I was just looking at my tube of lm13700’s from that seller last night and thinking that they have all the hallmarks of being genuine and that I should really make some VCA’s and filters with them! I think I did test them when I got them over a year ago and they tested good.
For reference I am about 90% sure the tl072 and tl074’s from the same seller are fake (lm324’s or whatever dirt cheap Opamp) but I haven’t done the through zero or slew tests to confirm (they still go boing in a bridge-T circuit) And the NE555’s are most definitely not genuine, package quality is sub-par, but I was expecting that for the price, they are most likely inferior clones.
2 Likes
A while ago I got shoddy 555s and just for a laugh I piggy backed 2 and got a result. This was for robots not synths.
Hi all.
Bit of a newbie here.
I’ve been trying to build the LMNC filter using the LM13700N I got from eBay.
5 of them have blown when powering the circuit.
I’m convinced my circuit is good.
I’m thinking I may have bought counterfeit chips.
The eBay seller had lots of complaints! Lol.
Is the LM13700 different to the LM13700N which I bought?
Would these be ok?
UK seller
I think that if you’re getting them from a parts supplier they’re generally more trustworthy than if you get them from ebay.
That said, be careful about the current that you’re feeding into the Iabc inputs especially. I’ve blown a few that way.
1 Like
Thanks.
What would I need to be aware about regarding current.
Would that be my power supply?
When powering up a circuit for the first time it is wise to use a power supply which allows you to turn up the voltage from 0V slowly and keep an eye on the current the circuit draws. If the current rises quickly to a value you did not expect, then turn the voltage down and check the circuit. Powering up at full voltage would in that case maybe have blown something. This is something I always do and in this way I can detect problems easily. If you do not own such a powersupply then you may want to consider buying one, or building one yourself.
I made a simple one using an old laptop power supply and connected a DC to DC converter to it which allows me to set the output voltage. There are plenty of converters on aliexpress and they are cheap.
4 Likes
Great advice. I have a bench power supply but it does not have negative voltage.
Then you may want to consider getting another one …
Or with the one you have you could test the positive bit of the circuit, then the negative bit and if you do not find a short or excessive current being drawn, only then connect it to your rack’s power supply and do a full functional test.
1 Like
Thank you. I’ll have a go.
Here’s the schematic by Rene Schmitz that the LMNC Performance Filter is based on. (Not sure if that’s the filter you’re building though, but if not, similar ideas apply.)
The control current is supplied by one of the transistors via two 10k resistors, one on each side. For the LM13700 (used instead of the even more obsolete CA3080) these resistors connect to the control current inputs, pins 1 and 16. In the worst case scenario the transistor output is 12 V. To avoid blowing up the LM13700 the control current must be below 2 mA. By Ohm’s Law the worst case is I = 12 V / 10k = 1.2 mA. So as long as those two resistors are really 10k, and there isn’t some short or other such problem, and there isn’t a 20 V voltage around that can somehow get onto those resistors, a genuine LM13700 will be safe.
There’s some discussion of this stuff here:
https://electricdruid.net/design-a-eurorack-vintage-vca-with-the-lm13700/
which is about designing a VCA, not a VCF, but as far as control current goes it’s a similar thing.
The letter suffix just indicates the package: M or MX for surface mount (SOIC), N for through hole (PDIP). So what you have is what you want — or at least it’s marked as what you want.
Unfortunately there are few reliable (non eBay, non AliExpress, etc.) mail suppliers, if any, that still sell the through hole LM13700. Pretty much any mail vendor that claims to have them might be selling legit new old stock, maybe used chips, maybe no-name copies, maybe downright counterfeits, and it’s near impossible to know until you buy some and test them. At this point buying the SOIC version and an adapter board as described above is probably your best bet, unless you’re fortunate enough to have a reliable local electronics parts store that maybe still has a box of them.
2 Likes
You can still get them from Tayda though they are pricey.
I had luck off of Amazon at the end of 2021 from:
https://www.amazon.com/gp/product/B08FLHNCCC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
I built the triangle/square vco from the data sheet to test them.
2 Likes
Right, I was misremembering about Tayda, thought they’d delisted them.
The Amazon listing is… interesting; from the photo it appears they’re not pretending they’re TI chips. No geographically dubious map of Texas, just numbers.
1 Like
This just went by on the Synth DIY mailing list:
2 Likes
If anyone buys and tests some of these I’d be interested in hearing their report.
Wow, that still exists!?
Yup, and full of people reminiscing about the good old days. From the same LM13700 thread:
My first ever transistors, a pair of 2G371, were 5 shillings each in 1965
Cheers
I subscribed for a while about 30 years ago. (If it’s the same mailing list.)
A while ago I reported problems in building Haillant’s VCA which features an LM13700. The IC got very hot and emitted a burnt smell, destroying a couple of capacitors in the process.
There was a discussion about whether there was something wrong with the build or whether the IC was the problem. As I could not find any shorts or possible mistakes (as the very helpful people in this forum suggested) and the build was done on a PCB by a reputable designer, I gravitated towards concluding that the IC was to blame.
Yesterday, I finally managed to put together an LM13700 tester circuit setup and tested the batch of ICs I got from ebay, against an IC that I bought from a reputable physical shop (many thanks to @SCGator for the pictures of his tester circuit and the resistor/capacitor values to build the blinker VCO).
The IC from the shop works as intended, while each of the ICs in the batch bought from ebay got hot, too hot to touch, within a few seconds (and did not produce the expected output). For what’s worth, here’s a picture with the working IC on the right, and one of the fake (?) ICs on the left.
To give a little more context, I looked up my files and found that I paid €3.57 for a single LM13700 in the shop, and $3.02 for 5 from ebay, so if the price looks a little too good to be true, then it probably is.
6 Likes
Buying cheap ICs from eBay is an expensive choice when they turn out to be worthless…
Consider also the cost of the time it has taken you, and possible damage to other components.
I find taydaelectronics.com pretty good, they sell 13700s for $4.
2 Likes
If they are faulty, just mark them as not as described and you get a refund.
1 Like