Bottlehead Forum
General Category => Technical topics => Topic started by: rockpassion on July 23, 2014, 02:05:00 PM
-
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
-
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.
-
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
-
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.
-
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
-
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.
-
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
-
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
-
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
-
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.
-
The 4uf are paper, I would change them out...John
-
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
-
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
-
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
-
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
-
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
-
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
-
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?
-
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
-
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.
-
Ok, I am not sure I am hooking up my DVM properly. I hook my DVM black lead to the chassis then use my red lead to check connections. I rechecked my C3 capacitor and got 91 VAC on one side and 89 VAC on the other, not millivolts. Switching to DC I got 14.5 VDC on one side and -7.5 VDC on the other. Once again the minute I touch either side on C3 the buzz stops. But when I test every capacitor connection I get almost the same results with the exception the buzz only stops when I test on C3. I desoldered the C3 capacitor and tested it on my DVM and got the proper reading. So, I desoldered two other capacitors and they all tested out properly. Also, there is a meter that tests the two 807 tubes and the VR150 tubes. The meter registers on the first 807 but not on either the second 807 or the power tubes. So I have something wired incorrectly I think.
Per Grainger I rewired the input to a three wire setup and I also shorted the input wires to ground and the buzz was a little less.
My plans are to start checking my rewiring connections and resoldering all the connections to see if I have a bad solder joint.
One last question. This amp is almost 60 years old and one of the resistors, R34, is encased in a metal case. The resistor is a 0.2 ohm, 2 watt wirewound type. It appears to be grounded to the chassis. Can I replace this with a current equivalent wire wound resistor and ground it to the chassis?
I will say this definitely test one's patience but at the same time it is fun. I just wish that it wasn't deadly.
Onwards and upwards and once again thanks for all the help.
Richard
-
I would suggest checking the resistance between the chassis and a known power supply ground. Something just isn't adding up here.
-PB
-
Paul,
This may show my ignorance but are you suggesting I find a power supply ground within the amp. Would that be a ground associated with the Power Transformer T3. Also, I see on the schematic a number of System Ground points off the Power Transformer vs Chassis Ground would it be one of these? Or, there is a black wire on the wiring schematic that is listed as a System Ground. I currently am using that wire for my negative speaker connection, could I use that to check for the resistance you are suggesting.
This is where I am confused. There is one wire listed on the wiring schematic as Out. I traced it back to the Output Transformers 8/16/32 ohm taps and am using that for my positive lead (red) to my speaker. The Common tap, listed as 0, goes through a resistor R34, and a couple of capacitors C9 and C10. From there a wire shows going out and is listed as the System Ground, I am using that for my negative lead (black) to my speaker. Am I using the wrong wire for my speaker. And if so will I have to run a wire from the O tap to my speaker.
Thanks for the long distance help.
Richard
-
C11 looks to be a multi-section lytic, it should have a single negative lug that is grounded. Start there, then see where else it goes. For convenience, you could head to the drug store, grab some green nail polish (should be an interesting checkout process), then dab a green dot where all the grounds are.
-PB
-
Paul,
It actually has two lugs and both of them are showing OL on my DMV. What am I looking for with regards to a reading.
One thing on the capacitor. When I unmounted the old capacitor it had a phenolic plate that went between the capacitor and the chassis body. The replacement capacitor had a phenolic plate and a metal plate. Should this capacitor be grounded to the chassis?
I will look for additional points tomorrow.
Richard
-
I'd take some photos.
You are looking for the ground leg of your capacitor. This is power supply negative, and should be ground. You should have power supply voltage (DC) across this capacitor. This is the place to start, to be able to actually measure your DC power supply voltage (not AC).
Once you can measure the power supply voltage, you can leave your black probe where it is and measure other DC voltages in the circuit.
So far, your voltage measurements have been erratic enough that this needs to be nailed down before looking for any issues.
-PB
-
I hope these photos are clear enough.
Richard
-
Paul,
A little confused on what you are saying.
When I look at the C11 capacitor there are three sections A, B, C, plus three lugs on the outer part of the capacitor can. Are these the power supply negatives you are referring to. Of the three only two are being used, one between A & B and one between A & C.
When I trace the wires from these two lugs there are three wires attached. The wire from the AB lug goes to the C8 capacitor, a second wire goes from the AC lug to the C10 capacitor, and a third wire goes from the AC lug to the V6 voltage regulator tube.
So am I measuring DC voltage here. If so am I leaving my black probe attached to the chassis and measuring these points or am I attaching my black probe to these points and measuring somewhere else?
I have been reading but I am still very confused on what a System Ground is. Sorry for the ignorance and I appreciate your patience. I hope you don't mind being a teacher here.
Thanks
Richard
-
Start by measuring the DC voltage across the first capacitor in the power supply. I think you swapped that out for a Solen?
-
Paul,
I was able to take some measurements. I measured all the Solen replacement capacitors. I had my black probe attached to the Chassis and used my red probe on each capacitor.
C8 - two red wires that are attached to R33 & R22 and one black wire that is attached to the ground lug on C!!. The readings were; on the red wire it started at 18.4 VDC and dropped to and stabilized at 14.3 VDC. On the black wire it started at -2.18 VDC and dropped to -1.37 VDC.
