View Full Version : Anyone care to look at a schematic and confirm or refute my diagnosis and strategy?
01-21-2012, 01:06 AM
I have this nifty little Bell 2521 mono receiver. I am about to burrow into it and give it some long overdue love. The tuner section works fine, however the 2 phono settings (RIAA and EUR curves) and the tape setting generate considerable noise and hum.
Referring to the schematic one notes first that C1 and C2 in the lower right corner are multi section can caps which ought to be replaced as a matter of course. Am I correct in calling these the "filter capacitors in the power supply"? These supply proper voltages to the various stages in the audio circuit, in this particular case 190v to the phono/tape preamp stage? I will be replacing all 4 sections of the one and the 3 sections of the other with individual capacitors of equal value with one exception being the 50µF/25v section of C1 ('c' with the triangle) which I will replace with a 39µF and a 10µF in parallel.
Looking at the audio circuit of the phono/tape preamp section the capacitors seem to have values noted in MMF. Am I correct in assuming that they are likely to be ceramic and therefore less likely to be in need of replacement? Once I get inside I will be able to figure out which are paper and which aren't. At any rate, the can caps and the 12AX7 in the phono section are being replaced straight away. Does any one want to lay odds that this will eliminate the cacophony in the phono section?
01-21-2012, 08:21 AM
Well... the phono stage and tape-head inputs are where you'd start hearing excess ripple (from decaying power supply caps) first. The outputs, being push-pull, inherently cancel out much of the hum within the output stage, themselves. OTOH, the early sections are all single-ended... making them much more vulnerable. And, the phono section would be the most vulnerable of all... since it has the most amplification, and deals with the lowest-level input signal (it's easy for hum to "swamp" the signal there).
There's probably a very good chance that replacing the power supply caps will at least greatly reduce the hum in the phono section.
BTW: Except for the very first cap from the rectifier tube (40uf, directly connected to the rectifier output)- which should stay the same value for safety (not overloading the rectifier)- all the other subsequent power supply caps can be increased in value somewhat, safely. Excepting that first cap, even up to double the original values for all the other sections, is OK. That can also help reduce residual hum...
And, yes... the caps labeled in 'mmf" (i.e, micro-microfarads, or picofarads), are probably ceramic caps. If they can be verified to be the flat disc type upon visual inspection, they can probably be left alone. Those almost never go bad with age.
01-21-2012, 09:00 AM
Pete -- The nature of your hum in the phono section will be the biggest clue as to the origin of your noise issues in that section. But first, C1 and C2 are often referred to as filter capacitors because that is one of the primary functions they provide. However, they also provide important decoupling functions between the various stages, so sometimes they are call decoupling caps as well. In reality, the caps closest to the rectifier are doing the lions share of the filtering function, while the caps at the end of the voltage dropping resistors are largely providing decoupling functions only.
If the hum is only present when the phono preamp section is selected (V9), then it is doubtful that a filter cap is causing the problem. By the time the B+ is dropped down to the level that supplies this stage, the required filtering is all but done, such that the actual cap at the power source to this stage (C2c) is really only providing a decoupling function as mentioned above. Therefore, if C2c is bad, the most likely effect would be to cause what is known as "motor boating", so named because of the putt-putt sound it makes. You did not indicate that this is the type of noise you are getting, which is why I am doubtful that replacing C2c will solve the problem. From a standpoint of reliability and absolute performance however, this cap should be replaced without question.
If any of the power supply caps are not doing their job regarding the filtering they need to do, the resulting hum will always be 120 Hz in nature, which is one octave higher than traditional 60 Hz audio hum, as caused by poor grounds, bad cables, and the like. It is my bet that your hum is the 60 Hz type, which -- when generated internally -- is usually caused by the sensitive preamp stage picking up hum from the radiated AC field of the wires supplying the heater power to the tube.
Some things you can check are:
1. Try a different tube at V9. Different 12AX7s can have differing noise levels they generate with regards to hum and other noises.
2. Adjust the hum level control provided. This is R3 on the schematic, and should be adjusted for minimum hum with the level control advanced with phono selected, and no input connected. It's effect can often be dramatic.
3. Lead dress. The wiring to pins 4,5 & 9 of V9 make up the heater wiring. Make sure these leads do not run close to those of pins 1,2 6 & 7. Often, simply moving the heater wiring around slightly can have a significant impact on the final noise level, as proper placement can cause the wiring to actually "buck" itself as a noise source, reducing the hum in the process.
