Gertsch RT-7 pics and fuse replacement - Page 1
Forum section: electronics / testgear

author: tatel
author_rank: Frequent Contributor
timestamp: June 02, 2025, 10:52:21 am
content: Hi, I'm going to order a bunch of different 6x32, mainly slow fuses, just to have spare parts, should shit happen. It turns out, Gertsch RT-7 manual specifies 1.5A slow-blow, but says nothing about fuse voltage rating. I checked what was on the device and these fuses are rated for 125V. First time I found that.Not going to look for 125V fuses, aiming at 250V instead. So I guess these should be 750 mA, not 1.5A. Am I right? Any nuances to be taken into account?I'm attaching some pics of the device.

author: J-R
author_rank: Super Contributor
timestamp: June 02, 2025, 10:38:23 pm
content: Unlike more complex, active test equipment where the fuse is also limiting the total power depending on the supply voltage, I'd think this should only care about the current flow. So any proper 1.5A fuse rated above the voltages it will experience (for safety reasons) should be fine.However, 750mA seems like it also would be fine as long as you're not expecting to push through more than that on a regular basis. Fuses are a balance between nuisance and safety, so smaller could be better in some cases.

author: srb1954
author_rank: Super Contributor
timestamp: June 03, 2025, 07:15:52 am
content: Quote from: tatel on June 02, 2025, 10:52:21 amYou should always observe the maximum input voltage rating of 0.35f volts RMS with an absolute maximum of 350V RMS. This means for a 50Hz input you should never apply more than 17.5V RMS to the primary of an RT-7.The maximum of 350V RMS can only be achieved at 1KHz input frequency, which would require quite a sizeable amplifier to provide this level of voltage. In practice such a configuration is unlikely and applied voltages are probably going to be well under 125V and the lower voltage fuses would be fine.The manual cautions against using lower current fuses which add additional series resistance in the transformer input and output and could degrade the maximum accuracy obtainable from these devices.

author: tatel
author_rank: Frequent Contributor
timestamp: June 03, 2025, 08:10:20 am
content: Quote from: J-R on June 02, 2025, 10:38:23 pmI don't think I'd be putting even 750 mA on this, let alone 1.5A. I have tested it with DMMChek, so I guess about 1 mA. I also gave 10-15V from a variac at 50 Hz but, of course, readings were jumping all over the place. It was just the first test and I didn't have DMMCheck at hand. But manual, while saying it should be feed .35xFrequency volts and stating it will work better at 400 Hz, also states that it could take up to 350V at 1 kHZ or faster frequency. Probably I should refrain from giving more than 48V or so anyway. But, to put 125V fuses on a device while saying it could take more than double that makes me wonder what's the reasoning.QuoteGenerally speaking, I see manuals that specify fuses with half the current for double the voltage in some cases, while others specify the same for both 115 and 230V. I'm also seeing devices from the US used in Europe with the original fuses. One example: I got two non-working Tektronix 485, both have been used in Europe, one came with a 3A fast fuse (as the manual says without one word about volatage), the other came with a 1.5A fast one. Both fuses are still working. So I think I'll get fuses for half the current even if manual specifies the same current for both voltages. I guess, that way, the worst would be fuse blowing up, which I feel more comfortable with, than letting the smoke out.I can easily get metric 5x20 fuses, homologated, fast or slow, locally. Say, from Schurter AG. But it looks, to get that imperial size, it's easier to get them from Farnell or Mouser. I want to put an order for any 1-1/4x1/4" spare fuses I could need on the future, and forget about that matter for hopefully a long time.

