[low_budget_listener]

Hello All,

My "new" RM-9 sent a relatively loud buzz through the speakers, which made near-field listening a less than perfect experience.  I found and fixed some loose solder joints on the circuit board, located around the tube sockets.  The buzz is significantly quieter now, and much louder in the left channel than the right.  It's only objectionable with the gain switch in high (so naturally, it's now in low), but I'd still like to fix it.

Additionally, the bias pots have obviously been replaced, but one is scratchy in use, and they are all misaligned relative to the holes in the top cover, so I want to replace them again.  I was going to use Bourns 3386-503K (or P, I don't know which has the correct lug spacing)  These are 50K, 1/2 watt pieces, which are similar to the Clarostat parts in there now.  Can someone confirm that they are adequate for the job?  If someone has a strong opinion on what to use here, I'd appreciate hearing it...

I took a (bad) picture of the guts.  The total capacitance for each of the two channels in the center of the circuit board (circled) is different (30 uF on one, 45 uF on the other).  Is there any practical significance to that?  Which value is correct?  Is this a place where swapping caps can make an audible difference?

Finally, it looks like someone fooled around a bit between the power cord and the circuit board.  Maybe it was just removed when the bias pots were replaced, but I was wondering whether anyone familiar with this amp could take a look and tell me whether it looks stock.  Ultimately, I'd like to incorporate the updates of the later RM-9's, but otherwise return it to stock condition.

Thanks to all for any help you may offer.

[Ericus Rex]

Looks fishy to me.  My RM-9 has two caps where yours seems to have four (in your circles).  Also, my two caps are equal value.  Looks like your power supply caps may have been replaced as well.

As for the bias trim pots, I replaced mine a couple of months ago.  Turns out all my noise (after repairing circuit tracings like yourself) was there in those pots (balance trim in my case- went ahead and replaced all four).  It's such a cheap and easy fix for noise and these particular pots seem prone to failure.  I replaced them with the original which I found at Mouser - Bourns 3386P-1-503LF.  A whole $1.28 each.  Mouser P/N 652-3386P-1-503LF.

Best of luck and keep us posted!

[low_budget_listener]

Hi Ericus, thanks for your response.  Your amp has one cap per channel there?  Any idea what value?  You're right, mine look fishy to me too.  One channel has three caps, 10 uF each, and the other channel has one cap, 45 uF.  Whoever did the work used three different types of caps.  Looks like whatever was on the bench got used.  That's why I wondered whether the correct bias pots were used.  Thank you for the part numbers.  The spacing on the K and P versions of the Bourns 3386 is very similar -- I couldn't tell which one would be a proper fit for the holes on the circuit board.

I'd like to order new caps at the same time.

I agree as well with your assessment of the power supply caps.  There are four on each side, and while they at least match, they're floating loosely inside the amp.  Only the wiring holds them in place.  Electrical tape was wrapped around the legs that are connected cap-to-cap.  All in all, not as cleanly done as the rest of the amp, so the work looked out of place.

There are a couple of wire jumpers installed where AC feeds the cb, which also look questionable, largely because the soldering is lumpy and looks sloppy.  Whoever replaced the bias pots probably removed the cb from the amp, though, removing the power wires in the process -- the mess may just be the result of putting the cb back in.

Two of the fuses were moved to holders mounted on the bottom plate, which is a mod I like for convenience.  It was also more skillfully done than the above-mentioned work.

Thanks again for your help!

Best,
Jim

[Ericus Rex]

I'll check on those caps for you Jim.  It may take me a couple of days though.

[low_budget_listener]

Ericus, thanks for your offer, but you've been a great help already, and I wouldn't want to put you to all that trouble.  Hoisting the RM-9 is no easy task, and dismantling your system for something so trivial hardly seems worth it.

I'm going to order the manual (with the schematic) and a few other odds and ends from Roger, but since he's very busy lately, it may be some time before I have it in my grubby little hands.

In the meantime, maybe someone out there who already has schematics, or needs to open his amp for his own purposes, can chime in with information on the value of those caps.

I'm inclined to replace all of them, while I'm in there, since they're getting pretty old (the amp is #113).  I don't know the truth of it, but I've read that the design and performance of recent caps offer an improvement over the parts that were available 25 years ago, and caps seem to be the part most people suggest replacing in older amps.

