Magnatone M10 Weak Output

[dpm309]

Working on a Buddy's Magnatone M10 that has very weak, distorted volume from Channel 1 (the reverb and vibrato channel). Channel 2 sounds great.  When the guitar is plugged into channel 1 and you turn up the volume on channel 2 most of the way, you get a stronger, cleaner sound. This must be some kind of bleeder over between the 2 channels. Tried cleaning the stereo input (both channels are tied together) with no success. One of the preamp tubes (V2 12au7) tested bad so I replaced it with no success. Rest of the tubes tested out fine. Won't be able to get voltages until I open it up. This amp also has a safety power shutoff when you pull the top off of this so I will  have to jumper it when I get it apart on my bench. Been on the Magnatone site and this amp looks like a bear to tear apart and repair so before I delve into this, does anyone have any suggestions or ideas. I have attached a schematic for reference though this may not be the exact one. I believe a schematic is pasted to the inside of the amp.
Thanks,
Dan

[terminalgs]

You need to figure out if you have an M10 or an M10A.  The M10A has a 12" speaker and 4 knobs per channel  (vol, treb, bass and  an 3 position contour switch) and the M10 has an 8" speaker and just three knobs per channel (vol, tone, and contour switch).


the circuits are similar, but the M10 has a transistor reverb recovery and the M10A has a tube recovery.


But wait.., for both M10 and M10A, the 2nd channel signal by-passes the vibrato and reverb and goes straight to the PI, so if you hear ch.1 through Ch.2's volume control, then I'd suspect filter caps in the B+ supply. see schematic, channel 1 & 2 both share "D" and "C" B+ nodes. your Ch.1 single is probably finding its way through either "D" and "C" b/c the cap that should be a virtual ground for the audio signal isn't working.


As for poor signal from Ch.1, it could still be a bad cap at "C" since Ch.1 uses "C" as does both of the vibrato's phase splitters. 

I'd alligator clip in a 10uf-40uf electrolytic where "C" is and see if any problems get resolved.  While the filter cap in the lower chassis at the bottom of the amp, you can do the alligator clipping on the upper chassis.

[dpm309]

Thanks T.   It definitely is a M10. I will try out your suggestions when I get back to it tomorrow. Will post an update.

[dpm309]

I replaced the 1/450V cap for the "D" power supply with a 22/450V. The schematic I found stapled inside the amp shows that "D" cap should be 30/450V. The attached schematic shows it to be a 1uf/450V. I also soldered a 33uf/450V cap across the "C" supply. Powered it up and still weak output on Channel 1 but I am not getting the bleed through between channels. The 1uf/450V cap was oozing waxy material out of the negative end so that could have been the problem with the bleed over. Going to have to go back to determine what is going on with channel 1. Channel 2 still sounds fine. Will need to go over the circuit in channel 1. Any ideas?
Thanks

[terminalgs]

Check voltages on Ch.1's 12AX7, and the two 12AU7s.  the schematic only shows B+,  but for each of those triodes make sure there is a voltage drop across plate resistors and cathode resistors. I don't have a lot of hope that this is the issue, but its easy enough,  and you never know, a shorted plate resistor can make things act funny.


So you can divide and (hopefully) conquer by removing the reverb circuit from the path the signal takes from Ch1. preamp to PI.  if you pull the reverb's 12AU7 driver tube and jumper it's pin 7 to  pin 7 on the 12DW7, you'll by-pass the reverb circuit (makes this an M9 of sorts...).  If the problem goes away, you know to dig into the reverb circuit, if not, the problem is in the Preamp or the vibrato.


If you look at that vibrato section with the varistors, you can see there are several coupling caps.  If one of the 820pf is open, you'll get the 'quiet' part of the vibrato cycle... i.e. low volume, all the time. 

do you happen to have a o-scope?

[dpm309]

I do have an O scope but am far from proficient with it. I have had good luck using Doug's listening amp and probe that I built a couple of years ago. Will check the voltages on Ch 1 tubes as well as check the plate and cathode resistors. This amp is 50 years old so it is possible that the carbon comp resistors have drifted significantly or just failed. Will get back to tomorrow.
 T - Thanks for the input and advice.
Dan

[dpm309]

I have posted my voltages on the Valve Data Page: Magnatone M10.   http://el34world.com/charts/valve/ValveData.php?e=view&f=18169

It looks like the voltage on pin 6, V3 is low as compared to pin 1. They both feed off the "C" power supply connected by a 22k resistor. I will check that first thing in the morning. Don't know if anything else looks off but am still getting the low, distorted sound on Channel 1.
Thanks

[sluckey]

I can't decipher your voltage chart. How did you determine what is V1, V2, V3, etc? They ain't marked on the schematic.

[dpm309]

The best I could determine: V1 is Chl 1 1st gain stage, V2 is Ch2 1st gain stage, V3 and V4 are vibrato, V5 Reverb and V6 PI. I will try to mark up a schematic and post it tomorrow.