author: bdunham7
author_rank: Super Contributor
timestamp: June 03, 2025, 08:13:24 am
content: The maintenance manual specifies a specific model of fuse and it is still available, so I'd just order exactly that.https://manuals.repeater-builder.com/te-files/MISCELLANEOUS/GERTSCH%20RadioTran%20Instruction.pdfhttps://co-en.rs-online.com/product/littelfuse/031301-5hxp/70184155/The fuses installed now may be fine, they could be marked 125VAC even though they have a 250VAC rating at much lower fault current. You wouldn't use a lower current fuse because you aren't changing or switching anything like you would with a power supply. Besides that, the 0.75A version of this fuse has a 3X lower breaking capacity and 3X higher DC resistance, both less desirable characteristics. In most use cases there should be very minimal current in the ratio transformer. The fuses are for things like the accidental application of DC, equipment malfunctions and gross user error. Plugging it into a wall socket shouldn't really be on the list of things likely to happen.Now, for today's AI humor, here's what Google's AI engine has to say about it:AI OverviewLearn moreGERTSCH ST-200B AC RATIO TRANSFORMER ERMS IN 35 f MAX PEAK 500V TESTED | eBayA Gertsch ratio transformer, or RatioTran, can be used to precisely scale DC resistance. While a transformer's fundamental function is to transfer energy between circuits via a magnetic field, and thus primarily operates with AC, Gertsch transformers are designed with toroidal cores and precise winding ratios to maintain high accuracy in voltage and current ratios, even at lower frequencies like DC.Elaboration:DC Resistance and Transformers:.While transformers typically deal with AC, the DC resistance of the windings is a critical parameter in their performance. This resistance contributes to power losses and can affect the overall accuracy of the voltage or current ratio.Gertsch Ratio Transformers and DC:.Gertsch ratio transformers, specifically designed for precision voltage and current scaling, can be used to measure DC resistance with high accuracy. They utilize toroidal cores, which help to minimize the effect of stray fields and ensure a uniform magnetic flux, leading to a more stable and predictable voltage/current ratio.DC Measurement with Ratio Trans:.The Gertsch RatioTran can be used as a voltage divider, allowing precise measurement of DC resistance within a bridge circuit. The transformer provides a stable and accurate scaling factor for the DC voltage or current, making it suitable for calibrating voltmeters and other precision measurement equipment.Accuracy Considerations:.While the Gertsch RatioTran is highly accurate, there are limitations. At very high frequencies (above 10,000 Hz), the accuracy can deteriorate due to leakage inductance and stray capacitance. Additionally, the use of different fuse types or incorrect fuse size can also impact accuracy.Advantages of Gertsch Ratio Transformers:.Gertsch transformers offer several advantages, including high input impedance, low output impedance, and mechanical ruggedness, making them reliable for use in various applications.Transformer Impedance and DC Resistance - EEVblogMay 6, 2019 — The current transformer specifies a 150 ohm primary impedance and a 150 ohm secondary impedance. I understand this is t...EEVblogGERTSCH RATIO TRANSFORMERS - Ciphers By RitterFeb 27, 2005 — The small toroids are most accurate at 400Hz, which would allow 140 volts RMS. ... DC cannot be ratioed in any transfo...Ciphers By Ritter28 PART III. ELECTRONICS COMES OF AGE 1946-1965 3.1 ...This bridge allows precise scaling of dc resistance over a wide range of resistance with the extreme accuracy and stability of a p...IET LabsShow allAI responses may include mistakes.

author: tatel
author_rank: Frequent Contributor
timestamp: June 03, 2025, 08:54:58 am
content: Quote from: srb1954 on June 03, 2025, 07:15:52 amI'm aware of that.QuoteHowever@mawyatthas posted some design that can do 450V peak?https://www.eevblog.com/forum/testgear/hv-buffer-amp-for-awg/msg4939390/#msg4939390If I'm right, that would be more than 300VRMS and, while he says it's for low frequency, I guess 1 kHz would qualify as low frequency. Don't get me wrong, I'm not going to put more than 300 VRMS on this Gertsch willy-nilly. But, since I have it, I guess it could be useful to check some AC volts ranges, so aiming in that direction, while learning at a slow pace. One of these juntek amps will be the first step. Then, at some time, I would wonder about how much of a high voltage I'm willing to deal with.QuoteThank you very much for this head-up. Thisdidgo under my radar. What a wonderful resource this forum is.Any advice against putting 1.5A, 250V, slow-blow fuses on this device?

author: J-R
author_rank: Super Contributor
timestamp: June 03, 2025, 09:07:49 am
content: I just checked my RT-5 and it has one Littelfuse 1.5A 125V (doesn't look like the photo, has the spring) and one NTE 1.5A 250V fuse. Resistance of each was approximately 0.17 Ohms.Using the original fuse or similar does sound like the best option if resistance will impact the reading.

author: tatel
author_rank: Frequent Contributor
timestamp: June 03, 2025, 10:01:45 am
content: Quote from: bdunham7 on June 03, 2025, 08:13:24 amRS doesn't like me, it seems, and I for sure don't like RS. I ordered some Cramolin cans from them and it was a nightmare. Even worse, as an organization, RS looks like brain dead to me.But I think this is the fuse you linked from RS, from Farnell/Newark instead:https://www.newark.com/littelfuse/031301-5hxp/cartridge-fuse-slow-blow-1-5a/dp/26K8487However, when I look for it on es.farnell. it gives me 0313015hxp, so 15A. Don't know whats going on.What I can get from es.farnell is this one instead (newark link, not es.farnell)https://www.newark.com/littelfuse/031301-5mxp/cartridge-fuse-slow-blow-1-5a/dp/42K9673Seems to be exactly the same ratings. Don't know about the difference from HXP and MXP

author: tatel
author_rank: Frequent Contributor
timestamp: June 03, 2025, 10:04:10 am
content: Quote from: tatel on June 03, 2025, 10:01:45 amIt seems, quantity code M=1000