ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Tomorrow I will start replacing the four electrolytic capacitors on the main board (PCB 1) and examine the tantalums. I will also replace the trim pots on PCB 1.There is an extra trim pot mounted on the trace side of PCB 1 that I don't see in the service manual. Other Beogram 4000 pictures also have this trim pot so was it a change that isn't on the schematic? It looks like it goes from ground to the 1C11, 1C13 and 1R37 node.Another trim pot I see in other Beogram 4000 pictures is the one next to the tonearm movement detector. It is a 100 Ohm trim pot. The one on my BG4000 deck has a very loose dial.
Jacques
sonavor: There is an extra trim pot mounted on the trace side of PCB 1 that I don't see in the service manual. Other Beogram 4000 pictures also have this trim pot so was it a change that isn't on the schematic? It looks like it goes from ground to the 1C11, 1C13 and 1R37 node.
There is an extra trim pot mounted on the trace side of PCB 1 that I don't see in the service manual. Other Beogram 4000 pictures also have this trim pot so was it a change that isn't on the schematic? It looks like it goes from ground to the 1C11, 1C13 and 1R37 node.
The extra trimmer on the solder side of the circuitboard was fitted to enable the deck to sense semi-transparent records (that wasthought to become a trend around this time).It's a 50K trimmer and it should be adjusted so that the deck can tell the difference from a transparent record to no record.The DR signal should be high when over the radial platter ribs looking through a transparent record, and low when over the radialribs of an empty platter.It was factory fitted from apprx june 1st 1973.
Martin
chartz:As regards the carriage worm screw, there should be a felt washer between the white pulley and the supporting bracket, just where you inserted a nylon one. Check it isn't somewhere inside the deck! My Beogram 4000 has a stainless steel worm screw by the way.
So you are saying where I show my nylon washer, the Beogram originally had a felt washer? There wasn't any washer when I received this turntable. The drive pulley was not attached when I got it so the previous owner must have lost it.For now I have other problems. I can't adjust the FO and RE voltage levels below 7 VDC. The tangential drive works as far as the arm assembly moving forward and backward but the voltage levels aren't what the service manual calls for. New trim pots for those didn't help. Regarding the trim pots, the 1VR1 trim pot was shot. It measured open circuit across all legs. The 1VR3 trim pot measured 10kohms while the service manual calls for a 5kohm trimmer. 1VR4 was a 20kohm pot but the service manual calls for a 50kohm trimmer.
I replaced the five electrolytic caps on PCB 1 as all of the originals were out of tolerance on the high end. Currently though, my platter motor drive has stopped working. It was way too low for a good sine wave but now it is zero. So it is time to take a break and look at it fresh tomorrow.
I can get the platter motor to spin again today after re-adjusting and measuring the drive signal using the motor trimmer (1VR3, 5kohm), the 33 1/3 RPM trimmer (1VR1, 50kohm) and the 45 RPM trimmer (1VR2, 5kohm). The problem was the motor wasn't getting enough voltage. Starting with the 33 1/3 RPM speed, I am able to go up to about 3 VRMS and adjust 1VR1 to get a 42.3HZ sine wave that looks pretty nice. The motor turns at that setting so that is good although I expected the voltage to be able to be higher. Any higher though, and the signal distorts from the nice sine wave.
When I switch to 45 RPM, the motor stops. It doesn't have enough voltage so I increased the 1VR3 setting until the motor turns for 45 RPM. I used the 1VR2 trimmer to set the frequency at 57.3 Hz.
Going back to 33 1/3 RPM, I now have a distorted, square wave looking signal and I have to re-adjust the 1VR1 trimmer to get 42.3 Hz.
So as it stands right now, it appears this poor Beogram is still ill. The forward and reverse direction drive voltages for the tonearm assembly motor are around 7.5 VDC. I cannot adjust those with the FO and RE trimmers.The platter AC motor works at the proper frequencies but the signal is not a clean sine wave in order for the motor to turn.I am tempted to take the AC motor out of my spare Beogram 4002 type 5503 turntable and see if it performs differently. Of course it could just be the platter drive amplifier having a bad component that is under-performing.
Looking back at my pre-recap pictures, the 33 1/3 RPM motor drive voltage could only get up to 1.5 VRMS and keep a good sine wave. After the recap and trim pot replacements I am able to get 3 VRMS and a decent sine wave. Of course 45 RPM doesn't function at that level but it seems like there was some improvement which is encouraging to see. On my earlier Beogram 4002 type 5503 (which also has the AC platter motor), I was able to get a nice sine wave for 33 1/3 RPM and 45 RPM at a voltage level of over 6 VRMS.