[sluckey]

V1 is Chl 1 1st gain stage, V2 is Ch2 1st gain stage, V3 and V4 are vibrato, V5 Reverb and V6 PI.

OK, I marked up a schematic based on that info. You have several readings that are obvious mistakes, either reading or writing. Things like 114v on a pin that's connected to ground, or different voltages on pins that are tied together.

Hard to say what is right or wrong at this point. You need to take another set of readings and post. Then I'll take another look.

[dpm309]

Sluckey, Thanks for looking this over. Will go back over the schematic and circuit with a fine toothed comb to see what I did wrong and will repost my findings. The schematic that was attached to the inside of this amp is actually for a M9. The one I attached for the M10 came off of the Magnatone website and neither of these schematics label the valves.

[sluckey]

I don't know why Magnatone did not take the extra 5 minutes to label the tubes on the schematics, especially in their more complex circuits with that crazy tube line up. Ever wonder about all the extra holes in the chassis? That's because they used the same chassis for every one of those suitcase style amps.

Here's a schematic with voltages for a working M10-A. I know they are different but the modulator circuit that you had some questions about  voltages are almost identical. May be of some use...

     http://home.comcast.net/~seluckey/amps/magnatone/Magnatone_M10A.pdf

[dpm309]

I have reposted my revised reading on the valve data page:   http://el34world.com/charts/valve/ValveData.php?e=view&f=18169
Looks like I had V3 and V4 reversed.  I have the M10A schematic from the Magnatone website and have been comparing it with the M10. Looks like pins 2 and 3 on V4 are low. Other than that, everything looks close. I have also attached a revised schematic with what I hope are the correct valve numbers.
Thanks,
Dan

[sluckey]

V4 pins 7 and 8 cannot be right. Pin 7 should be slightly negative, not 98v. Pin 8 is connected to ground. It can't be 114v.

V5 pins 1 and 6 cannot be different. They are connected together.

[dpm309]

Sluckey, I tried editing the valve data but it kept reverting back to the original. Have re-entered the tube data: http://el34world.com/charts/valve/ValveData.php?e=view&f=18178
Hope I have everything right. From what I can see, looks like Pins 2 and 3 on V4 are low. Other than that, it appears that the voltages are in line. Will check the resistor and cap values in that area. I am getting an open reading on the 8.2M resistor connected in-line with a 1M to pin2. All the other resistors in that area look correct. I don't have an 8.2M resistor on hand.  The highest one I have is 4.7M. Also noted that I am getting a bad/marginal reading on the 1st 1/2 of V4 (12DW7)  (using a B&K 747). Don't have any of these on hand but could I substitute a 12AT7 or 12AU7 to see if this is the culprit?
Thanks,
Dan

[sluckey]

V4 is not the problem. I only mentioned V4 because the voltages were obviously wrong and that made me doubt some of your other readings. The last set of readings do look OK, at least they don't raise any red flags.

Concentrate on V1, V3, V5, and the transistor. They are all in the signal path for the VIB channel. Tube substitution is always the first order of troubleshooting. An o-scope and sig gen would locate the point that the signal goes poop pretty quickly.

[sluckey]

Here's a fresh schematic that may be easier to read...

[terminalgs]

dpm309,  I mentioned by-passing the reverb circuit by removing the reverb driver 12AU7,  and jumpering  the pin 7 of that tube to the pin-7 of the phase inverter tube.  This is will eliminate all of the reverb including the transistor and the transistor's coupling cap (that non-polarized 4uf/15V electrolytic,, thats what this is right?).  that 4uf/15V cap jumps of the page at me as being a high potential failure item.

[dpm309]

I bypassed the reverb circuit and am now getting full sound. Also tried a couple of different 12AU7s with no luck so my next move is to change out the 4uf/25V non polarized cap. Will have to replace with either a 3.3 or 4.7uf since that is all that is available locally. Now that I have it narrowed down, I can concentrate on the reverb circuit. Will report back when I change out the cap.
Thanks T and Sluckey for all of your help and suggestions.
Dan

[sluckey]

OK, making progress. I don't want to distract you working on the reverb circuit but does the vibrato work with V5  and Q1 bypassed?

[dpm309]

Yes, the vibrato works with V5 and Q1 bypassed. Next step is to swap out the 4uf/25V non-polarized cap (have 4.7 and 3.3uf). I also picked up an NTE102A which is supposed to be their equivalent of 2N2614 to determine if Q1 might be the culprit.
Thanks
Dan

[sluckey]

Getting close now.