There are some Wonder Caps in there now, two on each side of the cb.  I had thought that Roger eschewed the use of "boutique" parts.  Are these stock, do you think?

Thanks again, Ericus, for your help.  I appreciate it.

Best,
Jim

[Ericus Rex]

It's no problem, Jim.  Helping each other is what we do here at Audiocircle.  The amp is already out of the system (I have three amps I rotate through).  I would just need to pop the bottom plate and take a look-see.

Mine has Wondercaps in the same location as yours and I assume they're original.  Those are the coupling caps and that would be the spot for good sounding caps. I guess where Roger differs from other manufacturers is the other guys put very expensive parts where they don't do much good.  Roger puts them only where they do the most good.

I went ahead and replaced all the circuit board caps (except the Wondercaps) when I did my overhaul a few months back.  I had two leaky caps and a few that tested just within tolerance so I went ahead and replaced them all while I had it apart.  Didn't cost too much as I did not use boutique caps in keeping with Roger's design philosophy.  Check the 1/2 watt metal film resistors around the tube sockets as well.  I had a couple of them test out-of-spec or inconsistently as well.

As for your power cord issue, mine has been monkeyed with as well (prior to me) so I don't know exactly what stock looks like.

[low_budget_listener]

It's no problem, Jim.  Helping each other is what we do here at Audiocircle.  The amp is already out of the system (I have three amps I rotate through).  I would just need to pop the bottom plate and take a look-see.

That being the case, I'll gladly take you up on your offer.  Thank you very much.  Given your findings when you worked on your amp, it sounds like a good idea to look over the other parts as well.  The circuit board is not heavily populated, so it shouldn't be too onerous a task, although I may go blind trying to read the color bands on the resistors.  I'll consider it my first lesson in electronics.  It may be time to buy one of those magnifying lamps...

[Roger A. Modjeski]

Hi,

From your picture I would punish the monkey who got hold of your amplifier. Here are some quick answers:

There should be a total of 4 main supply caps. We started with 800 uF and went to 1000 uF later. Those caps are not original. Sadly people replace caps that don't need to be replaced and they do it badly as in your amp. It's best to glue the caps down wtih Silicone. They will stay in placed forever. I recently worked on amp #31 which still has its original caps. There have been no major changes in electrolytic caps other than making them smaller.

There should be only 2 radial caps in the center of the PCB. The originals are 22uf 500V. They second stage filters for the driver B+.

As Ericus noted if I use a "botique" part I will use it where it matters. The Wondercap coupling caps are original and were not expensive at the time. The biggest cap ripoff these days are still the most expensive and you know who they are.

I've gotten my lab well enougn in order to accept some repairs. In general a minor repair/tune-up to return to spec for an RM-9 costs $650 plus parts. I also rematch the output tubes for best performance.  If the amp is in for a tune-up I can add the chokes and caps that comprise the sonic upgrade to MKII status for an additional $300 Write me for a return authorization and info on where to send.

[low_budget_listener]

Roger, thanks much for the info.  I ordered the manual yesterday through your website, because I'm averse to living without music, and want to resuscitate this amp as quickly as I can.  Although lower on my list of priorities, I'm also interested in getting some of the low-noise 6922 tubes you mentioned, the possibility of converting the RM-9 to class A operation, and one or two other things.  I was planning on calling after the first of the year to discuss it.  Would that be appropriate?

Best,
Jim

[Ericus Rex]

Jim,

Considering how monkeyed-with your amp is you may consider sending it to Roger for a complete look-over versus attempting to fix yourself.

[Roger A. Modjeski]

Roger, thanks much for the info.  I ordered the manual yesterday through your website, because I'm averse to living without music, and want to resuscitate this amp as quickly as I can.  Although lower on my list of priorities, I'm also interested in getting some of the low-noise 6922 tubes you mentioned, the possibility of converting the RM-9 to class A operation, and one or two other things.  I was planning on calling after the first of the year to discuss it.  Would that be appropriate?

Best,
Jim

Please contact me at [email protected]. I'm actually more available between Xmas and new years than most other times. If you want your amp converted to class A that is rather simple. If you have an 8 ohm speaker put it on the 2 ohm tap and you will be class A at about 25 watts which is enough for most listeners.

