Mc240 Low Voltages On 4 Tubes

drillher

drillher

Super Member
The power supply was rebuilt,and all of the coupling caps were replaced.Only about a dozen of the resistors were replaced so far.On V1 at pins 1,6 I am measuring 86v where it should be 103.On V2 pin 2 is 85v but should be 103,pin 7 is only 30v but should be 85v.On V4 pin 6 is 314v but should be 430v,pin 2 is -24v vs.-46v.Tube V7 pin 2 is -24 vs -46v.Pin 6 is OK at 423v vs 430v.The 6L6 tubes are measuring close to spec.voltages,as are the 12BH7s.Any advice on what to check first would be appreciated.
Dave
 
When I unplug 12ax7 ( V1) the pin 1 plate voltage goes from a low 87 v to 118v.It is the same drop with two different 12ax7s.Why would having the tube in cause a large voltage drop?
 
I guess when V1 is removed, then V1-1 and V1-6 are just connected to the nominal 125v power supply voltage, C26-B (and the grids of the next stage, which would not affect the DC voltage seen). When you install V1, then the plates are sinking some current, too much apparently. Are V1's grids (V1-2 and V1-7) at 0v as they're supposed to be ?
 
Yes,on V1 pin 2 is 0v and pin 3 is .7v.I replaced all of the coupling caps except the Black Beauties 4 yrs.ago and repalced the BBs this week with k40 pio Russian caps to no avail.The other low voltage is on V4 and V7 where pin 2 and 7 are only -24v that should be -46v. Thanks!
Dave
 
Check the power supply voltages with all output tubes removed. These should read high but the ratios will be the same as with the outputs installed.

Thanks,
Ron-C
 
I can try that this evening.The voltage measurements on all of the 6L6 output tubes measure very close to spec.
 
You can take measurements with the small tubes removed also. You could have a shorted tube. If the voltage is low without the small tubes you have other problems.

thanks,
Ron-C
 
In the original post you have 314 volts where you should have 430. If this is 430volts on the output tube but not the small tube you have found your problem.

Thanks,
Ron-C
 
With the snall tubes removed,The voltages measured on the small tube pins went up by a large measure over the specs.The only value that was still low was pin 7 of V4 and V7 which measured -27v and should be -46v.I left the 6L6s in and had a slightly higher plate voltage of 452v(was 426v with small tubes in) and the pin 5's on the 6L6s measured a very high -137v with the small tubes out where it was -44v with them in.Could I have damaged the output transformers with those readings on the 6L6s?
 
Do you have the schematic in front of you? The transformer feeds -150 volts to the line that the -46 comes from. You can now look at the amp with the output tubes removed.
You have bad caps or out of spec resistors that are causing the issues. If one side of the dual triodes is the correct voltage the problem will be on the low voltage side. In the case of V1 this is shared between channels left and right where all of the other small tubes are used, each in a channel.


Thanks,
Ron-C
 
Ron,

I will did replace both prs.of 1meg resistors that supply V4 &V7 and still only have -24v.I would suspect the .22 cap,but I just replaced that as well.I will recheck the power supply where the -48v leg comes off of a 100k connected to a 56k resistor.Pin 7 on tubes V2 and V5 were also about 20v low with the tubes out.Thanks for your help.
Dave
 
I would not assume that new old stock PIO caps are good. I always use Orange drops or Illinois Capacitors and never have cap issues.

Thanks,
Ron-C
 
Is there any way to test the new .22 Russian PIO caps without lifting one of the leads from a solder lug?I could also replace all of the resistors,but I'm not sure that the problems are resistor related.Thanks again!
Dave
 
ron-c said:
I would not assume that new old stock PIO caps are good. I always use Orange drops or Illinois Capacitors and never have cap issues.

Thanks,
Ron-C
Click to expand...

Agreed with Ron-C, you've eliminated quite a few items that _could_ be the problem. What's left is where the problem.

No easy way to test caps like these in-circuit, as well. Let's go back to the analysis so far:

- We know that there are a set of voltages that, according to the specs, are not correct.
- Somewhere along that part of the circuit, there's likely a leaky cap that is throwing those voltages off.

I haven't read the whole thread - yet - but suspect that the PIO cap just ahead of the first bad voltage in the signal chain is not right. You should be able to lift one leg of that coupling cap and see the previously wrong voltages return to more reasonable specs.

This is a way to find leaky caps, BTW.

Cheers,

David
 
David,

Thanks for looking at this thread.When I first measured the low voltages,it seemed logical that one or more of the 4 B.B. .22 caps were leaking.I was surprised to have the same readings after I replaced them with 4 Russian PIOs.I replaced about a dozen resistors as well.If there is not a easier way,I can lift leads on the .22 caps and then remeasure voltages.Would I need to lift all four of them?
Dave
 
I measured voltages from the power supply which were a little low.The 435v leg measures 423v,the 300v leg measures 291v,the 125v leg measures 117v.Pin 1 plate voltage on V1 measures 86v,but should be 103v.I replaced R14 and R15 as well as disconnected the .22 C4 cap which made no difference.Is it correct to have a 31v voltage drop due to the 100k R 14 in series with the 117v source? I am using line voltage.Thanks,
Dave
 
Just one channel has the low plate voltage on V1, correct? What are the plate, grid and cathode voltages on V2? Those 2 stages are direct connected so V2 will affect what you read on V1. R16 resistance should be 18K.

If it is original, heat it up and see if it is changing value. Use a soldering iron to heat it while measuring resistance, too.

The voltages on the power supply taps appear to be all about the same in ratio so that's likely ok at this point.

Cheers,

David
 
You must log in or register to reply here.

Similar threads

pete1729
MC-240. Seeing some squirrelly voltages on a front end tube.
Replies
5
Views
132
Brice
Brice
Pageplayer
Running Hot After Fresh Recap
Replies
12
Views
1K
Pageplayer
Pageplayer
MAXZ28
SX-1250 No Sound-No Indicator Lights-No Tuner
Replies
9
Views
2K
MAXZ28
MAXZ28
A
C22 tubes position
Replies
1
Views
2K
TSmith8605
TSmith8605
reverend13
HH Scott 299D - Power But No Audio
4 5 6
Replies
113
Views
4K
reverend13
reverend13
Back
Top Bottom