Winter Project - 3 MC-40's to restore / Update?

[chinacave]

Hmm -

After posting these and re-reading DCGillespies' comments above, I see that I did not test the amp correctly - with the input level control fully turned up - but only 1/2 way per the notes below / love my sophisto-mocated data logging system.

Guess I will run them again......

BTW the post-it should be pretty self-explanatory, "NI" is non-inductive.

 

[chinacave]

20 Khz -

and out of curiosity;

40 Khz

 

[chinacave]

Dis-assembly

The last one is in the worst shape - with more rust on the exposed part of the chassis: its on the left below.

Quick 'Before' tests.

Voltages were OK, but low across the board.....

 

[chinacave]

Ser # 01397 appears to be totally stock and un-molested. It also apparently has the choke enclosed in the power transformer, rather than separate as in the first two.....

 

[chinacave]

To keep myself sane and have a better chance of correctly attaching stuff - I found some stick-on number tape at the big box stores for marking wires - As the color code for the various transformer wires often is not that noted in the schematic, even in the original 1963 manuals that came with the amps.

Rather than kill myself unsoldering every wire, this time I snipped them close to the joint after marking. In the last two, there has been plenty of length, so my theory is that less heat is good, and 3/8" - 1/2" less length is OK, and I will have a cleaner connection with less-corroded wire thus exposed, and possibly a neater job.

Below, you MAY be just able to make out what I think is black marking on the tips of the terminals, indicating factory solder joints - IE marked after assembly to show if anything was disturbed.....

 

[chinacave]

Update

As I am finishing up the 3rd amp, I note with puzzled sadness that the FIRST two, while working and sounding fine, both have LOWER voltages than nominal, and in fact even when the voltage selector switch is on the 117v setting, they are all still low across the board- like 10-15% I can list exact readings if needed, but does anyone have an idea if A) I should be concerned, B) how I might correct this with (?) capacitor changes?

Thank you

 

[Rodeodave]

Hi,

Usually I would first suspect out-of-spec resistors, but since you replaced them it's likely not the cause of the deviations you're seeing. Second suspect would be the caps (leaky caps), but as you put in new ones this can likely also be excluded.
I remember reading in some service manual (unfortunately can't remember which one) that the voltages should/must be within 10% of the reference. But if yours are consistently low it's worth investigating.

I think it would be good if you posted some voltage readings, maybe it gives us a clue.
What's your input line voltage?
What happens if you swap around tunes? Do the voltage deviations change with the tube or do they measure (about) the same for a certain spot?

 

[chinacave]

Finished up the third amp total rebuild last night.

Actually finished the physical re-build 4-5 days ago, when I first ramped it up with a variac, it seemed fine, then started drawing to much current, and the new CL-90 inrush limiter started smoking.

So I thought I might hit the kill switch and sniff around for the gremlins that have been issues before - This time it was a missing connection to ground on the tag board.

Even having a tested, working unit next to this one on the bench for visual comparison, still took several hours of tracing each component & wire to spot the missing jumper.

Will run some sine & square wave tests tonight hopefully and also run full voltage checks on all three amps to see if anyone can suggest why all the voltages are under 10-15%...

-Also did a lot neater job on the can re-stuffing - another thing that benefits from practice with the JB Weld - run a bead on the inside rim of the open can and on the mating surface of the base, set right-side up for a while to cure and run together, then set in a warm place (like next to the shop amp) to set.

Also tried my hand at old-school lacing on the long wire bundle in the back - pretty happy with it as a first attempt.

 

[chinacave]

Voltage charts for all three amps:

See that these are slightly illegible - Will see what I can do to improve the readability

 

[chinacave]

 

[chinacave]

I am curious & hopeful that some may feel moved to comment on why my voltages are apparently consistently low -

All parts save transformers / sockets replaced with new matched to w/in 1% of those on the schematic -

- for #2500 it’s the numbers in BLUE
- Same for #2499
- For #01397 they are in RED


I thank you for your input to help with my output.

 

[dshoaf]

What is the setting for the primary tap on the power transformer? If these are different then there would be a difference in the power supply voltages. Lastly, I see the bias voltages vary, too. Once you can get the plate voltage on the output tubes up, then follow the procedure discussed in the Hints & Kinks to set bias voltage properly.

Cheers,

David

 

[chinacave]

Hi David -

All at 120V AC in to the plug and the selector on the 117V setting.
I'll hunt for that description in Hints & Kinks - Thanks.

Ian

 

[Rodeodave]

Hi, I just gave your measurements a quick glance.

The readings are symmetric L/R throughout, which is good. Also, they are consistent.

I would start looking for the cause elsewhere: How much voltage does the CL90 drop? What is the primary voltage right at the transformer?
Am I correct that the MC30 did not incorporate a thermistor originally? According to the service manual an MC30 draws ~100W idling, which amounts to ~0.8A AC current (assuming 120V AC) through the CL90 thermistor, which according to its datasheet has a residual resistance of ~4Ohms with that current. Hm, that should only drop about 3 Volts over the thermistor, likely not an issue.
But seeing that the voltages are already too low in the PSU section, there must be something up there. What about the secondary voltages of the transformers, and at the rectifiers?