The caps in the feedback networks on the selector switch associated with V9 will likely be ceramic, and therefore, will not be "bad" from a standpoint of function. Everyone has their own opinion as to how various caps sound, so whether you replace these as part of your repair/rebuild will be a personal choice.
The coupling caps should definitely be replaced as a matter of recourse. Given that the feedback in the preamp section is applied around the second stage only, and reinserted at the plate of the first stage, any leakage from the coupling caps in this section would certainly cause a general instability in the second stage, resulting in noise from the preamp.
I hope this helps!
01-22-2012, 11:14 AM
I swapped out the the 12AX7 to no avail. I ordered new caps for the power supply from Mouser. While monkeying around inside I noticed that the hum dropped slightly when I touched the top of the 6AU6 in the first FM IF amp section. I swapped around all three 6AU6's in the IF amp sections, no change. The PS caps need to changed anyway. I'll do that and check back in.
01-26-2012, 03:14 PM
Well, I did it, part it at least. I opened the unit up. Desoldered the C2 can and removed it. I restuffed it! I reinstalled it and now the phono section is fine. Yay! I listened to a Louis Armstrong 78... tears of joy, and relief. Pics to follow.
01-26-2012, 04:23 PM
As always, do the easiest, most obvious things first. Congrats on the radical improvement with the rplcmnt of the multisection C2, as expected.
Great news from my personal POV regarding the audio line-level and output section: The overall design appears to be 'sound' :thmbsp: including a decent looking tone ctrl section and the very simple, yet appealing Cathodyne phase inverter. I'm hoping the audio performance is pleasing.
Further thoughts: Oddly though, they must have had a surplus of that 6AV6 tube type, as they seem to have repurposed that tube for this task. Notice the unused pins 5 and 6; those are there for a type of variable bias used for automatic gain and volume control when this is in its more suited and typical role as the detector tube in a tuner.
Dave mentioned a somewhat different perspective and potential issues:
#1 was changing the V9 tube. A likely culprit and a good, easy procedure. What wasn't specifically mentioned was that HK leakage can cause hum that bleeds thru the tube from the AC that is present on the heaters. The more advanced units, or an advanced procedure for you to casually consider in the near future would be to convert the AC heaters to DC.
#2 I'll humbly and mildly somewhat disagree with Dave's take on item #2, as a good set of tubes with low or essentially no HK leakage will yield little in the way of hum changes when the hum ctrl is rotated. This ctrl would be essentially null and void once DC is present in place of the AC htrs. Things like filter caps and less expensive filtration technology have all improved since this amp was built, so what they did (AC htrs) was typical for the time.
#3 Although remotely possible, items such as lead dress are unlikely to change, so they're unlikely to suddenly be the cause for a hum issue. Possible perhaps, but highly unlikely as long as nothing has changed from the original wiring conditions.
Replacing the most critical couplers such as C51, 53, 55 and 56 should make the greatest, audibly perceivable improvement and these are arguably the most critical caps, both for line-level audio and for the elimination of cap-leakage potential. If you want to go deeper after that, you could consider tackling certain critical caps in the tone ctrl network next.
Dave: ...the feedback in the preamp section is applied around the second stage only, and reinserted at the plate of the first stage, any leakage from the coupling caps in this section would certainly cause a general instability in the second stage, resulting in noise from the preamp.
Gee, I dunno about that. Although I definitely see the standard issue, global feedback loop from the output transformer's secondary, it feeds the cathode of the second stage, V10B as it certainly should and has no bearing that I see on any plate or anything else regarding the first stage, V10A. I suppose it could go all the way back the the first stage's cathode instead, but that could well be overkill. This is the same old standard-issue global loop that is practically universal in its configuration and connection. The idea is to keep everything as minimal and straightforward as possible, so that is where it would 'have' to connect, as it does so at this location in so many other amps for all the right reasons.
01-26-2012, 06:41 PM
Stereo -- Thanks for your comments on my post! Actually you and I agree on the issues you raised:
#2 -- Hum balance controls will in fact have little if any effect on hum induced from H/K leakage concerns, because that is not what these controls are working to address. Their purpose is to balance the influence of the energy radiated from AC heater wiring, relative to the signal wiring in a unit in the most sensitive audio sections. They do not eliminate the radiated energy, but merely balance it to minimize the impact on audio circuits, and therefore minimize any audible hum generated in the output.