Ericus does have good advice for you and anyone else buying an amp that has been as worked over as yours. Who knows what else has been done by the monkeys. Once the amp is on the bench its not a big thing to replace all the trim pots, re-solder the sockets and whatever else is needed.

Dont forget to request the chokes and additional power supply caps that give you the added detail of the MK II

[low_budget_listener]

Roger and Ericus, Good advice, I'm inclined to agree with you.  I've just sent Roger an email.

As an aside, I was certain that the recent move would be reflected in slower response times at Ram Labs, but I was wrong.  I ordered the manual on Saturday, and have already received an order confirmation.

Thanks again to all!

Best,
Jim

[Roger A. Modjeski]

Not to disappoint, but keep in mind the confirmations on tubeaudiostore.com are automatic. Let's see how fast Sal fills the order. He works out of LA and comes up 2 times a month to get things he is out of or doesn't have. I do apologize that there is sometimes a delay in shipping. I am working on that. We offer a lot of things and some need restocking from time to time. However I try to respond quickly when one of my amplifier owners needs help.

 

[low_budget_listener]

Not to disappoint, but keep in mind the confirmations on tubeaudiostore.com are automatic. Let's see how fast Sal fills the order. /snip/ However I try to respond quickly when one of my amplifier owners needs help.

You guys did a great job.  It arrived today.  Wow, this is perfect.  Everything is in here, and it's easy to cross reference between the schematic and the drawing of the board layout.  Thanks Roger!

Happy Holidays to all!

Best,
Jim

[low_budget_listener]

It took me a while to identify and assemble the parts to replace, but now the soldering iron is hot and I planned to finish the repair today.  Which makes it the perfect time to notice something I previously overlooked -- using the drawing of the circuit board layout (from the owner's manual) as a reference, the bridge diodes in my amp are installed in reverse.  In the drawing, the anodes are closest to the fuse holders; the diodes in the amp have the cathodes closest.

I remind readers that I'm a complete novice, and know virtually nothing when it comes to circuits, so I'm completely lost.  My first thought is that this would reverse the polarity of the DC circuit.  Is this true?  If so, what is the practical impact on the behavior of the circuit?  The amp was functional before disassembly.

Just to cover all bases, it's also possible the drawing is incorrect.

My dilemma is that I don't know if I should rectify (bad pun) the situation by orienting the diodes to reflect the CB drawing, which seems most prudent, or leave them as is (sensible if the amp wouldn't function if they were, in fact, installed in reverse).

Much appreciation to all.  I'd love to get this completed and back into my system.

Best,
Jim

EDIT: Also weird -- all six of the light blue caps at the bottom of the circuit board in the above picture have their positive leads soldered to traces that go straight to the screw holes for the metal stand-offs (ground).  Maybe the person who worked on this amp considered the physical flow of electrons when thinking about positive and negative, instead of the conventional "negative to ground".  The largest pair, on the outside edges of the CB, appear to be original.  They were still bonded to the CB with silicone, and the leads hadn't been soldered to remnants of leads from previous caps, as had all four of the smaller caps.  Is it possible that the amp was built with positive ground?  Is the polarity of components important only in relation to other polarized components (they must all face the same direction regarding electron flow, but which direction is unimportant)?  Are the anodes and cathodes of tubes indifferent to the direction of flow?

Thanks for any input!

[muzak]

Man that amp really looks like a mess.  Here's a photo of mine.  I've had it for about 12 years.  A few weeks ago it developed a pretty loud hum in the left channel.  I tested all of the components on the circuit board.  Turned out I had a bad cap--the one I'm pointing to (22uf 500v IC brand).  I've since replaced it with a cheap cap I got at our local surplus shop and now it works fine.  I've order some better caps (the original & one similar) similar to the original.  Mine amp is number 305.  I'm pretty sure mine is original and never been messed with. My caps look a lot different than yours--all are axial type on the board except the two  larger light blue ones, at the bottom of the board extreme right & left sides, 100uf 60 volt. One concerning thing is that my board has those 6 small brown mica caps and yours has none.  Why?  That's a question for Roger-but check the schematic..good luck!