I wish I could be of more help, but that's all that comes to mind currently.

 

[chinacave]

Hi Rodeo –

Thanks for the note –
I can check the voltages I took that you suggested (as did the owner or the amps) tonight – I recall that the raw output of the PT was something like 167vac – and consistent for all three units.

I used 1N5408 3A, 1000PIV diodes for the voltage doubler – I am thinking there may be an issue there? As the filament and bias voltages seem good, (except for one bias tap on Unit #2499) I am zeroing in on either the caps or the diodes in the section between the PT and the choke. I ordered some UF2007-T’s as part of another order to swap in and see if that makes a positive change…..

I did think to check the voltage drop on the thermistor – and as you suspect, it was about 2-3 v. They did not originally have the thermistor installed.
The voltages BEFORE striping & re-building were closer to nominal.
I’ll post more detailed results tonight.
Thanks again for looking at it & your suggestions!

 

[chinacave]

Welp - no change.

All three units have 167 vac out from the power transformer.

That is with 120vac in with the selector switch set at the 117v setting - (yes, at 120v the fil. voltage is a bit high but all other readings have been at 120v in...)

I changed the diodes in the voltage doubler to the UF2007T's, and the B+ and bias voltages are exactly the same. Darn.

-Could it be the capacitors - I believe they are 137uf (rather than the 150uf speced...) at least that is what my notes show....seems well within normal tolerance
- Could it possibly matter which way the choke is connected???? Can't see how that would matter, but I am a bit at my end trying to suss this out.

I guess the other increasingly likely situation is that there is a common mis-wiring in all three units that is dropping the voltage in the B+ supply.

Can anyone confirm what the raw AC out of the power transformer should be? Just to check.

Thanks!

 

[Rodeodave]

Puzzling to say the least.

Well, from the input...120VAC in, 3V dropped over the thermistor, so we can imo safely assume(!) that the transformers see a reasonable primary voltage.

My estimate for the secondary voltage would be the following: The nominal voltage at the first capacitor is 470VDC, and the rectifier circuitry is a voltage doubler. We measure RMS (factor sqrt(2)=1.4...), thus the secondary voltage, sans loss, should be around (470/1.4)/2=168VAC, which is close to what you see.
Since there are smoothing capacitors before the choke I think it's justifiable to have 1.4 as the RMS conversion factor. Otherwise it would be 0.9 because of the high reactance of the choke at higher order AC components.
The diodes drop .7V each. My estimate for the choke, based on its size and reasonable current capability, would be 500mH and 10R. It should thus drop no more than a few volts. In total, choke and diodes, I would think no more than 10VDC.
I don't think that the polarity of the choke is of fundamental importance.

To conclude, your transformers a likely working fine, your circuitry might not, and I have no other clue. :scratch2:

 

[audiodon]

chinacave,

I don't know if this will help directly, but it may show a way to approach correcting the low voltage problem.

On my HK Chorale, the HK HotRod, I have also used a pair of CL-90 thermisters on the power transformer primaries. It does drop the voltage a few volts on the way in.
Here's what I did:
1. I run the variac up to 125VAC, which is close to the maximum I see here. Occasionally I've seen 127VAC, but it can go fairly consistently up to 124 or so. Then I measure B+. It's never been more than 15% off, but tends to run a bit lower voltage-wise than I'd like.
2. I adjust the variac to the point where B+ is where it should be according to the schematic. Then I measure the downstream DC voltages read at the different points after the voltage dropping resistors between cap sections.
3. I adjust those voltage dropping resistors by changing them out so that the voltage drops are within about 5-10% of spec when B+ is correct.
4. Retest at wall voltage. If B+ is a lot low (which it has never been), I'd readjust or reconsider the type of thermisters or whether to use two of them.

Maybe someone with more experience will punch holes in my method, and I'll be all the better for that, but this way strikes me as a decent way to get the unit dialed in for modern component tolerances as one of the last steps in the finishing process.

 

[chinacave]

Dave, Dave and Don -

Thanks so much for looking at my chicken-scribblings!

Short story is I think I 'fixed' it - the usual way, which was carefully re-flowing all soldier joints. A few looked suspect, so I did them all - On firing up - the voltages were closer to spec, and the square wave tests were a lot better-looking - I'll do some listening tonight.

The longer story was a pal I consult, an EE with 30+ years of repair experience as a job, pointed out what was then obvious to me - that the voltages were all within 7% of nominal -low, yes, but within the range of tolerance stack. I am still puzzled that when on the 117v input voltage setting with 120v in, that they should still be low - I will run them straight from the wall, where I see +/- 120-124VAC -

Don - I like you suggestion of tweaking the PS resistors and I may yet work on that...

I think I might have been chasing chimeras here, worrying about getting the voltages dead on, rather than asking "Is the unit working correctly & sounding right, putting out spec output power, low distortion seen on the scope, not drawing too much current, sounds nice" It does all of those and I am pretty sure it will sound nice tonight.

Am I OK in stopping here, or do you folks think i should look into this further?

 

[audiodon]

Hidden solder flow problems are a bear. We've all had 'em.
I've been looking for a little mirror (hopefully non-conductively) surrounded on the end of a telescoping arm to look underneath solder joints.