Author Topic: 1970 1200s restoration  (Read 196 times)

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Offline Tdlunsfo

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1970 1200s restoration
« on: April 20, 2017, 05:24:46 am »
I recently acquired a 1970 1200s for a song and trying to restore it. Here is where I am at currently.

Fuse holder replaced
Power cord replaced and grounded to chassis
Power supply cap can replaced.
50uf caps replaced
1000uf cap replaced.
2w resistors replaced on cap can
1w resistors replaced in bias circuit
Removed diode recitifier and wired up second 5AR4 socket. (This is the model with two sockets but sold with the diode)

Fired up the amp and it sounds nice and loud. The problems start with the hum and a weak tremolo. I checked the riveted grounds as they are known to be loose but they are tight. Will try some solder on them later today. My tremolo has the Sigma Datacell LDR module. I can hear it oscillate but its weak. I noticed mine is the -12 model LDR, but the schematic denotes 6 volts across the light. Should mine be the -6 model that I see being sold as NOS? All the old caps in the trem test ok as well as the transistors.

So I still need to check bias and reverb but wanted to get your opinions on the hum and tremolo first. Also, would like to know which LDR you guys have in your 1200s amps as information on this is limited.

Cheers!
DL

Offline Tdlunsfo

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Re: 1970 1200s restoration
« Reply #1 on: April 20, 2017, 06:20:54 am »
Forgot to mention that the death cap has been removed and the polarity switch bypassed. I did pull some tubes a minute ago and the PI tube had an effect, the others had no affect. PI tube tests good. Also tried grounding rivets here and there with no effect. I did remove and leave out the big 20uf orange Mallory caps. These appeared to be added but I cant verify it. The are not on my version of the schematic...but then again the cap doesn't match the schematic 100% either.

Offline _peter

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Re: 1970 1200s restoration
« Reply #2 on: April 20, 2017, 09:05:31 am »
Hi,

you aren't talking about the 600V 20 filter caps, right?

Cheers,
Peter

Offline Tdlunsfo

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Re: 1970 1200s restoration
« Reply #3 on: April 20, 2017, 09:13:32 am »
Yes and I think so or at least one of them. The other appeared to be in the bias circuit. Can't find them on the schematic though. The filter cap was a 30,20,20,20 model but schematic shows a 40,30,20,20.

Offline _peter

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Re: 1970 1200s restoration
« Reply #4 on: April 20, 2017, 10:49:33 am »
But those 20 @ 600V are the first two filter caps. You ripped them out and wonder why it's humming?

If I got you right, you have a huge ripple on the supply voltages now. That has less effect on the power amp,
as it works in push pull mode, but the ripple will modulate the preamp signal.

That's why the hum stops when you pull the PI.

I'll attach the schematic, so we know we are talking about the same thing. I guess you mean C29 and C30.

Cheers,
Peter

Offline Tdlunsfo

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Re: 1970 1200s restoration
« Reply #5 on: April 20, 2017, 12:53:53 pm »
You know what... your right. I have miss-counted the number of capacitors and didn't take into account that one of the 20uf on the cap can is being unused. I guess from working on these old tube radios, I was used to seeing these added after the fact and assumed the cap can took care of all the filtering. Thanks for helping me see the forest from the trees. This might even fix my tremolo issue as well. Will post back.

Offline Tdlunsfo

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Re: 1970 1200s restoration
« Reply #6 on: April 20, 2017, 06:27:39 pm »
Wired back in the old caps and then blew a fuse. Change fuse, fire it back up and notice a redplating 6550..uh oh! Quick check on the bias and I am somewhere in the -30v range instead of -55v. Re-biased the amp and the hum and slowly decreased. She sounds great. Tremolo is still a little weak but reverb sings. Amp lacks bass. Could be that the speaker is sitting out of the cab. Overall a very clean sounding amp. Threw a Big Muff Pi on it and she growled! This amp is a beast. If you can't cut through the mix with this thing then I don't know what you need. Lesson learned on the bias and swapping over from diode rectifier back to tubes.

Offline _peter

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Re: 1970 1200s restoration
« Reply #7 on: April 20, 2017, 10:58:43 pm »
Hi,

good to hear the amp is working again. Nevertheless, I would put in new electrolytic caps.
The  20 @ 600V might be hard to find. Here in Europe I got lucky on this weird page:
https://www.die-wuestens.de/eindex.htm
They have F&Ts with a 550/600V rating.

Cheers,
Peter

Offline Tdlunsfo

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Re: 1970 1200s restoration
« Reply #8 on: April 21, 2017, 05:10:08 am »
They are available but man they are expensive. $25 a piece. The ESR was pretty low on these at .64, almost half of my brand new can cap. I feel good leaving them in for now. Planning to dig into the trem today. Already suspecting this 22v zener and the 1 and 2uf caps. Not sure what's up with the bass. Could be tube related.