C9 - two red wires that are attached to C10 and T2 and the black wire attached to R4 and a bridge connection to C10. The readings were on the red wires; start at 40.4 VDC drops to 19.9 VDC. On the black wire starts at -3.06 VDC and drops to -1.35 VDC.
C10 - two red wires attached to C9 & L1 and the black wire connected to R34 and the same bridge connection that C9 . The readings were on the red wires start at 30.86 VDC dropping to 22.45 VDC. On the black wire starts at -2.96 VDC dropping to -1.82 VDC.
Richard
-
Please measure across the cap, not using the chassis as a reference.
-PB
-
Will do.
Richard
-
Paul,
To make sure when you say across the capacitor I am assumiung you mean to put my black probe on one side of the capacitor and the red probe on the other.
If so I got the following readings. C8 - 487.8 VDC, C9 - 585.4 VDC and C10 -584.8 VDC.
Richard
-
OK, that is more like it.
The chassis of your amplifier isn't grounded, this is a problem.
I'm guessing you don't have a 3-wire power cord on the amp?
This is a safety issue to address immediately.
-PB
-
Paul,
You are right. It is an old two prong. Where would I attach the ground wire in the amp for a three prong power cord.
When I look at my old BH Seduction the IEC connectors ground terminal goes to the chassis and also to the transformer. So would I run a wire from both the chassis and the transformer. If so, I think I know how to ground the chassis, but looking at the transformer I do not know what point to use as ground.
But I do have a question. If a capacitor is supposed to block DC voltage why am I getting readings showing DC voltage.
Richard
-
You are right. It is an old two prong. Where would I attach the ground wire in the amp for a three prong power cord.
When I look at my old BH Seduction the IEC connectors ground terminal goes to the chassis and also to the transformer. So would I run a wire from both the chassis and the transformer. If so, I think I know how to ground the chassis, but looking at the transformer I do not know what point to use as ground.
Yeah, the Seduction is a good model to follow. You probably have 2 prongs coming in, and one of the prong is soldered to the chassis (bad). You want to add a 3 prong cord. Hot and neutral go to the power transformer primary (through switch/fuse, add a fuse if there isn't one), the earth connection goes to the chassis at the closest possible place from where it enters the amp.
But I do have a question. If a capacitor is supposed to block DC voltage why am I getting readings showing DC voltage.
Capacitors store charge (good for a power supply), and offer a reactive impedance to AC (also good for a power supply, power supply noise is shunted through the cap to ground).
The DC blocking application works for coupling capacitors, where the cap offers relatively low AC impedance while blocking DC voltage from the next stage.
-PB
-
Paul,
Thanks!!! I checked the schematic and traced the 115 VAC wires to both the on/off switch and the fuse directly, one wire to each. By what you are saying I am going to find a point close to where the ground wire from the 3 prong cord originates into the chassis and connect it directly to the chassis.
I might not do that today but as soon as I do should I measure the same capacitors in the same way again?
By the way, I think I have learned more in the last 30 days than in the previous 5 years of just constructing the kits. This is great. I love this forum!!!
Richard
-
Yeah, you really want to nail down the whole power cord thing before doing anything else, I should've brought it up earlier.
You should find that one of the wires from the 2-prong cord connects to the chassis somewhere. It's also possible that they just come in and go to the primary, and that the chassis is floating (gasp!).
Once you have the power cord properly installed, then you can try measuring that cap voltage again. If you get something funky, you will want to connect power supply ground to the chassis as well. (I like doing this close to the RCA jacks or where the signal comes in to the first tube)
-PB
-
Paul,
These amps do not have a power cord. There are two wires that come from the power switch and one from the fuse. I attached a three prong cord with the ground attached to a screw on the chassis.
I rechecked the voltage on the three Solen capacitors and found the voltages very close to the previous readings. There were as follows. C8 First reading 487.5 VDC second 486.3 VDC, C9 first reading 585.4 VDC second reading 583.6 VDC, C10 first reading 584.8 VDC second reading 583.3 VDC.
Another interesting thing is, there is a meter on the front that measures the two 807's and the OD3/VR150's and shows whether they are good or bad. It registers for the first 807 that it is good but the second 807 and the OD3/VR150's does not even register. I have a few 807's and will try them along with another pair of OD3/VR150's.
I Look forward to your response.
Richard
-
OK, it's good that the chassis is grounded. I would recommend also grounding the power supply to the chassis. This is often best done up where the RCA input jacks are.
-PB
-
Paul,
I looked at my Seduction and see the transformer in it is grounded back to the ground connection on the IEC connector. Unless you tell me otherwise, I will run a wire from a screw connection where the transformer is anchored to the chassis and then directly to the ground wire on the three wire power cord. Does the transformer itself ground the external metal case surrounding the transformer? I assume it must. Looking at the schematic I also see the Center Tap is grounded to the bridge connection between C9 and C10, but I assume this is a System Ground and not grounded to the chassis.
There are no external connections on the amps. I only have exposed wires. If I can get these ampos up and running I will install all the connecting devices I need directly to the chassis.