[dpm309]

I pulled the 4uf non polarized cap and it only measured about .4nf (.0004uf) so I replaced it with a 4.7uf NP cap. Fired it up and when I turned off standby, there was a loud snap and spark (but no smoke or burn't smell). Didn't look like anything was burnt so I removed V5 and bypassed the reverb and the amp was acting like before - CH 2 was louder and the vibrato was definitely working. Pulled Q1 and measured in my multi-meter and am getting 423 hFE so it seems like it is OK. The 4.7uf NP cap measured over 6uf and I don't know if the higher value could have caused the issue. The 3.3 NP cap I have on hand measures 3.7uf which is pretty close to the original. Haven't changed it out yet. All of the resistor and cap values in reverb circuit seem in line except for the 3.3M resistor connected to the base on Q1 (reads 320K) and the 25uf ecap connected to the emitter (no reading).  I will pull these 2 components from the circuit and re-measure when I get a chance.  I am stumped. Any ideas?
Thanks,
Dan

[terminalgs]

Radio Shack sometimes has non-polarized electrolytics since they are used in audio cross overs..  in a pinch, you can put two +/- electrolytics back to back, so the (-) leads are connected and the (+) leads face out. To get 4uf, you'd need two 8uf's. for voltage ratings,,  i dunno,, err on the high side.. 40V ea?

[jojokeo]

I pulled the 4uf non polarized cap and it only measured about .4nf (.0004uf) so I replaced it with a 4.7uf NP cap. Fired it up and when I turned off standby, there was a loud snap and spark (but no smoke or burn't smell). Didn't look like anything was burnt so I removed V5 and bypassed the reverb and the amp was acting like before - CH 2 was louder and the vibrato was definitely working. Pulled Q1 and measured in my multi-meter and am getting 423 hFE so it seems like it is OK. The 4.7uf NP cap measured over 6uf and I don't know if the higher value could have caused the issue. The 3.3 NP cap I have on hand measures 3.7uf which is pretty close to the original. Haven't changed it out yet. All of the resistor and cap values in reverb circuit seem in line except for the 3.3M resistor connected to the base on Q1 (reads 320K) and the 25uf ecap connected to the emitter (no reading).  I will pull these 2 components from the circuit and re-measure when I get a chance.  I am stumped. Any ideas?
Thanks,
Dan

That's weird seeing that high of a value non-polar cap. Is the cap you pulled out labeled as such? It's only acting as a coupling cap so even a .47uF should allow plenty of signal through just as the caps going from the pi to the power tubes are 10x smaller being .047uf's. Unless it's completely bad, this should not be related to your issue?

Getting a hFE reading looks close and it appears that the PNP tranny is fine also. BUT the other two parts are VERY important. The 3M3 resistor (along with the 56K) help set bias of the tranny, while the 22uf/25uF cap gives it the gain needed in fully bypassing the signal. These two parts is most likely your problem.
 
*Don't forget to also check for the proper neg. voltage at point E/100k resistor

[dpm309]

The 4uf was labeled 4MFD. I replaced it with a 3.3uf/100V Non-polarized cap. Also checked the 3.3M and 4.7k resistors and they both measured within a few percent.  The 25uf e-cap was reading 50uf so I replaced it with a 25uf/25V e-cap. Fired it up and still getting weak, distorted sound from channel 1. I previously replaced V5 with a good 12AU7 ( the original read bad on my tester) with no luck. Seem to have the problem narrowed down to the reverb circuit but I am out of ideas on where to look next.

[jojokeo]

The 4uf was labeled 4MFD. I replaced it with a 3.3uf/100V Non-polarized cap. Also checked the 3.3M and 4.7k resistors and they both measured within a few percent.  The 25uf e-cap was reading 50uf so I replaced it with a 25uf/25V e-cap. Fired it up and still getting weak, distorted sound from channel 1. I previously replaced V5 with a good 12AU7 ( the original read bad on my tester) with no luck. Seem to have the problem narrowed down to the reverb circuit but I am out of ideas on where to look next.

Okay, those values measured are fine and within tolerance/range of what they're supposed to be so it appears that the tranny & circuit area are good to go. One more thing to check off the troubleshooting list.
Next is to check that the 5k pot out of the reverb tank is good (it likely is but just quickly check it to make sure). Next is to try to measure the resistances on both incoming & outgoing small transformers of reverb tank and check for faulty connections on these and also their RCA connectors they're attached with. Somewhere along this area there is likely something not connected properly, bad contact, etc. you may also very carefully push on wiring in these areas with a dental pick or similar to see if there's a bad contact somewhere.

[dpm309]

Was about to throw in the towel so I fired it up one more time and am now getting decent sound out of both channels! Replaced V3 and V4 with good tubes as well as the 4uf/25v NP cap and the 25uf/25V e-cap. This is a great sounding amp even with the 8" speaker. Great vibrato!

Want to thank everyone who has helped me on this amp. Was a great learning opportunity.
Dan

[jojokeo]

...One of the preamp tubes (V2 12au7) tested bad so I replaced it with no success. Rest of the tubes tested out fine.

Thought the tubes tested fine from your first post, but nice that it's at least fixed now and you're enjoying the results.