Don't trust the manual when it comes to the trimmer values.Trust what's fitted.
I take it that you did replace the bipolar start cap with the same type.
Here are the Beogram 4000 trimmer resistors - Service manual values, original values found in this unit and my current replacement values.I can try a 10k and 20k for 1VR3 and 1VR4, respectively. 1VR1 I just have to assume was 50k.
Tonight I changed some of the PCB 1 board trimmers. Here is the new chart.
The affect on the tonearm assembly motor drive seems to be an improvement. The forward and reverse normal speed voltages across the tangential drive motor are in the 5.5 VDC range. Playing around with 1VR5 and 1VR6, I think I have them set where the slow FO and RE (when lightly pressing the "<" and ">" buttons) is in that 2.5 to 3.0 VDC range. It was kind of tricky for me in making that adjustment though.
Changing 1VR3 (motor drive control) meant re-adjusting the drive signal. Again I was able to adjust a nice sine wave for 33 1/3 RPM but the result was insufficient to turn the AC motor for 45 RPM. Here is the initial adjustment for 33 1/3 RPM. I put my Channel 1 scope probe at the base of 1TR9 to see what the controlling signal was like there.
In order to get the AC motor to turn for 45 RPM I had to up the voltage output via 1VR3. That also results in a distorted control signal as shown here.
I moved my Channel 1 scope probe to an earlier point in the drive circuit. I placed it on the collector of 1TR8. Here is the 33 1/3 RPM selected. 45 RPM looked the same except for the frequency.
While on the 45 RPM speed I moved the Channel 1 scope probe to the base of 1TR8. The controlling signal at the base doesn't look too bad. The 1VR3 trimmer is after this point. Does this mean the distorted drive signal is a function of 1TR8 or 1TR9 ?
Today I tried swapping 1TR9 with 1TR11 (both BC144 NPN transistors). The result wasn't much change. I can adjust the motor drive to a little bit better looking sine wave but the voltage level is just over 3 VRMS. At that level the 33 1/3 RPM always works but the 45 RPM is right on the edge as far as the motor turning. From the service manual it looks like I should be able to go up to 6 VRMS on the motor drive with a nice sine wave signal. My Beogram is at half of that.
Looking at my rail voltage for the motor drive, I have 21 VDC when the motor isn't turning. When 33 1/3 RPM is on, the rail voltage drops to about 16 VDC and when 45 RPM is on, the rail voltage drops to 15 VDC.
Later this morning my power supply failed where the rail voltage no longer switches on through the reed relay. The voltage on the coil is not functioning. I checked the control circuit for the relay and can see that the voltage levels are off. The base of 2TR5 should be 0.7 VDC, I get 0.29 VDC.The schematic shows 11.5 VDC at the junction of 2R14, 2R15, 2R16, 2C4 and the collector of 2TR4. I measure around 22 VDC there.I am thinking I might have to halt work on this project and order new components to try and rebuild the power supply (and maybe the motor drive circuit). I have to consider the problem might stem from the transformer too. This diagram shows my bad relay control voltages as well as my input power. In parenthesis I show the voltages Martin gets on a working Beogram 4000. The 250 VAC is of course his line voltage so I expect mine to be my 125 VAC, which it is. So that part is okay. The next two are off though. He gets 24 VAC and 10 VAC on his secondaries where I get about 18 VAC and 5.4 VAC. I found a place in the USA that says they can rebuild the transformer for me. I am considering that but I think I still have a power supply board problem so I may need to rebuild that board. Can someone post a list of the modern replacements for the transistors needed for the Beogram 4000 ?
Power is okay on the Beogram I believe. I took a break, then went back and checked again. I had my probe on the wrong end of the resistor when checking the power board relay control. The node I thought I was seeing 20 VDC on was actually the rail (makes sense now). The relay does turn on and I have the correct voltages per the schematic +10.4 VDC and +0.7 VDC on the base of 2TR4.
In working with the motor drive circuit, I got it to start working again but it will disappear on me while adjusting the 1VR3 trim pot. So I either have a bad connection somewhere or some bad component(s) in the drive amplifier circuit. I will start resoldering and measuring those components to see if I can discover the problem.
I may have missed a step somewhere, but do you still have the transformer secondary voltages at +18 and +5.4V? I don't see much point in any downstream troubleshooting if the elementary supply voltages are at 75% and 55% of nominal!
--mika
tournedos: I may have missed a step somewhere, but do you still have the transformer secondary voltages at +18 and +5.4V? I don't see much point in any downstream troubleshooting if the elementary supply voltages are at 75% and 55% of nominal!
