just in case.

 

[2wo]

The 4uf are paper, I would change them out...John

 

[rockpassion]

Thanks, I am in the process of buying the parts as suggested. 

Any thoughts on the common/ground/negative connection for the speakers.  After reviewing the schematics again, I am leaning towards reattaching the System Ground as the proper Common/ground/negative connection.

It maybe awhile before I get back since I will have to wait for the capacitors to be delivered. 

Richard

 

[rockpassion]

Ok, I was able to change all the capacitors to exactly the same values as per the schematics with one exception.  The schematics calls for 3, 4uf 600 V paper capacitors.  I replaced them with 3 axial Solen Electrolytic 4.7uf 630 V capacitors. 

I am now getting a hum that increases as the voltage increases.  The hum does not start until 60 Volts and is really loud at 120 V's.  Also, the amp has a meter on the front that measures the 2 807's tubes with a pass/fail dial plus it measures the two regulators.  Before I made the changes the meters showed that the tubes were all good and the hum was a lower volume.  The meter has  three settings, one each for each 807 tube, and one for the regulators.  Now the first setting shows the first 807 as passing but the next 807 and regulator settting pegs the dial way beyond the pass setting.  I switched it off real fast when that happens. 

I will take some pictures tomorrow or Thursday and post them.  Any thoughts.

Thanks

Richard

 

[rockpassion]

I have been working on my amp and have made some progress.  I replaced all the Caps with new Caps.  I also replaced a couple of resistor that were not reading the values that were listed on the schematics.  I replaced the rectifiers with 1N4007 diodes.  I changed some wiring and resoldered a few connections. 

The results, the hum is now about half as loud as it was.  But I have noticed a few things.  I hope maybe someone can explain.  I have the amp hooked up to a Variac.  I start off a 60 volts and the hum is pretty loud.  But as I turn it up the hum actually decreases and is less loud at 120 volts.  Is that strange?

I also  used my  DVM to measure a few points to see what kind of voltage I was getting.  The interesting thing happened when I measured the connection at a couple of the capacitors the hum stopped.  I was getting AC voltage readings but no huim. The minute I lifted the DVM probe the hum returned.  Any ideas.  I still think it is a grounding hum but have no idea where to go from here. 

Thanks again,

Richard

 

[Paul Birkeland]

I have been working on my amp and have made some progress.  I replaced all the Caps with new Caps.  I also replaced a couple of resistor that were not reading the values that were listed on the schematics.  I replaced the rectifiers with 1N4007 diodes.  I changed some wiring and resoldered a few connections. 

Heck yeah man, I'm glad you didn't give up on it!

The results, the hum is now about half as loud as it was.  But I have noticed a few things.  I hope maybe someone can explain.  I have the amp hooked up to a Variac.  I start off a 60 volts and the hum is pretty loud.  But as I turn it up the hum actually decreases and is less loud at 120 volts.  Is that strange?

I don't remember the specifics of the amp, but there are certainly valid reasons as to why this would happen, and it shouldn't be something that alarms you.

I also  used my  DVM to measure a few points to see what kind of voltage I was getting.  The interesting thing happened when I measured the connection at a couple of the capacitors the hum stopped.  I was getting AC voltage readings but no huim. The minute I lifted the DVM probe the hum returned.  Any ideas.  I still think it is a grounding hum but have no idea where to go from here. 

Which caps?  Is it some or all of them?  Part of me wonders if you have something loose that's vibrating, does the hum go away with just one probe touching?

I feel like the last time I saw this, the big issue was that the audio ground and the chassis were not connected, but this is a source of some consideration for vintage amps where the neutral can sometimes be tied to the chassis. 

-PB

 

[rockpassion]

Paul,

Thanks for the response.  I am not near any of my schematics but it only happens on two specific capacitors.  I will get which ones they are and post them in the next couple of days.

Since none of my schematics have any output voltage readings will I have to attempt to calculate them?  I would like to be able to test specific connections. 

I am learning a lot and am getting more comfortable with this process.  Since I am so new to this and I do understand how dangerous it can be so I am taking my time and trying to read as much as I can.  It definitely helps me to stop and think about what to do next as I have figured out a couple of problems. 

Thanks again,

Richard

 

[rockpassion]

Paul,

I rechecked the capacitor and it is the C3 (.01mfd, 600v) capacitor on the schematic.  It connects the Grid of the 6sn7 to the plate of the 6sl7.  I have hooked up my DVM with ground to chassis.  When I touch the red probe to the 6sl7 I get 91 VAC and on the 6sn7 end I get 89 VAC and in both cases the buzz ceases. 

Also, when I hook up my DVM with just the ground connected and the red probe not touching anything and the amp is turned off I get 3.5VAC when I turn it on it jumps to 5.5 VAC.  Also, I get 87.5 VAC on the rca connector to the preamp when the amp is on.  Should there be any voltage on this rca connector?

That's it for now. 

Richard

 

[Paul Birkeland]

I rechecked the capacitor and it is the C3 (.01mfd, 600v) capacitor on the schematic.  It connects the Grid of the 6sn7 to the plate of the 6sl7.  I have hooked up my DVM with ground to chassis.  When I touch the red probe to the 6sl7 I get 91 VAC and on the 6sn7 end I get 89 VAC and in both cases the buzz ceases. 

Hopefully those are DC volts?  There should be no voltage on the 6SN7 side of the capacitor.  If the DC voltage is the same on both sides of the cap, the cap is shorted and needs to be tossed out.

Also, when I hook up my DVM with just the ground connected and the red probe not touching anything and the amp is turned off I get 3.5VAC when I turn it on it jumps to 5.5 VAC.  Also, I get 87.5 VAC on the rca connector to the preamp when the amp is on.  Should there be any voltage on this rca connector?

There shouldn't be any voltage on the RCA jacks, at all.  How is the AC line voltage coming into the amp?  Is it a 2-wire AC cord?

 

[2wo]

At C3 you say you are reading ~90 VAC, are you sure you are not reading millivolts ?

Set your meter to DC, 200v scale if not auto ranging and with the black to the chaise or a known good ground recheck both sides of C3. I would expect to see something like ~100VDC one one side and <5V or so on the other, compare with C4 which should be about the same .

This amp has an input transformer at the RCA's short the inputs together and to ground, try all of the combinations and see what that does for the hum...John 

 

[Grainger49]

You might want to add a new power cord with a ground to chassis.  That is the first thing I do with all old equipment I work on.