Russian LOMO`s - any experience?

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mikeyB

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Apr 12, 2005
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Location
Manchester UK
Any experience?
Me matey has been having an ebay buying frenzy! - managed to get 2 pristine units along with 2 dodgy(tell you later) units and one that looks older than my grandma! - who`s been dead for a long while!
Dodgy units had psu`s supplied! - oh my !- did i learn fast! - no earthing and no ht discharge - i`m just getting over the (actual) shock and my fear of working on them !!!

Dodgy units - supplier went to a lot of trouble removing the input transformers from the cans and re-assembling - wired the empty cans unbalanced - 1 unit with caps to grid, the other direct to grid! Outputs on both dodgy units had 470k pot strapped to earth - wiper out.
Earthed the supply and added bleeder resistors to the ht rail (345V ht!). Noticed on output that many 10`s of volts before heaters warm up and ht current is drawn! Thinking of using Carnhill 9600:600 output tx to isolate this voltage - any thought`s? There is about 40db of gain when using a SM57 (unbalananced) mic.

"Grandma`s" unit and the pristine units have the input transformers intact - they roughly have input dc res of 27R CT and 1k2 out res to the valve stage - ratio measures 1CT:14.5 - i`m guessing that`s well over 20dB of gain! I`m guessing low in z for ribbon mic`s.

Any guru advice as to what the pot network around the first valve is? - Matey reckons it,s a high freq rolloff - but i`m a deaf old drummer so i don`t hear it!?!

Looking at lifting the CT ground so as to phantom power for condensers and probably 20db pad(based on 200R impedance)

Hopefully looking to bypass the mixer pentode(looks like there is plenty of gain) and replace with duplicate output stage(is this a withe cathode follower?) for 2nd input - thus ending up with a 2 channel per rack system.

FOOTNOTE - this rather large output voltage is a bit of a worry - do all our valve projects need a delayed or slow start HT supply at turn on?
 

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Wow, I know what unit you speak of, but I doubt most people will.  There are a lot of different possible preamps one could mean when only saying Lomo.  The additional pots are simple equalizers. 

I had some of these at one point; never fired them up.  Considered the addition of second output stage, but decided it was just too much work.
If you build a second output stage, it'll surely sound different from the first.

Had I proceeded, I would have probably converted them to single channel units and used the other input transformer elsewhere. 

If you have 2 complete units, maybe make that 2 channels, and build 2 channels of something else.

Output transformer sounds right.  Not knowing what output load you had on, it's hard to comment on the output voltage.  It's basic cap leakage, maybe best to replace the output caps.  They might take out an output transformer if it's really bad. 
 
Thanks for the reply Doug - how do you test PIO caps?-(do you just apply dc one side and see the resulting voltage on the other terminal?)
I'll post some pics when i can get my hands on a camera.

Mike
 
measure the capacitance, and then fire up and see what DC on isolated side measures with a 10K load resistance to ground across output.  Normal to see initial higher value, but should drop quickly and stay near zero.  If not, replace. 
 
Looks to me like the pots round the first stage pentodes (EF86) control (mic) gain. Other pots are level. Output consists of another EF86 type plus White Follower with overall NFB acting as a virtual earth mixer. Quite a neat design overall.

Cheers

Ian
 
Thanks again Doug for the reply and thanks Ian. Any news/links to your friend, Leadbreath?
Doug - you mention tying the output side of the cap to ground with 10k - would this be a good value for a pot if running unbalanced - the racked units have a 470k pot divider for output - which could explain the higher  initial output DC that lasts for approx 15 secs before the valves warm up. You also mentioned The carnhill 9600:600 (4:1) may be a good choice ont he output- Colin (the fine) Slenderchap (Audiomaintenance.com) supplied both a gapped and ungapped version for me to experiment with - many thanks to Colin - maybe the gapped can handle the DC if the output cap fails?

i,ll try to post some pics of the units.
The newer units look good regarding layout (a few external jumpers around the valve pcbs look to be a bit risky to the unwary!)

Ian  -  i see what you mean with the loop around the 1st pentode - but turning the pot from one end to the other doesn`t seem to affect gain  - unless it`s a fine trim like on the redd47.

I`ll repost the schem with component values next - i`ll try to measure the gain  overall compared to the 1st stage tap above the level pot - will find out if there`s any gain in the mixer pentode stage - will the cathode output stage be around unity?

Mike
 
A Russian friend had translated the pot markings for me once, and I forget the exact wording, but I think it was high freq adjust.  EQ at any rate.  If I remember to dig it up, I'll clarify. 
 
mikeyB said:
Ian  -  i see what you mean with the loop around the 1st pentode - but turning the pot from one end to the other doesn`t seem to affect gain  - unless it`s a fine trim like on the redd47.

Looking at the circuit again I thiunk you are right - looks more like EQ than gain -depends on the component values really.

Cheers

Ian
 
Yeah, I remember C1 being pretty small.  I drew this one out by hand, the schematic confirms what I found other than the parts.  It's been awhile.  Will try to post my drawing with parts values. 
 
The 220K pot and 18nF turnover at 40Hz and the 6.2K and 7nF turn over near 4KHz. So it look looks like the 220K +18nF  reduces NFB below 40Hz perhaps compensating for low frequency roll of in the transformer and the 6.2K and 7nF give a shelving HF boost starting around 4KHz.

Be interesting to see what is measures.

Cheers

Ian
 
Thanks again Ian
Finished checking the circuit values annotated onto schematic(see original post)and some pics.....

nice lomo
 

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.... and great grandma's lomo....
Computer about to run out of battery and i haven't adaptor to hand - i will continue this later.
 

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Thanks Leadbreath - look forward to any info available.
Just noticed your info Doug - ties in pretty much with what I got - thanks for that.

Here's a schematic for the psu from the guy that supplied the dodgy pair of lomo's. Scary pics in later posts.
The transformer seems incredibly chunky! Primary res of 27R! Lots of windings(spare ones too - enough for 48 volts and relay control/lamps etc). HT choke rated at 80mA - hoping there's enough ht for 2 racks! For heaters - hoping there's enough voltage to split the windings and supply 2 heater supplies - using LDO regulators.
Voltages - on and off load along with winding resistances - heater windings read very low.
 

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