Thanks for that Mika. Those voltage levels have troubled me but the power supply has been able to get DC levels to operate the turntable. But I was wondering if the low secondary voltages were playing into the results I am seeing. So it might be time for me to start talking to the transformer repair shop about rebuilding this one. According to Frede and Martin, it is next to impossible to find a spare transformer for these turntables.
sonavor:Thanks for that Mika. Those voltage levels have troubled me but the power supply has been able to get DC levels to operate the turntable.
They may be fine at the state you have been measuring them, but I'd be surprised if the 6V supply could operate properly with 5.4 VAC input once the load gets higher, if it has been designed for 10 VAC.
I suppose you could test with another off-board transformer (which doesn't need to fit physically), but it can get tricky in practice.
I may be able to find a repair shop for transformers here in Mexico, I have gotten bigger transformers rewound, not pretty but worked perfectly.
Collecting Vintage B&O is not a hobby, its a lifestyle.
tournedos: sonavor:Thanks for that Mika. Those voltage levels have troubled me but the power supply has been able to get DC levels to operate the turntable. They may be fine at the state you have been measuring them, but I'd be surprised if the 6V supply could operate properly with 5.4 VAC input once the load gets higher, if it has been designed for 10 VAC. I suppose you could test with another off-board transformer (which doesn't need to fit physically), but it can get tricky in practice.
Yes, I measured the secondaries with the Beogram operating and the secondary that starts at 5.4 VAC drops down to 4.9 VAC when the tonearm assembly is moving. The 18 V secondary drops down into the 17 V range. I am trying to contact a transformer repair person I found in Maine, USA. Most transformer repair places I looked up only deal in gigantic, industrial transformers.
Søren Mexico: I may be able to find a repair shop for transformers here in Mexico, I have gotten bigger transformers rewound, not pretty but worked perfectly.
Thanks Søren, I may have to pursue that option if I can't find one locally. One concern I have on the transformer though is the base is in some sort of epoxy. I don't know how they will deal with that.
If you dont find a solution up there, lets see what I can find out here, but I will have to have the transformer here, the Mexicans are sometimes better copy cats than the Chinese
I found a second transformer repair shop up in Michigan that I have decided to send the transformer to. I could never get through to the shop in Maine (by email or phone). I was able to talk to the technician in Michigan and he sounded like he would be able to build me a new, replacement transformer for less than attempting the repair of the original. So I am going to put this project onto my backlog while I wait for a replacement transformer. Maybe I will start on a Beogram 8000 restoration while I am waiting.
After my last post the transformer repair shop in Maine returned my call. I sent them pictures of the Beogram 4000 transformer and they are confident they can repair it. So I mailed my transformer off to Maine this morning and the project will be on hold until I get the working transformer back.
Good idea, hope they will do a good job at a decent price. Even though it sort of worked with your rewiring, it might have failed completely soon if the remaining windings were working harder than normal.
Option 1 was a no-go. This option was my first choice as I wanted to give a shot at having the transformer repaired. Unfortunately, once the repair shop got the base off, they said the epoxy prevents them from being able to rewind and repair the transformer. The transformer was returned and no charges were incurred other than what I spent to ship the transformer to Maine.Now I move to Option 2. I have shipped the transformer to a shop in Michigan. This shop does not do repairs. They build replacement transformers. So I am hoping they will be able to duplicate the size and function of the broken transformer. I should know in a week or so if this option will pan out.
If it doesn't, my third option will have to be an attempt to build a whole new power supply for the Beogram. It could be fun but I am hoping Option 2 works.
-sonavor
UpdateOption 2 - The transformer shop in Michigan says they can build me a transformer with the primary and secondary requirements I need. However, the challenge for them is being able to build one that fits within the confines of the Beogram 4000 chassis. The shop doesn't build their transformers using the same form factor that B&O used back in when the Beogram 4000 was in production.
So I am in a holding pattern with this project until I hear if the new transformer is a go or not. At this point I am putting the odds at 50:50 that I can go with Option 2 (replacement transformer) or Option 3 (build a new power supply).
Eventually someone will start sourcing replacement transformers for classic, vintage equipment.
In the meantime John, why not try to have the BG work with adjustable external PSU units?
Good suggestion Jacques. I will look at where I need to tap in my external DC voltages.
Thanks
Tonight I rigged up my Beogram 4000 to get DC power from my bench power supply. The bench supply provided 24 VDC and 6 VDC to the Beogram positive and negative rectifier terminals are. I removed the rectifiers from the power supply first (the transformer is already pulled and sent off).The supply shows the current draw with the Beogram running (platter motor spinning).
This picture shows where I tapped in the external DC power to the Beogram 4000 power supply.