New 45 build has weird voltages - need guidance please!

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page361
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New 45 build has weird voltages - need guidance please!

Post by page361 » Sat Apr 14, 2012 10:42 am

I need some help on this one - some of the voltages seem to be pretty wierd...

When power is ON
Getting 420v DC at standby switch with rectifier installed.
V4 & V5 pin 5 reads -21VDC
Heater wires are all good - 3.19AC

When power and standby are on with preamp tubes installed I get these wacked out readings:

V1 1= 37 3= 0.3 4= 3.19AC 5= 3.19AC 6 =37 8= 0.3 9= 3.19
V2 1= 39 3= 74.3 4= 3.19AC 5= 3.19AC 6 =55 7= 39 8=39 9= 3.19
V3 1=51 3= 3.5 4= 3.19AC 5= 3.19AC 6 =52 8= 3.5 9= 3.19

Any suggestions on where the problem may be would be greatly appreciated...

Thanks!
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Re: New 45 build has wierd voltages - need guidance please!

Post by neikeel » Sat Apr 14, 2012 11:44 am

You are losing your B+ somewhere.

Either something is drawing it off (and getting very hot!) or you have a poor high resistance joint in the chain, early on by the look of it.

What voltages are you getting on the screens, PI terminals of your 8k2 dropper?

Where are you tying the HT terminals (choke, mains filter, rectifier HT) together post HT fuse?

Other than that looks really nice (I know where you got those nice 'RS' caps from too :wink: ).

What trannies did you go for in the end - I am still struggling with my RS PT from 'he who must not be named'!
Neil

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Re: New 45 build has weird voltages - need guidance please!

Post by page361 » Sat Apr 14, 2012 11:53 am

Screen grid voltage is almost non existent ... -0.7
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Re: New 45 build has weird voltages - need guidance please!

Post by page361 » Sat Apr 14, 2012 11:57 am

Yep SM does a great job on the RS caps - it's his offset type chassis and faceplate too... I used a Marstran 1202-55 PT.... RS OT from he who shall not be named :)
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Re: New 45 build has weird voltages - need guidance please!

Post by page361 » Sat Apr 14, 2012 5:30 pm

Here is a drawing of the power section - if anyone sees something out of kilter please let me know!!!! :)
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Re: New 45 build has wierd voltages - need guidance please!

Post by neikeel » Sat Apr 14, 2012 6:07 pm

That looks ok in the diagram.

So you need to check the feed from your choke to the screens (presume you measured the HT at the apex of the rectifier (pin 8) to be correct at 450v ? and after the standby switch to be the same and at the tie point of the mains filter cap etc.

Just work it through logically from where you have voltage to where you have none - does your GZ34 check out ok on another amp?
Neil

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Re: New 45 build has weird voltages - need guidance please!

Post by page361 » Sat Apr 14, 2012 7:09 pm

Thanks Neil - yes I measured 427v at both V6 and the standby switch...getting good voltage from the secondary. Ya know I was wondering myself if the GZ34 could be losing steam. I'm taking it over to a mate's house tonight and going to check...he has another 45 I built....but it works! LOL!
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Re: New 45 build has weird voltages - need guidance please!

Post by page361 » Sun Apr 15, 2012 10:33 am

OK so I replaced the rectifier tube with a new one this morning - but unfortunately still get the same low voltages.

Am still getting 420v DC at V6 and the standby switch. When Standby is off the voltage drops to 72v DC at V6, the standby switch, and choke/filters. Also V4 & V5 pin 5 still a read around -21VDC...

V1 1= 37 3= 0.3 4= 3.19AC 5= 3.19AC 6 =37 8= 0.3 9= 3.19
V2 1= 39 3= 74.3 4= 3.19AC 5= 3.19AC 6 =55 7= 39 8=39 9= 3.19
V3 1=51 3= 3.5 4= 3.19AC 5= 3.19AC 6 =52 8= 3.5 9= 3.19

So I think this rules out it being a bad tube..crap!
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Re: New 45 build has wierd voltages - need guidance please!