To minimize the H/K leakage concerns you mention, many manufacturers elevate the AC heater supply by ~ 15-75 vdc, so that the AC heater voltage cannot find its way across the diode path that is formed by the heater and cathode elements, and then into the audio signal path.
The use of these two approaches became so popular that many manufacturers began using two hum controls -- one for each channel -- as well as the positive heater bias mentioned, to maximize the reduction of noise in phono preamp circuits. I know (for example) that the controls on my Dynaco SCA-35 do in fact have a dramatic effect on the hum level produced as the controls are rotated either side of optimum, with the level advanced, phono selected, and no input connected.
Regarding my comments on the feedback employed, these specifically relate to the phono preamp section (V9), as all my comments did in that post. Clearly, I should have not used the general term "preamp section", and more properly called it the phono preamp section, to eliminate any confusion. Specifically, if C43 were to fail intermittently, it would surely cause the second section of V9 to become unstable and generate LF noise, given the frequency sensitive feedback applied around this stage.
In the end, it really comes down to the type of noise that was originally being produced by the phono preamp stage. If replacing the phono preamp filter cap cured the problem, it is most probable that the cap was allowing the phono preamp to become unstable, generating noise that was interpreted as general hum. If it had failed and was allowing 120 Hz power supply hum through (one octave higher than classic 60 Hz hum), then the high level stages surely would have been subjected to the same 120 Hz noise as well, meaning that the high level inputs would have also been noisy -- but that was not the case as I understood it to be.
In any event, I am glad the OP has his amp/receiver working again, and can enjoy the results of his efforts!
I hope I've have explained my comments more clearly this time!
01-26-2012, 07:01 PM
You're a good man DC, I never made it past the wide screen.:D
01-27-2012, 10:45 AM
Dave, thanks for your response in post #7 and for clarifying your POV as to the feedback in the phono section. That makes much better sense now!
As an aside, I'm also familiar with the elevated htr voltage trick, although my personal preference is to convert htr strings to DC whenever it's practical. I presume you know the key to the ctrl is its wiper; in this case, the 'center tap'. I mostly see it headed to gnd as opposed to an elevated potential, but either way will work to a degree, just as two resistors in most cases could basically work to take the place of the ctrl...
I hope the OP will seriously consider the recomended coupling cap upgrades I outlined in any event.
01-29-2012, 12:17 PM
The first steps.
Identifying the cap
Removed, I drilled out the rivets.
The top of the cap sawn through. I just used a hacksaw...
01-29-2012, 12:33 PM
The next steps.
The cap unsheathed.
The waxed paper spooled off. This takes almost no effort.
With the waxed paper off there were three aluminum tangs showing. Each had a symbol at its base; half circle, square and triangle. These correspond to the three different values required in the schematic and noted on the side of the can.
I broke off the three tangs and drilled 1/16" holes next to them. I also drilled holes that exited by the ground lugs. I favored the two lugs that had holes in them for ease of soldering.
I put the new caps' leads through the holes and and wrapped them through and around the appropriate lugs. I held off of soldering them.
01-29-2012, 12:48 PM
The last steps.
There's a close up of the leads wound through the lugs.
The original cap had three values. One of them, 50mf:25v, I replaced with two in parallel; a 39/50 + a 10/50.
I held it up for Louis to look at, he liked it.
Here it is resoldered. I soldered the new leads and the old connections at the same time.
The last photo is the resoldered cap screwed to the chassis and with the sawn off can placed on top. It could be permanenently attatched, I chose not to.
01-29-2012, 01:05 PM
With both of the can caps restuffed, it sounds great. At full volume with no input there is a very slight hum. I will definately take Stereotype's suggestion to replace caps 51, 53, 55 and 56, but not right away. They are ceramic, 10000mmf, should I consider silver/mica or film? As I understand the coupling caps are essential to good sound and, in this case the values are pretty low so maybe I should spring for something sort of high end?
For now... it really sounds swell. I listened to a couple of mono Chet Atkins records over coffee this morning. :D
Thanks everybody! You guys are great!
01-30-2012, 06:40 AM
01-30-2012, 07:01 AM
Very nice job, and so glad Louis approved as well!
01-31-2012, 04:12 PM
That was a great job ! Thanks for the pics. I might just have to try doing that for my next recap. JOHN