Rohde & Schwarz 400 to 800 MHz 2 kW TV broadcast amplifiers - Page 1
Forum section: electronics / testgear

author: alex.forencich
author_rank: Frequent Contributor
timestamp: September 04, 2014, 12:53:50 am
content: A friend of mine is working with an electronics recycler to get rid of a whole bunch of retired Rohde and Schwarz TV broadcast amplifiers (VH602A2). Apparently they have around 600 of these things that came out of over 100 complete broadcast systems.I'm not sure if they are all the same part number, but they cover the 400 to 800 MHz range (approximately) and they can source up to around 2 kW. They are designed to be water cooled and to run off of a 408 volt 3 phase supply, about 8 amps. The guys at R&S say they should be able to run off of 220. Each amp has 16 BLF861A transistors for the main output stage, two more for the output stage driver, and then a preamp with four more assorted amplifiers. The preamp alone can source around 20 watts or so. All of the BLF861A transistors are soldered on to four thin circuit boards that sit on a large aluminum water block. The output and driver stage are built from amplifiers consisting of two BLF861A connected with baluns and hybrid couplers. The output stage consists of 8 of these modules interconnected with an 8 way splitter and combiner tree. The output is a giganic coaxial connector on the back. A gigantic switching power supply that provides 32 volts at around 165 amps is bolted to the other side of the block along with a blower for cooling everything that isn't directly connected to the block. The preamp also contains circuitry to monitor the power supply, blower, temperature, amplifier state, and input signal level, disabling itself if an issue is detected. The preamp also has a varactor diode phase shifter.These amplifiers are a bit of a pain to disassemble, so the recycler is interested in possibly reusing them for something else if it means they are worth more than scrap value and/or don't have to be disassembled. They could make excellent 70cm band transmitters.Anyone have any ideas on what these are worth/could be used for? Or perhaps interested in one personally? It would be sad if all of these had to be scrapped.Edit: apparently the company has decided to try listing them on ebay here:http://www.ebay.com/itm/Rohde-Schwarz-Verstarker-Power-Amplifier-VH602A2-Liquid-Cooled-2082-9000-02-/321511774684?pt=LH_DefaultDomain_0&hash=item4adb9859dc. I believe the reserve price is set at 1k.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: September 04, 2014, 12:56:55 am
content: Here are some plots of the frequency response. Gain of the preamp is about 50 dB and gain of the driver stage (drives the power stage) is about 16 dB.

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 04, 2014, 01:04:09 am
content: Are they capable of digital (ATSC)? Perhaps that is why they are being scrapped instead of being re-used with new, digital exciters.I believe there may be a market even for analog-only equipment in Central and South America (and possibly other geographies?) where they haven't converted to digital yet.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: September 04, 2014, 01:06:15 am
content: I have not gotten the full story on why they are being scrapped. I think they are capable of digital transmission, but the digital transmit power is spec'd to be much lower than the analog transmit power, likely because of the linearity requirement.

author: German_EE
author_rank: Super Contributor
timestamp: September 04, 2014, 08:18:10 am
content: "A gigantic switching power supply that provides 32 volts at around 165 amps"Be still my beating heartThe HF Power Amplifier I have runs at 48V 20A and I had a lot of difficulty finding something suitable. Thirty two volts is a little on the low side but I'm sure that you could make a lot of radio hams very happy if you pushed this stuff in their direction.One thing though, those power supplies will take 5,3Kw from the mains supply on full load (and that is at 100% efficiency). That's 24A at 220V on a single phase supply so three-phase will probably be best.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: September 04, 2014, 08:34:17 am
content: The back panel says 5.7 kVA, 3 phase 400 volt +/- 15%, 47-63 Hz. Yeah, I think 3 phase is the way to go, but not everyone has access to three phase power. Since it's a switching supply, there is little point in building a phase converter if it will accept single phase, albeit at a much higher current.This ebay listing has pictures of the outside of an identical unit:http://www.ebay.com/itm/ROHDE-SCHWARZ-BD-IV-V-VH602-A2-2082-9000-2-POWER-AMPLIFIER-MFG-RFB-NO-D-O-A-/190631126748Looking at the back of the unit, you can see the two water cooling connections. These go straight into an aluminum water block. Everything above the water block is the actual amplifier, everything below is the power supply.I will see if I can take some decent pictures of the insides with the covers off.

author: German_EE
author_rank: Super Contributor
timestamp: September 04, 2014, 03:08:38 pm
content: OK, I guessed at 5,3 Kw and you say 5,7 KVA, close enough. Either way we won't be running these from a standard power plug but at least we could heat the apartment during the Winter.