Post by neikeel » Sun Apr 15, 2012 10:46 am

page361 wrote:OK so I replaced the rectifier tube with a new one this morning - but unfortunately still get the same low voltages.
Yes you will do, sorry I have not been here to say that if you have 420v on the standby switch there will be nothing wrong with your rectifier 8)
page361 wrote:Am still getting 420v DC at V6 and the standby switch. When Standby is off the voltage drops to 72v DC at V6, the standby switch, and choke/filters. Also V4 & V5 pin 5 still a read around -21VDC...!
Something is dragging your voltage down. If there are no other tubes in then there is a partial short.

First discharge your filter caps and unhook your PT centre tap from the common connection on your filter can - does the voltage plummet then? If so there is a problem with your OT wiring or the OT itself. Did you get a schematic for the OT - are the primaries/centre tap wired the correct way round etc. If the voltage still bombs rehook up your OT and disconnect the choke, same again, if an issue unhook your filter cap.
Other daft thing could be an error in wiring up the bus wire under the board but you have to go through it in a logical sensible way. I would take notes on a pad as you go through your list so you do not get frustrated and go around in circles as these things are wont to do :wink:
Neil

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Re: New 45 build has weird voltages - need guidance please!

Post by page361 » Sun Apr 15, 2012 11:02 am

OK checking now...here's what I have for the CM RS OT

Primary
• 6.6k/YELLOW to V4
• 6.6k/ORANGE to V5
• Center Tap / RED – combine with choke wire and terminate to filter cap


Secondary ( 8 ohm tap)
• Yellow & Blk/wht (8 ohm) solder to POS term speaker jack
• Spare purple POS term speaker jack- terminate to 27K Feedback resistor on board
• Blue & Blk/yellow wire - (4 ohm) heat shrink at tuck under board
• White (16 ohm) heat shrink and tuck
• Green - solder to NEG term of speaker jack
• Spare Black – NEG termto ground (V4)
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Re: New 45 build has weird voltages - need guidance please!

Post by page361 » Sun Apr 15, 2012 11:53 am

OK so hopefully I did this correct Neil...I removed the center tap wire to the filter cap (as noted in drawing below) ....when I did that and turned both power and standby to ON, I was getting 420vDC as opposed to the 72vDC when the wire was attached.
Do you suggest I reverse the OT primary (orange and yellow) wires? on V5 pin 3 ?
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Re: New 45 build has weird voltages - need guidance please!

Post by page361 » Sun Apr 15, 2012 12:31 pm

Double checked bus wiring under the board - no issues I can see. I had also taken a snapshot of it before installing - unless I am absolutely blind I just can't see anything amiss...do you?? Disregard the white and yellow wire tucked in holes - did that because they were simply getting in the way flopping around :)
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Re: New 45 build has weird voltages - need guidance please!

Post by neikeel » Sun Apr 15, 2012 6:22 pm

Ok so you have pointed the finger in the direction of the OT/OT wiring 8) .

Do you have a decent meter? If you measure the resistance between the centre tap and the 6.6k wires you will probably get about 90ohms on one and 100ohms on the other? Basically half but never quite the same, the total being measured across both 6.6k equal to the sum of the halfs. Now check you have not got a high resistance to the casing of the transformer or between you primary coil and any of the secondary wires. If ou have the OT is faulty.
A good opportunity to check that the unused wires on the primary side are folded back, insulated and protected too.
If this does not show up the issue I am out of ideas and you will need to consult a proper tech. Might be worth discussing with the OT maker having all your tests thus far to hand.
Good luck!
Please feedback when you are fixed..........
Neil

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Re: New 45 build has weird voltages - need guidance please!

Post by page361 » Sun Apr 15, 2012 10:29 pm

Thanks Neil - I too am now thinking the OT is suspect...also I'm curious... didn't you mention that you too were having some challenges with your RS OT ...of the same maker?

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Re: New 45 build has weird voltages - need guidance please!

Post by neikeel » Mon Apr 16, 2012 4:25 am

page361 wrote:Thanks Neil - I too am now thinking the OT is suspect...also I'm curious... didn't you mention that you too were having some challenges with your RS OT ...of the same maker?
No - not at all. My offset has an original 60's OT I am just waiting for the batch of correct early RS type PT to be made.
Neil

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