Regards
Richard
-
Paul,
I grounded the Transformer to the ground wire on my power line. I did notice a slight decrease in the volume of the hum. I also remeasured the C8, C9, and C10 capacitor. I noticed a slight increase in DC voltage to C8 from 487 to 493, C9 from 585 to 591, and C10 from 585 to 591.
Any thoughts on what to do next.
Thanks
Richard
-
Measure AC millivolts across the speaker taps and let us know what you see. It would be a good idea to short the inputs while doing this measurement.
-PB
-
Paul,
Are you saying the taps on the Output transformer? In this case the C and 8 ohm tap.
Richard
-
Paul,
Just to make sure on the input side, I have the two input wires twisted together and shorted to the chassis. Should I instead use a shorting RCA plug.
I took the measurement across the O and 8 ohm tap on the output transformer. I tried using millivolts AC on my Voltmeter but got an OL reading. So I switched to Volts AC and got 4.78 VAC.
Richard
-
Wow, that's a lot of hum!
Shorting the input wires together should be good enough, shorting them together and then touching the chassis is even better.
The next test is to measure the AC voltage between the output of C3 and ground, as well as C4 and ground. The output side will have 0V DC on it. If both sides of one or both caps have DC on them, then replace that cap.
-PB
-
Remember that he's measuring that hum with the output unloaded. Not saying it isn't very high, but it might be worth putting an 8 ohm output load resistor across the output when making the measurement to get a more realistic assessment of the hum level with a speaker load attached.
-
Doc & Paul,
I made this measurement with an 8 ohm speaker attached. Thanks for the info. I will do the other tests and get back. Does it matter which side of the capacitor I am measuring? Am I grounding the black lead to chassis or some other ground within the circuit?
Richard
-
Now that you know that the power supply, chassis, and incoming earth are all connected, you have some flexibility in terms of where you ground your probe. For the AC and DC readings, any of these will work.
I would recommend measuring the AC on the side of the cap with no DC voltage present.
-PB
-
Paul,
I made the measurements you suggested. I grounded the black lead and took my measurements. On C3 on the side going to V1 I get a reading that starts at 21.83 and drops and seems to stabilize at about 9.65 VDC. On the V3 connection I get -6.3 dropping to -4.5 VDC. I took the reading on the V3 connection for AC and got 85 VAC. On C4 on the side going to V2 it started at 19.05 VDC and dropped to 9.98 VDC and on the V3 connection it started at -6.4 VDC dropping to -3.92 VDC. Again, I checked the VAC on the V3 connection and got 89.3 VAC.
Since voltage is measured relative to ground does the negative voltage indicate bad capacitors? I do have a set of spares for these so I can replace them if need be.
Richard
-
You are saying "V" and numbers, are you measuring the caps in question?
These should show somewhat high voltage on one side, and something close to 0V on the other.
Some negative voltage would indicate either the existence of a negative voltage supply, or excessive grid leakage current from the next stage would make negative voltage.
Positive voltage would most likely be leakage through the cap itself.
-
Paul,
Sorry, I did measure C3 and C4, the V1 & V2 refer to the 6SL7's and V3 refers to 6SN7. The connections refer to the wires going to these tubes. I hope that helps.
So, it sounds like the negative voltage is not indicating a bad capacitor but something else?
Richard
-
On C3, the side that goes to V1 needs to be WAY higher than 9V. Without spending a lot of time modeling the circuit, there should be 100+V there.
On the V3 side, looking more carefully, there should be some bias voltage on that grid. You should see the same voltage on pin 1 and pin 4 of the 6SN7 socket.
In any event, if you have old 0.01uF caps and old 1M carbon comp resistors, spend $4 and remove them and toss them. For that matter, replacing R14 and R15 with decent metal film resistors should go a long way towards having a reliable amp.
For me, however, the next thing I would want to do is to start looking at the flow of B+, because it doesn't seem to be getting through to your driver stage. R6 and R9 are your plate load resistors for the 6SL7, what do you see for DC voltages where they connect together? (Also replace those if they are carbon comp resistors)
-PB
-
Not to sound like some old know it all fart, but if someone manages to talk me into restoring an older piece of gear for them (you're probably going to need to be female and rather charming), I just chuck all of the caps and replace them with modern equivalents. Ditto any carbon comp resistors. The amount of time spent wanking around with bad voltages and noise is cut down by about 90%.
-
Doc,
After the previous post by Paul it dawned on me to do exactly that. I have already replaced all the Caps and the two diodes. So, I might as well spring for all the resistors they aren't that much. So this weekend I will order all the resistors and install them before I continue any further.
Thanks for the feedback, being frugal has its place but not in this case. Resistors are cheap and I might as well replace them with newer metal film resistors. This is a real learning experience.
Richard
-
Paul,
You may not hear from me for awhile. I took Doc's advice and ordered replacements for all the resistors. As soon as I get them I will replace them and cleanup and resolder all the wire connections.
I do have one additional question but I will have to take a couple of pictures to explain myself. I will try to get those early next week.
Once again thanks for all the help.
Richard