author: Mr Simpleton
author_rank: Unknown
timestamp: September 04, 2014, 06:38:20 pm
content: Nice, some Earth-Moon-Earth capability on 432 MHz!Pity I do not operate on that band...

author: alex.forencich
author_rank: Frequent Contributor
timestamp: September 04, 2014, 10:02:34 pm
content: Picture time! Sorry for the high compression on these; I wanted to make them as large as possible (pixel wise) so you can zoom in, but the forum limits me to 1 MB per pic.These two are the top and bottom of the amp with the main case and preamp cover removed.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: September 04, 2014, 10:03:44 pm
content: Closeup of preamp and power stage driver.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: September 04, 2014, 10:12:40 pm
content: Closeup of power amp stage. The first is from one of the complete amps, the second is from the one that the recycler disasembled.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: September 04, 2014, 10:33:09 pm
content: Alright, signal path discussion.The input comes in the back through some sort of a coaxial quick connect connector. I don't recognize what it is. Anyway, that signal is passed through a hole in the waterblock to the front of the amp where it enters the preamp. This is the SMA connector, roughly in the middle on the left in the first photo. Inside the preamp, it passes through an attenuator, then a diode switch, then a Mini Circuits ERA-5SM+ amplifier. From there it enters one port of a hybrid coupler (large silver area with the 'fingers'). The two ports on the opposite side of the coupler are connected to what I believe are varactor diodes. The idea is that the signal enters the hybrid coupler, passes throuh one of the ports on the other side with no phase shift and the other with a 90 degree phase shift, reflects off of the varactor diodes, picking up a voltage variable phase shift, then passes back through the hybrid coupler, coming out of the other port (due to the 90 degree phase offset). A trimmer on the front adjusts the voltage on the varactor diodes and, hence, the phase shift. After the phase shifter, the signal passes through two mini-circuits baluns (ADTL1-12) and another mini circuits amplifier (HELA-10). From there, it passes through a matching network, a BLF1043 amplifier, another matching network, and finally a UGF09030 amplifier. The final amplifier is rated to 30W and 1 GHz.From there, the signal is sent through an SMA to SMP cable that connects to a pair of BLF861A transistors that drive the power stage. Each one of these packages contains two transistors, so there are four in total. The signal from the preamp is connected to the power transistors through a hybrid coupler and pair of baluns. Then the BLF861A transistors each amplify 1/4 of the signal, and the pieces are put back together with two more baluns and another hybrid coupler. The driver can probably source about 200W.The output of the driver is then connected to the main power stage. The power stage consists of eight pairs of BLF861A in the same configuration as the driver, but possibly with slightly adjusted biasing. Seven couplers split the signal into eight pieces which are fed in parallel into the power stages. Then the outputs of these are combined with directional couplers and sent to the gigantic coax plug in the back. The circuitry next to this connector contains more directional couplers to pick off the forward and reverse power for monitoring.All of the KDI devices on the board are loads that terminate all of the extra ports on the various couplers as well as the reverse paths on the directional couplers. These loads will absorb any RF energy that might be reflected due to mismatches.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: September 05, 2014, 02:11:20 am
content: So is that ebay link the one for sale , the ones your talking about , or an example of some like ?

author: alex.forencich
author_rank: Frequent Contributor
timestamp: September 05, 2014, 02:17:56 am
content: Quote from: KD0CAC John on September 05, 2014, 02:11:20 amThat's somebody else. They are the same amps, though (same part number). Those units are replacement units in road cases. These units are installed in complete transmission systems (loaded into racks with controllers, power combiners, piping, etc.).

author: alex.forencich
author_rank: Frequent Contributor
timestamp: September 05, 2014, 02:58:44 am
content: Looks like the circumstances have changed somewhat: the company has decided to list them on ebay as a trial run to see if there is much interest. Listing is here:http://www.ebay.com/itm/Rohde-Schwarz-Verstarker-Power-Amplifier-VH602A2-Liquid-Cooled-2082-9000-02-/321511774684?pt=LH_DefaultDomain_0&hash=item4adb9859dcI believe they have the reserve set at 1k.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 05, 2014, 03:12:27 am
content: Quote from: German_EE on September 04, 2014, 08:18:10 amMeanwell do 48V supplies------pretty certain they can do 20A,as their 24V ones will do 60A!!

author: German_EE
author_rank: Super Contributor
timestamp: September 05, 2014, 09:59:26 am
content: Meanwell did make the power supply I needed but the cost was amazingly high. I ended up purchasing three 48V 7A units and running them in parallel and for some reason this worked out cheaper.